Coilover Spring Rates for Toyota Tacoma & 4Runner

Does my Toyota Tacoma or 4Runner need 700lb springs?

We often get asked about whether or not someone needs 700lb springs for the front of their Tacoma or 4Runner. Usually this question is asked because the customer has added more weight to the vehicle or heard that a 700lb spring is needed. Every vehicle is different, and everyone has their own driving style. 700lb spring may work for one person, and be horrible for the next. For this article we will reference Fox and King OE fit shocks only.

Why do people recommend heavier springs?

More Weight
The best reason for recommending heavier rate front coilover springs is because the vehicle is substantially heavier than stock and heavier rates are necessary to prevent coil bind and to achieve the desired lift height.


Reduce Preload
Some people associate preload (loosely related to threads showing above spring nut) with a harsh ride because they are “cranking down” the spring. Increasing preload only raises ride height, it does not compress the spring more, and it does not make the ride rougher. The only time too much preload causes a rough ride is if you lift it too high and don’t have any down travel, or if the spring runs out of travel before the suspension bottoms out.

Compensating for Soft Valving
Fox, King and other OEM Tacoma/4Runner front coilovers come valved too soft from the factory. They can bottom out on small drop offs and bounce up and down on freeway rolling bumps. Some people add a stiffer spring to compensate for this feeling, and have some success, but cause other issues in the process. The proper solution is to tune the coilovers for the weight of the vehicle.


More Lift Height?
Its very common to think a heavier rate coilover spring will lift your truck more than the standard spring it comes with. Thats not entirely accurate. Ride height is achieved by adding preload, not swapping to a heavier rate spring. Before you swap to a heavier rate spring, make sure its the correct spring rate for your trucks total weight. Often times, you will just need to add more preload to get the desired ride height.

Why do we recommend soft springs on Tacoma’s?

Vehicles with soft springs tend to ride like Cadillac’s while vehicles with heavy springs tend to ride more like dump trucks. This is because softer springs compress from ride height more easily and force the tire to extend down from ride height more quickly to follow the terrain (see preload article for more). The result is that softer springs ride nicer.

Heavier springs are harder to compress and transmit more vibration into the chassis. Tacoma’s and 4Runners with heavier than necessary springs won’t ride as smooth on the road and will get more feedback on little bumps. On large bumps the heavy springs can be too stiff and feel jolting.


How soft can we go?

The next logical question is if softer springs are better, how soft can I go? In the perfect world I would like to see spring rates in the 450 to 550 lb/in range however that would require more preload and longer springs than can fit in the space provided by Toyota. The first limitation is the overall length of the spring, 14” long springs are the longest reasonable spring to install on OEM fit mid travel Tacoma & 4Runner coilovers. Next we need to consider how much travel the spring has relative to the travel of the shock and how much preload* is required to achieve the desired ride height. For the most 4Runner’s and Tacoma’s 600, 650, and 700 lb/in springs are the only 14” long springs that satisfy those requirements. Despite our desire to run softer springs it turns out the 600 lb/in springs are the softest that can fit and function, which is probably why that is the standard spring rate on most aftermarket 2.5 coilovers.

*Preload = (spring free length) – (spring length when shock is fully extended)


Why tune your shocks instead of installing heavy springs?

Shocks are velocity sensitive and are substantially more tuneable than springs, especially when using AccuTune Double Flutter Stacks. Springs only react to the frequency of repeating bumps or the size of large bumps. Shock valving when used with double flutter stacks can respond differently to small bumps, big bumps, slow rolling bumps, and large fast bumps. Fixing soft shock valving should be done internally where the root of the problem lies.


Spring Rate Recommendations for 2-2.5” lift:

600lb springs for Toyota Tacoma or 4Runner

13” x 600 lb/in Spring:
Standard on:

-2.5 Fox Coilovers – Tacoma

-Mods: sliders, skid plates

-Min installed length: 11” 1
(max preload = 2″)

14″ x 550lb Spring:
Standard on:

-2.5 King Coilovers for 03+ 4Runners

-Mods: sliders, skid plates

14” x 600 lb/in Spring:
Standard on:

-2.5 Fox Coilovers – 4Runner

-2.5 King Coilovers – Tacoma 

-Mods: sliders, skid plates, light bumper, & light winch

-Min installed length: 11-3/8” 1
(max preload = 2 5/8″)


650lb springs for Toyota Tacoma or 4Runner

14” x 650 lb/in Spring:
Optional on:

-All Fox & King

-Mods: sliders, skid plates, steel bumper, winch, dual batteries

-Min installed length:  11-3/4” 1
(max preload = 2 1/4″)

700lb springs for Toyota Tacoma or 4Runner

14” x 700 lb/in Spring:
Optional on:

-All Fox & King

-Mods: sliders, skid plates, steel bumper, winch, dual batteries, bed rack, roof top tent, additional armor

-Min installed length: 11-3/4” 1
(max preload = 2 1/4″)



1:         Min installed length:  Minimum length the spring can be with the shock fully extended before risk of coil bind

13” spring data not available, extrapolated between 12” & 14”, use at own risk

All data based on Fox shock and Eibach spring data, use at own risk



Heavy rate 650 and 700 lb/in springs are necessary to achieve 2+” of lift on heavily loaded Tacoma’s and 4Runner’s but can cause undesirable ride qualities on vehicles near stock weight. Bounciness and undulating motion should be fixed by re-valving. For most Toyota Tacoma and 4Runner owners, a 600lb front spring will be the correct rate to go with. Contact AccuTune Off-Road for help selecting your spring rates and for more detailed rate calculations.

What Size Tacoma/4Runner Shocks Do You Need?

When it comes to performance off-road shocks for Tacoma’s and 4Runners it can be difficult to choose which shocks are right for your Toyota and your type of off-roading. In the past you had to choose which shock would give you the ride quality you wanted, and you’d put up with the quality (or vise versa). But with the advent of AccuTune Equations, you can now get any shock custom tuned for your Toyota, your weight, your terrain, and your preferences.  So now you can choose which brand, size, and configuration you want, and still get the ride quality you’re looking for. With that in mind there are now three things you want to consider when choosing shocks: ability to make proper damping force, heat capacity, and durability.


Why Does Damping Force Matter on Tacoma Shocks?

A Brief Background On Shocks:

Damping force is the amount of force produced by a shock at a given shaft velocity. Shocks make damping force when oil flows through ports in a piston and is restricted by round disks that act as springs. When shock diameter increases so does the amount of oil that has to flow through the piston, so naturally the damping forces increase. As a result there are ranges of damping where each size shock is most adequately sized.

Tacoma Shock Size Piston Comparison

Damping Force & Front IFS Coilovers:

The front coilovers on Tacoma’s and 4Runners are mounted about half way up the a-arm, which means the shocks only move half the speed of the wheel, and half the speed means half the force. Being mounted half way up the lower control arm also means that only half the damping force created goes to the wheel.  So on the front of Toyota 4×4’s the front shocks only deliver 25% as much damping force to the wheel as if the shock had been mounted there. This means that front Tacoma and 4Runner shocks need to make approximately four times more damping than the rear shocks. Four times more damping force means that 2.0 shocks really aren’t big enough.

For the purpose of completeness, there are tricks that can be done to valving to make shocks stiffer than traditional methods allow, and there are ways to design shocks which make them naturally stiffer. But aftermarket 2.0 shocks are ultimately not designed to make the forces required for the front of a Tacoma or 4Runner. Yes, we can make it work, but the when shocks are made to be stiffer than they naturally want to be it can lead to harshness are reduced life span. For this reason many undersized shocks are very soft and give you a bouncy, Cadillac like ride.




Serious off-road driving requires serious damping forces, shocks that are too soft will bottom out frequently and will be bouncy off-road.  In the front 2.0 shocks are adequate for stock weight Toyotas doing on-road driving and slow speed off-roading such as hunting, fishing, and slower speed trail riding.  2.5 shocks are necessary for heavier Toyotas, and for high speed such as fire roads and desert running. In the rear 2.0 shocks provide enough damping force for stock weight Tacoma’s and 4Runner’s doing everything from on-road driving up to pre-running. With the addition of full overland gear such as skids, bumpers, and RTT it is necessary to step up to 2.5 shocks in the rear as well.


