Story by Doug Gore
The most important variable that determines how a race car handles is the traction produced by each of the car’s tires. Since the traction of each tire is non-linearly dependant upon its instantaneous vertical load and those loads are constantly changing as the car goes around the track, there is no ideal static weight distribution, and even there was, would constantly change over a long race.
Most short track racers rarely need to pit, so they do not have the opportunity to adjust their cars during pit stops. The set-up that they start with will determine what they will finish with. That fact can put an even higher importance on pre-race chassis set-up.
In addition to tire choices, spring and bar rates, shock dampening characteristics and roll stiffness distributions you also need to know what the static weight distribution of your race car is. Having a repeatable baseline weight distribution is one of the keys to achieving consistently good performance.
The first step in making repeatable weight measurements is acquiring a set of scales. While I have had good luck with a matched set of bathroom scales and a good set of weight dividing platforms, modern digital scales have become reasonably priced and very easy to use. A set of four grain scales will also work well.
Accurately weighing your race car is not about buying the most expensive scales. Inexpensive scales, including the ones that use force dividing platforms, can give you excellent results if you use them meticulously.
Regardless of the scales that you have, it is a good idea to check them for accuracy and consistency by weighing yourself on each pad and comparing the results.
The next step is one of the most important for consistent results. You need to select an area in your shop where you can weigh your car before and after every race. The area should be fairly open, have a solid, (preferably concrete) surface and be close to level. Bicknell’s Randy Williamson recommends that the floor by flat and level to within an eighth of and inch for dirt cars while Troyer’s Bill Colton recommends a flatness of better than a sixteenth, if possible, for pavement cars.
Label the scale pads and mark their locations on the floor so that they can be returned to exactly the same positions each time. Recheck the tops of the pads for equal height and levelness. Shim the scale pads if necessary. Remember that an out of plane amount of 1/8” at one pad will cause an error that is roughly equivalent to one round of wedge.
You are now ready to prepare your car for weighing. Begin by installing a set of tires that have known circumferences consistent with what you run and are inflated to the pressures used on the track. I prefer to use a dedicated set of tires for the set-up process that are not used for anything else. Again, understand that small changes in tire circumferences will make significant measurement differences. Fuel the car to a known and repeatable level and check the other fluids. Seating the driver in the car is optional but either way, be consistent. After setting your corner heights roll the car on the scales or lower it in place with jacks.
The next step is somewhat controversial. Rocket Chassis’ Mark Richards recommends that you always disconnect one rear axle from its driving hub, especially on dirt cars that use a lot of anti-squat like the current late models. His testing has convinced him that the extreme anti-squat of the four bar cars randomly changes scale readings when the tire stagger wraps up the axles as the car is pushed onto the scales.
Scott Bloomquist’s crew chief insisted that they never remove an axle when scaling their cars but that they do lower the cars down onto their grain scales. He also pointed out that since the grain scales were on wheels they were free to roll and could release any thrust caused by rolling stagger. I have not personally experimented with this effect but I can make a convincing argument either way. As homework, try the experiment yourself.
Bounce on each corner of the car a couple of times and check the scale readings. Use the same sequence each time. If they do not settle to repeatable scale readings you probably have a bind that will need to fix before proceeding. If the readings are consistent record them as a starting point.
Genesis Shocks’ Mike Farr suggested that if you are racing with shocks that have a very high rebound such as might be used on the right front, you should wait a minute after bouncing on the car before taking scale readings. High dampening forces can delay the chassis settling. Shocks with high gas pressures should remain connected but shocks without high shaft forces are best disconnected.
Once the car has stabilized, record the scale readings. If you have electronic scales also record the left side weight, the rear weight and the diagonal weight percentages. Otherwise, calculate the desired percentages by hand.
If this is the first time that you have scaled your car, compare your corner weights to those recommended by your chassis builder and/or cooperative competitors. If their recommendations and your measured weights are similar you are probably close. Then, if you have been happy with the car’s balance on the track, save these corner weights and percentages for future reference.
Once you have a repeatable static weight baseline, it is time to experiment with your car’s weight distribution. While the car is still on the scales, spend a few hours experimenting with its static weight distribution and its different adjustments. You will learn that the car’s left side weight percentage can not be changed by adjusting any of the jacking bolts, the tire air pressures or even the spring rates. The only thing that will make a left side percentage difference is moving weight (i.e. ballast) from side to side and changing wheel offsets.
The rear weight percentage is much the same. The jacking bolts will not change the rear percentage. At this point you might want to experiment with different amounts of fuel however. You will be surprised at how much adding a hundred and fifty pounds of fuel behind the rear axle changes the front to rear percentage.
As you work with your car’s static weight distribution you will see that the jacking bolts change is the car’s diagonal percentage, or the combined right front and left rear weights when compared to the car’s total weight. Increasing the car’s diagonal percentage, or wedge, will tend to tighten the chassis up. You should learn how much each jacking bolt affects the diagonal percentage, but remember that the exact amount depends upon the springs selected. As always, keeping good records is very important here.
Once you have adjusted the jacking bolts and the ballast to achieve the weight percentages that you want, you should re-check all of your ride heights since weight jacking also affects them. Try to maintain the desired ride heights.
One other word about moving ballast around. As you move ballast from side to side or front to rear the corner weights and percentages will change. Moving ballast up and down vertically will not change the static corner weights but it will change amount of weight that occurs transfer on the track. Higher ballast increases weight transfer, which generally tightens the car.
With all of your set-up weights and percentages recorded, along with your other set-up variables, it is time to go to the race track and fine tune your car. Then, when you return to your shop, and before you change anything on the car, re-weigh it twice. Once with the aired up tires that you finished the feature on and again with your set-up tires. These two new sets of corner weights will help establish a new baseline appropriate for that race’s track conditions.
Over time, you will be able to build a data base covering different track conditions (and tracks) that you can use to refine your baseline set-ups as well as for making informed weight adjustments at the track.