It sounds pretty simple, just change a spring, right? There are some things to know before you change a spring in a race car. Some of these are simple and a no brainer, while others are not that obvious or difficult to understand, but necessary in order to do the spring change correctly.
Ride Height – This first tip is known by almost every racer on the planet, but if you are new to the sport, here you go. When you change a spring, and that usually means you are changing the spring rate with a new stiffer or softer rated spring. You will then need to adjust the spring height so that the ride height remains unchanged.
There is a saying in racing that holds true most of the time. It is, don’t make more than one change at a time. That way, whatever happens to the cars handling can be attributed to that change. If you change more than one thing, you are left guessing which one, or if both, caused the change in handling.
At ride height, we have a load distribution on the four tires. We measure these loads with a set of scales, or at least I hope you do. We want those loads to remain unchanged after we change a spring. So, you need to measure your ride height, coil-over length, etc. so that you can adjust the spring height to bring the ride height back to what it was before the spring change. Then the tire loads will remain unchanged.
If you change the spring angle, such as in a dirt Late Model or Modified, in order to change the rate the car will feel and the spring base, you will have also changed the ride height and load distribution if you don’t adjust the height of the spring.
Putting more angle in the coil-over will lower that corner of the car, so you’ll need to raise the spring height to compensate. Putting less angle in the coil-over will raise that corner of the car and you’ll need to back off on the adjuster ring a bit to get the ride height back to normal.
Only change one spring at a time. If you don’t, things will get confusing really quick. After you change the spring, roll the car back and forth before you check the ride height measurements again to settle the tires.
Shock Change – When you change your spring rate, you will need a different rate of shock to control that spring. If you are making a significant change in spring rate, the shock change will also need to be significant.
Basically, a stiffer spring will require a stiffer rebound rate to control the stiffer rebound reaction of the spring. The shock can also be changed to have less compression because a stiffer spring resists compression more so than a softer spring.
An example would be going from a conventional setup to a bump setup. In these setups, the right rear spring you will need will be stiffer than what you ran for the conventional setups. Therefore, you’ll need a shock that is stiffer in rebound rate.
Balance Change – When you change your spring rates, you are not only changing the travel of that corner, you are changing the handling in a lot of cases, especially in the rear.
Say you need more heat in the left front tire, so you surmise that a stiffer RR spring rate would force more loading on that tire. This is the way many racer think, and they would be right, but for reasons they probably didn’t consider.
A stiffer RR spring, when the other springs have not changed, will alter the dynamic balance of the car. The rear will roll less and since it is attached to the front end by way of a stiff chassis, it will roll less too. This change will cause those two to now by out of balance, if they were in balance before the change.
This imbalance in this case does cause more load to end up on the LF tire at mid-turn, but at the expense of the dynamic balance, which we value a lot to make the car more consistent.
If you change the RR spring, or the LR for that matter, you’ll need to make a change to the panhard bar (rear moment center height) so that the dynamic balance stays the same. If you increase the RR spring rate, you’ll need to lower the panhard bar. If you increase the LR spring rate, you’ll need to raise the panhard bar.
Panhard Bar Change – If you change the RR spring rate, then the amount of travel that spring will experience through the turns will also change. A stiffer spring will travel less, all other things being equal to before the spring change.
If it travels less, the right end of the panhard bar (assuming the car has a right side chassis mount for the panhard bar) will then end up being higher that it was with the softer spring. If you can determine how much less the new spring travels than the original spring, you can then adjust your panhard bar height down so that it will end up at the same height as when the softer spring was in the car.
You can check the shock travels to find out how much different a new spring travels than the old spring. Use this difference as a measure of how much to change the panhard bar height. If there is a motion ratio between the end of the panhard bar and the spring, then allow for that ratio when changing the bar height.
Rear Steer – Many chassis are built such that they can rear steer when the chassis rolls. Most teams will adjust the trailing arm angles to eliminate the rear steer. If you change a spring, that can cause rear steer due to the change in spring movement.
If you have determined the ideal right side trailing link angle, and then change how much the RR corner of the car travels, then you must change the trailing arm angle so that your rear steer amount will remain unchanged.
A RR that travels less (from a stiffer installed spring rate) will need less trailing arm angle than before the change to a stiffer spring rate. That is because it will end up at a different height after the chassis rolls.
The RR corner of the chassis will be higher with the stiffer spring rate, so if you don’t change the height of the front of the trailing link, it will end up higher than before the change and that will steer the rear end differently.
Conclusion – In any change you make to your chassis, consider what other parameters might also change as a result. Think about how the chassis will move differently and then make sure you compensate for that difference. Your setup and handling will stay much more consistent if you do.
Sources:
DRP Performance Products
www.drpperformance.com
888-399-6074
Gale Force Suspension
www.galeforcesuspension.com
251-583-9748
Intercomp Racing
www.intercompracing.com
800-328-3336
Longacre Racing Products
www.longacreracing.com
800-423-3110
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