What is Heat Capacity and When Does it Matter?

Heat capacity is the ability for a shock to withstand rigorous off-road driving without fading, leaking, or substantially shortening their life span. When looking at heat capacity two things must be considered: amount of oil and speed heat dissipation.

The quantity of oil inside a shock relates to how much heat it can absorb, and heat dissipation is how quickly it can rid its self of heat. Larger diameter shocks hold more oil and have more surface area so it’s no secret they have better heat capacity. But Fox and Icon do make Aluminum Bodies 2.0’s with small 5/8″ diameter piston rods, which help them hold more oil, and dissipate heat more quickly.

When is Heat Capacity a Factor?

Heat capacity becomes a factor when running rough roads at speed for long periods of time. As an example washboard road causes extremely fast movement of the shocks which generate a lot of heat. On the other end of the size spectrum, large whoops also generate a lot of heat when run at speed because the shock is using its full stroke very rapidly. Any terrain which causes rapid wheel movement will be demanding on the shocks.


Tacoma’s and 4Runners work well with 2.0 shocks for slow speed off-roading and about 5 minutes of hard run time on demanding terrain. The five minute run time is a rule of thumb and assumes the terrain is constantly rough and the speed is constantly high. With the increased oil capacity 2.5’s are good for about 15 minutes of hard run time before they start fading and life span gets shortened. Adding weight further reduces run time because it requires more damping which creates more heat. If you’re a light wheeler out exploring the backwoods with your 4Runner 2.0’s will handle the heat. If you’re blasting the dunes for an afternoon in your Tacoma then you should step up to 2.5’s for their ability to handle the heat.


What Influences Durability?

Shock durability is dictated by the size of the shock, quality of material, and reservoir configuration.

Size Matters For Hard Use:

As discussed previously, larger shocks naturally make more force. Part of this is because damping is directly related to the cross sectional area, so larger diameter shocks operate at lower pressures. Lower pressures are easier on seals and increase the shocks life cycle. In addition larger shocks are less likely to reach temperatures past their designed operating range.  Running a shock over temp doesn’t always make them fail right away, but it does harden the rubber seals and reduces the lubricity of oil, which leads to a shorter life span.

Quality of Materials:

We aren’t going to dive into the specifics of each brand here, but there are differences between the quality of materials used in each brand.

See our Fox vs King: 2.0 Coilover Shock Comparison article for more info.

Remote Reservoirs – Not Just About Cooling:

While it’s true that remote reservoirs do increase oil volume and thereby increase cooling and heat capacity, that’s not their primary purpose. The primary purpose of remote reservoirs is to reduce the shocks operating pressure. As the shaft goes into the shock body the volume is displaced and the nitrogen gas charge increases in pressure. Higher pressures drastically increase the friction and wear on seals, as well as reducing the ride quality. (note that nitrogen pressure is not a tuneable element of the shock, do not change from the manufacturers recommended pressure).

In an IFP shock there is a single cylinder containing the piston rod, oil, nitrogen, and Internal Floating Piston (IFP – which separates the oil & nitrogen). All of these items are in line which doesn’t leave much room for nitrogen, meaning the original 200 psi charge may exceed 600 psi when fully compressed. The 600 psi is actually not too bad, but once the shock starts heating up the oil expands and the pressures can sky rocket. The inline construction of IFP shocks is often done on an assembly machine which also seals the shocks, making them a sealed unit which can’t be rebuilt.

Remote reservoir shocks have a separate cylinder just for holding nitrogen, which has substantially more volume. As a result the pressure is very consistent, even when the shocks get hot. The remote reservoir design requires separate nitrogen charging, making them rebuildable.

There is a whole article coming about this topic, so stay tuned.



**Note in these pictures, the nitrogen space in the IFP is usually much smaller, and the nitrogen space in the Remote Reservoir is usually much larger (90% of the reservoir). Because these shocks were cut away the parts have been artificially moved.

             Remote Reservoir Shock Cutaway



IFP shocks are a great budget solution for street use and slow speed off-roading but are not suitable for high speed use on Tacoma’s and 4Runner’s because the method of construction does not make them durable enough. Remote Reservoir shocks provide better ride quality and better long term durability than IFP shocks, and are well suited to hard use on demanding terrain.


What Do Compression Adjusters Do & Do I Need Them?

There are three types of compression adjusters on the market, low speed, high speed, and mid speed. Each one is designed to tune a specific performance characteristic of the vehicle.

Low Speed Compression Adjusters:

Low speed compression adjusters tune handling, g-out, and slow speed bottom out performance. In most cases low speed compression adjusters are a needle and seat which allows a small quantity of free flowing oil into the reservoir. Once oil reaches a high enough speed and pressure it flows through a separate set of spring washers into the reservoir, referred to as the high speed circuit.


High Speed Compression Adjusters:

High speed compression adjusters tune harshness and compliance on large hits such as potholes, whoops, and fast speed bumps. In most cases high speed compression adjusters take the high speed springs washers mentioned above and make it possible to tune the pressure at which oil will start to flow through the disks. By requiring higher pressure the high speed damping is increased.

Mid Speed Compression Adjusters:

Mid speed compression adjusters have some impact on low speed and high speed compression, depending where in the adjustment range they are. Fully open the mid speed adjusters tune high speed a little bit, and fully closed they tune low speed a lot (each adjustment is a large increment).  Mid speed compression adjusters are designed like a VERY LARGE low speed compression adjuster. They allow a large amount of free flowing oil when fully opened and allow none when fully closed. Oil not flowing through the adjuster passes through a set of spring washers.

King Mid Speed Compression Adjuster



Compression adjusters allow you to easily and externally change the damping of your shocks. They are very useful if you’re adding weight for trips, picky about ride quality, or want to firm up that on road handling. Low speed compression adjusters are useful for on-road driving, and all types of off-road driving. High speed compression adjusters are most useful for high speed off-road driving. Mid speed adjusters are less effective than low speed or high speed adjusters and are useful for all types of on-road and off-road driving.

You can read more about the Fox & King Compression Adjusters here:

How Fox DSC Dual Speed Compression Adjusters Work

How King Compression Adjusters Work

Are Upper Control Arms Necessary For My Tacoma or 4Runner?

Stock upper control arms on Tacoma’s and 4Runner’s are the first item to bind when the suspension extends. By replacing them with a tubular or billet arm the suspension is able to gain one inch of droop. One inch of travel may not seem like a lot, but many lifted Tacoma’s only have 2-3″ of droop, so that extra inch makes a big difference. When you don’t have enough droop the shocks will clunk hard against their extension stops and will feel very harsh on potholes, washboard, and any bump that requires more than two inches of movement. When the wheel has no down travel it can’t follow the terrain so it jerks the chassis down into the hole, then bangs back hard on compression when it hits the face of the obstacle. With sufficient down travel the tire smoothly follows the contour of the road.


If you are lifting your Tacoma or 4Runner more than 1.5″ it is necessary to install an upper control arm to get the full benefits of the upgraded shocks.

  • Icon Vehicle Dynamics 2005 & UP Delta Joint UCA

    Icon, Tubular UCA Kit, Delta Joint, 05-UP Tacoma





Do I need Extended Travel Rear Shocks?

With an upper control arm installed the front of Tacoma’s and 4Runners pull about 10 inches of wheel travel, and in the perfect world the rear would have more travel in order to keep up. It’s ideal to have more travel in the rear than in the front because it is more subject to height changes when on the gas, and because it “follows along” and bottoming out the rear causes bucking.

