cross weight

[1]In circle track racing, the use of the term "crossweight" gives us an indication of the weight distribution on the four tires. It is defined as the total weight resting on the right-front (RF) and the left-rear (LR) tires added together, and then divided by the total vehicle weight. That math gives us a percentage number to relate to, i.e., "51.4 percent of cross."

Dirt and Asphalt

The percentage number for crossweight is used for asphalt cars. For dirt cars, the terminology is a little different. Most dirt teams refer to the amount of "bite" or "left-rear weight" in the car, which is the number of pounds of weight that the LR tire supports over the right-rear (RR) tire. Subtracting the amount of weight supported by the RR tire from the LR tire's weight, we arrive at a number and call that the amount of bite in the car, i.e., "100 pounds of bite or left rear."

How Crossweight Changes

It is important to note that if we increase the weight supported by one tire, say the LR, we also increase the weight supported by the diagonal corner of the car, in this case, the RF tire. As we make that change, the opposite change takes place at the other diagonal because those tires (the LF and RR) experience a decrease in the amount of weight they will support. So, for example, when we increase the bite in the car (LR), we also increase the weight on the RF tire.

In The Past

Historically, these numbers meant little to the circle track racer as related to particular setups, different types of racetracks, and overall weight distribution. All we knew was that a certain range of crossweight or bite worked for certain conditions.

Today, we have an entirely new understanding of crossweight and the use of bite related to the setups of the cars. Beyond knowing generally that more crossweight makes the car tighter at mid-turn and less tends to loosen the car, we can now know exact numbers to be used with a balanced setup.

For simplicity, we will refer to the weight distribution of the cars, whether dirt or asphalt, as crossweight. Dirt racers need to understand the relationship of changing the LR weight and how that affects the other three corners of the car as well.

Multiple Ranges

There are different ranges of crossweight percentage that will make your car neutral in handling. The same car at the same racetrack will be neutral in handling in the range of 48 to 52 percent and also be neutral in a range between 56 and 60 percent of crossweight. Dirt racers often change the LR from near zero to upward of 300 pounds. That represents a range of 48-percent cross to 60-percent crossweight.

The actual number that will work for your car depends on the front-to-rear weight distribution, the type of track, and the setup. Generally, the flatter asphalt racetracks with less grip require a high range of crossweight percent with the associated high LR weight. The car will like the lower crossweight at the higher banked asphalt tracks.

Dirt teams will do the opposite. A low crossweight percent loads the RR tire more, and that will tend to produce more bite off the corners on dry slick surfaces. The general thought is that more weight on that tire helps to cut through the dry material lying on the track and plant the tire on the hard surface.

Many teams who run a series of asphalt tracks will try to run the high crossweight range they have successfully developed for flat tracks while racing at the higher banked tracks. They would do much better, as far as consistency, if they switched to the low crossweight range on the high-banked asphalt tracks.

Cross Is Related to Front-to-Rear Weight Distribution

The exact amount of crossweight that will make your car neutral is directly related to the front-to-rear weight distribution. The greater the rear percentage, the more crossweight is needed in a car to stay neutral in handling.

For an asphalt example, let's look at a particular car that was prepared with a balanced setup and was neutral in handling with 51-percent cross-weight. If the same car is changed to an unbalanced setup with a softer rear suspension, the crossweight percentage would have to be reduced for the car to be neutral. The need to use a lower cross-weight is an indication of an unbalanced setup.

How to Make the Cross Change on the Racetrack

Your car can be set up so that the amount of cross-weight changes as the car circles the racetrack. A stiffer RF spring (stiffer than the LF spring) will load the RF and LR tires upon braking, causing a momentary increase in the crossweight percentage. A softer RR spring (softer than the LR) will load the LR and RF tires upon acceleration. Differences in shock compression and rebound will produce a similar effect on entry and exit, but only while the shock is in motion.

Sway Bar Effect

We can cause the static cross-weight to change by preloading the sway bar. Preload on the sway bar adds weight onto the RF and LR tires, which increases the amount of static cross-weight percentage. You must remember, the larger the sway bar the greater the effect of preload on the car. If we want to preload the bar, we should know how much the cross percentage will change, then lower the static (before loading the bar) crossweight percentage by that amount. When we preload the bar, we will have the exact static crossweight we want. The preload on the sway bar mostly helps provide bite off the corners.

Most dirt cars do not have sway bars. The few teams that have experimented with sway bars on dirt have seen positive results in some cases. This would be where it is beneficial to load the LR tire as opposed to loading the RR tire.

Rear Geometry Changes Cross

A few racers have discovered a way to cause the crossweight to change by utilizing the rotation of the rear end under acceleration. If the car has a three-link type of rear suspension and has a moveable link like device, a pull bar or torque arm, or lift bar, the rear end will rotate as a result of the forces of acceleration. This rotation is caused by the pinion gear trying to climb the ring gear in the rear end and taking the rear housing with it. This is not related to the dirt cars with birdcages or locating the rear end fore and aft. The bracket used for attaching the trailing arms to the rear axle tube must be clamped solid to the rear axle tube.

In order for the crossweight to increase, the springs must be mounted to the rear axle tubes with the LR spring mounted in front of the axle and the RR spring mounted to the rear of the axle. As the rear end rotates with the pinion moving upward, the LR spring will compress and the RR spring will decompress. This causes more weight to be supported by the LR and RF tires, hence more crossweight percentage of distribution while the car is accelerating.

