If you are old enough, you might recall your Dad (or maybe you) throwing a few bags of sand in the trunk of the family sedan when the first snowfall arrived. That was it for Winter preparation in much of Canada, especially for those who drove American cars. Some, more forward thinking motorists installed snow tires, a pair of them, on the rear wheels. The idea was to prevent one from becoming stuck in the snow. It was all about forward movement, there was never any thought to steering or braking.
Over the years, our understanding of the laws of physics as they apply to daily drivers has grown and with it, the general wisdom regarding the placement of “the best” tires has changed.
As front wheel drive vehicles began to take over the market, the general wisdom was to mount a pair of snow tires on the front. The back tires are just along for the ride, or so went the public’s opinion at the time. For a while, even some so-called tire experts would say that while installing four snows was preferable, if you could only install two for whatever reason, then those two should go on the front.
In the past decade or so, the general feeling is that the tires with the most available grip should be installed on the rear axle and for good reason. It really has little to do with gaining forward momentum, or not getting stuck, and everything to do with providing consistent, predictable handling. One’s ability to motor away from a stopped position is of little importance if one can’t stop or turn out of the way to avoid hitting something.
When the two tires with the most grip are installed on the front wheels, turning and braking are accomplished up front all right, but what happens with the rear tires, the ones which have less grip? As the weight transfers to the front wheels, those back tires begin to lose traction. This causes a sudden change in direction that is called oversteer. In this condition, the back end of the car will try to pass the front end, meaning that you will hit whatever obstacle is in your way, backwards.
With the tires which have the most grip on the rear of the vehicle, the front wheels will lose traction first. The result in this situation is what is known as an understeer situation. That means the the car is going straight, even though you are asking it to turn and stop. In this situation, you hit whatever obstacle is in your way forwards.
Nobody ever wants to think of the specifics of a crash while driving, but this distinction between forwards and backwards is an important one. While auto manufacturers invest heavily in creating safety systems to absorb side and rear end collisions, the reality is that most of your car’s safety features are designed to work in a front end collision. You have a much higher chance of surviving a frontal collision than you do a side or rear impact.
So what does all of this have to do with installing my Summer tires?
I used Winter tires as an example, because most people have felt the conditions I have described above while driving in the snow, even if they do not know the terms to describe them. The reality is however that exactly the same rules apply on dry pavement, with all season or Summer tires. The big difference is that the speeds involved are generally much higher, meaning that the potential for danger is greater.
Despite our best efforts to minimize tire wear with regular tire rotation, it is inevitable that two of our matching tires have more tread than the other pair. Spring is the perfect time to inspect your Summer tires as you are pulling them out of the garage and give them the once over.
You want to look for cuts or bulges in the sidewalls and things like nails or screws stuck in the tread, that might lead to a leak. If you see a bulge or cut and aren’t sure if it is a safety issue, take them to your local tire dealer and get their opinion.
While you are inspecting them, make note of tire wear. Are all four tires worn evenly? If not, then the pair with the most tread should go on the back. Do they have smooth, even wear across the tread? If not, you might need a wheel alignment. Is there enough tread life left to get another season out of them or do you need to buy new tires? How can you tell that? If there is less than a couple of millimetres left before you get to the wear bars, then you should be thinking about new rubber.
Once you determine that your Summers are good to go, then install the best two on the rear axle. If you are not doing the change yourself, then explain to your tire shop that this is how you want it done.
Oversteer/Understeer images via Johnny O