4Runner Rear:

Stock height replacement 4Runner rear shocks have around 9″ of travel, which isn’t bad. 2-3″ of lift is very common in 4Runners and, when used with bump stops spacer allows the use of 9.5″ travel shocks. The most simple version are 2.0 diameter direct replacement 2-3″ lift shocks which require 0.5″ bump stop spacers. The next option is to use 2.5 Tundra rear shocks which we re-valve, clock the reservoir fittings, and swap bushings to fit on the rear of 4Runners. For extreme off-roaders we can order custom length shocks which net you 11″ travel and require 2″ bump stop spacers. When using the extreme length shocks be sure to pair them with a spring capable of longer travel lengths or they may fall out.


Tacoma Rear:

First gen Tacoma’s come with extremely short shocks that are different left to right, and it is impossible to pull decent wheel travel out of the rear using stock mounts. We recommend building custom rear shock mounts and installing universal fit 2.0 x 10″ remote reservoir shocks, also available with low speed compression adjuster.

Second gen Tacoma’s have taller rear shock mounts, and travel is largely dictated by what spring pack you choose. Stock replacement 05+ Tacoma rear shocks have roughly 8.5″ of travel and are about the most you can get out of a stock leaf pack. Mini pack  rear leaf springs work by replacing the overload with a small leaf pack which nets about 1.5″ of lift and an extra 2″ of droop. These leaf packs are an excellent fit with 2-3″ lift rear shocks mounted in the stock locations, using a 1″ bump stop spacer.

Full replacement Tacoma Leaf Packs are capable of much more travel than mini-packs and there are a lot of options for shocks depending on what your goals are. Because replacement packs are capable of a lot more travel than stock, they should be set to deliver at least two inches of lift. For direct fit bolt on 2.0 shocks you’ll want to run a 2″ bump stop spacer and 3.5-5″ lift for almost 10″ of travel (that’s correct, most shock companies incorrectly list lift heights, give us a call and we’ll get you the longest shock that’ll fit correctly). For direct fit 2.5 shocks you will want to run our Tundra shocks converted to fit the rear of a Tacoma, these net you almost 10″ of travel. Ten inches of rear wheel travel is about the most you will get in the stock location without lifting the rear higher than the front.

In order to use the full travel of aftermarket replacement leaf packs it is necessary to use a Tacoma rear shock relocation kit. The kits allow you to install 2.0 or 2.5 shocks with 12-14″ travel. For most overland setups we recommend heavier leaf springs (Icon & OME Dakar) and 12″ travel shocks. For lighter setups looking to go fast off-road we recommend lighter leaf packs (Deaver, Atlas, Icon) and longer 14″ travel shocks. When choosing leaf packs you want the lightest rate that will carry your weight. Heavy Dakar springs on an stock weight Tacoma will not ride well, and light Deaver springs on a heavy overland setup won’t hold the weight consistently and may change heights substantially as the weight changes. The ICON RXT Tacoma leaf pack is nice because it’s adjustable and tuneable so you can set it up to meet your needs. The other benefit of shock relocation kits are that you don’t have to run as much bump stop spacer so you can gain extra up travel and droop travel, both of which are necessary for a comfortable ride.

When it comes to oversize tires on Second and Third Gen Tacoma’s 285/70R16 is the most popular choice. In the rear 285’s require at least 1″ bump stop spacers to prevent the tires rubbing on the fenders. Your exact experience may be different based on wheel and tire combo so always flex test before hitting the trail.



There are a lot of options for 4Runner and Tacoma rear shocks depending on rear spring choices. Bolt in fitment is the only option for 4runners. If you’re lifting the rear of the T4R maximize the travel by using aftermarket springs, longer shocks, and bump stop spacers. First gen Tacoma’s require universal fit shocks and custom mounts to get decent travel. There are tons of options for Second Gen Tacoma’s, but in general stock springs and stock springs with add-a-leafs should use the stock mounts. Aftermarket leaf springs are best used with a shock relocation kit. At AccuTune Off-Road we will custom tune all of them to deliver the ride you’re looking for.



Conclusions About The Capabilities of Taco & T4R Shocks:

2.0 IFP Monotube

In the front & rear of Tacoma’s and 4Runners 2.0 IFP monotubes provide a more comfortable ride than twin tubes and OEM replacement shocks, and work well for street driving and exploring the backwoods at a normal pace.

AccuTune Valving: No (not available pre-tuned)
AccuTune Rebuild: No (we do not offer rebuild services for these shocks)

  • Fox Shock 03-21 Toyota Tacoma Front Coilover, 2.0 Performance Series, IFP, 0-1″ Lift, 0-2″ Lift



  • Fox Shock 05-ON Toyota Tacoma Rear, 2.0 Performance Series, IFP, 8.1″, 0-1″ Lift




2.0 Remote Reservoir

2.0 Remote Reservoir shocks are available for the rear of Toyota 4Runner’s and Tacoma’s. The added gas space improves heat capacity and durability making these shocks excellent for exploring the backwoods, blasting fire roads at speed, and prerunning.

AccuTune Valving: Yes
AccuTune Rebuild: Yes

  • Fox Shock 03-ON 4Runner, 07-14 FJ Cruiser, 09+ GX460, GX470, Rear, 2.0 Performance Series, Remote Reservoir, 9.6″, 2-3″ Lift



2.0 Remote Reservoir With Adjuster

2.0 Remote Reservoir shocks with compression adjusters increase damping capability, and make it easy to adjust the suspension for more control or more comfort. They’re also a great way to help your suspension cope with added weight when loading up for trips.

AccuTune Valving: Yes
AccuTune Rebuild: Yes

  • Fox Shock 05+ Toyota Tacoma Rear, 2.0 Performance Series, Remote Reservoir, 8.6″, 0-1″ Lift, LSC Adjuster



2.5 IFP Monotube

2.5 IFP Monotube shocks are only available for the front of Taco’s and 4Runner’s. The increased size makes them a great choice for heavier Toyota’s doing slower speed overlanding. This is also the first size shock available in extended length configurations for use with long travel upper control arms.

AccuTune Valving: No, due to complicated construction
AccuTune Rebuild: No, due to complicated construction

  • Fox Shock Kit: 03-21 Toyota Tacoma, 03-09 4Runner, GX460, GX470, Front Coilover, 2.5 Series, IFP, 0-2″ Lift, 0-3″ Lift



  • King Shocks Tacoma 96-04 (6 Lug) Front 2.5 Dia. Internal Reservoir Coil-Over



2.5 Remote Reservoir

In the front of 4Runners and Tacoma’s 2.5 Remote Reservoir shocks are the first shock on this list able to handle tough off-road terrain at speed. The large size provides the damping and heat capacity necessary, and the remote reservoir configuration ensures a long life and better ride quality. In the front these are great for exploring and overlanding on washboard at speed. These are also suitable for use on fast fire roads and whoops.

In the rear, 2.5 Tacoma Remote Reservoir Shocks are able to withstand hard use and heavy loads. If you’re carrying a lot of weight such as a roof top tent, weekend gear, and more then this is the appropriate size for you. You’ll be able to tackle tough terrain without slowing down and without blowing out your shocks.

AccuTune Valving: Yes
AccuTune Rebuild: Yes

  • Fox Shock Kit: 05-On Toyota Tacoma Rear, 2.5 Series, Remote Reservoir, 9.4″, 2-3.5″ Lift



  • King Shocks Tacoma 05+ (6 Lug) Front 2.5 Dia. Remote Reservoir Coil-Over



  • Fox Shock Kit: 05+ Toyota Tacoma, Front 2.5 Coilover, Remote Reservoir, UCA Required, 0-2″ Lift, 0-3″ Lift



2.5 Remote Reservoir With Adjuster

These are the go to shocks front and rear for people wanting to use heavy overland rigs on difficult off-road terrain at reasonable speeds with good reliability and a comfortable ride. On stock weight Tacoma’s and 4Runner’s 2.5 Remote Reservoir Shocks with Adjusters will let you tackle washboard, whoops and more at high speed for long periods of time.

