Bumpstops are starting to...
Bumpstops are starting to make a comeback. KEVIN THORNE
Every now and then, a new design or advancement in technology produces a new and innovative part for short track competition. At first glance, it might appear that bumpstops fall into this category, but in truth they are nothing new. Bumpstops first broke on the scene in the late '80s and early '90s. NASCAR quickly outlawed them because of the potential complexities and to help smaller teams that might be unable to adapt.
Bumpstops are pieces of rubber or plastic attached to front shocks to limit shock travel. They are applied to the front only, because this is where the chassis will experience the most travel in a corner.
This may all seem simple enough, but I can assure you the simplicity ends once you attach them to a shock. Why limit shock travel in the first place? The simple answer is that the shock and spring setup under a Cup car all work together and the team that could figure out the perfect combination would dominate until other teams caught up.
With the introduction of the Car of Tomorrow, NASCAR has allowed bumpstops back into the sport. As with most decisions that NASCAR makes concerning the cars under its sanction, the ripple effect can be felt all the way to the local short tracks. Bumpstops are now beginning to be allowed in series all around the country, which prompts SCR to give the grassroots racer a primer on bumpstops.
Again, the problem is that they seem so very simple, but this is where most teams go astray. Bumpstops usually fit into a small cup that slides on the shock, and there are a number of different sizes and levels of hardness to choose from.
The first time I used bumpstops was at Iowa Speedway last fall in a Hooters Pro Cup race, as we had trouble with the chassis and exhaust bottoming out and scraping the pavement in the corners. The car was experiencing way too much travel in the front. Without bumpstops, bigger springs in the front end would have been our only solution and this would have changed the chassis dynamic, requiring a new setup underneath the car. Instead, we measured our shock travel and put the bumpstops to where the shocks sat on the stops, limiting the travel in the front end. We qualified Sixth with the stops.
So, we learned, there is speed to be found in bumpstops. But there is no good reason to slap them on and go racing on Friday night. You need to test over and over again to see if you can find speed. The advantage is the ability to run softer springs in the front of the car without the worries of bottoming out the chassis or rubbing the exhaust. Once you place the stops on the front shocks, you then have to work to get them perfect.
The height of the bumpstop is vitally important to how the car handles through the corner. If you don't have the heights correct on the bumpstops, the shock can actually hit the stop and then bounce back up and try to rest on the other bumpstop. This is where it is extremely helpful to have a shock specialist helping your team. Our team uses R.E. Suspension out of Mooresville, North Carolina.
This is basic bumpstop setup,...
This is basic bumpstop setup, with a cup for the bumps to sit in, two sizes of stops, and two sizes of shims for spacing. JOHN GIBSON
R.E. Suspension has built...
R.E. Suspension has built shocks for every type of racing imaginable, from dirt to asphalt. JOHN GIBSON
Here is a 1/4-inch shim for...
Here is a 1/4-inch shim for raising or lowering a bumpstop. These shims are so crucial that they are now being produced as small as .o1-inch. JOHN GIBSON
Notice how the front of the...
Notice how the front of the car is almost completely sealed off from the pavement. This is one of the main objectives when running bumpstops. KEVIN THORNE
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...
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...
Notice how the weight on this car is being transferred to the right side of the car. With bumpstops, the team could limit the transfer of weight. KEVIN THORNE
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.