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B Need help understanding tires and adhesion

  1. Aug 25, 2016 #1
    According to physics F=mu×N. With tires there has to be more that applies. To increase friction, you have to run a stickier rubber compound tire (mu) or increase downforce like with aerodynamic parts (N). But why is it that if you install a new set of tires that is the same coefficient of friction, and you don't change the normal force, but you do go to a wider tire size, how is it possible that a larger contact patch increases traction?
    There are many factors at play here, first and most importantly is that the tires must reach the optimal operating temperature to achieve maximum grip.

    There are also tire slip angles which is the angle at which the tire is in relation to the direction of travel of the wheel/rim (optimal slip angles seem to be around 6-10 degrees). Then we have the factor of a wider tire with a shorter sidewall being able to have better steering response and the wider surface area more efficient in evacuating heat, which also leads to longer life.

    There are other factors like suspension setups and the pressure per area of the contact patch, and not to mention a vehicles center of gravity and body roll. There is alot of other things like the way the tires deflect under lateral load, and people actually get better lap times just from changing wheel widths to better fit the tires so the sidewalls can deflect properly.

    If you gain traction by putting on a wider tire of the same compound, why does it seem to defy this simple rule? I have read through tire load sensitivity and I'm not sure it answers my question, or maybe I don't understand it fully, as I am not a genius. Hopefully someone has the answer.
    Last edited by a moderator: Sep 13, 2016
  2. jcsd
  3. Aug 26, 2016 #2


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    That's not a law of physics. It is only a useful approximation, good for many situations but not all.
    Tire load sensitivity says that in the real world, traction is not directly proportional to normal force -- that for tires, there is no single μ so that F is equal to μN. This would apply if you reduced inflation pressure while increasing the tread width. The contact patch would increase in area, normal force per unit area would reduce and friction per unit area would not reduce by quite as much.

    Alternately, a wider tire with inflation pressure held constant would have a contact patch that is wider, but shorter. The tire would deflect less under load and tread squirm would be reduced. This should have an effect of improving grip slightly.

    I do not race cars or design tires for a living, so take this for what it is worth.
  4. Sep 13, 2016 #3
    It obviously doesn't apply to rubber or elastic things. I'm sure there are more equations at work here that I am unaware of. So it still stands that tread width/contact patch area alone is not the reason for increased traction? But that there are other factors that come about from having a wider tire that does help with traction?
  5. Sep 13, 2016 #4


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    As I understand it it's very difficult to calculate frictional forces from first principles. In many cases the best we can do is measure it and try and create a model that can help predict what effect changes will have. I note that many racing teams with huge budgets still manage to make wrong tyre choices.
  6. Sep 13, 2016 #5
    This is a topic which causes lots of arguments on motoring sites. Pressure = Force / Area so for a given pressure and weight of vehicle contact patch is constant. Or is it? As you have pointed out, and what was discovered in 1699 (????) is that friction is independent of contact patch area. Some argue that this is all true and what usually happens is that a wider tyre allows a softer compound and you get better heat dissipation and more material means tougher construction. These wider tyres usually are lower profile too which means they don't deform as much when cornering. as you pointed out. The true enemy according to this view is that heat and mechanical stress are what destroy the tyre and that is why you can't run bicycle tyres on an F1 car, same amount of traction but they are destroyed in a very short time.

    I did find a paper (and I will have a look for it) that actually tries to measure normal force and contact patch area, the results are all over the place. Any racer of course knows all the answers or at least think they do. I notice you assume that the larger patch increases traction for the same compound as most people do, but is that correct?

    I am quite interested in this topic but it is difficult to discuss especially on motor racing sites because everyone knows the answers, just like they can all feel a 2HP increase.

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