This is a controversial subject!
So I am going to try to lay out the case as best I can – and as neutrally as I can.
Setting the Stage!
Here’s the recommendation as it appears in the USTMA (US Tire Manufacturers Association) publication “Care and Service of Passenger and Light Truck Tires”.
“When replacing tires on a vehicle, it is recommended and preferred that all four tires be replaced at the same time for continued optimal vehicle performance. However, for those cases where this is not feasible, below are some general
guidelines to consider when replacing less than four tires for a light vehicle, whether it is one or two tires. ……. [some references removed here]
If the vehicle manufacturer has alternate recommendations, always follow their recommendations.
When a pair of replacement tires is selected in the same size and construction as those on the vehicle, the two new tires should be installed on the rear axle. Generally, new tires with deeper tread will provide better grip and evacuate water
more effectively, which is important as a driver approaches hydroplaning situations. Placing greater traction on the rear axle on wet surfaces is necessary to prevent a possible oversteer condition and loss of vehicle stability.”
Here’s a link:
Care and Service of Passenger and Light Truck Tires 2017_0.pdf
Please note that the version I am linking to was published in 2017.
You will find various, but similar, wordings of the recommendation all over the Internet.
Why the recommendation?
The recommendation is supposed to give the uninitiated advice they can count on. Putting it on record means that it can be referred to. This is useful for tire dealers when they need to explain to the consumer why they did what they did.
Let’s breakdown the whole recommendation:
Replacing all 4 tires is best: Even though the braking ability decreases as tires wear, having 4 tires with the same state of wear means the vehicle stays balanced, front to rear.
Same size and construction: To maintain that balance, all the tires on a vehicle should be as similar as possible.
What is the basis of this recommendation?
What is the most dangerous situation? A curve on a wet roadway! In this situation, you’ll want to have the front staying in front – that is, understeer. New tires on the rear maintains that.
Even though the front tires may be hydroplaning, when the car slows down, the tires will stop hydroplaning and the car will be steerable and pointed in the right direction.
In the days before ABS (Anti-lock Braking Systems), you could say the same about severe braking on a wet surface – you want the fronts to lock up first, because the opposite means the rear tires aren’t slowing that end of the car as fast as the front, so the rear of the car wants to become the front – and you could easily demonstrate that.
But ABS prevents the tires from locking up, so it is exceedingly difficult to demonstrate the difference.
Personal Experience:
The tire company I used to work for would put on a new product demonstration every year at our proving grounds, in mid to late February, when tire sales were slow. We had hundreds of dealers, journalists, and the like come down to our proving grounds and drive on the new products – against their competition. Wet, dry, etc. – especially wet!!
One of those demonstrations was about where to put new tires.
The setup:
We had 3 identical cars. The first car had 4 new tires on it. The second one had new tires on the rear, shaved tires on the front, just like the industry advocates. The 3rd car had new tires on the front, shaved tires on the rear – just like some advocate.
Please note: The shaved tires were shaved to 4/32nds (3mm). Not worn out (which would be 2/32nds = 1.6 mm!) but clearly low enough to cause wet traction issues.
We set up a large skid pad where the cars drove in a circle around the pad at about 45 mph. A section of the pad had a wet section with water flowing across the surface, about 1/8″ deep. It was shallow enough that you could walk through it without getting the tops of your shoes wet, but deep enough that the tires knew it was there!
We had each participant drive each car through the wet area. The first car would go through the wet area like it wasn’t there. That is, the new tires didn’t hydroplane!
The second car would hydroplane at the front, but not the rear. The car would drive off in a tangent with the driver not having steering control. HOWEVER, the front would regain traction when it left the wet area, just like what would happen as the car would normally slow down. In other words, the driver could regain control before the vehicle stopped.
But the 3rd car would exit the wet area SIDEWAYS!! EVERY TIME!! And the driver couldn’t regain control until the vehicle stopped. Both the 2nd and the 3rd car had the same trajectory, but the question was which end was leading. I hope it is obvious that you would want the car pointed in the right direction so that when it regained traction, you could steer it.
We put 100’s of people through that exercise every year. We did that for many years, so there are literally thousands of people who KNOW what happens, having experienced it!!
Comments on objections I’ve encountered!
Lawyers wrote those industry recommendations? Nope! It was engineers! I was one of them. I sat on several committees where we struggled to get the wording exactly right. We did not take this lightly! We wanted to make sure we were on solid ground. If we needed testing to be done, we had our companies perform what was needed.
Example: There was a question about what difference in tread depth is significant. The representative from Michelin volunteered to answer that question. He reported back that they tested tires with varying degrees of wear difference – and he said the results were that 1/32nd of an inch (0.8mm) was basically undetectable, but 2/3nds was.
Yes, we did run all this past the lawyers!
Regarding braking: This used to be easy to demonstrate as to where to put new tires, but the advent of ABS makes it much more difficult. That’s because slamming on the brakes used to lock up tires, but it doesn’t anymore.
Again, new tires on the rear kept the rear end BEHIND – where it’s supposed to be.
Dry traction? That’s not the issue here. There is normally plenty of dry grip and most people stay well within the limits. Where the problem lies is when the driver is caught by surprise – and that’s when it is raining or snowing! That’s why the industry makes these kinds of recommendations.
Rear tires follow in the tracks of the fronts? Most cars don’t have the same track width front to rear. Besides, the problem is in the corners where each tire is carving its own path. Check out videos in the snow where cars can leave visible trails!
Accelerating? This is probably why most “On the Front” people feel they are entirely correct. Yes, in low traction situations, accelerating in a FWD car is better with new tires on the front. But the real problem is when the vehicle is cornering – which is why the industry puts out videos showing vehicles on wet skid pads.
Race cars? Yes, a slight amount of oversteer is the fast way around a racetrack (except for drag strips – and that’s a whole different set of issues!) A bit of oversteer helps the car rotate, meaning it goes through the corner easier.
But that’s not the point of street driving. Safety is!! And oversteer is NOT the safe way! People find a bit of understeer more comfortable and easier to control.
It’s been at least 20 years since this recommendation was first made. In that time, it has been cited in court repeatedly. If there were some contradictory demonstrations, then this wouldn’t have lasted this long!
Videos that demonstrate this:
First the infamous Michelin Video:
I have heard these objections to this video:
- At 1:45, you can see the driver turn the steering wheel. Some folks claim this is the driver causing the car to spin out.
- First, do a frame by frame analysis and you’ll find the driver is REACTING to the skid, not initiating it!
- Second, why would Michelin do that? What advantage does Michelin get by causing this? I can’t think of a thing. They certainly aren’t trying to sell more tires as a result?
Here’s another one:
What’s weird about this video is that in the braking test, the car stops 17 feet shorter with the new tires on the rear – and he completely ignores that. Of course the acceleration of a FWD car would be better with the new tires on the front. But that isn’t the point. That’s not a safety issue.
But the one he didn’t try is cornering – that is, which end spins out first and what happens when it does.
Further, he conducted a slalom test, but kept changing the distance between the cones because the car wouldn’t turn enough for him. In other words, he proved absolutely nothing with the slalom test.
This is typical when a guy is trying to prove the point he already has made up the conclusion for.