AccuTune Valving: Yes
AccuTune Rebuild: Yes

  • Fox Shock Kit: 05-On Toyota Tacoma Rear, 2.5 Series, Remote Reservoir, 9.4″, 2-3.5″ Lift, DSC



  • King Shocks Tacoma 05+ (6 Lug) Front 2.5 Dia. Remote Reservoir Coil-Over W/ Adjuster



  • Fox Shock Kit: 05+ Toyota Tacoma, Front 2.5 Coilover, Remote Reservoir, UCA Required, 0-2″ Lift, 0-3″ Lift, DSC



Icon Vehicle Dynamics – 2.0 Digressive Piston Review

While rebuilding some Icon Vehicle Dynamics 2.0 FJ Cruiser rear shocks we took the opportunity to snap some pictures and explain more how they work.

Icon Vehicle Dynamics Digressive Shocks

Icon Vehicle Dynamics advertises that they use digressive pistons in their shocks for better chassis control (handling). But as you can see in our Piston Tech Article, there are some downsides to extremely digressive damping, and that digressive pistons don’t always mean digressive damping. So we dug into the IVD shocks to figure out what’s really going on.

Linear Compression

IVD uses linear compression with large bleed and valve disks in a pyramid stack configuration. The result is a linear damping curve with good comfort on small bumps, and control on large ones.

IVD Shocks Linear Compression

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Digressive Rebound

Icon shocks use pistons designed to be digressive in rebound. However the rear FJ Cruiser shocks we opened up are using methods to make them act more linear. These methods include low digressive preload (probably 0.003″) and large bleed holes.

Icon Shocks Digressive Rebound

Inside an Icon Vehicle Dynamics FJ Cruiser Rear Shock


Icon Vehicle Dynamics improves the ride and control of their Digressive shocks by using linear compression, and marginally digressive rebound. By reducing the digressive nature of the shocks they are able to mitigate a lot of the downsides to digressive damping. The result is improved comfort on washboard and more control on big bumps (than a very digressive shock).


Related Products:




Digressive vs Linear vs Progressive Pistons & Shock Valving

AccuTune Off-Road’s Experience:

Our Founder and author of this article, Ryan Raker, has been designing, tuning, and manufacturing shocks with digressive, linear and progressive damping curves since 2001. During that time he used state of the art shock dyno’s to design and/or tune premium shocks purchased by nearly every lift kit manufacturer. As a result AccuTune Off-Road is uniquely familiar with the real world pros and cons of each type of shock, how to tune them, and their proper applications.




Performance of Digressive, Linear, and Progressive shocks:

Shocks can be digressive on compression , rebound or both, and each has it’s own pro’s and cons. The terms Digressive, Linear, and Progressive refer to the shape of damping curve produced by a shock (as seen below). Most Fox shocks and most King shocks are linear, while most Icon shocks and entry level Bilstein shocks (5100 & 6100) are digressive. The graph below is data directly off a shock dyno and is used for illustration purposes only. If these were rebound loads, and the shocks were tuned for the same vehicle they would likely intersect around 10 in/sec or less. If they were compression loads they would likely intersect around 100 in/sec.

Shock design and tuning can impact performance on small bumps/washboard, large bumps/whoops, g-outs, and handling. Creating the perfect damping curve is all about making the right trade offs to achieve the correct performance across all suspension events. In the examples below it is assumed each of the shocks are well designed and very digressive or very progressive in order to explain the differences.

Digressive Linear and Progressive Pistons Valving and Damping Curves

Small Bump & Washboard:

Small bumps and small washboard cause the wheel to make very fast, but very small movements. Because the movements are so small the valve disks may not have a chance to open. As a result these forces often happen around 2 to 10 in/sec on both compression and rebound. From the graph we can see that digressive pistons would be stiff and possibly harsh while progressive shocks would be very smooth. Because the movements are small tire pressure and tire stiffness can play a large roll in ride comfort.

Large Bumps & Whoops:

Large Bumps and Whoops are high speed events on both compression and rebound and it’s important to think about what happens to both the chassis and the wheels.

On compression the wheel may move at 200+ in/sec and ideally the shock will do a combination of absorbing the bump and raising the chassis to prevent bottoming out. A digressive shock may blow off so much load that it can’t sufficiently raise the chassis. An extremely progressive shock on the other hand will feel smooth and controlled.

On rebound the wheel might move around 50 in/sec. A digressive shock will start by pulling the chassis into the series of bumps before hitting the speed to freely let the wheel move. For shocks with a very digressive damping curve the change in stiffness can cause the vehicle to feel as if it is loosing control. A shock with a progressive rebound damping curve will always feel like the wheels and chassis are disconnected, making for a potentially bouncy ride.

G-Outs, Towing and Overloading The Suspension:

On G-Outs and other slow speed bottoming events digressive shock valving will feel firm and controlled. Progressive shock valving will feel out of control because the suspension will bottom very easily and then spring back up very easily.


Handling is a tricky one, digressive valving will provide better handling than progressive valving due to the higher force at 2 in/sec. Although shocks and springs will impact handling, but that should not be their primary responsibility.  Shocks and springs are the only tuneable elements that control performance over the various suspension events and should be tuned to provide the best ride possible. Handling should be tuned by the sway bar since that is it’s primary responsibility.


Digressive shocks are bad at small bumps and large bumps, but good at handling and g-outs. Progressive shocks are good at small bumps and big hits, but bad at handling and g-outs. Linear shocks provide the best all around performance. At AccuTune Off-Road we design the shape of the shock curve to provide the performance you want on the type of terrain you will encounter. The result is curves that are slightly digressive, linear, or slightly progressive to deliver the some of the benefits without all the drawbacks.

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Piston & Shock Designs For Digressive, Linear and Progressive Damping Performance:

There are two traditional piston designs: Digressive and Linear. Digressive pistons primarily produce digressive shock curves (duh), while linear pistons can produce digressive, linear and progressive shock curves depending on their tuning.

Digressive Piston Design:

Digressive shocks are made by preloading the valve disks, typically between 0.002″ and 0.015″. The most common way to do this is to design the piston to have a raised outer edge where the piston seals. While some shocks have a clearly raised edge, others are machined in a convex shape that’s less obvious. The drawback to these designs is that slight variations in preload thickness produce drastically different damping loads.

Digressive Piston Design

Linear Piston Design For Digressive Performance:

Linear pistons feature perfectly flat faces so other techniques need to be used to create a digressive damping curve. If the shock curve does not need to be extremely digressive a linear piston can be made digressive by using a very small through bleed hole. This solution often only works for shocks with a lot of rebound damping.

Digressive Damping from a linear piston

Linear pistons can be used to create digressive damping curves by preloading the valve disks. This can be done with coil springs (sometimes used for through shaft compression adjusters), or by inserting a preload ring into the disk stack. Either way the results are the same.

Linear Piston Design For Linear Performance:

Shock pistons with a flat sealing surface for the disks and sufficient bleed hole size will produce a linear shock curve. Note that even linear shocks tend to have some “nose” or point on the curve around 2 in/sec. This is due to friction and other forces related to the transition of the internal parts from stationary to moving (or reversal of direction). For linear performance it is most common to use pyramid stacks (a stack of valve disks with decreasing diameter as shown below), although it is possible to have linear performance with a flutter stack.

Linear Piston and Linear Damping

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Linear Piston Design For Progressive Performance:

There are A LOT of ways to tune linear pistons to have progressive damping curves. All of these designs allow a lot of oil to flow unrestricted at slow speeds, but some techniques are better than others. The methods listed below are ordered from best to worst design.

Internal & External Bypass Shocks:

Internal and External Bypass Shocks have additional flow paths through which large amounts of oil can flow relatively unrestricted. Because the free flow is limited to certain parts of the travel they mitigate issues with performance on g-outs by firming up towards the end of travel. The best designed bypass shocks have multiple stages so the transition from soft to firm can happen closer to ride height and over a longer distance so the shock doesn’t suddenly get stiff.

Progressive to linear to digressive bottoming with Bypass Shocks

Directional Bleed:

Bilstein produces a 46mm shock that allows additional free bleed in the compression direction by using a check valve. This design can produce progressive damping curves on the compression side where they are most commonly desired.

