Aerodynamic gains outweigh other considerations in choosing shocks

Learn more about bump stops, which help keep the front end of the race car low to the ground by limiting the upward travel or compression of the front suspension.

Updated: September 12, 2008, 2:26 AM ET
By Bill Borden | Special to ESPN.com

The engineers, shock gurus and chassis specialists had pretty much perfected a way to keep the bottom of the old Sprint Cup car's nose stuck to the track through a combination of softer springs and special shocks that would hold the suspension down once it compressed either from braking or going into a corner. They are still working to perfect that accomplishment with the new car but they have made significant gains of late.

Holding the nose down allowed them to achieve more aerodynamic downforce by exposing more of the car's front to the on-rushing air. And it also limited the amount of air that could flow under the car. However, it made the old car a bear to drive because it was basically a solid or un-sprung front end once the springs collapsed and rested on the bump stop.

You could literally listen to the driver's voices quiver and shake as they talked on their radios because they were being violently bounced around by the uneven track surfaces and the only part of the suspension that could absorb any of the bump was the tires. It was faster but it was not a fun ride for the drivers.

One of the ways the teams are able to achieve lowering the front end to the precise level they want is to use the a "bump stop" to limit the upward travel or compression of the front suspension.

[+] EnlargeInside a shock absorber
Penske Racing ShocksAn inside look at a racing shock absorber.
Bump refers to the compression of the suspension under load. The spring carries or supports the weight of the car. A stiffer spring will compress less under load than a softer spring will so selecting a spring that is compatible with the desired suspension travel is very important.

However, teams cannot rely on the spring to stop compressing when the desired ride height is achieved so they build in a stop on the shock that limits the spring's travel. During practice and testing they can play with various amounts of bump stop rubbers or washers to adjust and tune the car's suspension travel to the ride height that they want for a particular setup.

While springs primarily control the compression or bump in a suspension the shocks primarily control the rebound as the spring tries to return to its normal length. If the spring did not have a shock or dampener then it would simply bounce up and down or pogo in increasingly smaller cycles until it returned to its proper height.

You probably have seen a street car going down the road bouncing up and down in an exaggerated manner because its shocks were worn out. Controlling the bump and rebound of the suspension is critical to making a race car fast and consistent in its handling characteristics.

The old Sprint Cup cars were experiencing five inches or more of travel in the front end suspensions. That is a lot of travel for a race car suspension even by today's standards.

The primary reason a race car requires stiffer springs than your street car is to limit the amount of body roll and lateral weight transfer in the corners. Race cars enter into the corners at greater speeds and experience a much greater load being exerted on their suspensions than a street car so it is important to keep the race car's nose up and not let it roll over.

I contacted Jim Arentz, the technical director at Penske Racing Shocks, to find out the latest thinking concerning the shocks that are now being used in NASCAR. Penske Racing Shocks are used by many of NASCAR's top teams, so Arentz is aware of all the latest tricks being used to gain a performance edge.

Arentz explained the difference between a Sprint Cup shock and your normal street or competition style shock.

[+] EnlargePenske Racing Shocks tech
Penske Racing ShocksA tech for Penske Racing Shocks scans the barcode on rear shocks to be used at Talladega and Daytona.
"The difference between NASCAR shocks and aftermarket shocks really comes down to the setup," Arentz said. "We supply many applications for street cars -- Corvettes, Vipers, Porsches, Mustangs, Ferraris, etc. -- and the same racing components found in NASCAR shocks are used there as well. The main difference comes down to the type of force characteristic the shock produces.

"Aftermarket shocks are usually built to be a compromise between a comfortable ride and the ability to have increased performance under aggressive driving conditions. NASCAR shocks are built specifically to address certain track conditions and in the case of the COT car, the front shocks have the responsibility to control the recoil of the shock off the bump rubber to maintain a constant front splitter height.

A comfortable ride is sometimes not the priority, Arentz said, because it is outweighed by the aerodynamic gains of keeping the front ride height stable.

"The main area of focus on the COT car when it comes to damper curves is what is called the 'zero-point,' " Arentz said. "The zero point is essentially the force value of the shock in rebound just prior to when the shock changes direction from rebound to compression or bump. It is a measure by which teams evaluate the load threshold of the shock required for each track to maintain their desired response from the bump rubbers.

"Basically, teams want the front shocks [and suspension] to compress quickly to the desired ride height [and splitter height] and once there, stay there through the corner for the most stable aerodynamic platform. This is why teams will typically run very soft compression forces and much larger rebound forces."

I think I've got it, Jim. That is why we hear the chattering of teeth whenever the drivers try to talk on their radios.

I also asked Arentz to explain the differences between linear, digressive and high-flow pistons.

"The main piston types run in NASCAR have various effects," he explained. "A linear piston creates an increasing amount of force as the shock velocity increases while a digressive piston characteristic has an increasing amount of force with shock velocity until a certain shock speed is achieved then transitions to a nonlinear force relationship.

"This is usually a 'blow-off' effect where the shock force 'flat-lines' and force no longer increases with velocity. This type of piston is useful for bumpy conditions.

"A high-flow piston is similar to a digressive piston as it has a large flow area to 'pop' the piston shims open with large shock velocities. This is a very popular piston in NASCAR due to driver feel and flow characteristics."

Uh, OK. He sure does know what he's talking about but now I have a headache and I'm almost sorry I asked. Who knew shocks could be so complicated. It's shocking to say the least.

Like most things in racing, the teams will take something that they find to be good and stretch it to the extreme. The loads that the shocks are now being asked to carry or control are much greater than ever before so durability and reliable performance has become a bigger concern.

As Arentz explained it, "Due to the use of bump stops in NASCAR, much higher loads are being transmitted through the suspension to the shocks than before. As data began to come in from teams who used load cells to measure these shock forces, it became apparent that some of the shock components could be made more durable for longer part life.

"We evaluated the major structural shock components and looked at ways to make a standard part with a finite lifespan live much longer under the new demands. The decision was made on behalf of safety to implement reinforced mounting ends on all NASCAR shocks as well as new adapters to allow teams to run a bump rubber cup on their shocks.

[+] EnlargeShock absorber
Penske Racing ShocksThe main reason a race car requires stiffer springs than a street car is to limit the amount of body roll and lateral weight transfer in the corners.
"The bump rubber cups are used to 'capture' the bump rubber to maintain a constant force vs. displacement characteristic as well as again increase part life.

"Because the bump stops are typically made from a rubber or urethane material, they are supposed to 'give' when a load is applied. If there is not a limit to how much these flexible components can be compressed, you may experience tearing or part failures," Arentz explained. "An added benefit of the bump rubber cups is that they help deflect radiant heat from the brakes away from the rubber or urethane components which again promotes a more consistent and durable bump stop package.

All shock manufacturers are required to submit components that are intended for use in NASCAR prior to the racing season, Arentz said.

"NASCAR looks at performance, safety, and price on any new component to deem if it is something that they want their competitors to use," Arentz said. "We work closely with teams to determine which components will be most useful to them in competition and then go through the submittal process with NASCAR. The overall objective is to encourage competition and a level playing field among the suppliers whose livelihoods depend on their NASCAR customers."

We now know much more about how the shocks with bump stops work to improve performance on the NASCAR race cars thanks to Arentz's explanation so therefore, I guess the bump stops here. Sorry, Harry!

Bill Borden is a former championship winning crew chief who operated David Pearson's Racing School for many years.