The basic rates of the springs...
The basic rates of the springs we use in circle track racing have changed a lot over the past few years. The labeled spring rates mean little unless we know something about the suspension systems in our race cars. Teams have found many varied ways to crutch the spring setups in their cars. Installing quality springs and knowing the true rates is just the beginning of understanding the effects of spring rates on the setup in your car.
In order to win races, our car must be fast, not only right out of the box, but at the end of the race too. In fact, many races are won by cars that may not be the fastest car 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 the car will be fast as well as consistent.
In a short sprint race of 25 or 30 laps, a jackrabbit setup may work relatively well. A team with a well-balanced setup that may start off a couple of tenths slower for the first 10-15 laps may not have enough time to take advantage of everyone else's lap time falling off as the laps count down. 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 car that is closer to balanced will have the best chance to win in any length race.
A spring rate's influence...
A spring rate's influence on the setup is dictated by the installation method. In a Formula car, we see a similar situation in that there is a motion ratio. The wheel rate is much different than the spring rate. In this installation, a push rod extends from the lower control arm to a rocker arm that is connected to the coilover spring. The spring moves much less distance for every movement of the push rod, which is at an extreme angle to the control arm. In a Stock Car there are motion ratios too.
Many odd arrangements of springs have been tried and right now we are seeing some racers across the country trying new methods to find a balance. For every car 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 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 at the range that they will be working in conjunction with the corner where it will be working.
It's a good idea to buy or...
It's a good idea to buy or borrow a good quality spring rating device so you can accurately determine the installed spring rate of each spring you might use. Compress the spring to the installed height and then rate the spring from there. Here we see a team measuring spring rate with the gas pressure shock installed to learn the combined spring rate that includes the coil and the shock spring rate caused by the pressure. 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 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. Check the rate up and down 2 inches from the installed height. For softer spring setups, this corner might travel up to 3 inches or more in compression.
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 much stiffer and travels much less than a conventional rate spring.
The LR spring reacts similarly to the LF spring in that it moves 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 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 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.