Understanding How Weight Distribution Affects Your Race Car
Mike Aguilar
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Image by Mike Aguilar
Improper weight distribution in your race car – strip burner, autocrosser, circle tracker, etc. – can make your car dive like a dump truck or a block of wood on ice. Get it right and your car launches down the corner, hits that hard hairpin just right and holds the corner at full throttle. We’re going to take a quick look at what several sanctioning bodies say about weight and how it is to be distributed throughout the car. We’ll then talk about static weight distribution and cross-weight percentage and how they affect how your race car handles.
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Weight is strictly enforced in all NHRA competition classes, with weight requirements and limits varying from class to class. The hard and fast rules that stay the same across all classes are:
• any weight added, ballast, may not extend over the front or rear of the car’s body or tires, and
• must be permanently attached to the vehicle, and
• there may be a maximum of 500 lbs ballast with a maximum of 100 lbs of that being removable.
NOTE: This information is from an NHRA Rule Book 2019 Addendum. The full rule book is only available to current members/racers.
The sanctioning body for most off road racing events (like Baja) is SCORE International. Each class has a minimum and maximum weight limit. Roll bar/cage requirements also vary based upon vehicle weight. On top of this, SCORE has one rule regarding weight that applies to all classes:
• Any ballast used must be installed as an integral part of the vehicle’s structure.
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Like other bodies, the SCCA sets a minimum weight requirement for all classes. As an example, all cars in the TA2/Trans Am 2 class cars must weigh at least 2830 pounds and any ballast used to meet this minimum must be mounted securely. SCCA also mandates that no more than 52% of the vehicle’s weight may be over the rear wheels and fuel may be removed during tech/weigh-in to meet that requirement. One difference with the SCCA is that they require that the car be in race trim, including the driver.
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Static weight distribution is the weight of the car on each wheel at rest set up and prepared for the race, including the driver. The easiest way to explain static weight is to have you step on a scale. Your weight on the scale is your static weight. Race cars can be weighed either by placing wheel scales under each wheel and combining the values from the four scales or by driving on a four wheel scale. When measuring static weight, you need to unload the suspension before taking your weight reading if you don’t have skid pads/turn plates on your scale pads, because not doing so can skew your cross-weight percentage, which we’ll talk about below.
Image by Enilda Aguilar
Cross-weight percentage is the percentage of vehicle weight on diagonally opposite corners. Most everyone is familiar with front-rear weight distribution – how much of your vehicle’s weight is on the front and rear axles. Let’s take the SCCA example of 2830 pounds from above. No more than 1,471.6 pounds of that weight, 52%, may be over the rear axle/wheels, leaving 1.358.4 pounds for the front wheels.
Image by Enilda Aguilar
Let’s say that the weight over the two axles is divided equally side-to-side. This means there are 735.8 pounds on each rear wheel and 679.2 on each front. To determine cross-weight we add the weight of the right front and left rear wheels and divide that sum by the total weight of the car. Our sample car has a cross-weight of 50, meaning it’s neutral. If the percentage was over 50 we’d have “positive wedge,” and negative/reverse wedge for a number below 50.
NOTE: It’s the rare vehicle whose weight is evenly distributed side-to-side, especially with a driver in the cockpit.
Above, you can see the result of improper weight distribution- a spin out.
Image courtesy Wikipedia
If your car has positive wedge, it’s most likely going to understeer, or “push,” through left corners. Reverse wedge means you’ll experience oversteer. Positive wedge loads the LR tire in a left corner, giving you better drive-off than if your car is loose (oversteering). This is helpful for circle track racers, but not so much for anyone who also has to make right turns in a race. Excess cross-weight for those who like to turn left and right while racing will cause excessive weight/load transfer when switching from hard left to hard right, which can cause things to “get interesting” for the driver trying to drive on the edge and maintain control.
If you don’t get cross-weight and front-rear distribution correct you’re going to have a very trying experience on race day. It’s a balancing act. Imagine yourself on a pair of shoes with a ridge down the sole. Now try and run as fast as you can without falling. You can if you get your weight distribution right. If not, road rash happens.
Image courtesy US Air Force
Static weight distribution can only be changed by adding or removing weight where needed, usually by adding or removing ballast from the front or rear centerline of the vehicle. If the class in which you’re racing allows it, you can also adjust weight distribution by moving components such as the battery and fuel cell or fire bottle. If you have to adjust your static weight, make sure you do so in a manner that fits in the rules.
When you need to adjust static weight distribution, make sure the ballast/weight you’re moving around is as far to the side or corner needing adjustment as possible. For example, if you need to increase the vehicle’s rear weight, place that weight as close to the bumper as you can and still mount/secure it firmly. Not getting that ballast as far into the corner or out over the edge as possible allows part of that weight to be felt on the other side. You don’t want the weight you’re adding to the left front to also add weight to the right front or either rear wheel.
Image by Enilda Aguilar
Adjusting cross weight percentage isn’t done by adding or subtracting weight in the vehicle. Cross weight is adjusted by raising or lowering one or more corners of the vehicle. For example, if your cross weight percentage is 52 and you want to get it down to 50, you need to lower the right front or right rear and/or raise the left front or right rear. Ideally, you want to raise or lower one side to get half of the desired adjustment and then move to the opposite side and lower or raise (opposite what you did on the first corner adjustment) an amount equal to the adjustment on the other corner.
In the example above of a 52 percent distribution, you can lower the right front or left rear an increment or two, then raise the left front or right rear an equal increment. The idea here is to get the ideal cross weight percentage while not changing ride height very much. Tire stagger, tire pressure and firmer springs will also change cross weight percentage measurements, as they modify ride height and cross weight distribution is a function mostly of ride height.
Load transfer in drag racing mostly deals with the transfer of weight from front to rear at launch.
Image by Enilda Aguilar
Up to now we’ve been talking about static weights – the car is sitting in a garage on a scale. That’s all fine and dandy for the garage, but the track is a completely different monster, my friends. Load transfer is why we don’t want a 50-50 front-rear or cross weight; we want something a bit different. Exactly what you want will depend on the type of racing you do.
The above photo shows examples of cornering loads NASCAR vehicles experience at tracks like Charlotte.
Image from NSF lecture
What is load transfer, though? It is how the weight or load of the vehicle moves around while you’re driving. Pay close attention at the drag strip this weekend. Watch the rear of the cars as they leave. You’ll see the rear squat, the front lift, and the torque will also cause the vehicle to twist and put more weight, or load, on the right rear. This can cause you to leave your lane or hit the wall. Not good.
Image by Enilda Aguilar
It’s the same with any type of racing. Take a look at the picture above of a NASCAR vehicle on the short track at New Hampshire. Notice the left front lifting? That’s load transfer. That’s lost grip. You need to eliminate as much of that as possible, preferably by weighting and lowering that corner.
As you can see, improper weight distribution and chassis setup can turn your enjoyable day at the track into sheer hell. Get it right and you’ll be celebrating in victory lane. Get it wrong and you may be going home on a wrecker. See ya at the track!
