Ask Kevin: Smoky Burnouts: A Good Tire-Wear Test?

Motorcycle tire burnout

Question: Tire life is a constant issue for motorcyclists. We as riders, or you as evaluators, don't always have time to see how long tires really last. But here's an idea to solve that: At a recent stunt competition, during the stationary burn-out-until-it-pops event, I saw a Sportster come out on top as he easily smoked the tire for about twice as long as his nearest competitor. Wouldn't this burn-til-it-blows test be applicable to testing the relative longevity of new tires? Aside from the entertainment factor, it would certainly be time efficient. And perhaps when the manufacturers see an iconic publication such as yours using this technique, they could then adopt the metric and print an "SB" (smoky burnout) rating on the sidewall where, say, one dot represents every 30 seconds it lasted (oh look, my current tire has an SB rating of only 2.5—no wonder I only got 1900 miles on it....)

Rick Horn

Tempe, Arizona

Answer: What a concept! In line with the usual Materiel Command jargon, we could call this "accelerated durability testing." Some thoughts: Rubber, an organic material, begins to lose its properties above boiling water temperature. The hotter you get it, the faster it deteriorates (turns to smoke?). So, it's possible that the reason the Sportster rider was able to burn his tire so long was because the Harley's low-end torque allowed him to controllably spin his tire more slowly than could riders on a peaky sportbike. Because it heated less, it lasted longer.

I've been at touring rallies and have heard grizzled high-milers say things like, “Now, this here Metzeler front's got twenty-three thou on it right now, but you shoulda seen the Dunlop I had before. That one…” Why is tire life so anecdotal? Because the conditions of use vary so widely. Dunlop technicians attending such rallies say that 40 percent of the bikes they check have under-inflated tires. Under-inflation means greater rubber flexure, which generates more heat and becomes a higher continuous operating temperature. The hotter the tire runs, the faster it wears. A touring rider I know does most of his long distance in the fall, because he likes cool weather. Others hustle up the Rockies with full luggage, passenger, and trailer at 85 mph in August. Rates of tire wear will differ in those cases. Another determinant of a tire’s operating temperature is speed—higher speed means more flexures per second as the normally round tread profile flattens at it enters the footprint, then becomes round again as it exits. Therefore, 10,000 miles of exploring back roads at 40 mph will wear tires less than 10,000 miles of 85-mph Atlanta rush-hour commuting.

Then there's another point. A tire maker can increase a tire’s mileage life in various ways. One is by making the rubber harder, which sacrifices grip. Another is by making the carcass thinner and more flexible, so it generates less heat as it flexes (this was the big step forward achieved when tires gave up bias construction and became semi-radial in construction). An old bias race tire in my collection weighs 13 pounds, but a modern radial tire in the same size weighs 7 to 10 pounds.

Yet another variable is the hysteresis, or internal loss, of the rubber. Before the advent of certain recent technologies, the best way to achieve good rain grip was to use a high-hysteresis rubber. But the losses in such rubber make it run hotter, shortening its life. Recent rubber compounds are able to deliver rain grip without such losses, and their lower hysteresis makes them run a bit cooler, which may extend tire life as well.

So it’s complicated, and I very much doubt that we can get much insight into tire road life through “accelerated durability testing!”

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