I Contact Patch and Friction: The Impact on Stopping Distance

[Mikesaa309]

Hi,

Just having a discussion with a friend about the contact patch of a tyre on a road and it's effect or lack of on friction. I know that contact patch has no effect on the friction as friction is to do with the pressure applied but surely having more of the surface touching the other surface you're increasing grip in some way.

Lets say for example you have two identical cars in terms of weight, size, tyre size, air pressure in tyres etc but one car has 4 wheels the other 3 wheels. Which one would stop quicker at the same speed logically the 4 wheeled car with more contact area should be able to stop quicker but then in terms of physics the 3 wheeled car would stop better as there is more pressure on each of the wheel so more friction. So which is the correct answer?

 

[mfb]

but surely having more of the surface touching the other surface you're increasing grip in some way.

Unless you have something like glue, or drive in loose material like sand, you do not.

4 wheels or 3 wheels does not matter. Well, 3 wheels might be less stable, but that is a different topic.

 

[rcgldr]

In an ideal world, coefficient of friction depends on the materials and not a load factor, but for real tires, the coefficient of friction decreases as the load increases. Wiki article covers this somewhat:

http://en.wikipedia.org/wiki/Tire_load_sensitivity

Some articles mention the sensitivity as the load per unit area of the contact patch, since the issue is related to sufficient flexibility in the tire at the contact patch which is related to how much the rubber in the tread is compressed due to the load per unit area.

Decreasing the pressure increases contact patch only so much depending on how stiff the sidewalls are. Run flat tires have very stiff sidewalls and can run indefinitely at zero pressure (depending on which run flat tire), with only a small increase in contact patch size.

Decreasing the pressure on a wider tires allows the contact patch area to increase more with less of the sidewall limiting factor.

For racing cars, there's the issue of heat dissipation, so larger tires dissipate more heat, but they weigh more, affecting the suspension reaction, so there's a point of diminishing returns.

 

[particlebrake]

Wouldn't the four-wheeled vehicle stop faster due to the 33% larger brake surface area? It seems like stopping power is more reliant on brakes than tires.

 

[rcgldr]

Wouldn't the four-wheeled vehicle stop faster due to the 33% larger brake surface area? It seems like stopping power is more reliant on brakes than tires.

Assuming brake fade has not occurred, then the brakes can always produce enough torque to reach or exceed the limits of traction provided by tires.