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Too much friction!

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Nov 16, 2008.


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    My dear lady purchased two pairs of shoes today one with a slender heel and one with a slightly larger heel contact area, both pairs were from the same range at the same shop (Autograph Marks and Spencer). The pair with the slender heels came with a warning sticker on the sole saying:

    "These shoes have slender heels and will suffer from wear due to the large amount of friction on such a small surface area."

    Were the manufacturers correct to only put this sticker on the shoes with the slender heel? What difference does contact area make to friction?

    Just a bit of fun to get you thinking
    Last edited: Nov 16, 2008
  2. Johnpod

    Johnpod Active Member

    Surely the same friction applied over a greater area causes less apparent wear on the broader heeled pair - isn't it?

    The M&S disclaimer must be designed to avoid the claim of apparently excessive wear caused to the narrow heels by friction?

    Were they correct to put the sticker only on the slender heeled pair? Strictly, no. But sensible in the present climate? I reckon 'Yes'
  3. twirly

    twirly Well-Known Member

    Feeling afraid but brave so I will enter this debate.

    Surely a smaller area of pressure will show advanced signs of wear more readily than a heel with a larger surface area.

    So....... in my not so proven opinion of wisdom I would expect the said shoe to become potentially more unstable more readily, thereby potentially rendering the wearer more likely to suffer an inversion sprain.

    I could be wrong & I anticipate being shot down aflamed so I am wearing a flame retardant nightie. :eek:

    With the skin of a heffalump. twirly ;)
  4. Steve The Footman

    Steve The Footman Active Member

    I was taught that friction is a result of the interaction of the properties of the two surfaces and has nothing to do with surface area. I think this is what Simon was getting at.

    Perhaps they should be charged with false advertising. While the slender heels may wear more than the wide heels this may have nothing to do with friction.

    Also does more friction always cause greater wear?
  5. twirly

    twirly Well-Known Member

    Would the increase of pressure not also be a factor?

    I would have thought pressure + friction would increase the wear over a small surface area as at heel strike the body weight would drive a greater force.

    Back to my trench with my head down.

  6. Boots n all

    Boots n all Well-Known Member

    "Also does more friction always cause greater wear?"

    Yes, rub most things together long enough and you will wear one of them down,....something has to give, this is why we have lubricants for moving parts of machines...so there is as little friction as possible meaning as little wear as possible

    Also the smaller the heel top piece the harder the compound has to be to keep it stable, which tend to wear down faster....due to friction:empathy:

    Friction caused by the action of dragging one surface over another, Yes.....next time you see a worn down heel/shoe turn it over and you will see the longitudinal groves this will be more noticeable the harder the compounds

    Lets see how we go here, unlike Twirly l am not wearing a "flame retardant nightie":D
  7. Steve The Footman

    Steve The Footman Active Member

    But is the wear caused by friction in a machine the same as on the outsole of a shoe?

    There are other factors in wear that are related to friction, for example how much heat the friction produces and whether that heat has time to be conducted away or whether it changes the properties of the two surfaces.

    If there is enough friction between the shoe and the surface there will be no shearing motion once they touch. That can translate to much less wear.

    The type of action that creates the greatest wear in running shoes is a sliding contact phase with the force gradually applied to the ground. This scrapes away the outsole. This is more common in midfoot or forefoot strikers. Extreme heel strikers and pounders do not necessarily have the greatest wear in their running shoes.

    The direction and timing of the force may be more important than the friction when it comes to wear.

    Another interesting factor in shoe wear is that the hardness of the outsole has a big factor in the amount or speed of wear. While it might be counter-intuitive the harder the outsole compound the more quickly it wears. Replacing the normal carbon rubber outsole with a plastic for instance will not help with the durability of the outsole. The softer carbon rubber gives a little at contact, has more friction with the ground, and reduces the duration of movement between the two surfaces. A good example where more friction reduces wear.
  8. David Smith

    David Smith Well-Known Member


    Friction is a function of the normal force x coefficient of friction. The coefficient of friction between two surfaces is found by dividing the force required to overcome frictional force by the normal force (normal force = gravity for instance). Usually there are two coefficients of friction for a given material.