Flutter Stack Valving:

Flutter stack valving is much like directional bleed and is the most universal method for making linear pistons produce progressive damping curves. Because of how they work, flutter stacks can be added to any existing linear piston to produce the exact desired shock curve. By investing years into field testing and dyno testing we have perfected flutter stack valving.

Progressive Valving From A Linear Piston With Flutter Stack Valving

Soft Piston Valving With A Rate Plate:

A rate plate works by having soft initial valving which eventually hits a stiff plate that prevents the disks from continuing to deflect. The result is that the main valving starts to work like an orifice and build exponentially more force. These can work well with a lot of tuning and a lot of testing to make sure they never get harsh.

Progressive Damping From A Linear Piston With Rate Plate Valving

Pistons With A Lot of Free Bleed:

By drilling a lot of free bleed holes in the main shock piston the damping curve can become progressive. As slow speeds oil flows unrestricted through the large holes. At high speed the holes hit their maximum flow rates and the main piston valving has to start working, making the shock stiffer on the high end. This design makes both compression and rebound progressive which is not always desirable.

Progressive Damping From Linear Piston With A Lot Of Bleed

Step Off Disks:

The final method to making a linear piston progressive is to insert a thin, small diameter disk against the piston which allows oil to free flow before making the main disks deflect. This method results in inconsistent damping on both sides of the piston and should not be used (although we see it far too often).

Progressive Damping From A Linear Piston With Step Off Disk


There are a lot of options for piston design and damping curve design but anything too extreme tends to deliver undesirable performance in traditional shocks used for off-road applications. Digressive pistons can be advantageous for on-road use or for those not adverse to a rough ride. Linear pistons with progressive damping curves can provide an exceptional ride as long as the tuning and methodology is done correctly and bottoming is well controlled. Linear damping curves provide the best all around performance in standard designs. No matter what you choose AccuTune Off-Road will tune the damping curve to deliver the performance you’re looking for.


Related Products:


How King Compression Adjusters Work

King Shocks offer Compression Adjusters on 2.0, 2.5, and 3.0 coilovers, smoothies and internal bypass shocks. King compression Adjusters have a single knob to quickly make adjustments to mid speed compression, resulting in changes to ride comfort and performance. In addition to helping you dial in the perfect ride, they improve the function of the shock and can also save you countless hours of suspension tuning. Compression adjusters add damping force, and require special tuning, at AccuTune Off-Road we will custom tune your shocks to put you in the useable range of adjustment.

How Does The King Compression Adjuster Work


How Does The King Compression Adjuster Work

King compression adjuster has three flow paths: a large adjustable through bleed, a high speed non-adjustable disk stack, and a rebound check valve. By using a large diameter free bleed with adjustable orifice King has turned a design typically used for low speed compression adjustment into a design used for mid speed compression adjustment. The result is a very simple to use adjuster that affects mostly mid speed compression. We find the most useable range to be more than 4 clicks open. At 4 clicks and below the adjuster starts to function more as a low speed adjuster, but with large adjustments between clicks which may quickly jump from comfortable to harsh.



As you turn the knob on the King Compression Adjuster the flow path increases or decreases in size. The unique shape of the adjuster aims to make each click a similar amount of adjustment. You can see the flow volume is substantially larger than the Low Speed Fox adjustment. The increase volume is what makes this a mid speed adjuster.

King Compression Adjuster Adjustment Circuit


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How Much Adjustment Does The King Compression Adjuster Have

The downside of the high flow, mid-speed compression adjuster design is that it doesn’t impact the damping curve in very useful ways. If we want to control road feel, handling, and g-outs then we want to adjust low speed damping. If we want to control performance on big bumps or whoops we want high speed adjustment. Having a mid speed adjuster does impact both of these, with one knob, but the amount of adjustment is not as great as if they were separate adjustments.

Mid Speed Adjuster Design Impacts Low & High Speed, Just Not Very Significantly

At low speed the King compression adjuster can add up to 60% damping load across the full range. Nearly all of low speed damping adjustment happens in the first four clicks making it hard to control, and making it less useful for fine tuning. If your adjusters are open four clicks or less and it’s not harsh, then it’s time to re-tune. Due to the low amount of adjustment and the uncontrollable nature in which it happens, King Compression Adjusters don’t offer useful low speed compression adjustment. (King does not advertise their adjuster as a low speed compression adjuster, but it was important to review the functionality).

At high speed the King compression adjusters can add between 25% (solid axle and standard IFS) and 40% (light IFS) more damping depending on the vehicle. High speed adjustment is delivered in manageable increments across the last 24 clicks. The adjuster is more effective on IFS vehicles because the shock operates at slower speed, where the mid speed adjuster is more effective. The downside is that IFS shocks also operate at higher loads, making the adjustment range a smaller percentage.

King Compression Adjuster Range



Other Benefits of Compression Adjusters:

There are many other lesser known benefits of compression adjusters that improve the performance of your shocks such as increased damping, better cooling and more consistency.

Compression Adjusters are basically a second damping piston which works off the displacement of the piston rod and meters oil flow into the reservoir.  By adding a second damping piston the damping area and damping force are automatically increased. This is important for 2.0 shocks on solid axles and 2.5 shocks on IFS, which can run out of damping force. Because the compression adjuster piston is located next to the reservoir it does a better job putting heat into the oil where it is more easily cooled. Finally the compression adjuster piston significantly reduces the ability for the shock to cavitate and loose damping. Small amounts of oil can easily flow into the reservoir, but compression adjusters generally don’t flow large amounts of oil well, meaning they’re forced through the main piston, making it do it’s job.  These benefits by themselves are often worth adding a compression adjuster to your shocks.



The simple design of the King Compression Adjuster makes them very easy to use, however the adjusment range is not as good as dedicated high speed and low speed adjusters.  If King Compression Adjusters fit your needs AccuTune Off-Road can put you in the middle of the adjustment range and help dial in the ride you’re looking for.

King Compression Adjusters are very easy to use and offer many benefits besides damping adjustment. Because of how compression adjusters work and the limited high speed adjustment range they require special valving which is best done with AccuTune equations. Dial in the perfect ride with AccuTune Valving and Compression Adjuster equipped King shocks.

Related King Shocks With Compression Adjuster:


How Fox DSC Dual Speed Compression Adjusters Work

Fox offers Compression Adjusters on 2.0, 2.5 and 3.0 coilover, smoothie and internal bypass shocks. Fox calls their adjuster a Dual Speed Compression (DSC) adjuster because it has separate knobs for low speed and high speed adjustments. Fox DSC Compression Adjusters combined with AccuTune Valving are a great improvement to off-road shocks because you’ll be in the middle of the adjustment range, allowing you to quickly make adjustments to fine tune your ride comfort and performance. When it comes to shock tuning they will save you countless hours and greatly improve your results.

Fox DSC Shock Upgrade

How Do Fox Compression Adjusters Work

Fox compression adjusters have three flow paths: low speed free bleed, a high speed disk stack, and a rebound check valve. The Fox Low Speed Compression circuit works by using a needle and seat to change oil flow through the free bleed path. The Fox High Speed Compression Adjuster circuit uses deflective disks to control flow. The high speed circuit is externally adjustable and works by increasing preload on deflective disks. The rebound check valve allows oil to easily return to the shock when the shaft extends.

Fox DSC Adjuster Flow Paths

How Much Adjustment Do Fox DSC Compression Adjusters Have

Fox Compression Adjusters have a substantial range of adjustment that can transform your vehicle from a cadillac to a race car.  The Fox low speed adjustment ranges is huge and can add between 120% (solid axle and standard IFS) and 150% (light IFS) more low speed load depending on vehicle/tune. The high speed adjuster can add between 30% and 60% more high speed force depending on vehicle/tune. The adjuster is more effective on IFS vehicles because the shock operates at slower speed, where the low and high speed adjusters work together when fully closed. The downside is that IFS shocks also operate at higher loads, making the adjustment range a smaller percentage.

