Common sense may seem to dictate that new tyres should be fitted to the front where the powertrain is driving and steering the vehicle with lower tread tyres being rotated to the rear. But Michelin North America and many others advise that “two new tyres should always be placed on the rear axle and the older tyres moved to the front”. This advice has been proven in laboratories and test tracks and is all about the consequences of understeering, oversteering and the physics of a vehicles centre of gravity.
Hydroplaning occurs when water cannot be channeled through a tyres pattern and the tyre loses contact with the road. Worn tyres hydroplane more readily that deeper treaded tyres. Loss of traction at the rear axle causes oversteer which can cause a vehicle to sway and go into a tailspin. Loss of traction at the front axle results in understeer which causes the vehicle to go in a straight line. Understeer is easier for a driver to bring under control, oversteer is usually much more hazardous. The salient point here is that it is safer to lose traction up front rather than at the back.
When drivers encounters problems on the road, they tend to take their foot off the accelerator and apply the brakes hard. The vehicles centre of gravity moves forward loading up the front axle and reducing the rear axles load. The transfer of load to the front axle maintains good traction by pressing the front tyres to the ground helping the driver in steering and maintaining some control of the vehicle.
If the worn tread tyres are at the rear with new tyres in front, the centre of gravity moves to the front with the front wheels maintaining good contact with the road with the deep tread pumping water away. But the worn rear tyres, which are not clearing water as efficiently off the road surface, with little weight on them to help maintain good road surface contact, will lose traction and slide around causing the driver to lose control of the vehicle. Conversely, when the worn tyres are on the front and the driver brakes heavily, the front tyres still maintain good road contact, helping the driver with steering and maintaining some control of the vehicle. The deeper tread tyres at the rear will pump surface water efficiently and maintain good road contact with much less chance of the vehicle spinning out of control
Michelin maintain the same dynamics apply equally to rear, front and four wheel drive vehicles.
Carefull wheel rotation is crucial to even tyre wear and can be safely conducted in the traditional manner, i.e. front to back, when all the tyres have approximately the same tread depths. But new rotation rules must be adhered to when there is significant tread depth variation between front and rear tyres. In this situation, the lower tread tyres should always remain on the front and the tyres should be rotated side to side only while always remaining on the same axle.
This research clearly underlines the desirability of replacing tyres in sets of four Whenever this is not possible, tyres should be replaced in pairs. Failure to do so can result in an imbalance of tyre treads which can cause unpredictable handling in emergency braking situations on wet roads.
Clearly the demands of todays high performance vehicles demand a reappraisal of our attitudes to tyre maintenance and safety; always replace tyres in sets of four or pairs and always put tyres with the deepest treads at the rear.