Tire width and friction

[Papa_D]

I've read that tire width will not increase friction or traction on dry pavement but will only increase cornering stability. If you heat the tires enough to the point they become sticky, then and only then will you increase traction.
any thoughts on this?

[Goofus]

Completely false. Tires are a bit peculiar in the fact that on pavement, the heavier you load them, the lower the coefficient of friction becomes (for most other materials, the coeff does not change). By going with a wider tire, you are distributing the load over a larger area, which in turn, increases the coefficient of friction.

[PHOEBISIS]

Goofus,I don't completely understand your statement.
Does it mean that in the ranges of load that tire contact patches work-that the actual coef decreases as the weight(force or psi) increases?Why-?Does the higher load flatten out-crush- the ridges and valleys of the tire surface?These ridge-valleys probably interlock-like gears-with the ridges-valleys of the road surface.There is also some chemical bonding that takes place.(maybe H-bonding and other not quite chemical bond-bonds) I have no idea how bonds would react to decreased loads.
Narrow tires probably have some advantages-lower rolling resistance-maybe a bit more resistant to loss of traction on a wet road-the increased psi at the patch might push thru the water a little more quickly and get to the actual road surface.
Let me know if the above is right.Thanks.Charlie

[Goofus]

Whoops, it appears I've been caught trying to sound smarter than I acually am. I've had this same "tire width doesn't affect cornering" argument with two different physics professors and was never able to convice them, but here goes:

Does it mean that in the ranges of load that tire contact patches work-that the actual coef decreases as the weight(force or psi) increases?

Yes, that's it exactly. Here's an example of what I'm saying.

Take a block of wood and push it against the road. Say with 10 lbs of downward force, it takes 1 lb of side force to break free from friction. Push down with 20 lbs of force and it will take 2 lbs to break free.

Now take a block of rubber and push it against the road. Say with 10 lbs of downward force, it takes 1 lb of side force to break free from friction. Push down with 20 lbs of force and it will only take 1.9 lbs to break free.

Why-?Does the higher load flatten out-crush- the ridges and valleys of the tire surface? These ridge-valleys probably interlock-like gears-with the ridges-valleys of the road surface.

I have know idea why. The first place I picked this up was in the book Race Car Vehicle Dynamics by Miliken & Miliken. One of the authors used to be active on rec.autos.sport.tech a few years back. You could probably get a good answer there. Race Car Engineering Magazine also frequently has related articles.

The ridge-valley interlocking is what is happening and is probably the reason tires don't follow the Coulomb friction model and behave this way. I believe it's also how some cars can pull more than 1 lateral G without any extra downforce. That was the original argument with my profs.

There is also some chemical bonding that takes place.(maybe H-bonding and other not quite chemical bond-bonds) I have no idea how bonds would react to decreased loads.

You're way over my head now. If you find something more conclusive, be sure and post it here.

[IQ9]

Man...very heady stuff but for sure a good read. Thanks for site info.

[Papa_D]

Dug this up on some physics site. Who would have guessed?


Traction Friction of Tires
by Ron Kurtus (revised 20 May 2005)

Traction friction concerns the ability of a wheel or tire to start, stop, and not skid sideways. Automobile tires have treads to improve their traction and decrease the chances of a skid. The treads are shaped differently for various weather conditions. Race car tires normally don't have treads and use adhesive properties of the rubber for their traction.

Questions you may have include:

How do treads help a car stop or start?
Why should tread design concern the weather?
Why do race cars have wide tires?
This lesson will try to answer those questions. There is a mini-quiz at the end of the lesson.

Treads increase traction
With most vehicles, you want the rolling friction to be at a minimum. But you also want to be able to effectively start and stop without slipping, as well as to prevent skidding when going around a corner. For example, train wheels have very low rolling friction, but they also have very poor traction when starting or stopping.

Treads help traction
Treads are added to automobile tires to provide them with good traction. Although rubber provides relatively good static and kinetic friction, automobile tires have treads to further increase their traction and to account for different weather and driving conditions. The edges of the treads add to the coefficient of friction when stopping or starting.



Tire treads give more traction, plus help
move water outward in wet conditions

Zigzag shape
Most of the treads on an automobile tire are somewhat zigzagged. This increases the traction in starting and stopping the car, plus this type of tread also helps to prevent the car from skidding sideways when it goes around a curve.

Treads for different weather conditions
Some tire treads are arranged to help in wet conditions, as they channel excess water out from under the tires. Snow tires have deeper, wider treads to improve traction in the snow.



Some tires have wider treads
to improve traction in snow or mud

When a tire becomes worn and the edges of the treads become rounded, there can be considerably less traction. The danger then is that the car may go into a skid when going around a corner or may not be able to stop in a sufficient distance in an emergency.

Treads on race car tires
Tires on race cars usually have at the most 1/8 inch of tread. The treads aren't really used for traction as in a passenger car. Instead they help dissipate heat, which is the major reason for tire failure, particularly at the high speeds attained in the race. Some tires may get so hot that the rubber blisters and the tire blows out.

The aerodynamics of the race cars push them downward the faster the car goes, adding to the traction.

Controlled slide
Treads aren't really needed in race car driving since the cars tend to skid somewhat when going around a curve. This is a controlled slide, as opposed to an out-of-control skid in a passenger car. Also, race cars don't get involved in the stop and go traffic of a passenger car, so they don't need the treads for starting and stopping.

Soft rubber used
Instead of standard tires, race cars use rubber that is soft and almost sticky to the road. This is a form of molecular friction, and it is related to the surface area on the road. When a race car slides, it is more controlled with this type of tire than with a tire with treads, which may skid suddenly. The warmer the tire gets, the better its traction. This is provided the tire doesn't get so hot that it blisters.

Slicks
Although race car tires have only 1/8 inch tread at most, drag-strip tires have no tread at all. They don't need it, since there is no sideward sliding and the race is not long enough to cause the tires to overheat. But they do get plenty hot, as the car spins its wheels. The type of tire used in drag racing is often called a "slick" because of no tread.



Some race car tires are called "slicks"
because they have no treads

No good in rain
The big disadvantage of having minimal or no treads in race car tires is if there is rain, the tires slip so much that it can be dangerous for the drivers and their cars. The adhesiveness of the rubber is lost when there is a layer of water between the tire and the pavement. Most automobile races are called off or delayed if there is rain.

In conclusion
Treads are added to tires to improve their traction friction. This makes them more effective in starting and stopped, as well as to help prevent skids when going around a corner. Special tread combinations also provide better traction in various weather conditions. Race car tires use the stickiness of soft rubber for their traction. This is good for controlled slides but not good for wet surfaces.