Fox DSC Compression Adjuster Range

Fox DSC Really Is Dual Speed Compression Adjustment

The Fox Low Speed Compression Adjustment give you the ability to find the optimal trade off between ride, handling and bottoming control. The Fox Low Speed Compression Adjuster has a large usable range and give each click an appropriate amount of adjustment. The Fox DSC Adjuster has 9 low speed clicks. While adjustments do spill over into high speed, the percentage change is heavily weighted towards low speed.

Fox DSC Dual Speed Compression Adjuster Low Speed Adjustment Range

The separate Fox High Speed Compression Adjuster gives you a dedicated adjustment for tuning on whoops and high speed hits. The adjuster has 11 clicks and offers a great range of adjustability which has almost no impact on low speed compression.

Fox DSC High Speed Adjustment Range

Other Benefits of Compression Adjusters:

There are many other lesser known benefits of compression adjusters that improve the performance of your shocks such as increased damping, better cooling and more consistency.

Compression Adjusters are basically a second damping piston which works off the displacement of the piston rod and meters oil flow into the reservoir.  By adding a second damping piston the damping area and damping force are automatically increased. This is important for 2.0 shocks on solid axles and 2.5 shocks on IFS, which can run out of damping force. Because the compression adjuster piston is located next to the reservoir it does a better job putting heat into the oil where it is more easily cooled. Finally the compression adjuster piston significantly reduces the ability for the shock to cavitate and loose damping. Small amounts of oil can easily flow into the reservoir, but compression adjusters generally don’t flow large amounts of oil well, meaning they’re forced through the main piston, making it do it’s job.  These benefits by themselves are often worth adding a compression adjuster to your shocks.



Fox DSC Adjusters really work and are an excellent choice if you’re changing weight, changing terrain, or want the best possible ride. Because of how compression adjusters work and the limited high speed adjustment range they require special valving which is best done with AccuTune equations. Dial in the perfect ride with AccuTune Valving and DSC equipped Fox shocks.



Related Fox Shocks With DSC Adjuster:

Choosing The Right Length Jeep Wrangler JK Shocks

While tuning our Go Fast Fox Jeep JK shocks we realized that many people are selecting the wrong length shocks and leaving significant travel unused.  It is common to choose Fox JK shocks by looking at lift height.  While lift height is a great starting place, it commonly results in people choosing shocks that are too short.  In order to maximize wheel travel you want to run the longest Fox JK shock possible without it becoming the bump stop.  This means that shock collapsed length is the best way to look at which Fox shocks will fit your JK.  Maximum shock collapsed length is determined by the length of the bump stop spacers you’re running.


The best way to measure for Fox shocks is to install the shortest bump stop spacers that prevent the tires from rubbing on the fenders and prevent the steering & drag link from hitting the oil pan.  At ride height measure from the top of the bump stop spacer to the metal bump stop mount.  Next measure from the bottom of the upper shock mount to the center of the lower shock mount.  For front JK shocks subtract 0.70″ from the shock length for the stem bushings.  The difference between the measurements is the minimum collapsed length of your shocks.  Select shocks with shorter collapsed lengths than your measurements.

For those who don’t want to take measurements we have assembled a shortcut guide below (select Fox shocks which have a bump stop spacer height less than your bump stop spacers). Double check that 40 to 60% of the shock travel is up travel. By using our method you will end up with the most travel possible for the best offroad performance.


SKULift HeightSideShockExtended LengthCollapsed LengthTravelOEM FRONTMin Spacer HeightMakeModelYear
FOX-985-26-0151.5-3.5"FrontFox 2.0 Performance Series, Remote Reservoir, CD Adjustable24.815.29.614.350.9JeepWrangler JK2007-2016
FOX-985-26-0161.5-3.5"RearFox 2.0 Performance Series, Remote Reservoir, CD Adjustable24.4514.859.614.550.3JeepWrangler JK2007-2016
FOX-985-24-0151.5-3.5"FrontFox 2.0 Performance Series, Remote Reservoir24.815.29.614.350.9JeepWrangler JK2007-2016
FOX-985-24-0161.5-3.5"RearFox 2.0 Performance Series, Remote Reservoir24.4514.859.614.550.3JeepWrangler JK2007-2016
FOX-985-26-0114-6"FrontFox 2.0 Performance Series, Remote Reservoir, CD Adjustable28.316.711.614.352.4JeepWrangler JK2007-2016
FOX-985-26-0124-6"RearFox 2.0 Performance Series, Remote Reservoir, CD Adjustable28.4516.8511.614.552.3JeepWrangler JK2007-2016
FOX-985-24-0114-6"FrontFox 2.0 Performance Series, Remote Reservoir28.316.711.614.352.4JeepWrangler JK2007-2016
FOX-985-24-0124-6"RearFox 2.0 Performance Series, Remote Reservoir28.4516.8511.614.552.3JeepWrangler JK2007-2016
KRS-25001-1800-2"FrontKing Shocks JK 07+ Front 2.5 Dia. Piggy Hose Reservoir For 0-2.0_ Lifts25.90215.83110.07114.351.48JeepWrangler JK2007-2016
KRS-25001-1810-2"RearKing Shocks JK 07+ Rear 2.5 Dia. Piggy Hose Reservoir For 0-2.0_ Lifts26.2816.18810.09214.551.64JeepWrangler JK2007-2016
FOX-883-06-0682.5-4.0"FrontFox 2.5 Factory Series, Internal Bypass, Reservoir, DSC Adjustable25.9715.6110.3614.351.3JeepWrangler JK2007-2016
FOX-883-06-0692.5-4"RearFox 2.5 Factory Series, Internal Bypass, Reservoir, DSC Adjustable27.8617.1210.7414.552.6JeepWrangler JK2007-2016
FOX-883-02-0682.5-4.0"FrontFox 2.5 Factory Series, Internal Bypass, Reservoir25.9715.6110.3614.351.3JeepWrangler JK2007-2016
FOX-883-02-0692.5-4"RearFox 2.5 Factory Series, Internal Bypass, Reservoir27.8617.1210.7414.552.6JeepWrangler JK2007-2016
FOX-883-26-0152.5-4.0"FrontFox 2.5 Factory Series, Reservoir, DSC Adjustable26.0515.9310.1214.351.6JeepWrangler JK2007-2016
FOX-883-26-0162.5-4.0"RearFox 2.5 Factory Series, Reservoir, DSC Adjustable28.0417.210.8414.552.7JeepWrangler JK2007-2016
FOX-883-24-0152.5-4.0"FrontFox 2.5 Factory Series, Reservoir26.0515.9310.1214.351.6JeepWrangler JK2007-2016
FOX-883-24-0162.5-4.0"RearFox 2.5 Factory Series, Reservoir28.0417.210.8414.552.7JeepWrangler JK2007-2016
KRS-25001-2233-5"FrontKing Shocks JK 07+ Front 2.5 Dia. Piggy Hose Reservoir For 3-5_ Lifts27.2716.89310.37714.352.54JeepWrangler JK2007-2016
KRS-25001-2243-5"RearKing Shocks JK 07+ Rear 2.5 Dia. Piggy Hose Reservoir For 3-5_ Lifts28.1417.26310.87714.552.71JeepWrangler JK2007-2016
KRS-25001-2856"RearKing Shocks JK 07+ Rear 2.5 Dia. Piggy Hose Reservoir For 6_ Lifts32.2819.56312.71714.555.01JeepWrangler JK2007-2016
KRS-25001-2846"FrontKing Shocks JK 07+ Front 2.5 Dia. Piggy Hose Reservoir For 6_ Lifts31.04318.6512.39314.354.30JeepWrangler JK2007-2016
FOX-883-26-0174-6"FrontFox 2.5 Factory Series, Reservoir, DSC Adjustable28.8517.3311.5214.353JeepWrangler JK2007-2016
FOX-883-26-0184-6"RearFox 2.5 Factory Series, Reservoir, DSC Adjustable29.2417.811.4414.553.3JeepWrangler JK2007-2016
FOX-883-24-0174-6"FrontFox 2.5 Factory Series, Reservoir28.8517.3311.5214.353JeepWrangler JK2007-2016
FOX-883-24-0184-6"RearFox 2.5 Factory Series, Reservoir29.2417.811.4414.553.3JeepWrangler JK2007-2016


Related Jeep JK Wrangler Shocks:

Fox vs King: 2.0 Coilover Shock Comparison

Contrary to popular belief, the differences between Fox 2.0 Remote Reservoir Coilovers and King 2.0 Remote Reservoir Coilovers run much deeper than the color of their anodizing.  In order to provide you a full comparison we have evaluated the real world performance and durability of each coilover to help you decide which is right for you.  For our comparison we used a Fox 2.0 x 14″ Travel Factory Series Remote Reservoir Coilover and a King 2.0 x 14″ Travel Performance Series Remote Reservoir Coilover.  These are two of the most popular off-road shocks we sell.

