In the never ending search for speed – bump springs have become an important part of chassis tuning and performance for a wide variety of racing disciplines on both dirt and asphalt. If you are not at least trying to tune with bump stops then you are falling behind the teams that are tuning with them and gaining valuable experience with each race. In the grand scheme of things bump springs are a relatively cheap tool that can have a big effect on the speed and overall balance of your car.
“Bump Stops in General”
The term “bump stop” in general can be misleading to say the least. When you hear the term “bump stop” your mind naturally thinks of something relatively hard that an object “bumps” against to stop it from moving any further which is correct in some aspects but the misleading part is that the mind associates the stopping of the object to always being rather harsh or violent when it contacts the bump stop but several factors come into play that determine the overall harshness or lack of harshness when an object engages a bump stop. The spring rate of the bump stop, the weight of the object and the speed at which it is traveling when it engages the bump stop are all factors that dictate the “harshness” or “feel”. The words “harshness” or “feel” refers to the stopping motion or deceleration rate of the moving object. A faster deceleration rate creates a harder more pronounced feel and a slower deceleration rate creates a softer less pronounced feel to the driver. Harder materials create faster rates and softer materials create a slower rate. In the case of a racecar - the moving object is the chassis as the car’s suspension absorbs the inconsistencies in the surface of the track, the banking of the corners and the aerodynamic forces on the body and wings. By changing the configuration and spring rate of your bump stop you can adjust and tune the deceleration rate to create a softer or harder feel depending on the application and desired feel by the driver.
Bump Springs are the latest thing we have been using to tune the chassis – the bump spring is superior to any type of foam or rubber material in that it has a true linear spring rate and can be used as a 2nd stage suspension along with the torsion bar or coil spring. Bump Springs are a bit more forgiving as well – meaning they are not as sensitive on the actual travel or gap used before the car engages the stop as other types of stops. They give a smoother feel to the driver by minimizing the change in rate as the bump stop compresses. We’ve had a bit of success with teams using them on the RR as well as the LR.
“Advantages & Application”
Although bump springs and stops are being used in nearly every type of racing their applications can vary greatly between the different types of racecars using them. Everything from quarter midgets to road race cars have adapted the use of bump springs to increase performance. Asphalt late model racing is easily the most vested in the use of bump springs and stops since widespread use of them became normal in the NASCAR Cup Series many years ago.
The real advantages is the things it allows you to do with your chassis that you wouldn’t be able to do without one along with creating significantly higher loads on the contact patch of the tires when the car is engaged on the bump stop thus creating much more bite. If you can think outside the box – the sky is the limit for what you can do.
For example - Asphalt late models run an A-frame type front end geometry which allows each tire to move independently. This type of front end geometry is tuned to control the camber and caster angles of each tire at maximum travel in the corners in order to create the optimum contact patch for the front tires and give them the most grip in the center of the corner where they need it to turn. It is critical that the nose of the car stays planted to maintain the proper angles for front tire grip as long as possible. The caster and most importantly the camber changes as the nose of the car moves up and down. As the nose rises the tires lose camber which changes the contact patch and takes away grip creating a push condition. With the use of bump stops they can run much softer springs in the front end than normal so the nose stays planted much longer - maintaining the best contact patch for the longest period of time to increase corner speeds - the bump stops act as a 2 stage suspension with the much softer springs by keeping the car from over traveling and hitting the track while at the same time adding load to the contact patch of the tires when the chassis is engaged with them.
