I'm considering experimenting with a new set of wheels. I came across this article which some of you may find useful:

http://www.mazda6tech.com/index.php?...d=20&Itemid=50


"You've probably read how important it is to put more rubber on the road. However, like most things in life, the solution is not so simple. Wheel widths, tire pressures, and contact patch shape will vary depending on which car the tire is being used on. Learn how these variables effect your tire's performance, how to maintain a good ride quality, and what tradeoffs are involved in raising and lowering tire pressures.

Are Wider Tires Better?

Basics:

The more rubber you can put on the ground, the better. Why? Because tires grip best when their load is distributed over a large area. The larger the contact patch your tire has with the ground, the better. This is why drag radials are specially developed to be run at ultra low tire pressures. The lower your tire pressure, the more rubber you will have on the ground.

The other way to get more rubber on the ground is to use a tire with less void area. The tread in your tire is all void area- it's area where there is no rubber with which to grip the pavement with. These grooves are needed for traction in the rain and snow, but they take away from dry road performance. A racing slick, for example, has virtually no void area. A racing slick also uses super soft, sticky rubber not suitable for the street- they tend to throw rocks and easily puncture. The more aggressive street tires use a semi-soft rubber as a compromise.

Back to tire pressures.

There is a huge caveat to reducing your tire pressure. Lowering your tire pressure decreases your tire's sidewall strength (not to mention ruins gas mileage). With a lower tire pressure, the tire's sidewall does not have the same reinforcement stretching it outwards for reinforcement. A strong tire sidewall is extremely important for cornering, otherwise the tire will deform. When a sidewall deforms, the tread shifts out from under the wheel enough that the tire begins rolling onto its side. When this happens, you are losing not just grip, but reaction time to steering inputs too. Sidewall strength is extremely important, as you want to stay on the tire's sticky tread! It is not uncommon to run 50psi in the tires while autocrossing to firm up the sidewall!

So while lower tire pressures can increase a tire's grip, for proper handling (and safety) it is MORE important that the sidewall not deform. You will gain more cornering grip by increasing your sidewall strength than by decreasing your tire pressure.

Where the wheel comes into play:

The easiest way to prevent a tire from deforming (besides buying a more aggressive tire) is to give the sidwall a little leverage. This is done by increasing the wheel width. By making the rim WIDER than the tread, the lateral pressure on the tire can be more directly transmitted to the wheel. Since the wheel is rigid, it will not deform, and will allow the sidewall to brace itself against the wheel.

Every tire has a spread of rim widths that it is designed to fit over. Let's say a size 225/45/17 tire will fit on a 17 inch rim from 7 to 8.5 inches wide. While the wheel is designed to fit on a 7" wide rim, the actual tread of the tire would likely be as wider (possibly much wider) than the rim itself. Because the sidewall is ballooning outwards beyond the wheel, it has little leverage to resist side-to-side movements. When the sidewall cannot resist these side-to-side movements, the tread begins to roll over onto the sidewall, and the entire sidewall deforms. In extreme cases, the tire could loose its bead and put the driver in danger.

Most experts will recommend that for aggressive driving, even the minimum wheel width listed by the manufacturer for a tire is still too small. It is possible for a skinnier tire to perform better on a skinny wheel than a wide tire on a wide wheel. Rather than getting 225/45/17 tires and putting them on 7" wide rims, try a 215/50/17 on a 7.5" wheel. You may find that combination more responsive, and as a bunus the slightly smaller tread may fit under the fenders more easily. More extreme, try fitting those 225/45/17 tires on an 8" wide rim, and roll, bend, or cut the fenders!

Why is this important? Because sidewall deformation robs traction, overheats tires, and causes them to wear unevenly! In some cases, lateral stability might be so good that the tire pressure need not be as high. If we can lower the tire pressure without worrying about the sidewall deforming, then we can increase the contact patch!

Of course, if cornering is not important- then neither is sidewall strength. But you wouldn't deprive your Mazda6 of twisty roads, would you?

The shape of the contact patch:

You'll notice going to a wider tire was not listed as a way to put more rubber on the ground. Are wider tires even any better, then? Yes, but not because they put more rubber on the ground.

Wider tires change the shape of the contact patch. It changes the shape from long and skinny to short and wide. There is an advantage to this. The sidewall of that tire is actually supports the car a minor amount- moreso in aggressive tires and almost entirely so in run-flat tires. Due to the circular shape of the tire, this amount is greatest directly below the tire, and smallest at the front and back of the contact patch.

See this example of what the compact patches- the shaded areas are the contact patches:



The sidewall of the tire must bend a little to create the contact patch. As seen in the diagram, this happens less on the wider tire. Because the sidewall has less bend, less of the car's weight is being supported by the sidewall. Instead, it is being supported by the tire's air pressure, which is constant among the entire contact patch. Since the even distribution of weight among the tire's surface will allow the tire to function better, the wider contact patch has more overall grip.

Just remember that for good lateral grip (cornering), preventing the sidewall from deforming is the greatest priority. Worry about that first- don't get hung up on wide tire sizes that, in the end, make only marginal improvements.

More Miscellanious Stuff:

Because a wider tire has less bend in its sidewall, a wider tire will have a marginally higher ride height. Look again at the diagram- the ride height of the tire is the distance between the tire's center and the contact patch. This is a smaller measurement than the tire's unloaded radius! In drastic width changes, you may compensate by decreasing your tire diameter! Decreasing your tire diameter will offset the increased inertia from having more rubber. And, if you really drive the car hard, you'll appreciate that your tires will resist overheating a little more since there is more rubber with which to distribute heat.

Another advantage of both wide tires and wide wheels is that the they contain a greater amount of air when inflated. This is especially important when changing to very low-profile tires. Reducing the volume of air it takes to inflate the tire will increase ride harshness. This is why Porche makes wheels with hollow spokes- to increase the volume of air it takes to inflate the tire. Of course, having too much cushion can lead to a lofty tire, and it is usually best to simply try to stay close to the what the car came with. The car's suspension is specially calibrated for that."