  • Fox 2.0 Remote Reservoir Coilover

    Fox 2.0 Coilover, 7/8″ Shaft, Remote Reservoir, 14″ Travel



  • King 2.0 Coilover, Remote Reservoir – 14″ Travel





Externally there are some significant differences which impact value, appearance and durability.  Both shocks use 1/2″ ID COM-8T teflon lined bearings, plastic dual rate sliders, aluminum remote reservoirs and anodized aluminum caps.  The differences begin with the reservoir hose, Fox uses a -8 stainless steel braided hose and King uses a -8 cloth wrapped hose.  Fox 2.0 Coilovers also include remote reservoir mounts which are not included with King coilovers.

Fox vs King 20 Coilover



In order to analyze durability we will assess the quality of the components used in the shocks.  Shocks can fail quickly due to damaged piston rods and seals, or over the long term due to seal wear and dirt intrusion.  The piston rod, seals, and oil are the key components which dictate a shocks durability.


Piston Rods:

There is a substantial difference in the quality of the piston rods used in Fox and King coilovers.  Fox 2.0 Factory Series coilovers use a 17-4 H900 stainless steel piston rods with hard chrome plating.  Fox stainless steel piston rods are very corrosion resistant and extremely strong at 190 ksi of tensile strength.  King 2.0 Performance Series coilovers use 1040-1060 high carbon steel piston rods with hard chrome plating and are NOT heat treated for surface hardness.


Piston rod hardness is also very important to consider.  Rockwell hardness is measured by pressing a ball into the material at a preset force, and measuring the depth of deformation.  This very closely mimics flying rock debris and the amount of damage it may cause to the piston rod.  Damaged piston rods are the leading cause of shock failure, harder rods are more resistant to rock debris and therefore offer substantial durability advantages.  Half the hardness means half the durability. Results from 3rd party Rockwell Hardness Testing:

Fox: 45 HRc

Sway-A-Way: 32 HRc

King: 23 HRc


Fox vs King 20 Coilover Piston Rod


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Seals are responsible for keeping the oil inside the shock across harsh conditions without contributing large amounts of friction.  Fox 2.0 Factory Series coilover shocks come with viton seals, viton is the benchmark material for high temperature and low friction seals. King 2.0 Performance Series coilovers use industry standard Buna seals which provide excellent sealing, and viton seals are available as an optional upgrade.  From the outside in, Fox uses a heavy duty wiper, then a u-cup with a secondary wiper lip, and finally a viton o-ring.  King uses a heavy duty wiper, u-cup and o-ring seal.  Fox’s use of the u-cup with secondary wiper offers extra protection against dirt intrusion in order to help the shocks last longer.


King vs Fox 20 Coilover Seals


Oil is responsible for providing consistent damping across a range of temperatures, conveying heat, and lubricating the seals.  When oil breaks down shock failure isn’t far behind.  Fox shocks use a conventional oil rated by the manufacturer for use from -65 to 275 F and has a viscosity index of 370 (more on this below).  King shocks use a conventional oil rated for use by the manufacturer from 14 to 230 F with a viscosity index of 150.  It is common for oils to operate outside their rated range in shock absorbers due to the unique nature of the application (Fox rates their oil up to 400F).  Fox oil offers substantially better lubrication across a wide range of temperatures based on the manufacturer ratings. Viscosity index (VI) is a measure of how much the damping will fade due to heat.  Fox’s VI of 370 is very high and provides exceptional performance.  King’s VI of 150 is an average rating.



Friction is the most important thing to manage in a shock, especially for vehicles with low weight and shocks mounted near a 1:1 motion ratio.  Higher friction is correlated to increased suspension harshness and reduced durability so managing friction is priority one for a shock engineer.  In some tunes, particularly those with flutter stacks, it is possible for friction to be more than 10% of the damping at low speeds.

Fox vs King 2.0 Coilover Shock Friction

Rebound Damping:

Low speed rebound damping impacts handling and harshness while high speed rebound impacts control through whoops.  Fox 2.0 Coilovers use 2 x .070″ bleed holes and King 2.0 Coilovers use 3 x .056″ bleed holes, despite these differences performance between the two is relatively similar.  Fox uses a smaller diameter (1.425″) rebound disk than King (1.50″) without any differences in performance.  On compression Fox uses a 1.60″ diameter disk which appears to limit port inlet flow more than King (1.50″ diameter), however rebound speeds are generally very low when compared to compression and the low flow path does not impact performance.  Both Fox 2.0 Coilovers and King 2.0 Coilovers offer excellent rebound damping, particularly for lighter vehicles.

Fox vs King 20 Coilover Rebound FlowFox vs King 2.0 Coilover Piston Rebound Port

Fox vs King 2.0 Coilover Piston Rebound Port Inlet


Compression Damping:

It is important for off-road shocks to provide a linear damping curve and to have very little hysterisis in the curve.  Linear damping with pyramid stacks (decreasing OD through stack) gives control of the damping curve to the shock tuner to build progressive rate flutter stacks for improved feel over small bumps and hard hits.  Both Fox 2.0 Coilover Shocks and King 2.0 Remote Reservoir Coilovers provide good, linear, consistent damping as you would expect from a high quality race shock.  In the graph below, the shock starts making 40 lbs of force at 0 IPS (left side, top lines), and increases to 520 lb of force before slowing back down to 0 IPS and making -5 lb of force.

King vs Fox 20 Coilover Compression Damping Performance

Fox vs King 2.0 Coilover Piston Compression Ports

Fox vs King 2.0 Coilover Piston Compression Port Inlet


It is important for shocks to resists cavitating on high speed events because cavitation leads to a loss in control and can damage the shock.  Cavitation is a function of remote reservoir ID and nitrogen pressure.  Fox 2.0 Coilovers and King 2.0 Coilovers have nearly the same ID (1.813 vs 1.750, respectively) but Fox recommends 200 psi and King recommends 150 psi.  The increased pressure in 2.0 Fox coilovers results in improved resistance to cavitation.  It is possible to run higher pressure in 2.0 King coilovers but higher pressures increase friction and seal wear.

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Factory Tuning Limitations

While both pistons are capable of providing excellent performance King has limited tuning options. King will only build a pyramid stack (decreasing OD through stack) and add one flutter disk. While a single flutter disk is industry standard, and we used to tune that way (and still do in the rare instance it delivers the desired performance), we’ve found that a single flutter disks does not give you full control of the damping curve. We now use AccuTune double flutter stacks which deliver vastly superior comfort on small bumps and more speed in the whoops. As a result we pull apart every King shock and re-valve it in house. Fox’s stock valving is considerably worse than King’s stock valving, however Fox will valve shocks however we’d like.


Fox and King both build race quality 2.0 remote reservoir coilovers whose performance and durability will exceed that of a conventional sealed monotube, but Fox really goes the extra mile to deliver a highly durable shock.  From the piston rod, to the seals, to the oil, Fox delivers superior short term and long term durability.  When it comes to valving neither Fox 2.0 Coilovers nor King 2.0 Coilovers deliver optimal performance without being re-valved. Whether you’re working on a daily driver or hardcore race truck, we can help you select the right shocks and set you up with the right springs and valving to get the most out of your Fox 2.0 Coilover shocks or King 2.0 Coilovers shocks.


