Getting Tire Camber Right Means More Tire Contact Patch
By Group R Motorsports owner Bob Bogwicz
Camber is probably THE most important suspension set-up parameter to maximize a tire’s contact patch as a race car corners.
Last month, I introduced you to the prime goal of race car handling: optimizing the grip of all four tires so the car will turn to the left as quickly as possible. With that in mind, we talked about the tire contact patch: the connection between the track and the race car.
Our first discussion of maximizing the tire contact patch centered on tire air pressure and how it affects handling. Another set-up measure that can be adjusted by a race team to maximize a tire’s contact patch is called camber.
Camber is probably THE most important suspension set-up parameter to maximize a tire’s contact patch as a racecar corners. Camber is the inward or outward tilt (as viewed from the front) of the top of a tire with respect to a vertical line. If the top of the tire is tilted towards the center of the car, it is negative camber. If the top of the tire is tilted away from the center of the car, it is positive camber. Figure 1 shows the front view of a right front tire with negative camber (the top of the tire is tilted towards the center of the car). The camber is measures in degrees of tilt from the vertical.
So, how does camber improve the contact patch of a cornering race car? Figure 2 shows the front view of a right front tire with no camber as if the car where standing still (or at least going in a straight line). Note that in this figure, the contact patch is fully on the racing surface. However, Figure 3 shows this same tire, with zero camber, while cornering at high speeds. Note how the force of friction tries to keep the contact patch of the tire in contact with the racing surface (this is called side-bite) while the cornering forces tries to lean the tire to the right.
The result, as you can see, is a tire that has very little of its contact patch on the track surface. This is counter to our prime aim of optimal tire grip. In this case, the cornering forces will overcome the force of friction between the tire and racing surface due to a minimal contact patch, the tire loses grip and the car does not corner effectively. In racing terms, the car wants to go straight instead of turning left. This is called a push, like the car is “pushing” the tires instead of them rolling smoothly.
Figure 4 shows the tire of Figure 1 (the tire with negative camber) during a high-speed corner. Note how the maximum amount of the tire is in contact with the racing surface. In the very simple terms, the tire starts off with a “tilt” but will “stand up” during hard cornering to optimize the contact patch. This is a VERY good thing!
Remember, a racing tire goes from the condition in Figure 1 to the condition in Figure 4 only during a corner and at racing speed. This makes the sidewall construction of a racing tire a vital part to the overall design of a tire because of the tremendous loads placed on it on the corner.
The camber of an independent front suspension vehicle (like a Small Block Super, asphalt modified or Clyde Booth’s supermodified) is made by inserting a spacer between the upper control arm (the suspension part that attaches the top part of the spindle assembly to the cars frame) and its mounting point on the frame. The thickness of this spacer determines the amount of camber the tire has. A car with a straight front axle, (like a most wing and all non-wing supermodifieds, a DIRT mod, or NASCAR Whelen Tour Modified), can also have a camber adjustment, but it is a little more complex. Many straight axle cars have the camber built into the axle. Once that’s done, one cannot change camber.
So, we’ve talked about the camber of a right front tire. But, what about the other three tires? On the left front tire, there needs to be camber for the same reasons as the right front tire however, just not as much. And, as you look at the front of the car, the tops of right front and left front tires are tilted in the same direction. However, since we defined camber as the tilt of the top of the tire with respect to the center of the car and a tire that tilts to the center of the car has negative camber, the right front will have negative camber as we describe earlier, but the left front will have positive camber because it’s top is tilted away from the center of the car. A typical set-up on a supermodified at Oswego Speedway will have -4° of camber on the right front and +1° on the left front.
For the rear tires, both the LR and RR have some camber. However, camber in the rear tires is due to tire stagger (the difference in circumference of the rear tires – a topic for another yet another Supermodified Tech Knowledge article!). When the RR has a greater circumference than the LR, the RR will be a little taller than the LR. This difference in rear tire height will cause the entire rear axle/wheel/tire assembly to be at an angle where the right end of the rear axle is higher the left end, as shown in Figure 5. This angle due to stagger will put a small amount of negative camber into the RR and a small amount of positive camber into the LF. This is also a good thing but rear tire camber is more of a by-product of another factor- stagger.
Camber is initially adjusted in the garage according to a race teams notes or the chassis builders recommendation and then “tuned” at the track via tire temperatures. I know, I know, I’m teasing you again about tire temps. Be patient, we’ll get to it. In the meantime if you have questions, comments or topics you would like to see covered in Supermodified Tech Knowledge, you can contact me at motosports@groupRtech.com. Follow me @groupRmtrsports and submit questions via Twitter as well.
Do you have a burning supermodified tech question or comment? Leave a comment below or Hit the Wailbag and The Bogwan will answer them once he figures out which way his tires tilt.