Rick's Free Auto Repair Advice

New tires on the rear

Why do tire shops install new tires on the rear of vehicle?

If you buy just two tires, the tire shop will install the new tires on the rear of the vehicle. That may seem counter intuitive since the front tires do all the steering and most of the braking. But there’s a very good reason for installing the new tires on the rear. Here’s what’s going on.

Which tires wear out first?

In front wheel drive (FWD) cars, most of the vehicle weight is on the front wheels because the engine and transmission are mounted in the front of the vehicle. In addition to the weight issue, the transmission and axles deliver all the driving force to the front wheels. That factor alone causes them to wear faster. But it doesn’t stop there. The front wheels also perform up to 80% of all the braking (although that ratio is changing a bit on cars equipped with stability control) because vehicle weight shifts forward during a stop. Finally, the front wheels do all the turning, which causes the outer edges to wear faster.

Based on all these factors, intuition would tell you that when the front tires wear out first, you should replace them first, while leaving the rear tires in place. This is an instance where your intuition is wrong. Here’s why:

Worn front tires cause understeer

When the tread wears out on front tires, they can lose grip in turns

understeer

Understeer—turn right on wet road or with excessive speed or heavy braking and the vehicle continues to go straight

and understeer causing the vehicle to head in a straight line when you really want it to turn right or left. That would argue for replacing the front tires first, right? However, if you replace the front tires and leave worn tires on the rear, you create an oversteer condition where the front tires maintain their grip in a turn while the lower tread depth rear tires lose grip. The front tires then continue to steer the vehicle in the intended direction, but the lower traction on the rear wheels causes the vehicle to breaks away and travel in a straight path, essentially turning your vehicle far more than you planned. Think of this like being struck in the rear by another vehicle while you’re partway through a turn. In that case, the accident would pivot the rear of the vehicle around the front wheels. That’s exactly what happens in an oversteer situation.

oversteer

In oversteer, the rear tires lose grip before the front tires and the rear breaks away and literally twists the vehicle around the turn from the rear

Understeer and oversteer are both bad

But oversteer is worse. It’s much easier to recover from an understeer condition by backing off the turn or slowing down than it is to recover from an oversteer condition when you’ve lost grip on the rear tires. Sure, you can apply the brakes, but what good would that do—you’ve already lost traction on the rear tires and the rear end is swinging out.

That’s why you should always install new tires on the rear

©, 2018 Rick Muscoplat

 

 

 

Posted on by Rick Muscoplat

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