If you answered up front Michelin says no; put the new tyres on the rear axle and the least worn of the remainder on the front.
Michelin UK’s technical manager, Brian Porteous, regularly demonstrates why at skin pan safety tests, inviting AutoExpress along for the ride.
“The ones who get it wrong have a belief based on what they think are reasons related to the braking or traction of a vehicle,” he said. “But industry advice is that it’s most important to keep the vehicle stable in a straight line, because that’s where the risk is greatest.”
Michelin’s demonstration proves the evidence is clear cut.
“Once the vehicle is stable, then you can start thinking about braking and traction,” says Porteous. “And what you actually find is there is sufficient braking and traction from the part-worn tyres at the front. Part worn tyres still have a decent level of capability. If they’re insufficient for braking and traction, then replace the fronts as well.”
At the demonstration Michelin had three identical VW Golfs equipped with its Cross Climate tyres. One had new tyres fitted at the front, with rears down to 2.6mm of tread, while another had new rubber fitted to the rears with the worn 2.6mm treads up front. A third Golf had worn tyres on three corners, and a brand new tyre fitted to just the right front wheel. A tyre must have 1.5mm of tread across the main tread area to pass a warrant of fitness in New Zealand.
“Car manufacturers design their vehicles with gentle understeer,” said Porteous. “It’s a condition that a driver can understand quickly, and an instinctive response to slow down brings the situation under control.
“That’s why we advise fitting tyres with the best grip at the rear, because then the tyres will always generate that same understeering condition, and will be working with the car to deliver a safe, predictable response.”
AutoExpress drove all three cars on the wet-handling circle at MIRA. With a low-friction concrete surface it magnifies handling defects.
The Golf with new tyres at the rear behaved as expected, with the nose slipping gently wide as speed increased. It immediately came back on line as the driver backed off the throttle.
In the car with new tyres at the front – contrary to guidelines – the difference was huge. The car’s innate stability had disappeared, and gently increasing speed caused the back end to break away suddently into oversteer as rear-end traction was lost.
“Oversteer is unpredictable, and most drivers are not trained to understand it. They may experience it first in a moment of panic, for example swerving to avoid someone stepping off a curb, or a car coming out of a junction,” said Porteous.
Driving the third car fitted with just one new tyre on the right front was “alarming”. Turning right was okay as the evenly worn left-side tyres held traction until the car’s nose drifted wide into gentle understeer.
Turning left, however, was dangerous. The new front tyre gripped better than the worn rear, launching the car into oversteer. In this scenario, even the corrections become unpredictable.
The conclusion then is that any new tyres must go on the rear axle with partly worn treads moved to the front.
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