The Science of Wet Grip

By Kevin Ash

Pictures: Double Red, Francesc Montero

As you’ll be very aware if you ride through the winter, roads are more slippery in the wet (and even more slippery if it’s frozen, but we’re only talking about rain here...). You probably think it’s too obvious to wonder why but that’s exactly what an Italian-German academic team has been doing. And they’ve not only come up with some astonishing facts, they’ve pointed the way to making big improvements in wet-weather grip.

There are times when you need all the grip you can getAt speed in the wet – anything above 40mph (65kph) or so – aquaplaning becomes an increasingly dominant factor. This occurs when the tyre is moving too fast for all the surface water to be pumped out of the way, so a layer is squeezed between tyre and road. Mostly this only happens in small parts of the tyre’s contact patch, and all you notice is reduced grip, but at higher speeds it can eventually lift the whole tyre clear of the road. Then you notice you’re sliding along on your ear.

But at lower speeds, even with no aquaplaning, grip is still reduced by around 30 per cent, at least doubling your stopping distance. One explanation suggests that the water still manages to form a partial barrier between tyre and road, and another says that the water weakens the molecular bonds which form briefly between the tyre’s rubber and road surface.

Grip is reduced by 30 per cent in the wet, which almost doubles braking distances
The problem is, although these might be the case, they don’t explain the big loss of grip. Incredibly, because of the roughness of the road and tyre surfaces, only about one per cent of a tyre comes into direct contact with the road, and water on the surface doesn’t make much difference to this, certainly not enough to cut grip by the amount it does.

One of the team, Erio Tosatti, says the microscopic irregularities in the road surface set up vibrations in the tyre which make a big contribution to its friction, they believe by ‘settling’ the tyre down onto the surface. When the road is wet, tiny pools of water form in the valleys between the irregularities, like microscopic puddles, which have the effect of smoothing out the road surface. The tyre’s rubber wouldn’t have reached down into those valleys anyway, but because it now slaps onto the water’s surface, some of the vibrations which help it grip are damped out. Which puts a whole new spin on the phrase damp conditions... Tosatti and Bo Persson from Germany calculated by how much this effect should cut friction, and came up with a figure of... 30 per cent. Looks good for their theory.

No matter how ugly your bike it still obeys the laws of physics
All very well, you might be thinking, but the end result is just the same: Brake too hard in the wet and you fall off. Ah, but now the scientists understand why a wet road is slippery, they can start to do something about it. Now they know they should be looking at ways of reducing the formation of these tiny pools of water or maintaining those helpful vibrations in tyres. As Tosatti says, to find a cure, first you must know where to aim.

Meanwhile, research into how to stop buses gushing out diesel will help too...