The vertical loads on your tires have a huge influence on how fast your car can go around the track, and how it responds to your driving style. Those tire loads go up and down from static values that can be measured and adjusted, and those adjustments can dramatically change your car’s behavior. Let’s dig into how and why some quality time in the garage can sharpen your performance at a nice price.
If your car always understeers when turning left and oversteers when turning right, or if the same tire always locks up first while braking, those are signs that you may have a crossweight problem that can be fixed by measuring and adjusting the static tire loads. You can also learn a lot about why your car handles the way it does if you know what the four static tire loads are.
The easy way to change the static tire loads is with adjustable-height spring seats. A harder way is adding or removing shims or rubber pads between the springs and their seats. When you compress the left front spring more by adjusting its spring seat, the left front and right rear tire loads will increase, and the other two will decrease. It can also be done by making one of the anti-roll bar end links shorter or longer. That pre-loads the bar, which is frowned on by purists, but it is commonly done by budget racers.
If you have to run ballast, you usually have some freedom in where it goes. The same goes if you plan to move the battery or the driver, or if you plan to move the engine. Moving those weights will change the static tire loads, sometimes by a lot. A 1 percent change in front weight percentage has a significant effect on the way a car handles, and there is an optimum for each car.
When you are measuring tire loads, the configuration of the car should include everything that is there when it’s on the track, including the driver and a typical fuel load. So, you need a helper to read the scales for you, or a long wire between each scale and its display so you can see all of them when you are in the driver’s seat.
Unless the driver’s seat is in the middle of the car, you won’t be able to equalize both the two front tire loads and the two rear tire loads, no matter how much adjusting you do. So, what’s the right thing to do about that? It depends on the handling issues you need to work on.
If the car understeers most of the time, and you want the same behavior when turning both directions, then the right thing to do is to equalize the two front tire loads and let the rear tire loads end up wherever they end up. Likewise, if braking performance is a key to your lap times, then equal front tire loads are the right goal.
On the other hand, if the cornering balance of your car is fairly close to neutral, you will end up with about the same balance when turning both directions if you balance the diagonal weights. To do that, add the left front and right rear tire loads together, and compare that number to the sum of the right front and left rear tires. Adjust the car until those two sums are the same.
If power application is more important than cornering balance, then the right goal is to equalize the tire loads on the drive axle. That will produce about the same corner exit acceleration while turning either way.
A key measurement to keep track of when adjusting the corner weights is tilt: that’s the roll angle of the car when it’s sitting still. If the tilt angle ends up off level while you are adjusting the corner weights, it will change a lot of other suspension alignment values. It’s also important to keep the steering wheel centered while measuring corner weights, because steering usually changes the tire loads.
Because corner weights are fundamental to the handling of the car, every professional racing team measures and adjusts corner weights before and after every track session. With a little time and a little money, you can dial in your setup just like the pros. There is a lot more on how to make your own low-buck suspension alignment kit in my book “Think Fast: The Racer’s Why-To Guide to Winning.”
Measuring Tire Loads
High-quality setup pads and wheel scales usually aren’t in an amateur racer’s budget, and they don’t need to be. A little creativity and a smooth garage floor can be viable substitutes for those pricey items. In the age of the Internet search engine, affordable digital shipping scales with enough capacity are easy to find. You can shim them level with just the right thickness of junk mail under the low side, and some scrap lumber will make it easy to roll the car on and off.
How can you tell if one side is low? Look through your tool box for two sockets that are the same height. Set one of them in the middle of each scale for the front axle. Next, set a square tube that is reasonably close to straight on top of the two sockets, and mark the top face of the tube with an arrow pointing forward. Finally, set a bubble level on the tube and mark its position. The bubble will move away from the low side. Grab an old newspaper, magazine, vinyl floor tile, or whatever and slide it under the low scale. When you are close to level, you can use a magazine that is open so that just the right number of pages is under the low scale. Try not to get distracted by what’s in the magazine!
Now you have the front scales leveled. Do the same thing to the rear scale pads, and make sure that the orientation of the square tube and the position of the bubble level are the same as when you leveled the front scales. If you do that, the tube doesn’t have to be perfectly straight. You may end up with a small height difference due to a slightly bent tube, but if that difference is the same on the front and rear axles, it won’t matter enough to bother with. If you want to know, you can find out if your square tube is bent by turning it around to see if the bubble level shows a difference that way.
If you really feel the need, you can shim the scales so that all four scales are at the same height by turning your square tube and placing it on one front and one rear scale. However, that step is rarely worth bothering with. The center of gravity of your car is probably fairly close to the plane passing through the axle centers, so if the car is nose-high or nose-low on the scales, the tire load measurements won’t change very much.
Whatever type of scales you buy, they probably weren’t designed to handle much side load. So, it’s not a good idea to drive the car on and off the scales. You are much less likely to damage the scales if you push the car on and off.
While you are making the wood rails to roll the car on and off, make sure that there is a flat section at the end of each ramp. The flat sections will let you roll the car off the scales and on again without it trying to roll down the ramps. If your car has a very low ride height, a low air dam/splitter, or low side sills, you will need long ramps to avoid contact on the way on and off.
Every suspension system has some friction, so it’s a good idea to use a consistent method to make sure that the friction is acting in the same direction on all four corners before you write down the tire loads. To do that, just push down in the middle of the car at the front, then again in the middle of the car at the rear. Now the car is ready to be measured.
You may be surprised by how much the tire loads change with the wheel turned. You can find out just how much they change by making your own turn plates. All you need is a pair of thin steel sheets under each tire, with a thin layer of grease between them. However, there is a risk that the car can slide sideways off the scales, so you will need chocks in front of and behind both rear tires when you are using home-made turn plates.