Wheel and tyre sizes

Some interesting bits here regarding wheel sizes :

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Quote from another forum:

Found an excellent article here : http://www.sportcompactcarweb.com/te...ht_wheels.html

Edited highlights follow...

Going up an inch in wheel diameter and running a lower profile tire is a good thing. A lower profile tire has shorter, stiffer sidewalls, which improve response to steering inputs and hold the tread flatter to the road surface during cornering load. However, it can be overdone.

Ultralow-profile tires are more sensitive to suspension tuning and camber changes. And stiff sidewalls don't conform to road surfaces easily. This makes ultralow-profile tires sensitive to shock, as the short, stiff sidewalls have very little compliance. Harsh surface inputs can make these tires skip and hop across the surface instead of digging in and finding grip. Large wheel and tire combinations also increase rotating and unsprung weight.

For example, most enthusiasts driving small-bore four-cylinder front-wheel-drive cars run 17x7-inch wheels with a 205/40-17 tire. The big wheel and low-profile tire look cool, but this combo is too large and too heavy for optimal performance. Hard-core track geeks driving these same cars almost always fit a lightweight 15x7-inch wheel with a 205- to 225/50-15 tire.

Huge wheels also increase your car's final drive. Their added weight increases the flywheel effect, slowing the car's acceleration and increasing load on the brakes. This means wheels larger than 18 inches are rarely used for performance, so even for large cars, 18 inches is the maximum practical wheel diameter.

Huge wheels can greatly increase unsprung weight-the weight of the components that aren't suspended. This includes the suspension arms, brakes, half of the shock absorber and the wheel and tire.

For the suspension to work well, the ratio of sprung to unsprung weight must be kept low. The best example of unsprung weight hurting handling is found on monster trucks. Even though they have wheel travel measured in feet instead of inches, it doesn't provide a significant measure of driver control.

This is why monster trucks have nearly as much unsprung as they have sprung weight, thanks to their huge tires and axles. This makes the shocks work much harder to damp wheel movement. Reduce unsprung weight and the suspension doesn't work as hard, the ride improves and the tires stay in contact with the ground.

The obvious way to make up for the disadvantages of increased wheel diameter and width is to use a light wheel. Light wheels are easier to accelerate and brake. They also reduce unsprung weight. Some examples of our favorite light wheels can be found in the table below. The mostly expensive wheels listed on this page are forged or semi-solid forged. These processes improve the grain structure and provide superior strength for the weight.

The value wheels are all cast. Casting is a less expensive way to make wheels and nearly all low-priced wheels are cast aluminum. Some of the Enkei offerings have MAT (Most Advanced Technology) formed rim sections. MAT helps improve the metal's mechanical properties using a spinning process that yields almost as much strength as forging for a fraction of the cost.

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