Wheel rate is determined by using the installed ratio (the position of the spring on the control arm), as well as the angle of the spring in relation to the line between the center of rotation of the ball joint and the inner pivots of the control arm. The motion ratio is the distance from the inner pivots to the center of the spring divided by the rotational length of the control arm.
We then need to square that number. The "installed ratio squared" number is then multiplied by the square of the cosine function of the spring angle measured from perpendicular to the lower control arm. That answer is multiplied by the spring rate in pounds per inch. The result is the wheel rate at that corner of the car.
If you have a racing buddy and want to run his setup, make sure you know the relationship of the motion ratios and spring angles when comparing the two cars. His motion ratios and spring angles may well be very different than yours and the setup may not translate well.
5 To determine a wheel rate, we need to measure the spring location, the lower control ar
6 For a big spring, or stock type lower control arm, the motion ratio is such that the wh
7 For a swing arm rear suspension, we also need to know our motion ratio to determine wha
Rear Spring Installation
In the rear for a solid axle suspension, the car "feels" the spring base as the distance between the top of the two springs. This is all the car knows and it's as if the chassis were sitting on a pair of springs that are resting on the ground. The rear solid axle assembly is a solid base for the springs to sit on and the car doesn't know, nor does it care, where the wheels are located. Wheel rate calculations at the rear serve no function whatsoever for a race car.
The lateral forces acting through the center of gravity will try to roll the car and the resulting angle of roll for this effect is determined by a combination of six things: 1) the spring base width, 2) the spring split if they are different, 3) the rear moment center height, 4) the stiffness of the springs, 5) the track banking angle, and 6) the magnitude of the force.
Spring split can enhance rear roll or restrict rear roll, even when the combined spring rates of both springs are the same. A stiffer RR spring rate over the LR rate will restrict rear roll. A softer RR spring rate over the LR rate will enhance rear roll. These affects are quite large compared to the affect of raising and lowering the Panhard bar or just changing the overall spring rate. So, be careful when making spring split changes, they will have a significant effect on the balance.
To make changes to compensate for the tendency for excess roll in the rear suspension, we need to look at altering the spring base (distance between the tops of the rear springs), the installed spring angle and/or the moment center height. The narrower the spring base, the greater the tendency for the rearend to roll for a given set of springs and moment center height.
A narrow spring base can be a real problem for some types of cars. The four-link, Dirt Late Model cars sometimes have the springs installed at high angles with the top of the coilover positioned well inside the framerails. This severely limits what we can do to eliminate excessive rear roll in the car.
Many teams have moved the top shock/spring mounts out closer to the wheel to increase the rear spring base. The swing-arm types of dirt car have a very wide spring base by virtue of the springs being mounted straight up, but are affected by motion ratio. The spring is mounted directly on the trailing arm, similar to the front spring mount, and that reduces the rate that the car "feels."
We can do a calculation similar to the front wheel rate calculation to find the effective spring rate. In most cases, we see an effective spring rate that is about half the installed spring rate. So, if we install, say, a 200-lb/in spring in the RR, the car will react as if it had a 100-lb/in spring mounted directly to the rearend.
Stock classes, as well as sedan touring cars based on stock dimensions all suffer from a too-narrow rear spring base. To overcome this deficiency, some teams have deviated from conventional spring rate layouts and gone to a stiffer RR spring. For this very reason, the BBSS setups tend to favor these cars. This "excess" spring split greatly reduces the roll tendencies of the rear suspension and causes a more balanced setup in the car.
Now, we see this same method used in many more series combining softer front springs with a higher rear moment center (Panhard bar). What these teams have done is work out a solution to their balance problems associated with the narrow rear spring base. The key objective still remains to balance the roll tendencies of the two suspension systems.
8 Here is the formula for determining your wheel rate. This is helpful when changing from
9 Once you find your old wheel rate, use this calculator to find your new spring rate tha
10 Remember that spring rubbers and bump rubbers will affect the spring rate and increase
The Relationship Between Springs and Shocks
Once we load our springs by placing a heavy race car on them, they will tend to remain motionless unless we add or subtract load from them. When we do disturb them, such as adding loading or weight that they must support, they will take some time to reach a steady position, oscillating up and down.
The first use for shocks or dampers was to control this oscillation cycle for ride comfort in passenger cars. In racing we need the same control, but more than that, we need to time the motion of the springs and suspension that the springs control.
The springs are hard to compress and easy to rebound. So, shocks must control the compression less than they control the rebound. The stiffer the spring rate is, the more we need rebound control and the less we need compression control.
So, when we change our spring rates, and over the period of the past 10 years many teams have made significant changes to their spring rates, we need to reevaluate our shock rates as well. A prime example is when a team decides to go with soft front springs and then allow the shock to rest on bump rubbers through the turns.
In this scenario, what needs to be controlled are not the soft ride springs, but the very stiff bump rubber/springs. So, applying the overall theme, the shocks compression control would need to be very little when using stiff bumps for front springs and the rebound control would need to be very high to control the high spring rate of the bumps.
Racers sometimes refer to the shocks on the front of the cars that run on bump rubbers as tie-down shocks since then basically keep the shock down on the bump rubber. But in reality, they are built with a high rebound rate so that the soft ride spring will not easily push out, but the stiffer bump rubber will, and that is what counts.
In a video I recently shot showing the left front shock running on a bump, the shock stayed in contact with the bump, but the suspension worked nicely through the lap with the bump compressing and rebounding naturally.
The point to be made is that when choosing or changing spring rates for your car, always consider that the shocks must work together with the spring rate to control the movement associated with that spring rate.
If the spring and the shock are mounted in different locations and have different motion ratios, then the work the shock must do to control the spring must be in relation to the difference in motion ratios.
Make sure that you know your true spring rates, replace bent or fatigued springs, and always know your wheel rates when altering the mounting points or transferring the setup from one car to another. The car basically rides on four springs and the more we know about how they affect the setup in our cars, the more accurately we can develop a winning setup.