Crutches: Weight Distribution And Weight Transfer

Weight distribution and its relationship to our setup is more complicated than many choose to believe. We have learned how weight distribution affects our setups and how we can improve our setups to take advantage of different weight distribution ranges related to specific types of race cars and various racetracks.

Weight distribution can be defined in several ways. It can mean the placement throughout the car of the physical weight and can be read in percentages, such as left-to-right-side percentage, front-to-rear percentage, and sprung and unsprung weight. It can also be read as center of gravity height. These numbers are fixed for a particular race car until we move weight around in the car. We need to know the dynamic weight distribution, which means reading the weight of the car with the driver and all fluids included just as it would be on the racetrack.

The other measure of weight is related to how much each tire supports the total dynamic weight of the car in percentages. The amount of weight that is supported by each tire can and does change when we physically adjust the heights of the springs and as the car travels around the racetrack.

Weight jacking bolts on big-spring cars and the adjuster rings on coilover shock/spring types of cars are the tools used to change the static distribution of weight. This changeable distribution of weight is what we often use to tune the handling balance of our race cars.

Weight distribution in our cars affects the handling balance. It is an adjustment tool for both finding the best handling balance and tuning the setup to a driver's preference. Some drivers like a car that is somewhat freer while some cannot drive a loose car. The reading of the distribution of the total vehicle weight on the four tires is known to pavement racers as crossweight percentage or wedge, and left-rear weight to dirt racers. Both tell us basically the same thing.

As the car begins to push, the driver turns the steering wheel more until the front has developed more traction than the rear and the car starts to go loose. This usually happens just as we are applying the throttle to get off the corner and the car goes very loose-quickly. The tight condition has led to a loose-off condition that may heat up the right-rear tire and cause it to wear excessively and lose grip. We are now abusing two tires because the car has a tight setup.

We can tell a lot about how a race car is set up and how balanced it is by analyzing the crossweight percentage that will work with the setup to make the car neutral in the turns. We must remember that a neutral-handling car is not necessarily a well-balanced car dynamically from the perspective of how the front and rear are working together.

Here, we will use the term crossweight percentage to represent the distribution of weight among the four tires. We find the crossweight percentage by adding the weight on the right-front and left-rear tires together and dividing that number by the total vehicle weight. The most important thing to know about crossweight is that we cannot build the setup around a particular number, but rather tune the crossweight to a balanced setup.

If the setup is truly balanced, then the neutral handling that we tune to with crossweight will be consistent and stay that way for many laps. If the car is unbalanced in the setup, the neutral handling will go away in a short time, usually from 10 to 25 laps.

Unbalanced setups go away because one tire is doing too much work. It used to be the right-front (RF) tire, but with the super soft setups we see on asphalt race cars, the right rear (RR) could be the one that is being abused.

If the setup is unbalanced to a great degree, with the RF tire working too hard, we may end up with a tight/loose condition. This is where, as we have discussed in past articles, the rear suspension is wanting to roll more so than the front suspension. This condition causes excess weight to transfer to the RF tire, and the front pair of tires becomes less equally loaded. Less equal loading causes less overall traction from a pair of tires, and a push develops.

To compensate for the push, we turn the steering wheel more to the left and the front tires gain more of an angle of attack. In past articles, we learned that the greater the angle of attack a tire has, the more traction it develops-to a point. The bottom line is that the RF tire is working very hard to turn the car and it will heat up and wear excessively. In a short time it will lose a lot of its ability to grip the racetrack, and the car will start to push.