Springs are one of the primary components we use to set up our car. We are fortunate in circle track racing that we need only to turn left. So, we can manipulate the spring rates on the four corners to whatever will suit the setup. When we have chosen the correct combination of spring rates to go along with the other parameters, we get what we want--a balanced and fast race car.
The car is supported by the springs and in modern day racing, the spring that supports the car in the turns might be assisted by a spring rubber, a bump rubber, or the tire in the case of coil-bind where the spring rate goes to infinity.
In every case, it's important to know the spring rate you're running on. It has been one of the frustrations of modern racing that finding the loading on the bump rubbers and spring rubbers is very hard to discover.
And if you moved over to this article from the shock article, you will remember that we told you how the shocks and springs work together to determine the chassis balance. Balance is our primary goal in setup and it has a special meaning that once understood will help you formulate your goals for setup.
The Balanced Setup
The key to chassis performance, whether you are running on dirt or asphalt, is to work toward a more balanced setup. The word balance means that the two suspensions systems, front and rear, are working together and that the four tires are working as hard as they possibly can.
1 Each spring is mounted in the car with a motion ratio to the wheel movement. It is this
This balance can be found by trial and error (by far the most commonly used method) or by giving the car what it wants based on input we get from the car. In the past, T&R took a lot of time and the teams really had no idea when the proper balance was achieved. We now have discovered indicators that we can use to help determine when we have truly achieved a balanced setup. Many odd arrangements of springs have been tried and right now we are seeing some of the racers across the country trying new methods to try to find a balance. For every car there may be dozens of combinations of spring rate, moment center locations front and rear, and weight combinations that will balance the car. Here are some considerations concerning springs when trying to balance your race car.
Knowing the True Spring Rate
The very first thing to consider when talking about what rate 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 within the range that they will be working in conjunction with the corner it will be working with.
The RF spring is mostly in compression, or bump, so we would compress that spring to the installed height and then further compress it 2-3 inches to find the average rate within that specific range. Remember that there is an installed motion ratio and the spring is moving less than the wheel, so we don't necessarily need to compress the spring as much as the wheel moves.
The RR spring is also mostly in compression and would be compressed initially to ride height and then up to 3-4 inches further, depending on the spring rate. For big bar and soft spring setups on asphalt, the right rear spring is usually stiffer and travels less than a conventional softer rate spring.
2 This is the correct way to measure for motion ratio for the lower control arm in a doub
3 The angle of the spring inwards toward the centerline of the car, as well as fore and a
4 A car with a solid axle suspension will “feel” the spring base at the top of the spring
The LR spring reacts similarly to the LF spring in that it moved less compared to the springs on the right side of the car. It's in some rebound on entry and some compression at mid-turn. This spring would be rated by compressing to ride height and then moving it up to 2 inches up and down.
For dirt car applications, the left side springs may rebound quite a bit as the left side jacks up for some slick track setups. In this case, the spring may well be very near or past the free (unloaded) height at mid-turn and compressed down the straightaway.
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 for some situations, to a softer right front spring than the left front spring.
Dirt cars can benefit from a softer RF spring on flatter dry and slick tracks. For asphalt, on the flatter tracks, corner entry is enhanced when running a softer RF spring.
For high-banked tracks, the front spring rate must be increased and it is often necessary to stiffen the RF spring more so than the LF 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 soft spring setups tend to over do the stiffening of the RR spring.
On flatter tracks, we have seen where, with some rough tracks, the very stiff RR spring just doesn't work and will upset the car due to excess bouncing going over the ripples or bumps. Remember, 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 mechanical downforce created by the banking will overcome the light spring rate and either the springs will go into coil bind 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 on dirt. Some teams running the conventional setups will go to extremes with this spring split and end up with 25-50 pounds of spring rate difference. Most of the time we don't need a high amount of spring split to achieve the desired effect. I have personally set up winning dirt cars in major series on flat tracks while using only 10-15 pounds of rear spring split.
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 on dirt 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 it was 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.
To do this we need to know how to calculate the wheel rates in each car. We first calculate the old wheel rates and then try different springs in the calculations until the new car has the same wheel rates. If we're 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 and probably have different motion ratios.
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.