One of the primary components we use to set up our car is the springs. When we have chosen the correct spring rates, as well as the corresponding Moment Centers and weight distribution, then we get what we want – a balanced and fast race car.
To achieve this balance, we need to know a little about our springs and how they work to affect the setup in the car. The way they are mounted, their rates as opposed to the other springs in the car, especially the opposite spring on the same axle, and how the rates might change with the motion of the chassis are all things we need to understand.
It is 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 coil-over 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.
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.
We 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 vertical, and that answer is multiplied times 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.
Finding the True Spring Rate
The very first thing to consider when talking about which springs to use in your race car is knowing exactly what the spring rate is for each of your springs. You must test each spring and do it in the proper way. We need to know the spring rate of our springs at the ride height and range that they will be working, in conjunction with the corner it will be working with.
Theoretically, we should rate a spring based on which corner of the car it will be used on because of the different ranges of motion. For the LF spring for example, we would measure the height of the installed spring and compress the spring in the spring rating device to that exact height before rating. Then, for a conventional setup, this corner might bump and rebound up to an inch or more, so be sure to check the rate up and down 2 inches from the installed height. For softer bump spring setups, this corner might travel up to 3 inches or more in compression.
Think about the motions of the other corners when checking the spring rate. Remember that a spring mounted at an angle will decrease its effective rate as it is compressed and the angle increases. That is why mounting a spring at an angle will soften that corner of the car.
Front Spring Split
The overall trend in circle track racing, for both dirt and asphalt racing, has been to reduce the rates of the front springs. There is even a move in some situations, to a softer right front spring.
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 rates must be increased and it is often necessary to stiffen the right front spring more so than the left front spring rate due to the high forces on entry.
No matter what the stiffness is for our front springs, we need to compensate at the rear in order to balance the cars 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 soft bump setups tend to over-spring the RR spring.
Rear Spring Split
It is necessary to balance the setup of the car by using different spring rates and spring split. For a car with a high rear Moment Center, the RR spring might need to be softer than the LR spring such as with stock cars running the Metric four-link.
For soft front bump setups, the bumps create high front spring rates and a low roll angle. So, the rear springs need to be split with the RR stiffer to reduce the rear roll. This can be anywhere from 50 to 100 lb./in. or more difference depending on the front stiffness.
Rear Spring Installation
In the rear for a solid axle suspension, the car “feels” the spring base at the top of the two springs. This is all the car knows and it is 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 does not know, nor does it care, where the wheels are located.
Overturning moments acting through the center of gravity are affected by the resistance to roll created by the spring base width, the rear Moment Center height and the rates of each spring. To effect changes to the rolling tendencies of 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 the Moment Center height. The narrower the spring base, the greater the tendency for the rear end 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 coil-over positioned well inside the frame rails. This severely limits what we can do to eliminate excessive rear roll in the car.
Many teams have moved the top 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 being mounted straight up, but suffer from another variable – 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 about half the effective rate as the installed spring rate. So, if we install, say, a 400 pound spring in the RR, the car will react as if it there were a 200 pound spring mounted directly to the rear end.
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.
Make sure that you know your true spring rates, replace bent or fatigued springs, and always know your front 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.