The wheel rates for most coilover...
The wheel rates for most coilover designs is affected by both the motion ratio and the spring angle. Both the angle and installation ratio effects are squared to find the wheel rate.
Dirt cars can benefit from a softer right front spring on flatter dry and slick tracks. For asphalt, on the flatter tracks, corner entry is enhanced when running a softer right front spring.
For high banked tracks, the front spring rate must be increased and it is often necessary to stiffen the right front spring more so than the left front spring rate.
No matter what the stiffness is for our front springs, we still need to compensate at the rear in order to balance the car's suspension dynamics. Dirt Late Model teams are running much stiffer right rear springs than ever before and seeing a lot of success. Asphalt teams who are trying the BBSS setups tend to overdo the stiffening of the RR spring.
Coil spring (stock big spring)...
Coil spring (stock big spring) cars are only affected by the motion ratio because the spring moves very closely to perpendicular to the motion of the arm. We still need to make sure that the supports at the ends of the springs are flat and perpendicular to the centerline of the spring when the spring is compressed after dive and roll.
On flatter tracks, we have seen where, with some rough tracks, the very stiff RR spring just does not work and will upset the car due to excess bouncing going over the ripples or bumps. There is a limit to how stiff a corner can be.
On the higher banked racetracks, teams that try the soft front springs will quickly change to a higher rate when the car bottoms out. If we use our common sense, we will know that the added downforce will overcome the light spring rate and either the springs will go into coil bind and/or the chassis, and hopefully not the oil pan, will contact the track surface first.
Rear Spring Rate Split When racing on flatter tracks, we can often run a softer RR spring rate so we can help increase the amount of bite off the corners. Some teams running the conventional setups will go to extremes with this spring split and end up with 25 to 50 pounds of spring rate difference. Most of the time we do not need a high amount of spring split to achieve the desired effect. I have personally set up winning cars in major series on flat tracks while using only 10-15 pounds of rear spring split.
Be sure to measure your control...
Be sure to measure your control arms correctly. We need to have accurate distances for the position of the spring in relation to the rotational radius of the control arm. The rotational radius is the shortest (at 90 degrees) distance from the ball joint to the rotational axis of the inner mounts of the control arm.
It's very hard to balance the front and rear suspension systems when using a high amount of rear spring split, so we try to go with as little as possible when trying to gain bite off the corners on flatter tracks.
With conventional setups on the higher banked tracks of more than 12 degrees, we can run a rear spring split with the RR spring rate higher than the LR spring rate to help balance the front and rear suspensions. This helps to control the rear roll of the car so that we do not need to raise the Panhard bar to excessively high levels.
Improper Switch Between Installation Ratios It's common for a team to buy a new car from a different manufacturer or move to a new class where the cars are constructed differently than what we are used to. What often changes is the installation ratio at the front of the car and spring base at the rear.
Suppose we have run a class using stock spring rates at the front with stock lower control arms and now decide to run a class that uses a fabricated front clip and coilover shocks and springs that are mounted differently on the lower control arms. If we had the setup figured out on the old car as far as wheel rates were concerned, we then need to duplicate those wheel rates (assuming the overall car weights remain about the same) in order to stay on track with our handling.
For a solid axle rear suspension,...
For a solid axle rear suspension, the rear spring base is felt by the car at the top of the springs. The wider the tops of the springs are mounted from each other, the less roll tendency the rear suspension will have. Narrow spring bases promote suspension roll and that may be detrimental to achieving a balanced setup.
To do this we need to know how to calculate the wheel rates in each car. We first work out the old wheel rates and then try different springs in the calculations until the new car has the same wheel rates. If we are going from a perimeter car (symmetric from left to right sides) to an offset chassis, the problem is compounded because the lower control arms are different lengths.
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 length of the control arm.