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.
The cars that use stock springs...
The cars that use stock springs have weight jacking screws and a housing welded to the frame to adjust the height of the top of the spring for changing the crossweight. The screw has a square hole to fit a 31/44-inch drive extension. A mark should be painted on one side of the screw so the number of turns can be counted easily.
This car started out neutral, but went to proverbial hell. That is why unbalanced setups do not win most races. To be perfectly honest, 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. That said, the balanced setup will still be as fast or faster than one that is unbalanced.
Another point of interest 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 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 (roll) center locations front and rear, etc., the car will be neutral at, say, 51.2 percent and 58.6 percent. The area 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.
If there are at least two ranges from which to choose, how do we know which to use? We have found 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.
This is how the weight would...
This is how the weight would be distributed at mid-turn 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 can happen with a balanced setup.
The very best dynamic (while in the turns) weight distribution is when we have two sets of equally loaded tires at mid-turn. In the low range, the outside tires will be equally loaded in the turns, and the inside tires will be equally loaded as well. In the high range, the RF and LR tires will be equally loaded, and the left-front (LF) and RR tires will be equal. With this weight distribution, we have more equally loaded pairs of tires at the front and rear at either range to provide the most traction available, allowing us to go through the turns as fast as the car is capable of going.
We seek a balanced setup because we need a setup that will provide the best weight distribution at mid-turn. The balanced setup causes the weights to transfer correctly and predictably at each end of the car. Unbalanced setups are highly unpredictable due to weight transfer caused by tire wear or the driver changing lines up and down the racetrack where the track banking is different or the track surface changes its grip characteristics. Handling balance may also change from afternoon (warm) to night (cooler) with an unbalanced setup, whereas a balanced setup stays more consistent.
This is an example of how...
This is an example of how the weight would be distributed at mid-turn 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 are supporting the same weight. The setup must still be balanced to achieve this result.
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. The greater the rear percentage, the more crossweight necessary to run in order to be neutral for each range.
If a car needs 52.2 percent crossweight to be neutral based on the front-to-rear percent, and if the team is running 49 or 50 percent and the car is neutral, the setup must be unbalanced. The setup must be a tight one in which the RF tire is working too hard and the car is tight. Reducing the crossweight will make the car more neutral but won't solve the problem with the unbalanced setup, and the RF tire will continue to do too much work. Even worse, the car will probably be loose off the corner, prompting the crew to make changes that will tighten it, thus intensifying the original problem.
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 by a car, the more crossweight percentage we need. If our car tends to fall off a lot on lap times over a fairly short run, the g-forces drop off also (less turn speed means less g-force).
When we change the front-to-rear...
When we change the front-to-rear percentage of total weight in the car, the crossweight needs to change to keep the car neutral in handling. This car is a 2,800-pound Late Model with a 66-inch track width, 56 percent left-side weight, and a 16-inch center of gravity running on a 12 degree banked track pulling 1.7 g's in the turns. The numbers in the chart were calculated using standard weight transfer dynamic formulas available in many books. The car is considered to have a balanced setup.
To stay neutral in handling, the required crossweight also needs to come down. Since it cannot change, the car will become tighter by having too much crossweight percentage as the lap times continue to fall off. One advantage of a balanced setup is that lap times stay more consistent, hence less change in the g-forces, as well as the required crossweight, and the more neutral the car remains.
* Lap times fall off considerably after 20 or so laps, while the leaders stay consistent.
* The RF tire shows excessive heat and wear compared to the other tires.
* The LF tire shows considerably less heat and wear than the other tires.
* The RR tire shows extreme heat and wear from spinning off the turns.
* The driver uses excess steering input at mid-turn compared to faster cars.
* The car "snaps" loose just when you get into the throttle.
As you make changes to make the car more balanced, the crossweight percentage will need to increase to keep up. If you raise the rear Panhard bar or stiffen the RR spring to reduce the rear roll angle to match the front, the car will become loose if you do not also increase the crossweight.
The process of balancing the car involves making the LF tire work harder. As that happens, the car will turn better with more traction available up front, meaning that we must also tighten the car with crossweight. Too many teams make adjustments that make the car more balanced, only to back off because the car gets too loose.
Pro-4 race cars are real race...
Pro-4 race cars are real race cars. They have all of the adjustments of the big Late Model cars. Weight management is just as important to these teams.
* Do weigh your car with the driver to know your exact weight distribution.
* Do level your scales, roll the car to take out all binding in the control arms, air up the tires to race pressures, and add all fluids, etc. before you measure the weights.
* Do make changes to the crossweight in the shop to know how much difference a turn of the screw or ring makes in the crossweight percentage.
* Do change all four corners when making changes to the crossweight percentage to maintain the correct ride heights. To put cross in, put turns into RF and LR springs and take turns out of the LF and RR springs. The opposite takes cross out.
* Do make records of your weights. Use the weights taken on racetrack scales for reference only. Do not ever change your crossweight to match the track scales.
* Don't build your setup around a particular crossweight percentage just because you have always run that number. Be open minded in order to find a better combination if that is what you need.
* Don't guess at the crossweight percentage in your car. The old way of jacking up the rear end with a large socket between the jack and the rear differential pumpkin and seeing how far off the ground the RR tire is when the LR tire is just touching is way out of date.
* Don't run the high crossweight range on high-banked tracks unless the banking falls off abruptly and there is a problem getting traction off the corners. And don't try to run the low range on the flatter tracks with bite-off problems, like I did at one time. That is how I learned about the different ranges.
* Don't move weight around in the car to effect changes to the crossweight percentage. Move the weight jacking devices or shim the springs in a stocker.
* Don't try to match your crossweight to a competitor that is running well. His overall weight distribution and setup may be different than yours, especially the front-to-rear percentage of weight. These setup components all work in combination, and each car's combination is necessarily different.
It has been said that knowledge is power, and as we all know, it can also mean speed. Knowing your weights is a part of that bundle of information you will need about your car in order to be competitive. Don't be shy about experimenting with items such as weight distribution, and by all means, think outside the box.
Once you find that perfect combination, you won't share it, just as no one shared with you. Then you'll know why it takes so long to be competitive. You must reach the point where you quit listening to your competitors and start thinking for yourself, no matter how long you've been racing.