Cross-weight Changes for Dirt Cars

Many dirt teams use a similar effect by mounting the springs with the LR spring behind the axle and the RR in front of the axle. One or both spring mounts are clamped to the birdcages. As the car moves vertically, the birdcage will rotate, creating the effect of changing crossweight percentage, or bite as it is called in this case.

Lateral Placement of Third Link Changes Cross

Lateral placement of the device used to control the rotation of the rear end under acceleration or deceleration can affect the distribution of weight and cross-weight during the transitional periods of entry and exit. The closer we move the bar to the LR tire, the more weight will be distributed to the LR tire on exit. The reverse is true during entry.

Unbalanced Setups Change Cross

An unbalanced setup will cause the cross-weight percentage to change as the car rolls through the turns. If the rear suspension is too soft compared to the front suspension, then the back end will roll over, causing excess weight transfer at the front. The RF tire will end up supporting excess weight and that causes an increase in cross-weight percentage with weight added to the LR also. This is exactly why a car with an unbalanced tight setup must run a lower static cross-weight percentage than a balanced car.

Front Geometry Causes Small Changes

Front geometry settings have a small effect related to changes to crossweight percent. Caster, camber, kingpin inclination, and antidive have their own influence on changes to cross-weight. These effects are relatively minimal because the changes are both small (in the case of camber/caster) and momentary (in the case of antidive).

The full jacking effect of caster and camber occurs at 180 degrees of steering input. Because we usually steer less than 10 degrees on the racetrack, the net effect would be 1/18 of the total. Camber change is the net result that causes the jacking effect, and normal changes to camber from normal steering inputs are in the range of less than 0.30 degrees of camber change.

Balanced Setups Make It All Work

Balanced setups are becoming more popular with both dirt and asphalt race teams. Crossweight percent can be an indication of the car's balanced setup. If the correct crossweight for a particular application is 51.4 percent, and we find we need to reduce crossweight to 49.5 percent to be neutral, our setup must not be balanced.

As we change our setup to balance the front and rear suspen-sions, we will also need to increase the crossweight percent or the car will be loose. If your team is stuck on using a certain low crossweight percent that you "always ran," your setup cannot be improved until you decide to change the crossweight, along with positive chassis setup changes.

Work toward setups that will plant the LF tire. On asphalt, a balanced setup will show the left side tire temperatures to be equal, as well as the right side tires being equal front to rear (not side to side).

For dirt cars, study the wear on all tires to get some idea of the work being done by each tire. As the setup becomes more balanced, the LF tire will show much more wear and we will be able to feel the heat after a good run.

With the setup balanced and the proper crossweight percentage set, your car will be fast and consistent and that is the combination that wins races. CT

What Is a "Balanced" Setup?

In this and future articles in Circle Track, we will be referring to a "balanced" chassis setup. This term denotes a condition that racers have always sought. All of the trial and error methods that have been developed over the past 30 years were aimed at achieving a balanced setup.During the past 10 years, racing technology has advanced to the point where we now know how to get to the balanced setup in an easier and quicker way. That will be a major technical theme of Circle Track magazine articles in coming issues. Dirt and asphalt racers will benefit from this new trend in racing technology. Balance in a setup is not just a neutral handling car, but also one where we know that both suspension systems, front and rear, are working together. When we achieve this goal, the car will be neutral in handling, all four tires will be working for maximum traction, and the setup will stay consistent over a longer period of time.The fastest cars do not necessarily have setups that will be fast beyond 20 laps. Gains in mid-turn handling performance over many laps represent the biggest gains in performance you will experience. Horsepower cannot equal these gains.Join us as we explore the many existing ways to make your car faster through the turns. Read the results of our research and testing as we discover new methods that will improve your racing effort and help you become successful.

To find our cross-weight percent number, we add the RF wheel weight and the LR wheel weight. Then, we divide that number by the total of all four wheel weights, which, in this case, is 2,600 pounds. Use wheel weights that represent the total weight of the car, including the driver and all fluids, just as it will be raced on the track. When weight is moved around on the car, the crossweight must be calculated using the new corner weights. The bite for this car would be 192 pounds of LR, which is determined by subtracting the RR weight from the LR weight (746 - 554 = 192).

Coil-over shocks have an adjusting ring on the shock body or a sleeve that slides over the shock. Turning this ring up or down changes the weight distribution on that corner of the car as well as the other three corners. To prevent errors in counting the number of turns you make, paint a mark on the ring as shown above.

We can change the distribution of the weight on the rear tires by using the antisquat effect of the third link. By moving the third link laterally, we will be redistributing the weight. More weight will be distributed to the closest wheel. Also, the pull bar allows the rear end to rotate under acceleration. We can take advantage of this rotation by altering the location of the spring mounts, one in front of the axle and one to the rear.

As the rear end rotates upon acceleration when using a pull bar or other type of moveable link, the clamp holding the shock/spring combination will also rotate. This action compresses or decompresses the spring. The rotation and vertical movement of the end of the shock/spring serves to add or remove weight from that corner of the car if the springs are mounted at opposite sides of the axle. This will change the crossweight distribution for the whole car.

As we move weight around in the car from front to rear, the amount of crossweight percentage the car will need to be neutral will change. After any changes to front/rear percent, make sure to adjust crossweight percent to maintain a neutral handling car. The higher rear percent example relates to many dirt late model cars while the lower rear percent tends to relate to stock-type cars on both dirt and asphalt.

1. "Crossweight Discoveries - Featured Vehicles - Circle Track Magazine" Circle Track Magazine (Feb. 11, 2010)
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