If you had to replace only two tires, it must be these

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Here's one for the four-wheeled fleet crowd (or your personal vehicles): if you had to replace only two tires, which would it be, front or rear? Michelin answered that question pretty dramatically last week on a sprinkler-soaked skidpad course.

First off, it's really a trick question — it's always best to replace all four tires together, not just two. But read on: it might be easier to get yourself into this situation than you think, and the answer's not very intuitive.

While the test was done with front-wheel-drive cars, Michelin reps noted this situation holds true for rear-wheel-drive, all-wheel-drive or "any-wheel-drive" vehicles. They asked a group of reporters the question, and many suggested replacing the front pair or drive wheel tires, noting that many cars today are front-wheel drive.

Nope: it's the rear pair that's most important, and you don't want to lose grip there before the front. Drivers took cars with a (1) worn front/ new rear and (2) worn rear/ new front pair of tires onto a Michelin Laurens Proving Grounds polished concrete oval track being showered with sprinklers and pushed the cars faster and faster to force them to hydroplane.

"The tire that has less tread depth is going to hydroplane first," noted Kevin Fuller (photo at left), senior subjective test driver at the Michelin facility. "Every tire will hydroplane if you go fast enough — even a brand-new tire will. You just don't ever want the rear tires to hydroplane before the front tires do."

(And that's another lesson here: don't drive too fast in slippery conditions.)

When the rear tires broke loose first, forget it; the car went into an uncontrollable spin. But when the front tires began to slide first, simply lifting off the accelerator brought the car back under control quickly. So long as the rear tires were still getting traction, indeed, it was much easier to detect when those front tires would break loose and even — don't try this at home — to almost "ride" the hydroplaning intentionally.

"There are two good things about having the better tires on the back," noted Evan Sanders, another Michelin test driver at the proving grounds. "The main thing is that [the front tires hydroplaning] is easier to control, it's safer, it's more stable and the car won't spin out," he told Fleet Owner.

"But the other good thing is, you can feel through the steering wheel when the front tires start to hydroplane. So you're getting some warning and you know you're going too fast when you feel that loss of traction."

"Can't happen to me"

Why is it easy to get into this situation of more-worn rear tires vs. the front ones? For one thing, drivers often have a blowout and replace only a pair of tires if the blown tire can't be repaired, and like many of the reporters figured, may have a front-drive vehicle and put the new pair on the drive tires. They'll leave more worn tires on the rear or shift the front pair to the rear if the blowout was on a front tire.

Or drivers often wait too long to rotate tires, which typically is recommended every 7,500-8,000 mi. Let's say you've got 50,000-mi. tires on a front-drive vehicle, so the drive/steer tires wear more quickly than the rear; you forget to rotate the tires until 25,000 or 30,000 mi. go by. You have a eureka moment, remember you meant to rotate those tires, put the fronts on the rear and voila: you've got the "worn rears, less-worn front tires" situation that could land you in trouble, as in the video.

Or maybe you've got different tire pressures on the front and rear tires, the Michelin representatives pointed out. Drivers may forget about that and rotate tires front-to-rear, and with incorrect pressure on the rear tires, you could end up with the same rear-tire hydroplaning/ breakaway problem.

"If you lower the pressure in a tire, you're flattening out the footprint, which makes it hydroplane sooner," noted Sanders. "So keep that in mind. We could have the same tread depth on all four tires and lower the pressure on the rear and have the same situation" of the rear tires hydroplaning or sliding first — which is just what you want to avoid.

"Grip, pressure and tread depth can all get you in trouble in the wet and the snow," Sanders added, so it boils down to this: You don't want rear tires with less grip, less tread depth or too-low pressure vs. that front pair.

 

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