For racing, choosing the widest wheel within the range recommended by the tire manufacturer is best. That way, you get the widest and most consistent contact patch and the best seating on the bead.

It is important that the proper wheel width be used for a given tire size. Tire companies provide data for each tire they make, including wheel-width recommendations. The range is usually about 2 inches in wheel width, with the middle width a good compromise for highway use. Using a rim outside the recommended range can be dangerous. If the rim is too narrow, the tire contact patch will crown and the sidewall beads will not seat properly. This could cause the tire to unseat from the wheel rim, causing the tire to deflate. This will ruin the tire at minimum and cause a crash in the worst-case scenario. Putting a tire on a rim that is too wide can cause the same problem, though the tire will never seat properly on the rim bead and may never inflate.

Within the range of recommended wheel widths for a specific tire, the wider wheel is the best choice. When a tire is on the widest recommended wheel, the tire contact patch is the most equally loaded. For highway driving, the middle wheel width will work, but for competition, the widest wheel is best. For a specific competition class where wheel width is limited, using a tire with a smaller cross section width is often better than trying to use the widest tire that will fit in the fender well on a rim that is narrower than optimum.

For performance applications, using the narrowest recommended wheel width is not desirable because the tire contact patch is slightly crowned. This is not dangerous relative to the tire bead staying seated to the rim, but it does hurt overall traction. Additionally, a rim on the narrow side of the recommended range can cause the tire contact patch to chatter or skip during cornering as the limit of traction is approached. For this reason, select the widest rim recommended for a specific tire application.

But you can go too far on wheel rim width, a trend becoming popular in several forms of motorsport. First the practice is dangerous since a tire bead can become unseated from the rim bead, causing a loss of pressure and a sudden change in handling. This can often lead to a crash.

This trend has become popular as a way to overcome handling issues caused by excessively stiff spring rates, another popular practice. The job of the springs is to allow compliance of the tire contact patch with the racing surface. It is not the job of the tire sidewall to act as a primary spring because the suspension springs are too stiff. While tire sidewalls are designed to flex proportionally to tire pressure, they are not designed to work as a cantilever, which they must do on a wheel that is too wide. By angling the tire sidewall on a too wide wheel, the effective spring rate of the sidewall is decreased, causing it to flex more under the same load. This flexing improves tire contact patch compliance, which is why handling is improved — but it also accelerates tire sidewall wear, makes the sidewall more susceptible to damage and punctures and increases the likelihood of the tire bead unseating from the rim. It is much better to utilize the proper spring rates for the optimum suspension frequency for the track and vehicle characteristics. In the long run, the ultimate performance is enhanced and the cost is greatly reduced, considering that, sooner or later, a tire will unseat from the rim and a costly crash is likely.

Spring Rates of Tire Sidewalls

Over the last 40 years, I have been involved in several tire comparison tests, some for DOT race tires. In every case, the methodology was flawed. I was always adamant that the test vehicle be tuned to the tires, but this was too costly and time-consuming. But not doing so invalidated the tests. I have also encountered many racers who change tire brands, either by choice or because of a change to spec tire rules. In most cases, the racer complained about how bad the new tire is compared to the existing tire. In fact, the new tire could have been better. Here are the issues and why retuning the platform for the new tire is crucial.

The sidewall of a tire is flexible. If it weren’t, it would not steer around corners with very much speed. Every tire has a sidewall spring rate, which is affected by the design, construction method and the pressure used in the tire. Changing from one brand of tire to another, or when the manufacturer comes out with a new tire design, you can be pretty sure that the sidewall construction is different, and that means that the spring rate of the sidewall will be different at a given tire pressure. In fact, you can take two identical tires from the same manufacturer and the will likely have slightly different sidewall spring rates.

If you have been contemplating this as you read, you may already understand the issue here. If the tire sidewalls have a spring rate, it must work in conjunction with the springs and antiroll bars, and therefore will have an effect on the roll couple distribution, or handling balance of the platform. And this is true and the core of the problem.

Since most of the roll couple on a car is distributed to the front, usually in the 60 to 85 percent range on a production-based racecar, then a change in tire sidewall spring rates that is the same front to rear will alter the roll couple percentage front to rear. This will change the handling balance. And this will require a change to the antiroll bar rates and possibly the springs rates as well.

When changing tires from one manufacturer to another, even though they are the same size, the differences in the spring rates of the new tires can have an effect on roll couple distribution. A different aspect ratio also will change a tire’s spring rate.
When changing tires from one manufacturer to another, even though they are the same size, the differences in the spring rates of the new tires can have an effect on roll couple distribution. A different aspect ratio also will change a tire’s spring rate.
Image courtesy of Brett Becker

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