High-Banked Setups
For a setup that uses higher rated springs in the front and rear for a higher-banked racetrack, the compression and rebound numbers would change to reflect the spring change. We might use a pair of 6.5/4.5 shocks on the front and a pair of 5.5/3.5 shocks on the rear. That is because of what we said before about the higher rating in the springs promotes rebound and therefore we increased rebound resistance. The higher spring rates also resist compression and so we can reduce compression resistance in the shocks.
When the banking of the racetrack is very high, say 18 degrees or more, we still need split valving, but we might go up on both the rebound and compression rather than up on rebound and down on compression. The reason is that the turn speeds are much higher on the very high banked tracks and the shocks will move much faster in both the transitional areas of entry and exit, and also as the car goes over bumps and dips in the racetrack. Without the higher level of control, the suspension might well bottom out with obvious negative results.
A more realistic layout for a very high banked track might be a pair of 6.5/5.5 shocks on the front and a pair of 5.5/4.5 shocks on the rear. We have increased the resistance for both the rebound and compression as well as incorporating split valving to a degree.
Low-Banked Racetracks
For low-banked racetracks, we would necessarily spring the car much softer to increase the shock travels and slow the change to a new chassis attitude in order to help maintain traction. One of the basic ingredients needed to maintain grip at low banked tracks that have less grip is to soften the whole setup including spring rates as well as shock rates. As we soften the springs, we would also soften the front shocks to say 5/4 valving and the rear shocks to a pair of 4/3 shocks. This is the general way shocks can be matched to the stiffness of the spring setup.
One exception is when we go beyond the normal softening of the front springs into what has become known as the soft spring setups that have become popular in short track asphalt racing. These setups often use super soft springs in the front and, to help control the car on entry, employ bump devices on the shock shaft to overcome the tendency for the car to bottom out as we brake and turn left on entry into the corners.
4 A good shock dyno is essential...
4 A good shock dyno is essential in our use of shocks. It’s very important to know the exact rates for compression and especially rebound so that we can match those to our spring layout. With bump devices, we need to match our shock rebound to control the high rate of the bump, not the ride springs.
5 With the soft spring setups...
5 With the soft spring setups we now see teams running, we need to put a lot of preload into the spring on the shock in order for it to hold up the car to ride height. A shock ratchet like this one from RE Suspension helps tighten the spring adjuster against that force.
6 Most stock installations...
6 Most stock installations place the shock at quite an angle and so it has less affect on dampening. Here the top is mounted almost the same distance from the centerline of the chassis as the spring, but the angle diminishes the effect it has. So, we need to run a slightly stiffer shock to compensate for the angle. An 18-degree angle reduces the rate by about 10 percent.
Here is where we might go with a shock that has very little compression resistance to allow the front end to dive and a much greater rebound resistance, not to tie-down the corners of the car, but to control the high spring rate of the bump rubber.
Many teams want to promote left front travel with pro-dive, soft compression numbers, and a high rebound rate at the left front corner. They are not just trying to tie down that corner to reduce the height of the left front valance, they are also using the high rebound shock to control that corner.
This lower attitude reduces the amount of air that travels under the car and helps promote downforce. Most of the gains from this arrangement are associated with reduced drag, more aero downforce and a lower center of gravity which reduces load transfer.
In Part 1 of this story, we talked about how the shock regulates the speed at which each corner of the car moves as load is transferred on to and off of the four corners. If we split the rates between the four corners, we can effectively change the load distribution on the four tires while the suspension is in motion.
So, to utilize the resistance of the shocks to redistribute load, we must have load transfer taking place and that, in-turn, causes the needed suspension movement. If the suspension moves, then so do the shocks.
Rate Split Changes Loading
Compression rate differences between a pair of shocks helps to redistribute load between the four corners of the car where load is being transferred onto a particular end of the car. At the front of the car, compression rate differences can change the load distribution on entry to the corner while load is being transferred to the front due to deceleration. At the rear, compression rate differences can help to redistribute load that has transferred on to that end from load transfer that is caused by acceleration off the corners.
Rebound rate differences help to redistribute load between the four corners of the car when the suspension is in motion as load is being transferred off of a particular end of the car. In the front of the car, rebound rate differences can change the load distribution on exit while the suspension is in motion when load is transferred off of the front and onto the rear. At the rear, rebound rate differences between the LR and RR shocks can help to redistribute load on the four corners of the car again while the suspension is moving and adjusting to the load being transferred off of the rear on to the front due to deceleration of the race car.
We need to think out the motions of the car related to how it is driven and how the track is configured, as well as the desires we have for improvement. Tuning for corner entry can affect corner exit. Do not tune with shocks for problems that can be fixed with basic setup parameters.
7 In a coilover installation,...
7 In a coilover installation, the spring and shock move in a one-to-one ratio. Therefore, the shock only needs to have an equal rate to the spring in order to control its force. Remember that the spring promotes rebound and resists compression. So, the shock must have much more rebound resistance than compression resistance.
8 For a big spring, or stock...
8 For a big spring, or stock spring installation, it’s harder for the shock to resist compression and easier to resist rebound forces. We only need to translate the spring rate out to the shock mount to know the forces and influence the spring has on the shock. In a typical big spring installation, a 400 ppi spring will rate only 200 ppi at the shock when we take into consideration the shock being mounted closer to the wheel and the shock angle.
9 This shock graph was taken...
9 This shock graph was taken from a shock we used in testing for bump setups and is useful for creating the high rebound rates needed to control the 1,000- to 1,200-ppi spring rates we often see with bump devices. It is adjustable within a range of 1,000 pounds to 1,300 pounds at 3.0 inches per second of movement. There is a high nose designed in for the rebound side so that the shock will keep the front of the car down on the bump and where the ride spring cannot move it very fast. A 200-pound force equal to a ride spring will only move the shock about 0.200 inches per second.
A car that is tight on entry should not be fixed with the installation of a high rebound LR shock that will reduce the loading on the LR tire on entry. A car that is tight on entry most likely has geometry or setup problems. Or, the rear may be out of square, the front steering geometry is not designed correctly, or the setup is just plain tight.
Remember that the more you have to trick up your shocks and shock selection, corner to corner, the harder it is going to be to be consistent. We are getting many reports of teams going to more research and matching their shock rates to the springs and bump devices they are using and winning races due to consistency.
Conclusion
Again, think out how shocks work with the springs, how bump rubbers or other devices that limit spring and suspension travel increase the spring rate the suspension feels and what direction we need to go to control our total spring rates.
I used to think that shock companies were over-shocking the front shocks with very high rebound rates, but I came to understand that they are only controlling the very high spring rates exhibited by the bump devices. In Part 3 of our shock series, we will learn how to fine tune our shocks for different setups and track configurations.