Nowadays, most cars tend to be front wheel drive. On a front wheel drive vehicle the front tyres are working much harder than those on the rear axle. On a front wheel drive car the tractive forces, steering forces, cornering forces and most of the braking forces are transmitted through the front tyres. Additionally most of the vehicles weight is carried by the front axle because the heavy engine and transmission components are centred there.
Assuming the vehicle geometry is within the vehicle manufacturer specifications, this concentration of workload usually results in the tyres on the front axle wearing quicker than the rear tyres. It would be wasteful, not to mention uneconomical, to replace all four tyres just because the front tyres have worn close their legal limit. So a lot of people find themselves having to replace the front tyres on their car before the rears.
There is a lot of confusion as to which axle the new pair of tyres should be fitted.
With a vehicle that has same size and fitment of tyre on the front and rear axle, the easiest (and some might think most logical) thing to do would be to just replace the front tyres. After all, they are going to wear quickest again so it makes sense to put the tyres with most tread on the front so that they last longer. For reasons of safety this is the wrong thing to do.
On wet road surfaces, the amount of grip a tyre has is directly linked to its ability to clear surface water so that the rubber in the tread can make contact with the road surface. One of the principal functions of the grooves in a tyre tread is to provide channels for the standing water on the road surface to drain away through. One could easily imagine that a tyre with worn tread grooves will not be able to clear as much surface water as an equivalent new tyre with the full tread depth.
So why is it better to have more rear grip than front grip?
To understand it’s necessary to talk a little about vehicle dynamics. During cornering, a vehicle will usually experience one of 3 dynamic states, which are understeer, oversteer or neutral cornering.
This condition is where the tyres on the front axle do not follow the exact angle prescribed by the steering, but ‘slide’ slightly wide. See below.
This condition is the opposite to understeer, whereby the nose of the vehicle follows a tighter line than the angle prescribed by the steering. This is caused by a lack of rear axle grip. See below.
And this condition is where the nose of the vehicle follows the exact line prescribed by the steering angle.
It is very difficult to build a car with neutral handling, especially a front wheel drive car where the weight distribution is front biased. So, most manufacturers build their vehicles with inherent understeer as it is the safer handling characteristic. If a vehicle is understeering, (sliding wide in a corner), assuming maximum front tyre grip has not been reached or exceeded, adding steering angle to the wheels will result in the vehicle negotiating the corner safely.
However, when a vehicle is oversteering, most of the things one would do to try to reduce oversteer and regain control will actually exacerbate the situation.
Braking will reduce the load on the rear axle, making the rear tyres more likely to lose grip. Likewise, reducing power abruptly will transfer weight from the rear axle onto the front, again reducing rear tyre grip. The only way to counteract oversteer is to steer the front wheels out of the corner, in other words reduce steering angle. This is something which is counter-intuitive and difficult to do whilst maintaining control of the vehicle.
To give the best possibilities of a vehicle handling safely when fitting new tyres to a vehicle in pairs, it is advisable to fit the new tyres to the rear axle.
Hero photo credit : revivalsportscars.com