Equivalent weight = Actual weight + Tire [Radius.sup.2]

Now, substitute our formula for the rotational inertia of a cylindrical tire, and you will quickly find that any tire will have an equivalent weight that is two times its actual weight. So how does this affect you? Let's use an E36 3281 as an example. In base trim, these came with 205/60-15 Michelin MXV4 tires that, according to the Tire Rack, weigh in at-20 lb. Upgrading to the tires from that model's sport package--Michelin Pilot HX MXM tires sized 225/50-18--adds 2 lb to each corner. Factoring their rotational inertia brings that to 4 lb per corner, for a relatively negligible 16 lb of effective weight. Stepping all the way to the M3's 235/40-17 Michelin MXX3s, the weight goes up to about 25 lb per tire, an effective increase of 40 lb all around.

While this difference is worth noting, it is not earth shattering, and it won't affect your tire shopping much. Most tires of a given size weigh about the same. Of course, the heavier tires are only part of the equation, as the larger wheels are not only heavier, they also carry that weight farther from the center. Because the weight of various after-market wheels varies tremendously, you do have the power to minimize rotating weight through careful wheel selection. Though calculating rotational inertia (I) for a wheel is prohibitively complicated, you can easily see that none of the wheel's weight is as far out as the edge of the tire, so the effective weight will be significantly less than two times the actual weight of the wheel/tire combo. A good rule of thumb, then, would be around 1.5 times the actual weight.

What does all this mean? Sure, the extra weight of larger wheels and tires may penalize acceleration more than you thought and braking performance, too, can be hurt slightly, but, the payoff in ultimate grip will more than recover the braking performance and pay dividends in cornering and responsiveness far beyond acceleration loss.

The moral of the story is to keep your wheel and tire choices within reason. Twenty-inch wheels weighing in at over 40 lb each will hurt performance more than they help, regardless of what you put them on, but a sensible Plus 1 or Plus 2 will almost always make life better.

COPYRIGHT 1999 McMullen Argus Publishing, Inc

COPYRIGHT 2000 Gale Group