When are Tender Coils ok to use?
April 14, 2020
What are Tender Coils?
Tender coils are very short, light rate springs that you see at the top of some coilovers. Their purpose is to keep the main springs tight when they come loose due to negative preload. This is typically only found on light weight vehicles where spring rate options are limited.
Why don’t we always use tender coils?
In the ideal world springs will always have positive preload. Positive preload forces the axle to droop giving you traction when articulated and helping the tires stay in contact with the ground over rough terrain and whoops. Positive preload is great for off-road performance and on-road comfort. When you have positive preload the springs don’t come loose and tender coils are not necessary. Tender coils are only necessary when a coilover has negative preload.
When do we recommend tender coils and negative preload?
Setting a suspension up with positive preload results in lower spring rates than negative preload. Stiff springs do not compress enough to provide adequate force at full droop, so they are loose, requiring tender coils. Stiff springs and tender coils are required when the spring rates recommended by the common “positive preload” setting are too soft. The springs may be too soft because of the spring choices available or because the light rate springs will not function correctly for the vehicles intended use.
The most common causes of the needing low spring rates are that the vehicle has too much droop, or that the vehicle is very light.
1: Bob installs springs with zero preload
2: Bob installs coilovers on vehicle. With full weight on the coilovers, vehicle is sitting 2″ too tall.
3: Bob lowers preload 2″ to get correct ride height (because lighter springs are not suitable for this vehicle).
4: Now springs are loose at full extension.
5: Bob installs tender coils to keep springs tight at full extension.
6: Ride height is correct with tender coils.
*Installing lighter springs are the preferred option on most vehicles
Installing Tender Coils?
Installing Tender coils also require a Tender Coil Slider. In order to install both of these, the top portion of the shock assembly (Top Cap) will need to be removed. Once removed, the slider and Tender Coil Spring can be installed. This work should be done by a professional since it requires the shock to be taken apart.
Are lightweight vehicles more prone to needing tender coils?
Lightweight vehicles are more prone to needing tender coils for several reasons:
-Sometimes light enough springs just aren’t available.
-The load carrying capacity may not be adequate on lightweight buggies with positive preload.
-Lightweight vehicles, especially rock crawlers with low gearing also frequently have problems with torque lean.
To combat these issues on light weight vehicles we recommend running sway bars front and rear (help with stability), limiting droop to 6” or less (keep rates high), have more up travel (if you want more total travel), make sure your suspension geometry is well refined (prevent body roll and torque lean), and mount the shocks to the links (improve spring selection & prevent bowing).
Why aren’t light rate springs available?
When springs get light so does their sensitivity to changes in rate. The typical 25 lb/in increments are not fine enough increments to have a light rate spring that works for many applications. Manufacturers don’t make more rates because the light rate springs have low demand and high warranty rates due to bowing.
Why are light rate springs more prone to bowing?
Light rate springs are made using small diameter wire. Small diameter wire is not very stiff and results in a spring that isn’t very sturdy, and is much more prone to bowing. To compound the issue, most vehicles needing light rate springs, also need them to be very long and even more likely to bow. Even when made correctly it is common for people to have issues with bowing light rate springs that can’t be fixed.
Why won’t light springs function correctly?
Springs are a critical part of the suspension which affect load carrying capacity, handling, and how the suspension reacts under load.
Load carrying capacity is important for any vehicle which will see fluctuations in weight. If the spring rates get too low the vehicle will sag way too much with any additional load. If your vehicle sags too much you may give up critical up travel resulting in a vehicle that is much more likely to bottom out.
Handling and stability are severely impacted by low spring rates. We rely on the overall spring stiffness and dual rate nuts to provide some handling and stability. If we did it all in the shocks they would be way too stiff (plus we don’t tune for handling).
Springs help control the suspension movement and impact how the vehicle reacts under load. Off-road vehicles with high hp or low gearing can cause the suspension to torque over. Suspension geometry, shock valving, and spring rates are the three things that resist this force, and all three need to be correct for the vehicle to work correctly.
Why don’t we recommend having more than 9” of droop for most vehicles?
We have found that for most application 9” of droop is where the spring rates really fall off a cliff and start to cause the issues listed above. More than 9” of droop on most common builds does not improve off-road performance and can cause issues.
Performance is not increased because you will probably have tender coils which don’t provide any downward force on the axle. Without downward force the tire doesn’t have much traction and isn’t providing much stability.
By moving some droop travel to up travel it will provide a more comfortable ride and more balanced suspension. With more up travel the tire can absorb the bumps before hitting the bump stops. Under articulation the tire can come up higher before it starts to raise the chassis, creating a more balanced and stable vehicle.
With too much droop on solid axles it can to swing too far side to side, and the tires start getting into the body and chassis. By having more balanced up and down travel the axle will not swing as much and you may be able to achieve more travel overall.
Summary
Tender coils are required when the springs can’t be soft enough to provide positive preload. Tender coils help prevent the springs from coming loose and damaging the shocks with negative preload. Common causes of negative preload are too much droop or lightweight vehicles needing greater spring stiffness than allowed for with positive preload.
