One of the most important setup tools for any race team is a good set of scales, which allows you to accurately determine how many chassis changes affect the car.
No matter how powerful your engine, how talented your driver, or even how advanced your chassis, stock car racing always comes back to how well your tires are able to maintain traction on the racetrack. The more traction you have available at all four wheels, the more speed the driver will be able to maintain through the turns, and the more acceleration he or she will be able to apply from that high-dollar race engine on turn exit.
The trick is to get all four wheels working for you. This is especially true in asphalt classes, but also the case in dirt racing, where setups are becoming more balanced and rolling everything to the right side of the car is a thing of the past. Of course, we are talking about a race car that's balanced through the turns, when inertia has a large influence on the chassis. The result can be a chassis setup that seems quite unbalanced when it is sitting still on level ground, and the only way to know exactly what you have in many critical areas is through the use of a set of racing scales.
After selecting a level spot on your shop floor as your setup area, jack up the car, position the correct pad underneath each tire, and slowly lower the car.
Modern setup scales can make a racer's life a lot easier. They are essentially four pads connected to a central readout/controller. The car is set with one wheel on each of the pads, which relay the individual weights to the controller. The controller not only displays the weights at all four wheels and total car weight, but it also calculates and displays other important information, such as front/rear percentages, side percentage, and crossweight (also known as "wedge"). More advanced models, such as the Intercomp Quik Weigh scale used in this article, also help you calculate center of gravity, where weight needs to be added or removed in order to reach target percentages and even rear bite for dirt track racers.
Setting Up
Most racing scales are relatively easy to set up. In the case of wired scales, such as the ones we are using here, it is simply a matter of plugging the wires into the scale pads and setting each pad in the correct location.
Using a set of scales is relatively easy. For example, all the wires and scale pads on the Intercomp Quik Weigh are color coded, so you can be certain the pad underneath the left-front tire is actually showing up as the left-front tire on the control panel.
Just as important, however, is picking the correct location to scale your car. If at all possible, all four scales should be absolutely level individually and in relationship with each other. This shouldn't be too difficult to accomplish on a concrete shop floor. Once you do, this becomes your permanent setup location. Tape or spray paint your marks on the floor so you can set up your car in this exact location week after week. If you need to scale your car at the racetrack for some reason, it can be much more difficult to find a level area. Teams that travel to different tracks often need to scale their cars and invest in a leveling rack to create their own level surface for the four pads.
Once you have chosen your area, roll the car into position, jack it up, slide the pads underneath the four wheels, and lower the car. Once the car is where it should be, gently push down on each corner to make sure the suspension is settled into its proper ride-height position. Also, it is important to make sure the car is in the same condition it will be in on the racetrack. All your lead should be in place, and you should make sure you have water in the radiator and oil in the engine. And don't forget the battery if you have removed it from the car for charging. You want to replicate anything that will affect the weight balance of the car. This also includes the driver. Have him or her sit on the doorsill when you are ready to take your readings. Of course, this isn't always possible if your driver isn't around or you are the driver, so you can also use a couple of sticks of lead, or bags of sand that have the same weight as the driver. When you are ready to scale the car, just set them in the driver's seat.
Ride height affects your crossweight percentage. To guarantee your car is at static ride height, push down on the car at each end to flex the suspension.
Total Weight and Percentages
Total weight is the amount of weight on all four wheels. That's the easy part. Of course, percentages are also relatively easy. Front percentage is the weight on the front two wheels divided by the car's total weight. Knowing your front percentage is important because it can be adjusted to help the car's reaction under braking. For example, on our dirt car (which is only a random setup and not a target we are recommending), the weight over the front wheels is 542 and 499 pounds. The car's total weight is 2,088 pounds. This makes the front percentage 49.9 percent. Left-side percentage is much the same. It is the weight of the left-front and the left-rear wheels divided by the total weight of the car. On an asphalt car, you want all the left percentage you can get, and the rule book will often limit how much you can have. Weight distribution is different in a dirt car, but it is still critical to know left-side weight. In our example, the weight on the left-side wheels is 542 and 547 pounds, for a total of 1,089 pounds. That number divided by our total car weight of 2,088 pounds gives us 52.2 percent of total weight on the left side of the car.
On our Dirt Late Model, the Intercomp readout shows the weight on each of the four tires, left-side percentage, rear percentage, crossweight, and total weight, which is easily readable in a single glance.
The only way to change total weight and either the front or side percentages is to move weight around in the car. Obviously, the only weight you can move around in the car is your lead. The lighter the race car, the more lead you have to use to meet minimum weight standards. This is an advantage because the more lead you have, the easier it is to adjust your weight percentages.
Crossweight
Crossweight is also known as wedge. It is simply the percentage of the total weight on the right-front and left-rear tires compared to the total weight of the car. Going back to our example, the total weight on the left front and right rear is 1,046 pounds. One thousand forty six divided by 2,088 gives us a crossweight of 50.1 percent. Adding crossweight makes the right front and left rear handle more of the load through the turns. Generally, more crossweight will tighten a race car. Too much, however, can make it unbalanced.
On a race car with coilover shocks, ride height can be adjusted by spinning the adjuster nut up or down on the shock body, which controls the location of the top of the spring. Pushing the top of the spring down raises the car's ride height at a specific corner.
When it comes to dialing in crossweight, you have two options. The first is moving weight around, as you must do to affect your front and side percentages. The second way you can adjust crossweight is to change the ride height at any of the corners. This is generally preferable to moving weight around because it avoids upsetting your percentages. On a coilover car, you can adjust ride height by turning the adjuster nut on the shock body that controls the location of the top of the spring. Most big-spring race cars use a jacking bolt that presses down on the top of the spring to control ride height.
Pushing down on the top of the spring raises the car's ride height. Likewise, raising the perch for the top of the spring lowers the car. When you raise the ride height at either the right front or left rear, that increases the crossweight, or wedge. Lowering the ride height at either the left front or right rear also increases crossweight percentage. Doing the opposite at either corner decreases crossweight. You have to be careful, however, if your rule book has minimal frame heights, because changing wedge without tracking how your frame heights are affected can make you unexpectedly illegal. Also, you don't want to race with the car higher than necessary. If you need to add a round of wedge to the right-front wheel to increase crossweight but cannot because it raises the frame too high, you can accomplish the same thing by putting a quarter round into both the right-front and left-rear wheels, while also taking a quarter round out of both the left-front and right-rear wheels.
A big-spring car works in much the same manner, except it uses a jack screw to control the height of the top of the spring instead of using an adjuster nut that threads on the shock body.
Also, remember that whenever you make a percentage change, you have also changed your crossweight percentage. Since everything on a race car is tied together, it is very difficult to make one chassis setup change without affecting something else. That is why it's nice to have the car on a set of scales, where it is easy to track the effect of any changes on the central readout. Just keep in mind that whenever you move lead, you need to check your front, rear, and side percentages, and also crossweight percentage. Anytime you move the adjuster nut on a coilover shock, or the jack bolt on a big-spring suspension, you need to check both crossweight and ride height.
Tracking Ride Heights on Scales
We already know that changing crossweight can also affect ride height. Tracking ride height changes when your race car is sitting on scale pads can be frustrating. One trick is to run a string between the tires and the scale pads on each side of the car, from front to back. Then, pull the strings until they are taught. Afterward, lower the car onto the strings. The weight of the car holds the strings (which simulate the ground) in place. You can now measure between the bottom of the frame and the string to accurately judge inspection ride height.