The distribution of weight...
The distribution of weight is critical to the handling of our race cars. We must know exactly how much weight is supported by each tire. Many track scales are not level, therefore cannot be relied upon to tell the correct wheel weights and should be used as a reference only. This information is best gathered at the shop on a level floor or surface plate.
There is a lot to know about weight distribution in a stock car. In the past few years, we have learned why weight distribution affects the setup 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 of the physical weight throughout the car and can be read in percents such as left-to-right-side percent, front-to-rear percent, 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. This should be dynamic weight, which consists of the driver and all fluids, just like when you are racing the car.
The other measure is the total weight of the car, in percentages, that each tire supports. Depending on how we mount or adjust our spring heights, the amount of weight supported by each tire can change. Weight jacking bolts on big spring cars and the adjuster rings on coilover shock/spring-type cars are the tools used to change this 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 finding the best handling balance and for 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 best known as the pavement racers' term-crossweight percentage, or wedge-or as the dirt racers' term-left-rear weight. Both refer to basically the same thing.
Coilover spring/shock combinations...
Coilover spring/shock combinations have adjuster rings on the threaded portion of the shock to set the spring height and regulate the amount of weight supported by each tire. It is a good idea to mark the ring so you can count the number of revolutions as you are turning it.
We can tell a lot about how a race car is set up and its balance 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 from the perspective of the front and rear working together.
For simplicity, we will use crossweight percentage, a universal term, to represent the distribution of weight among the four tires. We find the crossweight percentage by adding the weight on the right-front (RF) and left-rear (LR) tires 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, rather tune the crossweight to a balanced setup.
If the setup is truly balanced, the neutral handling 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.
Unbalanced setups go away because one tire in an unbalanced setup is doing too much work. It is usually the RF tire, but with the super-soft setups we've seen in the past year or so, the right-rear (RR) could be the one that is being abused.
The term balanced setup refers...
The term balanced setup refers to a combination of spring rates, moment center locations, and other setup parameters that will result in both ends of the car trying to do the same thing in the turns. That is defined as desiring to roll to the same angle. Once we can match up the two ends, the weight transfer is predictable and consistent.
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 when, as we have discussed in previous issues, the rear suspension wants to roll more than the front suspension. This condition causes excess weight to transfer to the RF tire, making the front pair of tires less equally loaded. Less equal loading causes less overall traction from a pair of tires, resulting in a push.
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. Also in previous issues, we learned that a tire with a greater angle of attack has more traction 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 much of its ability to grip the racetrack, and the car will start to push.
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. The tight condition has led to a loose off-condition that may heat up the RR tire and cause it to wear excessively and lose grip. We are now abusing two tires because the car has a tight setup.