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Wider tires, Friction questions

  1. Mar 4, 2014 #1
    I was reading my textbook, and I came across the old formula F friction = mu N . It also explicitly stated that force of friction is independent of area. Of course I immediately wondered if this were true then why do wider tires have move grip, So googled searched for the answer. I tried a couple of different searches, and read about 2 dozen different resulting pages. Here's the problem. They all had different answers, and every answer had another response that completely contradicted it.
    After all that reading I actually became less certain about what exactly friction is and how it works, so here is a list of questions I have.

    1. Is the force of friction independent of area for all different types of friction

    If Friction is caused by the molecular forces between two objects in close contact, and said objects are relatively FLAT than why wouldn't an increase in area cause greater friction.( more molecular bonds/ attractions)

    If the friction is caused by the interlocking of teeth of the two objects together, and the only way to start moving is to knock down the teeth (instead of lifting or sliding one object over the teeth of the other) then shouldn't an increase in area cause a great increase in friction? An even better example would be Velcro, surely an increase of the area of Velcro would cause a greater force of friction.

    2. If the force of friction is independent of area, then Why do wider tires have more friction or better grip?

    I searched for the answer but I found many different responses. Most of the responses were contradicted by a different answer given by someone else. Some claimed the tire didn't actually have more grip, but instead would wear down less and thus provide better traction. Some said the tires when hot act like glue or scotch tape, and thus increasing the surface area would be like adding more glue, others claimed that the Contact patch didn't even increase with larger tires. I can't even list all of the different answer here.
    So, I'm hoping you guys can shine some light on this .
    Last edited: Mar 4, 2014
  2. jcsd
  3. Mar 4, 2014 #2
    So how do you edit the title of a thread...
  4. Mar 4, 2014 #3


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    Your textbook is not talking about real-world friction. It is talking about a simple mathematical model of friction, proposed by Coulomb about 250 years ago, which it is fairly accurate for hard materials (wood, metal, glass, etc) sliding at fairly slow speeds. That is the sort of friction experiment you would do in a ihigh school physics lab. The big advantage of Coulomb's "law" (or model) is that you can use it to learn how to solve mechanics problems, without needing a computer to crunch the numbers.

    There seems to be a lot of mis-information because some people have never learned anything except Coulomb's law, and think that is "everything" there is to know about friction.

    I'm not going to try to "explain" friction of car tires, but they certainly do NOT obey Coulomb's "law" of friction.

    The contact area of a tire does not depend on the size of the tire. It mainly depends on the air pressure. The total contact area of all the tires, times the air pressure, = the weight of the car. If you want to demonstrate that, just let some air out of one of your own car tires (but make sure you pump it up to the correct pressure again afterwards!!!)
  5. Mar 4, 2014 #4


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    I took care of it for you. :smile:

    BTW -- any time you want a thread or title edited, just click the Report button and ask the Mentors for help.
  6. Mar 4, 2014 #5
  7. Mar 5, 2014 #6


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    This is true, however the shape of the contact patch (which affects grip available) and the ability of the tire to withstand high heat loading (from high performance driving) are both affected by the size of the tire, which is part of the reason why wider tires tend to be preferred for high performance cars.
  8. Mar 5, 2014 #7


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    I'm sure a full explanation would get far over my head pretty quickly. However, is there a simplified version that would help a layman understand the difference between the forces on a real car tire and those that would be predicted by Coulomb's?
  9. Mar 5, 2014 #8


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    Tires deform, which changes the surface structure and therefore how much grip they have.
  10. Mar 5, 2014 #9


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    An extreme example is the tire friction of a top fuel dragster. They can accelerate much faster than 1G even before the aerodynamic down-force begins to help. They burn the tires to make them sticky. But there are new things being discovered even now. Google gecko feet adhesive if you are interested. It has something to do with getting a lot of pad area very close to the surface.
  11. Mar 5, 2014 #10


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    This article discusses briefly the physics of tires:

    http://www.physics.sc.edu/~rjones/phys101/tirefriction.html [Broken]
    Last edited by a moderator: May 6, 2017
  12. Mar 8, 2014 #11
    Interesting, In that link it states that the contact area for wider tires increases (assuming all else is equal). I've heard that stated before, but I have also heard the opposite (the contact area doesn't increase for wider tires). The very first reply to this thread states the opposite.
    Last edited by a moderator: May 6, 2017
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