Wisdom of the ancients has it that when fitting a new pair of car tyres, the new ones should go on the rear axle to help prevent oversteer. Never having had much of a problem with oversteer, but wanting to lose excessive understeer and have the best boots for braking in an emergency, I always fit my newest tyres on the front.
This policy almost backfired a few days ago, when exploring the margins of my car* on a twisty, damp and autumn-leaf-slippy long uphill road at 5AM I caught a handful of oversteer and fishtailed violently about 4-5 times before regaining control. This got me thinking about standard road tyre tread, and about how much of the grip seems to be lost as soon as the tyre is only part worn. They never quite grip the same as when they’re brand new, mould-release compound notwithstanding.
What I’d like to know, is: What mechanisms are at work to provide cornering grip on a road tyre (ignoring the extra complication of aquaplaning)? How much is down to contact patch area, and how much down to the scrubbing action of the tread? It’s a common misconception that slick tyres provide the best grip on a dry surface. Yes, but only with very sticky racetrack rubber that is designed to work at high temperatures. Road tyres need the tread even in the dry, and when they’re part worn they lose a lot of grip.
At what point (say as a percentage of original tread depth) should the tyre be replaced? At the moment I run them down close to the legal limit before replacing, but I may have to change my policy. For the record, all 4 tyres are Pirelli P6000, which is quite a soft compound for a road tyre, and as grippy as it is quick-wearing. Usually grippy, anyway…
*Something I do from time to time when there’s no chance of hurting anyone or damaging property if it goes wrong. How else is one to know the operating limits of the car? The only times I’ve ever managed to get all 1.5 tons of my car to fishtail before has been on deep loose gravel and sheet ice.