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Homework Help: HELP! I have a Physics question!

  1. Jan 23, 2005 #1
    I had an interesting question pop into my head.

    Why would one benefit from small tires on their car while driving on a wet surface? I thought, oh it's easy, because the smaller the surface area on teh ground, the more pressure the tire has on the place that it is touching the ground, therefore holding the tire in the same spot so it won't slip.

    But then I thought, why wouldn't it be beneficial to have really narrow tires on a dry surface as well. Why don't race cars have skinny tires, so they will have more traction around turns, and also won't burn as much rubber upon starting from a standstill (thus hem further)?

    I don't understand the proof (using physics) behind smaller tires helping you maintain traction on a wet surface so if someone would be kind enough to explain that to me, it would be much appreciated.

    any help on this topic would be great -- I would really be greatful if you could use formulas (the simpler the better) to prove the answer, otherwise I find it hard to comprehend the material.

    Thanks a bunch!
  2. jcsd
  3. Jan 23, 2005 #2
    I have thought about the whole tire traction thing before as well. Generally, wider tires are considered to provide more traction than narrower tires. From a purely frictional perspective, this is not true. We can take "traction" to be simply a measure of the total static-frictional force opposing a certain applied horizontal force. As such, this force (the amount of traction) is proportional to the normal force exerted by the ground on the tire, [itex]F_{N}[/itex], since

    [tex]F_{s}\leq \mu_{s}F_{N}[/tex].

    Notice that total static-frictional force is not ultimately dependent on contact surface area. If you place a long cylinder vertically on a table, the greater pressure (same mass, smaller surface area) pushes the two surfaces together more closely, but the surface area over which they are in contact is decreased. (Static friction is an electrostatic attraction between the particles of the two surfaces). The two factors cancel each other out to make net static frictional force not dependent on which surface it is placed on the table with. Therefore, wider tires do not actually increase traction in that way.

    Other factors may be at play (side wall flex, specific considerations on dimensions and other things dependent on tire width, etc.), but you may be better advised by a tire specialist of some sort.

    Smaller tires do, however, increase traction when the surface is not solid. For example, rally cars racing on packed snow (which gives way a little under the weight of the car) have extremely narrow tires. The reason for this is that the increased pressure pushes more into the snow (causes it to give way more), digging deeper into it and increasing the contact between the side of the tire and the snow, which is a directly opposing force to lateral motion (what they try to avoid).

    EDIT: I can't see why narrower tires would be better in the wet (pavement) except that the increased pressure and reduced surface area may make it easier to evacuate water from underneath the tire, reducing aqua-planing.

    Hope this helps.

  4. Jan 23, 2005 #3


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    The increased contact pressure makes sense to me for the rain case.

    Regarding racing tires being wider and having a larger contact area, it's correct that the equation doesn't seem to indicate any benefit from larger tires, but I believe the reason is because of shear stress at the tire surface. If you have a soft compound which gives better traction, it probably is weaker in shear, so the area is increased to reduce shear stress.
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