My tyres always wear at different rates and I never change all of them at the same time. If they all have legal tread depths, does it matter if they are not equally worn? And when I put new ones, should I fit them on the front or the rear?
Chris
Whether your tyres are old or new, or a mixture of both, the general rule is to always put those in best condition on the front wheels. Whether the car is FWD, RWD, 4WD or AWD, the front tyres do 100% of the steering and at least 70% of the braking. Always. So they do the most work and are the most important to your control of the vehicle. Any loss of grip or puncture of any tyre presents the driver with a control challenge.
If failure is at the rear, the handling will misbehave but you will still have good steering and braking to cope with the situation. But not if a front tyre has failed. Both the handling and your two most important control remedies will not operate properly. Symmetry is important to handling integrity. Ideally, all your tyres should be identical – same brand, same ply pattern and rating, same tread pattern etc.
If that is not always practicable, then at least the tyres on the same axle should not be a mixture. Motoring at a steady speed inflicts very little wear. It is accelerating, braking and cornering that causes the tread to “scrub” on the road surface, even if the forces are well short of spinning, locking or skidding the wheels.
The rate of wear also depends on the road surface, of course, and is affected by heat. All of these factors are not minor influences. I think I have mentioned before that the same tyres on the same car can be “worn out” at 100 km if driven with extreme aggression and pace on a coarse surface, or can last many tens of thousands of kilometres with gentler treatment and conditions.
Gentler (not necessarily slow) driving will also make the wear more symmetrical on all four tyres, but not completely so. There will always be some differences. If you want to equalise wear (and later replace them all simultaneously) you need to rotate them to different hubs at every service. Wear at steady speeds is minimal. Acceleration (especially in more powerful cars) affects the tyres on the drive wheels more.
Braking has more effects on the front tyres because when you slow down the kinetic weight of the car moves towards the front (with enough force to squash the front suspension and lift the rear if you brake hard). If equal brake pressure was applied front and rear, the rear wheels would tend to lock up too soon.
So brake systems are designed at about 70-30 to apply more force at the front That is why these days almost all cars have disc brakes (more expensive but more effective) at the front, but sometimes old-fashioned drum brakes at the rear (because their efficiency is less crucial).
