Every ounce of weight your car carries on the track is a dynamic, moving force through the turns. The question is what is it doing to your tires’ ability to hold the track?

By Paul E. Boehm
You’ve spent countless late nights shaving every possible ounce off your car until it’s at the minimum weight limit-but have you thought about how to make the most of what’s left?

One of the most important things for a racer to consider when setting up a chassis is the weight transfer characteristics of the car. The amount of weight on a tire at any given time dramatically affects the traction that tire can generate. Therefore, weight transfer can be your friend, and it can be your enemy. If your engine is prepared to the maximum extent of the rules and you’re still looking for quicker lap times, chassis tuning is the answer. In fact, it is oftentimes more economical per dollar spent than engine tuning and it makes the car easier to drive. To reduce the negative effects of weight transfer, there are four options:

1.) Increase the track width.
2.) Reduce the center of gravity (CG) height.
3.) Reduce the vehicle’s total weight.
4.) Redistribute weight within the chassis.

Any time a longitudinal (or lateral) force is applied to a vehicle, it collectively acts at the center of gravity (CG). See figure 1. The vehicle’s tires resist these forces. Since the CG is located some distance above the ground, a twisting force, or torque is created which tries to overturn the car.


The higher the CG, the more leverage centrifugal force has to transfer weight. The farther the tires are from this torque, or moment arm, the less effect it has on them. Reducing the center of gravity height diminishes the effect of the moment arm as does widening the track. So, if you are still looking for more stability and have your car as low as you can go, try widening the track. If you are as wide as rules allow, look at your ride height.

Vertical Load and Traction

Notice in the graph of vertical load vs. traction force, as the vertical load on a tire increases, traction goes up, but the percentage of traction force decreases, effectively reducing the g’s a car can generate. For example, if this tire has 500 lbs. of vertical load on it, it can generate 660 lbs. of traction force, or 1.32 g’s (660 / 500 = 1.32).

If the load were 800 lbs., the traction force would be 925 lbs., but only 1.16 g’s (925 / 800 = 1.16). This is the tire’s traction potential. As you will see, for any of several reasons, it will usually be less. This line can move around relative to the graph based on several conditions such as camber, ambient temperature, tire temperature and track surface condition, and obviously the type of tire. It will never be straight, however because the relationship is not linear.


Weight Transfer during Cornering

During cornering, centrifugal force transfers weight from the inside tires to the outside tires. The total weight on all four remains the same – it’s just distributed differently. This reduces the overall traction the front and rear pairs can generate because the outside tires do not gain as much traction force as the inside tires lose. For ease of explanation, let’s look at a car with equal weight distribution on all four tires. As illustrated in figure 2, this vehicle has 800 lbs. of static weight distribution on each tire at rest. Let’s assume this car transfers 620 lbs. from each inside tire to each outside tire in a left-hand turn. This would put 1420 lbs. of weight on each outside tire and reduce the weight of each inside tire to 180 lbs. Checking our graph we see that a tire with 180 lbs. of load can generate 260lbs. of traction force and a tire with 1420 lbs. load can generate 1250 lbs. of traction. So each pair of front and rear tires will generate 260 lbs. + 1250 lbs. = 1510 lbs. of traction force or .94 g’s.


Chassis Tuning for Improved Weight Transfer Characteristics

Lowering the CG can be as simple as relocating the battery from its factory position to a major re-design of the chassis. With the exception of a few heavy items, most of the components will be small and will only make a minor amount of difference. Increasing track width can be accomplished with wider wheels and/or wheels with more offset or longer control arms. Let’s look at how it helps. The formula for calculating lateral weight transfer is:

Lateral weight transfer = tire potential g’s x weight x CG height
Track width

On our Saturday night circle track car, let’s use a 20” CG height and a 60” track width and plug in some numbers. On a typical day, our tires are potentially capable of 925 lbs. of lateral traction force or 1.16 g’s (925 / 800 = 1.16). Plugging the numbers into the formula we get (1.16 x 1600 x 20) / 60 = 619 lbs. of load transfer which equals 1510 lbs. of traction force or .94 g’s. If we increase the track width by two inches: (1.16 x 1600 x 20) / 62 = 599 lbs. of load transfer which equals 1540 lbs. of traction force or .96 g’s. If we lower the CG two inches, then (1.16 x 1600 x 18) / 60 = 557 lbs. of load transfer which equals 1585 lbs. of traction force or .99 g’s. Finally, let’s put the two together; (1.16 x 1600 x 18) / 62 = 539 lbs. of load transfer which equals 1620 lbs. of traction force or 1.01 g’s.



Left-side Weight and Right-side Weight

The purpose of weight biased to the left side of a circle track car is to get the car closer to the ideal 50/50 weight distribution for the front and rear pairs of tires in the corner. Let’s redistribute the load so that the left side tires weigh 200 lbs. more each. Static weight distribution will be 1000 lbs. on each left side tire and 600 lbs. on each right side tire. We’re still going to transfer that 620 lbs. back to the outside tires in the corner. Now the inside tires will be at 380 lbs. each and the outsides 1220 lbs. each. Checking our chart again we see that each inside tire can now generate 525 lbs. of traction and each outside tire can generate 1195 lbs. traction which gives a total cornering force of 1720 lbs., or 1.08 g’s. Finally, let’s reduce the weight transfer to 540 lbs. by widening the track two inches and lowering the CG two inches as in the example above. This leaves 460 lbs. on the inside tires and 1140 lbs. on the outside tires. The inside tires now generate 610 lbs. of traction force and the outside tires generate 1160 lbs. of traction force which equals 1770 lbs. for each pair of tires or 1.11 g’s. See figure 3.

Longitudinal Weight Transfer during Braking

During braking, weight is transferred off the rear tires onto the fronts. The long wheelbase resists this weight transfer rather well, much better than the narrower track width. However, the front brakes still do approximately 65 to 75 percent of the slowing because of the extra grip they get during weight transfer. Adding weight to the rear of the car will help the dynamic weight distribution during braking just as preloading the inside tires in a corner.



Longitudinal Weight Transfer during Acceleration

So far we have been talking about how weight distribution can help by preloading a vehicle to get the proper dynamic weight distribution for best traction when working with a pair of tires. When setting up a front engine/rear drive car for best acceleration, the more weight that can be re-distributed to the rear, the better, since only the rear tires are supplying traction to accelerate the car. Many purpose built race cars, i.e., CART, Formula One, etc., have the engine in the rear for best rear tire traction off the corners. These cars operate with about 60% of the total weight on the rear wheels. In the case of a four-wheel drive vehicle, weight biased toward the front would enhance dynamic weight distribution during corner exit. But with a rear wheel drive car, the more toward the rear the better. This technique is also supported by the fact that drag racers transfer 100% of the vehicle’s weight onto the rear tires. We can’t because we need a low CG to reduce lateral weight transfer, plus we need some weight on the fronts for exiting the corner. Accordingly, with a front engine/rear drive configuration, anything that can be relocated aft will help corner exit performance on cars with lots of torque exiting slow corners. Most Winston Cup teams place as much weight as possible as far back as possible on their short track cars, where traction out of the turns is critical.
To make the most of the information we’ve presented, take a long hard look at the tracks on which you race. If you are racing against competent competition, it’s almost impossible to be the fastest car on all areas of the track. Look at your strengths, the layout of the track, and determine the areas you are most likely to make a pass. Then tune your car’s weight balance to give you the best possible performance there.

Reprinted by permission of Circle Track Magazine

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Other Resources: Psychology of Stock Car Racing, Street Stock Setup Manual

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