Taking that type of thinking to a sprint car application – years ago at a test with a sprint car team on a 3/8 semi banked, pretty fast track that was dry slick - we had been making laps - trying things with our normal set-up in the car LF, LR, RR 1.000 and RF 1.025 bar – we then put a .950 LR bar in the car flat off the block without changing anything else and made some laps, we came in and kept winding rounds out of the LR and making more laps. We ended the at 4 rounds out of the LR with that .950 bar – the car was 1 ½” low at ride height and the car drove fine and was just as fast with the 4 rounds out as it was with our normal set-up in the car because the car sat down and basically rode on the bump stop. The LR ride height was a non-issue because you make your speed in the corners where the car is down into the suspension. With the bump stop maintaining the proper attitude and height of the car in the corner the ride height sitting in the pits was irrelevant. The goal of this was to show the team how a “good” bump stop on the car can allow you to run different bar combinations and set-ups than you ever thought possible by letting the bump stop act as part of the suspension and do more of the work instead of just being a travel limiter - getting them to think outside the box a little. This test was done utilizing “Game Changer” bump stops from Slade Shock Technology. This type of thing can only be done with a good bump stop that has the correct spring rate, compression and rebound properties. The molded foam bump stops that so many teams use would not work well in this example.
Winged sprint cars have used bump stops on their LR shocks as a simple travel limiter for decades and they mostly use a very primitive form of material that doesn’t have very good predictable compression/rebound properties for a bump stop. There is much that can be gained by using some of the better materials that are available on the market today. It is key to have some data on the materials you are planning to use so that you can make intelligent decisions when tuning or changing your bump stop gaps and configuration.
“Tuning with Bump Springs”
Winged sprint cars are unique in the fact that they load the left side of the car as the wing catches the air when the car yaws on corner entry. The LR corner of the car is the most important corner of a winged sprint car because it controls the attitude of the car on entry. It is desirable to have the car just barely engage the bump spring at the end of the stretch so when the car enters the corner it is already engaged and can’t get a running start at it when the car wings over. This creates the most consistent feel on entry because when the car wings over it simply loads and compresses the bump spring but you won’t get the transition of the car traveling down and then engaging the spring.
On smaller tracks the car will actually only engage for a short period of time as the car enters the corner – in most cases by the middle of the corner the car is off the spring and transferred weight to the right side of the car helping the car square up and change direction.
Larger tracks with high wing speeds will actually sit the car on the bump spring from corner entry to corner exit because of the sustained left side force on the wing through the corners.
By tuning your gap (the distance the car travels until it engages the bump spring) you can have a dramatic effect on the balance of the chassis for different conditions. To simplify it you can allow your car to travel farther on the left side for heavier or wetter conditions which will take some load off the right side and allow the car to slip across the tacky dirt but as the track goes away you can adjust or change your bump spring to close the gap some and take travel away from the left side which will not allow the car to unload the right side as much for slicker conditions.
Non winged cars are quite the opposite – with no wing they naturally load the right side in the corners as the car is sliding. The most effective use for a bump spring on a non-winged car is the RR corner.
One thing to watch out for when tuning working with bump springs is real rough tracks – I don’t recommend bump springs or stops on rough tracks. What can happen is the car can hit the bump spring hard when the car goes through the roughness and when it hits hard it can actually compress the sidewall of the tire causing the tire to bounce when it unloads and generally when they start bouncing they don’t stop until the red flag is out. As with anything in racing – common sense is your best tool.
“Looks can be Deceiving”
When choosing a different bump stop or purchasing a new bump stop looks can be deceiving. There are multiple different colors and shapes and sizes of bump stops on the market. None of this means anything. The only thing that should really come into play when selecting a bump stop is the actual stiffness of the material as it is compressed and rebounds. If the retailer who is selling the bump stops doesn’t have any data on them I would highly recommend you find one who does. You are kidding yourself if you think squeezing them with your hand is any way to judge them. As important as bump stops have become it is only smart to have some data on what you are putting on your car.
To generalize – bump springs and stops are a tool that can be used to make you faster but not every person is cut out to understand the data and applications of tuning with them. By no means am I saying that it is a must to be aggressively tuning and changing bump stops all the time to be able to compete but to point out the possibilities that are available should you choose to get deeper into working with them. If you have any questions or comments about what you have read here you can feel free to contact me.