Two new tires -- on the front or rear?

Thursday, October 14, 2010 by Mac McNabb

This question comes up almost every day, and as winter approaches, it comes up even more.

Yes, mounting two new tires on the rear of your car truly is counter-intuitive. If you never turn a corner at speeds over 5 mph your best tires could go on front. But since most of us turn, have on/off ramps and sweeping corners in our commutes, when replacing two tires they need to go on the back. It's all based on physics. Three things give your tire traction. They are not all equal. Remaining tread depth/void area typically is more than 1/3rd the equation when in adverse weather. Your tread compound and tread shape are the other two things contributing to traction.

If your tires are worn, your ability to resist hydroplaning is greatly reduced. Watch this video.

If your front tires are new and your rear tires are at 30-50% tread depth and it is raining and you are going around a sharp curve at 45 miles per hour. Your rear end starts to slide. What do you do?

1. Lift off the throttle.
2. Tap the brakes.
3. Other.

If you lift, the slide gets worse. If you brake, the slide gets worse. By braking or lifting you have shifted weight to the front, enlarging the contact patches on the front tires and decreasing the contact patch on the rear tires. There really isn't a good way to handle it. Steering through it while keeping the throttle constant is about the only way to have a chance at saving it.

I had the opportunity to drive a skid car at the Mid-Ohio Driving School. It is seriously tricky. The car simulates loss of traction on any specific corner of the vehicle the instructor wants, and simulates a much faster speed than you are actually going. It gave us a good picture of this scenario. Any lift or brake with the wheels turned or in a corner is ineffective.

Tire Rack always recommends that when replacing only two tires, the two newest tires should be installed on the rear axle of the vehicle. Read more.

Comments on Two new tires -- on the front or rear?

Sunday, December 26, 2010 by mike rider:
as a consumer is it my right to ask for the 2 new tires to go on the front of the vehicle or is it only recomended that the 2 new ones go on the rear
Saturday, February 4, 2012 by bryan:
This new to the rear campaign is quite possibly the most idiotic thing in recent memory...my father is a mechanic and has been most of my life and he has never put new tires on the non-powered wheels on any of the dozens of cars he's owned...quoting handling physics is counterproductive when you mention on-ramps and sweeping turns in the same sentence...as an autocrosser I can tell you that SPEED and BRAKE CONDITION have far more to do than tire placement....also living in Iowa its ridiculous to put tires with better tread-depth on the wheels that aren't using traction to pull the car forward

Don't believe everything you are told...its YOUR car and you are PAYING them to set it up how you wish...if they refuse spend your money elsewhere
Thursday, February 9, 2012 by Tire Rack Team:
Tire Rack agrees with the automotive industry’s recommendation to place new tires on the rear axle if only a pair of tires is replaced. We also recognize it may not be intuitive: because the deficiency of combining new tires on the front axle with half-worn tires on the rear axle will typically occur when driving through puddles of water or slush when some of the tires begin to hydroplane.

As mentioned in our Tire Tech article “Where to Install New Pairs of Tires,” Tire Rack team drivers have driven cars with both combinations on a wet skidpad that was used to replicate an expressway ramp.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=52

And while the drivers knew what to expect, new tires on the front axle combined with half-worn tires on the rear resulted in snap oversteer or spins when hydroplaning caused them to reached their traction limit. Mounting worn tires on the front axle and new tires on the rear resulted in easily controllable understeer at the limit.

As an autocrosser, you know that you want your car to handle the same on every corner all of the way through the course. It would be difficult to drive if it understeered on the first corner, was neutral on the next and oversteered on the third. It would also be hard to choose a line and approach a corner with the confidence needed to run fast times.

While placement of new pairs of tires wouldn’t be as much of a concern if all roads were perfect at draining water and slush, we all know they are not. Puddles and pools of water and slush can lead to hydroplaning on a highway straight, a sweeping corner or an expressway ramp. This is where maintaining balanced or predictable handling is needed. Autocrossers are used to pushing the limit. Less skilled drivers tend to panic and lose control if their vehicle begins to oversteer unexpectedly.

Finally, the simplest way to eliminate the need to mismatch tire tread depths is through proper inflation pressure, periodic rotation and alignment as necessary. Properly maintaining a set of tires will typically result in equivalent wear at all wheel positions and allow all four tires to be replaced as a set.
Friday, March 2, 2012 by boston_102030:
The reason for better tires in the back is to ensure the center of gravity (center of mass) is in front of the tires that have traction. A good analogy is the rudder on a boat - if the rudder is in back the boat will stay in control - if the rudder were to be be put in the front the boat would immediately spin 180 degrees. Same is true for a car - if the back tires hydroplane first the car will swap ends.
Thursday, March 15, 2012 by Tim:
I've read all your information regarding placing tires with better tread on the rear. You have reported test results of hydroplaning around a corner. Is that the only driving condition where better tread on the rear is safer? What about hydroplaning while driving on a straight section? In all non-hydroplaning situations, is it more helpful to have better tread on the front? What about heavy braking on dry roads? Will better tread on front reduce the stopping distance or improve steering control? The reason I ask this is I've been driving 45 years and can recall only one or two instances of hydroplaning (mostly going straight). Yet, there have been dozens of times when hard braking and quick steering have avoided mishaps. If there are good reasons for better tread on the front, might we be playing a game of odds for safest overall approach? Thanks.

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