Tuesday, January 22, 2008

Getting the Right Rubber for the Road

We have already said that you may not want the wheels and tires that the MINI dealer has to sell you. To understand why, we need to start talking about performance.

Performance, in gear head terms, is the general measure of how well your car does the four things it is supposed to do: start, run, turn, and stop. To win on the autocross or race course, you need to get up to speed as quickly as possible, go as fast as possible, get around corners as rapidly as possible, and stop in as short a distance as possible.

While many of the components on the car contribute to one or more of those goals, your wheels and tires contribute to all four. More than any other component on your car, the tires and wheels you choose will make significant differences in performance.

Let’s be clear about one thing from the very start: there is no such thing as the best tire and wheel for all those jobs. But there are some features that you should consider when buying any tire and wheel. They include weight, flexibility, and grip.

Weight is the most important aspect to consider. When we measure weight in a performance car, we divide it into two categories: unsprung weight, and sprung weight. Unsprung weight is the weight of all those things that stay attached to the road, or at least should stay attached to the road, when you hit a bump or pour the car into a turn while the rest of the car, the sprung weight, goes up and down on the springs. In other words, the wheels and tires are most of the unsprung weight.

In order to improve performance, we want to reduce our unsprung weight to the minimum required to get the job done. The first issue with heavy tires and wheels is that it takes more torque to get them up to speed. Further, Since heavier wheels have more momentum than lighter wheels when they are spinning, more braking effort is required to slow them down.

Finally, the more unsprung weight you have, the more difficult it will be to adjust the handling of the car. Changing springs, shock absorbers, and anti-sway bars will change how the weight above the springs moves around, but it won’t change have any effect on movement of unsprung weight.

For these reasons, you really want the lightest wheels and tires you can buy within your budget that are still strong enough to do their job.. The problem with the original stock or factory-optional wheels is that they’re heavy. There are many choices of wheels on the aftermarket that are much lighter because of their materials and construction, but that will still provide all the safety and functionality needed.

Tires are another issue where we’re going to go to the aftermarket to get better performance. Your MINI can be purchased with two different types of tires: performance run-flats and all-season run-flats. Notice that both types are “run-flat.” That’s a nice engineering feature, assuring that a flat tire won’t leave you parked beside the road. They also allowed the designers to avoid having to figure out where to stick a spare wheel and tire, which saved weight and cost.

But there are two problems with run-flat tires. In order to provide the run-flat capabilities, the tires are heavier than standard radial tires and they are stiffer than regular radial tires. As a result of the weight, they require more engine power to turn and have more inertia when stopping. Because of the stiff sidewalls, they don’t flex as well, so they don’t stick to the pavement as well in turns. They also give a rougher ride than most standard radial tires, which may not affect performance, but certainly detracts from comfortable motoring.

Mini Cooper Forum

So here is where we stick our neck out for the first time and suggest that if performance is your goal, you can improve the performance of your new MINI by buying from sources other than the dealer. Reputable aftermarket dealers offer an extensive variety of wheel designs to choose from at a wide range of prices and varying weights, as well as tires with different performance and behavior characteristics from several different sources.

Tire Rack (www.tirerack .com) for example lists 42 different wheels, and several different brands of tires for a high-performance MINI Cooper S for you to choose from. The MiniMania catalog shows 25 different choices of wheels, which are supplied with Kumho Ecsta Supra 712 Z-rated tires, a tire choice that they’ve tested and liked on their own cars.

Making the change won’t even cost too much. A good set of four aftermarket wheels that are much lighter than the MINI wheels, shod with a set of proven performance tires, can be put on the car for between $1000 and $1,300. Of course, you can get much fancier, and more expensive, tires and wheels but at least that gives you a ballpark idea of your costs.

Bottom line, if you don’t need or want the other components in the Sport Package, such as the Xenon lights, save your $800 and put it towards a good set of tires and wheels. It will be the single best investment you make in improving the safety and handling of your MINI.

3 comments:

Bilbo Baggins said...

Great article, you got everything together. But I have a few questions.

How come you only list the weight of one set of wheels in your web site?

How come you do not list any 15" or 16" wheels?

By using larger and larger diameter wheels while maintaining the same, or close to the same, rolling diameter, you push the weight of the wheel and tire further away from the center of rotation. Thereby increasing the potential inertia of the wheel/tire combination.

Two wheel tire combinations both the same weight and same outside diameter but one with a 16" rim and the other with an 18" rim the smaller rims would give better performance due to the fact that they could accelerate and decelerate more quickly.

In the real world smaller rims generally weight less than larger rims of a given strength and width. For Auto-X smaller rolling diameters also offer the advantage of lowering the overall gearing and lowering the C.G. So ultimately the ideal rim would be one that just clears the brake calipers.

Anonymous said...

Great article, you got everything together. But I have a few questions.

How come you only list the weight of one set of wheels in your web site?

How come you do not list any 15" or 16" wheels?

By using larger and larger diameter wheels while maintaining the same, or close to the same, rolling diameter, you push the weight of the wheel and tire further away from the center of rotation. Thereby increasing the potential inertia of the wheel/tire combination.

Two wheel tire combinations both the same weight and same outside diameter but one with a 16" rim and the other with an 18" rim the smaller rims would give better performance due to the fact that they could accelerate and decelerate more quickly.

In the real world smaller rims generally weight less than larger rims of a given strength and width. For Auto-X smaller rolling diameters also offer the advantage of lowering the overall gearing and lowering the C.G. So ultimately the ideal rim would be one that just clears the brake calipers.

IMHO
Bilbo Baggins

Anonymous said...

If you replace the run-flat tires with regular ones, then where would you have space for a spare tire in case of a flat??