Notice how the front of the car is almost completely sealed off from the pavement. This is
Like every other adjustment on a chassis, there is no magic number for setting bumpstop height in order to gain a couple of tenths. It will all depend on how much travel you are experiencing. The ideal way to test bumpstops is to make sure all of the travel measurements have been reset with the car sitting on the ground before hitting the track. These are usually determined with a black piece of rubber that slides up the shaft of the shock, showing how far the shocks are traveling in the corners.
Once you have the values reset, run five or six laps at race speed and then come in and measure shock travel. Pull off the shocks and put the bumpstops in place on the shocks. The choice of stop will depend on the track. If you are racing with high speed and lots of banking, a stiffer bumpstop, like the black one shown in the photos, will be required. However, if you are racing at a typical short track that is flat and without much grip, you will need to attach the softer, longer stop like the yellow one shown in the pictures.
This is how a bumstop looks once it is placed on the shaft of the shock. JOHN GIBSON
Once the bumpstops are attached, their height on the shaft needs to be set. This is done with the measurements from the five-lap run referenced earlier. The stop has to be placed where it will be useful. If it barely touches the shock as it travels, it will not improve the setup. Begin by taking the top of the bumpstop an inch higher than the amount of travel that the car experienced the first time out.
This is where driver feedback is crucial. The driver needs to feel how the weight is transferring and where the weight is resting. As the car enters the corner it will more than likely sit on the left-front bumpstop. But as the wheel is turned and the weight starts to transfer to the right side, it will rise off the left-front stop and sit on the right-front until the gas is applied. If one of the stops is too high or too low, then it will throw off the weight transfer.
The height of the stops can be controlled with shims. The ones shown in the photos are 1/4-inch shims, but they are now being produced in increments of .01-inch. They are that important. As a rule of thumb, if a chassis is transferring too much to the right-front wheel and is trying to lift the left-front tire, this can be controlled by raising the right-front bumpstop or by lowering the left-front stop.
Notice how the weight on this car is being transferred to the right side of the car. With
Bumpstops were created to assist the coil-bound setups that were being used across the country, where one of the front springs will be completely compressed. The bumpstops benefit this type of setup; if the spring is coil-binding too quickly and actually causing a tight condition, then set the bumpstop to where it doesn't stop the compression of the spring, but rather where it slows the compression down to a controlled rate.
I can't over-stress the importance of testing this type of shock setup before trying it at the track. Bumpstops can cause a lot of headaches if not installed properly. But most importantly, read your rulebook. Most divisions and series do not allow the use of bumpstops, but, again, following NASCAR's lead, some have crumbled recently and are allowing them.
Any racer who decides to try bumpstops should consult with someone who has experience using them. There is speed to be found, but it takes a skilled hand.