One of the primary components used to set up our car is the spring. When we have chosen the correct spring rates, as well as the corresponding moment centers and weight distribution, we get a balanced and fast race car. All too often we crutch the car using inappropriate spring rates that serve to balance handling but don't yield consistency.
In order to win races, our car must be fast, not only right out of the box, but also at the end of the race. In fact, many races are won by cars that may not be the fastest in the early laps of the race, but prove to be faster than the field at the end of the race. The key is to make sure we give the car what it wants so it will be fast as well as consistent.
In a 25- or 30-lap sprint, a jackrabbit setup may work relatively well. A team with a well-balanced setup that starts off a couple of tenths slower for the first 10 to 15 laps may not have enough time to take advantage of everyone else's lap times falling off as the race progresses. But in a 50- or 100-lap race, the car that falls off less as the laps move past Lap 25 or so will have a much better opportunity to win. A more balanced car will have the best chance to win a race of any length.
The Balanced Setup The key to chassis performance, whether you are on dirt or asphalt, is to work toward a more balanced setup. This can be done by trial and error (T&R), which is by far the most commonly used method, or by giving the car what it wants. In the past, T&R took a lot of time and teams had no idea when the proper balance was achieved. But we've discovered indicators that can help determine when we have truly achieved a balanced setup.
Many odd arrangements of springs have been tried, and some of the racers across the country are trying new methods to find a balance. There may be dozens of combinations of spring rates, moment center locations (front and rear), and weight combinations that will balance the car. Here are some considerations concerning springs when balancing your stock car.
Knowing the True Spring Rate The first thing to consider when deciding which springs to use in your race car is the spring rate for each of your springs. Each spring must be tested properly. We need to know the spring rate of our springs at the ride height and range in which they will work in conjunction with the corner where they will work.
Theoretically, we should rate a spring based on which corner of the car it will be used because of the different ranges of motion. For the LF spring we measure the height of the installed spring, remove it, and place it in the spring rating device. It is then compressed to the previously measured height before rating. For a conventional setup, this corner may bump and rebound up to an inch or more. The rate is checked up and down 2 inches from the installed height. For softer spring setups, this corner may travel up to 3 inches or more in compression.
The RF spring is mostly in compression, or bump, so we compress that spring to the installed height and then further compress it 2-4 inches to find the average rate within the 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, so it is 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 much stiffer and travels much less than a spring with a conventional rate.
The LR spring reacts similarly to the LF spring and is in some rebound on entry and some compression at midturn. This spring is rated by compressing to ride height and then up to 2 inches up and down. For dirt car applications, this spring may rebound quite a bit as the left side jacks up for some slick track setups. In this case, the spring may be near or past the free (unloaded) height at midturn and compressed down the straightaway.
Front Spring Split The overall trend in circle track racing, for both dirt and asphalt, has been to reduce the rates of the front springs. For some situations, there is even a move to a softer right-front spring.
Dirt cars can benefit from a softer right-front spring on flatter dry-slick tracks. On flatter asphalt tracks, corner entry is enhanced with a softer right-front spring.
The front spring must be increased for high-banked tracks, and it is often necessary to stiffen the right-front spring more than the left-front spring.
No matter what the stiffness is for our front springs, we 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 try the BBSS setups tend to overdo the stiffening of the RR spring.
The very stiff RR spring does not work on flatter tracks that are rough. The car experiences excess bouncing going over ripples and bumps. There is a limit to how stiff a corner can be.
On the higher-banked racetracks, teams trying soft front springs quickly change to a higher rate when the car bottoms out. Common sense tells us that added downforce will overcome the light spring rate and either the springs will go into coil-bind or the chassis will contact the track surface.
Rear Spring Rate Split When racing on flatter tracks, we can often run a softer RR spring rate to help increase bite off the corners. Some teams running the conventional setups 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.
It is very hard to balance the front and rear suspension systems when using a high amount of rear spring split, so we go with as little as possible when trying to gain bite off the corners on flatter tracks.
With conventional setups on 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 control the rear roll of the car so that we can avoid raising the Panhard bar to excessively high levels.
Improper Switch Between Installation Ratios It is common for a team to buy a new car from a different manufacturer or move to a new class in which the cars are constructed differently. 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. We decide to move to a class that uses 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 work out the old wheel rates and then try different springs in the calculations until the new car has the same wheel rates. The problem is compounded if we are going from a perimeter car (symmetric from left to right) to an offset chassis.
In an offset chassis, even if we install the same spring rates on both sides in the front, we will have different wheel rates due to the different motion ratios at each wheel.
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, and that answer is multiplied by the spring rate in pounds per inch. The result is the wheel rate.
If you want to run your racing buddy's setup, make sure you know the relationship between the motion ratios and spring angles when comparing the two cars. His motion ratios and spring angles may be different from yours.
Rear Spring Installation In the rear of a solid axle suspension, the car "feels" the spring base at the top of the two springs. This is all the car knows; it's as if the chassis were sitting on a pair of springs 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 change 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/or 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 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 mounts closer to the wheel to increase the rear spring base. The swing-arm types of dirt cars 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 to the trailing arm, similar to the front spring mount, and that reduces the rate the car feels.
We can do a calculation similar to that of the front-wheel rate 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 a 200-pound spring in the RR, the car will react as if there were a 100-pound spring mounted directly to the rear end.
Stock cars and sedan touring cars based on stock dimensions (such as the Hooters Pro Cup, Craftsman Truck, Busch, and Nextel Cup Series) all suffer from a rear spring base that is too narrow. To overcome this deficiency, some teams have deviated from conventional spring rate layouts by using a stiff RR spring. For this reason, the BBSS setups tend to favor these cars. The excess spring split greatly reduces the roll tendencies of the rear suspension and causes a more balanced setup in the car, especially when a large-diameter sway bar is used.
Many more series have teams combining softer front springs with a low rear moment center (Panhard bar). These teams have worked out a solution to their balance problems associated with the narrow rear spring base. Balancing the roll tendencies of the two suspension systems is still the key objective.
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.