Archive for July, 2010

Why do you go round corners?

You are accustomed to going round a corner if you’re driving on a rubber pneumatic tyre.
But would you if it was, say, a steel railway wheel fitted to your car? The answer- not likely!

So why does the tyre make things happen, that the steel wheel won’t?
The easy answer is “because it’s rubber”, though more than a little inadequate for this site if you’ve been reading our “All About Tyres”.

Rubber distorts where it meets the road. You’ve all seen the graphic shots of tyres in motion on particularly the V8 race cars, where they have lots of lovely places to hang a camera.

The tyre is shown flexing, distorting sideways in both directions, and generally having a hard time of it. Makes good viewing! But the camera can’t show what is happening where the tyre meets the road. So we have to revert to a diagram to explain.
OMG- that looks complicated! So use a bit of imagination to interpret what you see in front of you. I’ve drawn a wide race type tyre. Imagine now that you are in a pit in the ground- (shades of Sir Jack Brabham forty years ago in an old Goodyear tyres advertisement for water clearance tread patterns) looking up as the tyre passes over a glass plate set in the road surface.

Because rubber is elastic, under steering angles generated by cranking a steering angle into the wheel, the whole tyre, except where it is anchored to the wheel rim, distorts sideways too. As it rolls through the footprint, it generates a cornering force. This is not instantaneous- it has to roll through the patch to generate the force. In the adhesion zone- let’s call it the leading half of the footprint- the tread elements (rubber particles) move sideways. As they leave the footprint (the sliding zone)- they “snap back” to their original position, because rubber is elastic. Then up into the wheel well, and down again on the other side, ready to do it again! Like around 30 million times before it’s worn out! (But not if you corner ALL the time!)

The path taken by one tread element is shown by a row of crosses on the diagram. This movement generates friction, which generates grip, which generates cornering force, which makes the car change direction, hopefully somewhat in the direction you pointed. If speed or steering angle are too high, the whole tyre slides; or the grip available from the road surface may be inadequate also. You lose adhesion, and so don’t generate grip/friction/cornering force etcetera, etcetera.

So you have three directions all happening at once- the steering angle, the angle the tyre wants to travel at in its distorted shape. The angle between the two is the slip angle. The resultant angle is where the car eventually goes.

The less the slip angle generated by a steering force, the less the tyre wears out. This can be achieved by putting a steel belted radial carcass under the tread, or by changing the tread compound or pattern, or by putting more rubber on the road to distort less (called a slick, or a “widey”). There’s other ways too- even out the contact pressure across the face of the tyre, so that all the surface acts as efficiently as possible. And hang a spoiler on the car, front, or back, or both, which at Very High Speeds will generate down-force on the tyre, which lengthens the footprint, so that there is more available to do its work on the ground. Spoilers look spiffy, but don’t do much at road legal speeds. You could also make the car heavier to lengthen the footprint, but this carries other penalties.

Taking this a bit further, the same sort of tread movement is taking place when you accelerate or brake, or indeed, just roll along; but the tread elements are moving fore and aft. When you accelerate hard, you might hear a chirp as they protest their way back to their normal configuration. They also wear out quicker than a free rolling tyre. Think of the difference in wear rates front to back on a front wheel drive car- which is most of them today. If the same tyre is braking, steering, and accelerating, it wears out twice as fast as one that is just rolling along.

Yet even the free rolling tyre eventually wears out- because it too needs to grip the road.
My mate John regularly gets 90000 km out of his Calais tyres, when most of us would be happy to get 60000. He is a very conservative (read “slow”) driver. Trucks that travel to Perth from the East Coast regularly get 200000 from their trailer tyres– once on the Hay Plains it’s a straight run. Their tyres are worn smooth “ as a babies”- no abrasive pattern at all.


July 7, 2010 at 7:09 am 1 comment