Tires have directional treads for a reason

In brief, upon discovering a tire mounted in reverse to prescribed rotation, what should one do? This raises the salient question: what does directional tread actually do?

The measurable differences in hydroplaning seem to relate to coefficient of adhesion, pavement fluid depth, fluid viscosity, fluid density, tire inflation pressure, tire load (normal to the track), tire tread pattern (size of lugs and voids) and tire-tread depth. As far as I can find, the rotational appearance of tread pattern seems to have more to do with effective marketing suggesting “higher designed performance” is thereby attained.

Are directional tread patterns benefits — myth or fact?

You have certainly researched this topic and know a great deal about it.

First of all, when a tire is engineered with a directional tread pattern, it is strictly for water evacuation. Since water cannot be compressed by the car’s tire, it needs grooves to get out from the contact patch. The open area of the tire (the grooves) is called the void area. The bigger the void area, the more water can be removed.

But there comes a point of diminishing returns. Too much void area and the tire’s tread blocks become unstable, so the tire is terrible on dry roads. The tread blocks squirm, overheat and break apart. To get more water out from under the contact patch, the solution is directional tires. They can be designed to move the water faster, by angling the grooves. Faster removal then makes for more water being extracted.

I have tested directional tires to find out just how much the directionality affected grip. In the dry, running them in the correct direction and then in the opposite direction produced virtually the same capability. Braking distances, lateral load and steering response were within fractions of correct rotation.

That, of course, is because a tire on dry pavement does not need tread for grip. It is simply the size of the contact patch that makes the difference. Smaller void area equals better grip. The shape of the void area makes no difference. Note that dry track race tires have no tread; they are slicks.

Repeating this test in the wet was a different story. Testing took place at 60, 80 and 100 km/h. When the tires were reversed, wet traction dropped by about 18 per cent at 100 km/h. There was less grip lost as speeds went down.

Obviously, the faster the tire goes, the more water it has to get out from under the contact patch. My guess is if we had made the speeds higher, the performance difference would have been even higher.

Modern ultra-high performance tires use not only the actual flow of the direction grooves, but also some neat aero tracks. Some tires have the tread pattern sides designed to create turbulence, so the water is literally sucked up off the pavement in addition to being displaced by the tread blocks.

So why aren’t all tires directional? They can be noisier than other tires, a huge consumer concern. If they are directional and also asymmetric, they are limited in rotating them on the car for even wear.

And if the car — like many highperformance cars now — has a staggered fitment (wider at the back), you end up with four unique tires on the car.

I just reread your article on allweather tires. I want to buy four Hankook Optimo 4S tires, but this size seems to be unavailable. They are for a 2011Honda CRV, not an uncommon car. Any suggestions?

Your CRV is actually considered a small SUV and the tire size reflects that. The Hankook Optimo 4S is not available in SUV sizes.

However, the Nokian WRG3 SUV is available.

To the best of my knowledge, it is the only all-weather tire made in that size. Nokian is sold exclusively through Kal Tire. Send tire questions to: thetireguy_1@hotmail.com. Mail volume prevents personal replies.