A spring rubber changes the...
A spring rubber changes the rate of the spring as the spring compresses and decompresses. This changes the weight distribution among the four tires. Because this is a variable, depending on the amount of spring compression, it is not an advisable method for increasing spring rate.
This car started out neutral but became very unbalanced. That is exactly why unbalanced setups do not win most races. To be perfectly honest here, a short race of 25 or 30 laps can be won by an unbalanced setup. These short runs do not provide enough time for the car to abuse the tires to the point that it slows down sufficiently to lose the lead. That being said, the balanced setup will still be as fast or faster than one that is unbalanced.
Another aspect of crossweight is that for every race car and setup there are several ranges of crossweight that will cause the handling to be neutral. This is not universally known, but through literally hundreds of cases, it has been proven that one car may have as much as three ranges of crossweight that will make the car neutral.
For most short track cars, given a particular vehicle weight distribution, there is a low and a high range of crossweight that will make the car neutral. With the same springs, moment center locations front and rear, and so on, the car will be neutral at say 51.2 percent and at 58.6 percent. In-between is "no man's land." The truth is that we can find a certain percentage of crossweight in each range that will make the car neutral in handling.
A tool that facilitates changing...
A tool that facilitates changing springs and doesn't alter the weight distribution is this single-wheel scale by Intercomp. The weight is read with the old spring. Then, with the new spring installed, the jack screw is adjusted until the same weight is read.
If there are at least two ranges to choose between, how do we know which to use? What we have found is that the lower crossweight range works best on the higher-banked tracks of 12 degrees or more. For the lower-banked tracks, the high range works better to provide better traction off the corners under acceleration by increasing the loading on the LR tire. On the high-banked tracks, the banking usually creates enough mechanical downforce to provide sufficient traction so the tires will not lose grip under acceleration.
The very best dynamic weight distribution (while in the turns) occurs with two sets of equally loaded tires at midturn. In the low range, the outside tires will be equally loaded in the turns, with the inside tires being equally loaded as well. In the high range, the RF and LR tires will be equally loaded and the LF and RR tires will be equal. In this way, at either range we have more equally loaded pairs of tires at the front and rear to provide the most traction available so we can go through the turns as fast as the car is capable of going.
This is how the weight should...
This is how the weight should be distributed at midturn when we use the low range of crossweight. After the weight has transferred as the car negotiates the turns, the same amount of weight is supported by both right-side tires as well as both left-side tires. This is how it should happen with a balanced setup.
The need to find a setup that will provide the best weight distribution at midturn is the reason we seek a balanced setup. The balanced setup causes the weights to transfer predictably at each end of the car. Unbalanced setups are very unpredictable because the weight transfer changes as the tires wear or the driver changes driving lines.
Different track banking or surface changes that alter the track's grip characteristics can cause a handling balance change with an unbalanced chassis. Handling balance may also change as the track surface temperatures increase or decrease. From afternoon (warm) to night (cooler), the handling will change with an unbalanced setup, whereas a balanced one stays more consistent.
We can predict the best crossweight for a car based on its weight layout, or where the weight is placed in the car. The front-to-rear distribution of weight dictates the crossweight percentage to use. As rear percentage increases, more crossweight is necessary in order to be neutral for each range.
This is an example of how...
This is an example of how the weight would be distributed at midturn when we use the high range of crossweight. After the weight has transferred as the car negotiates the turns, the same amount of weight is supported by the RF and LR tires and the LF and RR tires would be supporting the same weight. The setup must still be balanced to achieve this result.
If we know a car needs 52.2 percent crossweight to be neutral based on the front-to-rear percentage, then running 49 or 50 percent in a neutral car means the setup is unbalanced. The RF tire is working too hard and the car is tight because reducing the crossweight makes the car more neutral. This change does not solve the problem with the unbalanced setup, and the RF tire is still doing too much work. What is worse is that the car is probably loose off the corner and the crew makes changes that will tighten it, making the original problem worse.
Another interesting phenomenon is that as the applied g-forces change, so does the crossweight requirement for a particular car. The higher the g-forces generated, the more crossweight percentage needed. If our car's lap times tend to fall off a lot over a fairly short run, the g-forces are dropping off also (less turn speed means less g-force). So the crossweight we have chosen will not be sufficient as the car slows.
To stay neutral in handling, the required crossweight should come down as well. Since it cannot change, the car would become tighter by having too much crossweight percentage as the lap times continue to fall off. One advantage of a balanced setup is that the lap times stay more consistent, hence less change in the g-forces, as well as the required crossweight. The car remains more neutral as well.