Why do tire shops install new tires on rear wheels
If you buy just two tires, the tire shop will install the new tires on rear wheels. 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.
Front tires wear out first?
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
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 and the vehicle fishtails. 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.
“The two new tires should always be placed on the rear axle and the older tires moved to the front.” The reasoning has been proven on test tracks and in labs. It’s all about oversteering, understeering and the physics of a vehicle’s center of gravity. Worn tires will hydroplane more readily than tires with deeper treads. A loss of traction on a rear axle causes oversteer, which could cause a vehicle to fishtail and kick into a tailspin. A similar loss of traction on the front axle creates understeer, causing the vehicle to keep going in a straight line. For the driver, it’s easier to compensate for understeer; oversteer usually is much more hazardous. It all adds up to the fact that if you have to lose some road grip, it’s better to lose it up front, rather than in the back.” — Ron Margadonna, senior technical marketing manager for Michelin North America.
Understeer and oversteer are both bad
But oversteer is far more dangerous. It’s much easier for drivers to recover from an understeer condition by backing off the turn, backing off the pedal and applying the brakes. Braking shifts the center of gravity and weight towards the front wheels, giving them more traction. But it also causes the rear to lift and that further reduces rear traction, which accelerates the understeer problem.
That’s why you should always install new tires on the rear
What about rotating tires?
This is the most common question I hear, “If I put the new tires on the rear, don’t they just wind up on the front wheels in 5,000 miles when I do a rotation?” Well, no. Because the recommended rotation in this situation is to keep the new tires on the rear and just rotate the tires side-to-side. Continue this rotation pattern until the front and rear tread depths are withing 2/32″. Then you can return to normal front-to-back rotation.
©, 2018 Rick Muscoplat
Posted on by Rick Muscoplat