May 4, 2021
- How do Bypass Shocks Work?
- Shock Zones & Basic Tube Layout
- Bottom Take Off Reservoir
- Off the Shelf Options
- Multiple Compression & Rebound Tubes
- Staggered vs Overlapping Tubes
- Piggyback vs Remote Reservoir
- Shock Diameter
- Tube Layout around body
- Tube Heights & Diameter
- Custom Setup Recommendations
- Fox vs King Comparison
- Check Valve Design
- Bypass Adjusters Screws
- Piston Rods
- Oil Seals
Bypass Shocks PART 5
- Bypass Shock Setup
- Up Travel
- Motion Ratio – Coilover and Bypass
- Total Weight
- Weight Distribution (front/rear)
- Unsprung Weight
Up travel gives the shocks room to absorb bumps without bottoming out. Bypass shocks are position sensitive and each zone needs room to do their job. Without adequate up travel you may find the suspension is either too stiff or too soft (or both), without the perfect spot in between. Without enough up travel a bypass shock may get into the bump zone on normal size bumps. Attempts to make it softer will result in bottoming out too easily. From our experience 5” of up travel at the wheel is the minimum for a bypass shock to function correctly, and the more the better.
Down travel is necessary to let the axle droop into the bumps and consequently soak them up comfortably. But too much droop is unnecessary and can cause sloppy handling and other issues with off-road performance.
The primary method for selecting springs is to look at preload and make sure the springs never come loose, and adequately push the axle down. There’s more to it than that, but for this illustration let’s stick to that requirement. If you have too much droop it will drive the spring rates way down. If you start with the vehicle drooped out, shocks fully extended, the spring rates have to be soft enough to compress all the way to ride height. The more droop you have, the more the springs have to compress to get back to ride height. Low spring force can reduce handling, g-out performance, drive shaft torque resistance, and load carrying capacity. For most vehicles 9” of droop is plenty.
On vehicles with very little droop the tires aren’t able to follow the terrain properly which can
cause poor ride quality. In addition, vehicles with too little droop may require spring rates that
are way too high which can lead to more ride quality issues. Alternatively vehicles that have very
little droop as well as too much preload on the springs can run into coil bind before bottoming
out. For most vehicles running bypass shocks 5” of droop is the minimum amount we
Motion Ratio On axle
If you’re mounting shocks on the axle, but looking for more travel than they provide it is common to lay them over at an angle. Laying them over at an extreme angle reduces their effectiveness and the quality of the shock damping. As the shock compresses the motion ratio usually gets more extreme. Having a position sensitive bypass shock does help, but does not totally compensate for it. We recommend keeping the bypass shocks within 15 degrees of vertical. Gaining more travel at the expense of the quality of the travel doesn’t get you ahead. If you have to make sacrifices, odds are you can give up some droop and never notice it’s gone.
Motion Ratio on Trailing arms
In the search for more travel it is tempting to put the shocks very far up the links in order to pull big numbers. As leverage increases on the shocks it can make them a lot more difficult to tune because the shocks quickly get too soft. Not only are they moving slower, but less of the force is getting to the chassis. Typical prerunners, U4 racers, and other vehicles with similar weight (650-1,000 lb/corner chassis weight) and running shocks in the 2.5 and 3.0 size should shoot for a motion ratio of 0.6 on the coilover, and 0.8 on the bypass, or less (higher numerically). Light weight vehicles can run more motion ratio, and vehicles with bigger shocks can run more (numerically less) motion ratio. This is not an exact science and there is a lot more to it than meets the eye. Most experienced chassis builders have it pretty well figured out. Another source of reference can be proven vehicles with similar weight and shock package which are running similar terrain. And of course you can always ask for our input when ordering shocks.
Coilover & Bypass Orientation on Trailing Arm
For a coilover/bypass setup on a trailing arm, you will find the coilover mounted closer to the chassis pivot and the bypass closer to the axle. Traditional shocks such as coilovers are better at low speed control and in this combo that is what we use them for. Bypass shocks are better at high speed control and that’s most of what the bypass tubes adjust. With the coilovers mounted closer to the chassis they move slower than the bypasses, so each shock is being more accurately used for what it’s good at.
We tune shocks in such a way that each shock has a purpose and a specific job to do. Purchasing too small or too large of a shock can prevent them from working well together. If your shocks are too small the vehicle will always be too soft and bouncy. If your shocks are too large it will be very harsh. For on axle mounting (650-1,000 lb/corner chassis weight) a 2.0 coilover and 3.0 bypass, or 2.5 coilover and 2.5 bypass are ideal. For the trailing arms discussed above a 2.5 or 3.0 coilover and 3.0 or 3.5 Bypass are good choices (the big shock package should be on a heavier vehicle). These are recommendations intended to get you started; your use case, budget, vehicle setup, and other factors should be considered as well.
In shocks there are inherent forces caused by flow restrictions. These are forces we generally don’t have control over, and can’t tune, but in most cases help the shock function correctly. For lightweight vehicles with on-axle mounting and big shocks, these forces can be too large relative to what the vehicle wants. The opposite goes for very heavy vehicles, it can be very hard to get the control we need out of the shocks. If you are building light you should consider trailing arms, especially if you want big shocks. If you are building a heavy vehicle plan for big shocks and having them closer to the axle.
Weight distribution is fundamental to having a performing vehicle. If one end is very heavy and the other is very light it throws off the balance of the vehicle. When the balance is off it is going to make it very difficult to tune. A 50/50 weight balance on the chassis is preferred for most vehicles.
The weight of the axle/tire/wheel combo matters. Getting heavy axles, tires, and wheels moving while keeping the chassis stable is very difficult. If your unsprung weight is similar to the chassis weight, it’s going to be difficult to make things work right. If you want to reduce the weight of your vehicle, start with the rolling weight, then axles. You’d also be surprised how much performance can be gained by losing weight on the wheels and tires.
Suspension geometry has a large impact on how the chassis reacts to bumps. If your vehicle squats a lot when accelerating the chassis is going to squat every time you hit a bump or whoop. This can load up the rear suspension and contribute a lot to bucking issues. The more travel you have, and the softer your setup, the less important this becomes.
Q: Why do trophy trucks have such poor chassis control?
A: Trophy trucks are incredible in the open desert floating through huge whoops, but they are ungainly beasts on road. More recently there has been renewed effort to improve their handling, but it is an uphill battle. Trophy trucks are very heavy and mount their shocks very far up the links in order to get more travel. All the droop also translates into very soft spring rates. Even though they are running the biggest shocks available, they are no match for the weight and motion ratio. So out of the five things we evaluated four of them are contributing to the poor handling.
Motion Ratio: Extreme, shocks very far up the links to gain travel
Shock Size: Biggest available
Total Weight: Very heavy (over 1,300 lb/corner chassis weight)
Weight Distribution: Good
Unsprung Weight: Good
Suspension Geometry: Good (lots of variation, but not a critical factor due to travel)