Related Fox and King 2.0 Coilovers:


How To Select The Right Length Coilovers

Check Collapsed Length

Off-Road coilovers are not meant to be used as bump stops because the wiper seal is usually exposed externally.  Follow these steps to make sure your shocks don’t become the bump stops.

  • Collapse your suspension until it hits the bump stop
  • If you have rubber bump stops measure how far they can collapsed until they’re metal to metal
  • Measure eye to eye length on the shock mount
  • Subtract the metal to metal distance from the eye to eye shock mount distance, this is your maximum collapsed length

Check Extended Length

Shocks need to have enough travel for a reasonable amount of down travel but without allowing drivetrain, steering or other components to bind.

  • Droop suspension while checking for bind on the drive shaft, cv joints, steering, or any other part
  • Test articulation and steering to be sure nothing hits or binds in every kind of motion
  • Measure eye to eye length again, this is the minimum extended length
  • Select shocks that are at least 1″ longer than extended length to allow for limit straps to set full extension

Fox 2.5 Factory Series vs 2.5 Performance Series Coilovers

*Updated 4-20-16 to reflect newest version of Fox 2.5 Factory Series Shocks.

In 2014 Fox introduced the 2.5 Performance Series line of coilovers to their offroad division in order to better compete with lower cost King and Sway-A-Way shocks.  Fox 2.5 Performance Series Coilover shocks use industry standard components to deliver a lower price point than the Fox 2.5 Factory Race Series coilover shocks.  Now included in this article is a review of of recent upgrades to the Fox Factory Series coilovers.


Fox vs Performance Series Coilover & Smoothie Shocks



Friction is one of the most important elements in a shock, it causes harshness and reduces damping control.  The effects of friction are most significant on lightly sprung vehicles because friction is a larger portion of the damping.

In 2016 both Performance Series and Factory Series Fox 2.5 Coilovers received upgrades to reduce friction and increase dampening consistency.  Both shocks now report 18 lb (+/- 3 lb) of friction, which is good for a large bore off-road shock.  Furthermore, the friction appears to be stable across a range of gas pressures (tested up to 300 psi) which will result in more consistent performance.


Piston & Damping

In 2015 both the Fox Performance Series and Factory Series 2.5 Fox coilovers received new pistons.  The Fox 2.5 Factory Series piston is a high flow race design with 3D CNC porting to eek out every bit of ride quality, while the Performance Series is an industry standard, low flow piston designed with manufacturing in mind.  For $50 we can upgrade the 2.5 Performance Series shocks to use the 2.5 Factory Race Series piston.  Both have 2 x .098″ bleed holes from the factory, although we can install one or three bleed pistons as necessary.


The new Fox 2.5 Performance Series coilovers have much smaller compression ports which greatly reduce the flow and increase load.  The changes in compression damping are very dramatic and may require some unique valving tricks on light vehicles with 1:1 motion ratio to avoid harshness on compression.  Luckily for you we know all the tricks necessary to make these perform.

The new Fox 2.5 Factory Series coilover pistons use a high flow design with 3d CNC ported inlets which reduce harshness and improve the ability to fine tune the shock.


Fox 2.5 Piston Factory vs Performance - Compression Outlet


The Fox 2.5 Performance Series coilover piston features small round rebound ports which can require tuning tricks on lightly sprung vehicles.  The new 2.5 Fox Factory Race Series coilover pistons have larger round rebound ports which have further improved tuning capability on lightly sprung vehicles.  Both pistons should perform well on heavily sprung applications.


Fox 2.5 Piston Factory vs Performance - Rebound Outlet


Wear Band

Both Performance Series and Factory Race Series Fox 2.5 Coilover Shocks use a new upgraded wear band with o-ring preloader.   The new wear band uses a lower friction, less abrasive, and more durable material which is coupled with an o-ring  to improve damping consistency.  Over time wear bands loose material which ends up in the oil, reducing dampening consistency and leading to increased wear, the new wear band is a great improvement on each of these characteristics.  The pre-load o-ring also increases consistency by reducing oil blow-by around the piston.   The new wear band is a big improvement and a great upgrade to both shocks.

Fox 25 Performance and Factory Piston Wear Band



There are some substantial durability differences between the Performance Series and Factory Race Series Fox 2.5 Coilovers.

Piston Rod

Piston rods are one of the most important parts of a shock in terms of durability.  Damaged rods from flying debris are the leading cause of leaking shocks, having a harder and stronger piston rod reduces the likely hood of a leaky shock.  Factory Series shocks use a 17-4 H900 stainless steel piston rod that is extremely strong, resistant to flying debris and corrosion resistant.  The Fox 2.5 Factory Series coilover piston rod is the best in the off-road industry.  Fox 2.5 Performance Series coilover shocks use a high carbon steel piston rod that is likely 1040-1060 case hardened steel.  High carbon steel rods are standard in the industry and are similar to King Performance Series shocks.  Below you can see that the Factory Series piston rod has twice the resistance to flying debris (HRc) and is almost twice the strength.

Fox Performance vs Factory Series Piston Rod Hardness and Strength



Fox Performance Series coilovers inherited a urethane wiper and Buna main seal from the original blue Fox 2.5 coilovers.  The main seal in Fox Factory Series shocks have been upgraded to Viton.  Viton is known for its superior durability and heat tolerance.  After many cycles at high temperature Buna can get hard making it more susceptible to leaking.  It is hard to speculate on the exact temperature range for the Fox seals but common Buna is rated for 225 F while common Viton is rated for 350 F.

Fox 25 Performance and Factory Seals

Oil & Valving

Fox 2.5 Factory Series & Performance Series shocks both use the same JM92 oil and the same valve disks on compression and rebound.


External Parts

Bearings & Hardware

Both Factory Series and Performance Series 2.5 Fox coilovers use COM-10T (5/8″ ID) bearings and both come with spacers to fit a 1/2″ bolt at 1.50″ wide.  Fox 2.5 Factory Series spacers are made from 304 SS while  Performance Series spacers are zinc plated steel.  Both spacers have o-rings to control rotation, simply remove the o-rings to gain full misalignment.

Fox 2.5 Factory vs Performance Series Bearing Spacers


Dual Rate Stop Nuts

Fox Factory Series dual rate coilover nuts are made from steel, and Fox Performance Series dual rate coilover nuts are made from aluminum, both use the same dual rate slider.  No functional differences have been noticed between the two.

2.5 Factory Series Piston on 2.5 Performance Series Dual Rate Nuts


Remote Reservoir & Hose

In 2016 the Factory Series shocks were upgraded to use the new Aluminum reservoirs.  The new reservoirs are lighter and feature redesigned internal parts which improve dampening consistency.  The Factory Series shocks use a high quality -10 braided hose with re-useable style aerospace fittings while Performance Series shocks are made with industry standard -8 crimp fittings and rubber hose.

Fox 2.5 Performance Series vs Factory Series Remote Reservoirs & Hose


Rod Ends & Lower Spring Perches

Fox 2.5 Factory Series coilovers use a long rod end which fits well into trailing arms and keeps the spring perch out of harms way.  Fox 2.5 Performance Series coilovers use a shorter rod end and may require careful fitment inside of deep trailing arms.  The longer Factory Series rod end also explains some of the length difference between the two shocks. Factory Series 2.5 Fox coilovers use a clam shell design to hold the spring force while the Performance Series 2.5 Fox coilovers use a snap-ring.  The snap ring used is very heavy and prone to scratching the rod end. Fox 2.5 Factory Series coilovers are available with a 1″ shorter rod end, which makes the shocks nearly identical in length.

Fox 25 Performance and Factory Lower Spring Perch and Rod End



Fox 2.5 Performance Series coilovers use industry standard parts, and a lower performance piston to deliver exceptional value for recreational users.  Extreme off-roaders, racers and anyone with in corrosive environments should use the Fox 2.5 Factory Series coilovers.


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