Special equipment allows Bridgestone engineers to visualize a tire footprint, using different colors to represent the different pressures and their distribution.

Friction is sometimes a blessing, sometimes a curse. While engine and lubricant manufacturers constantly try to reduce friction, there are places where friction is essential.

In fact, if it weren't for friction, you couldn't start, stop or stay on the road.

WHAT IS FRICTION, REALLY?

Friction is the force that prevents two surfaces in contact from sliding past each other. There's friction between ball bearings and their raceways, and there's friction between tires and the road.

AND THE SMOOTHER THE SURFACES, THE LESS THE FRICTION?

Not necessarily. We think of smooth surfaces as being slippery, but it all depends. In fact, it is possible to polish two pieces of steel so finely that if they touch they actually weld together.

And, if smoothness didn't favor friction, Indy cars wouldn't use slicks.

"Slick" doesn't always mean slippery.  Indy cars use smooth tread tires to get maximum torque transferred from engine to road.

SO WHAT’S THE SECRET?

It’s the way forces are transferred from one surface to the other, and especially the way that forces are distributed at the surfaces.

With slicks, you must transmit the enormous torque of a racing engine to the road as fast as possible. You must maximize contact between road and tire, so you use a huge tire with no grooves or voids.

Because the contact patch is huge, force is distributed very evenly. It’s no concentrated on any part of the patch.

WHY NOT?

If the force were concentrated, either the tread surface or road surface might break away, allowing the tire to spin. And rapidly spinning tires don’t produce forward motion.

SO WHY NOT USE SLICKS ON EVERYTHING?

Because roads aren’t always hard or dry. If we were on gravel, our racing slicks might just spin and throw gravel everywhere, without pushing us down the road. Gravel doesn’t hold together enough to push back against the tires.

In a way, you can say something similar about a wet, muddy or snowy surface. It’s almost as if you had really, really tiny "gravel"; in reality, molecules of water and granules of dirt.

Aggressive block pattern drive tires like the Bridgestone M726 concentrate forces to dig deeply into soft surfaces.

AND THAT’S THE REASON FOR BLOCK AND LUG PATTERNS?

Sure. With a block or lug tread pattern, the force is concentrated in a smaller area. That gives the tread power to penetrate soft surfaces. (Imagine poking a hole with a pointed end of an ice pick-then imagine trying to do it with the handle end.)

This concentration of force allows the tire to dig through the soft surface to more solid material underneath. The grooves and voids fill up with soft material and carry it away, letting the tire dig deeper. Very deep grooves, like the 30/32" ones on the Bridgestone M726, dig deeply into snow and soft surfaces.

By concentrating forces into an even smaller area. When you put chains on a tire, the chains ride on top of the tread, and all torque and weight is concentrated in tiny areas of contact between chain and road surface.

A road may be so ice-packed that tread blocks can’t cut through its surface, but enormous forces at the chain links break through the ice.

YOU’VE BEEN TALKING ABOUT THICK LAYERS OF SOFT MATERIALS. WHY DOES A THIN LAYER OF WATER CAUSE A PROBLEM.

Partly because water is a remarkable substance. If you drive on gravel there’s not much holding individual pieces of gravel together.

But with water, surface tension and forces between molecules bind everything together. In a hydroplaning situation, the tire can actually be separated from the road surface by the tiny film of water, so the tire isn’t touching the road at all.

HOW CAN THAT BE PREVENTED?

Concentrating pressure helps slipper films. And that is one reason correct inflation pressure is critical to good traction.

WHY IS THAT?

Because inflation pressure relates to the size and shape of the footprint, and pressure distribution within it. As a general rule, the higher the inflation, the smaller the footprint. And, the smaller the footprint, the more concentrated the pressure and the greater the tendency of the tread to cut through water and soft surfaces.

BUT IF THE FOOTPRINT IS BIGGER, THE TRACTION WOULD BE GREATER, WOULDN'T IT?

On dry pavement, or in a race, that might be true, but on wet pavement, a big footprint increases the possibility of hydroplaning.

Too LowNormal Pressure diagrams show that when inflation pressure is too low the footprint enlarges, reducing the pressure against the road and increasing risk of hydroplaning in wet conditions.

WHAT ELSE CAN BE DONE TO IMPROVE WET TRACTION?

If one way is to cut through film, another is to dry up the road.

A clever invention by an early tire designer, John Sipe, does bother: "Sipes" are slits cut into the surface of the tread. Unlike grooves, sipes are usually narrow, and may actually close up when not in contact with the road.

As the tire rolls, turning forces open the sipes, presenting and enormous number of "edges" to the the wet surface. These edges can cut through water, gripping the pavement.

In addition, because sipes are very thin, water can be drawn inside them by capillary action, helping to dry the road.

If sipes extend to grooves and are properly angled (as in the unidirectional treads of Bridgestone R226 and R227 radials), water may actually be pumped from the road surface into the grooves and expelled.

ARE YOU SUGGESTING THAT WET PAVEMENT IS DIFFERENT FROM SNOW AND MUD?

Definitely. In fact, fleets who run exclusively in the south and southeast, and never encounter snow, sometimes find that rib tires are ideal for all axle positions.

In the north and west or on north-south rotates, block-type treads may be necessary for drive axles, to deal with snow. Some fleets find that no matter how aggressive the tread pattern, the law sometimes requires chains on mountain passes.

SO OUR WINTER TIRE CHOICE DEPENDS ON THE WEATHER AND WHERE WE DRIVE?

Absolutely. And if we can give out one piece of advice; when roads are wet, snowy or icy, slow down a little. A few minutes extra time on the road beats a huge delay (or a worse fate) caused by an accident.

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