    1) The static coefficient

    2) The sliding Coefficient.

    The first is the force required to start sliding or translation and the second is the force required to continue sliding, where the later is always lower than the former.

    Wear is a function of distance travelled by one point on the surface relative to another contacting surface.

    The rate of wear can be measured in grams per hour. The rate of wear is reduced by reducing the friction. This is achieved by reducing the frictional force by introducing lubricant, reducing normal force, reducing particulates or engineering the contacting surfaces.

    The types of wear are categorised as friction or adhesion, erosion, corrosion and abrasion. In the case of the shoe sole only adhesion and abrasion and perhaps erosion would be of interest.

    The surface area of two contacting surfaces has no relationship therefore with either the total friction or the total wear.

    However, if the wear rate is constant at (X) grams per hour, then X grams removed from a relatively smaller volume will result in greater percentage wear. IE large heel has 200gram of material and small heel has 50grams of material then at a wear rate of 0.02 grams per hour then, after 1000hours the total loss will be 20 grams. The large heel will have lost 10% of its volume whereas the small heel will have lost 40%. It will appear then that there is more wear on the small heel when in fact there is the same amount of wear on both.

    Interestingly this is an important consideration in joint replacement. Which would be a better choice for the size of a hip ball joint, large or small? If a large replacement ball joint were fitted in a hip how would the wear compare with the same hip replaced with a much smaller ball joint. Consider that it is very important to reduce to a minimum the amount of foreign particulate that is produced by an artificial prosthesis inside the body.

    The answer is that the large ball joint would produce more wear than the small ball joint. This is because the large joint has a greater radius. Therefore to displace thru the same arc or hip RoM the distance travelled by a point of interest over its contacting surface area would be much greater than on the small ball joint. As the amount of wear is a function of this relative distance of translation there would be more wear and more particulate from the large hip joint.

    Cheers Dave
  9. Steve The Footman

    Steve The Footman Active Member

    So Dave it sounds like the surface area is more significant than the friction when it comes to wear. However even that does not explain wear adequately in all situations eg your hip joint example.

    Perhaps they should have just said "These shoes have slender heels and will suffer from wear due to such a small surface area."
  10. David Smith

    David Smith Well-Known Member

    Well no not really Steve, The surface area has no relation to the amount of wear except where the surface area has some bearing on the way material properties change due to apllied forces. IE On a car a large wide tyre, in certain circumstances, will give more grip for longer because a thin one will start to melt and spall earlier, due to the applied forces.

    These are simple models if we consider the nature of the mechanical properties of the heel material and the nature of the applied forces there are other possibilities.

    For instance if the heel material is the same for both a small and large heel then clearly for the same subject and therefore applied load, the force per unit area (pressure ) will be greater on the small heel. Therefore the material of the small heel can (will be forced to) deform to the shape of the ground irregulaities better then the large heel. In this case the lateral force required to cause skidding may be greater for the small heel. This can be known as the deformation coefficient of friction (DCeF). To overcome the DCeF the heel material will need to fracture or deform. As noted in my last post wear cannot occur until there is a relative movement between two points on the contacting surfaces. In the above case the DCeF can increase as the surface area decreases with a constant load, therefore and under certain circumstances it may be possible that the small heel does not skid and wears less than the large heel. (Unlikely but possible in theory)

    In the simple model where the rate of wear is constant regardless of surface area it is the volume or mass of simmilar materials and not how it is distributed that is important when considering the intuitive amount of total wear.

    So if one of the heels was small in surface area but very deep and the other had the same surface area but was very thin, which one would intuitively wear the most. I would imagine that most would answer that the wear was equal.
    However now if you took the deep heel and spread it out with a larger surface area and the same thickness as the thin heel, then because the depth of wear would be less on the large surface area heel then intuitively it may appear to have worn less.

    Perhaps the label should read ' these heels are slender and so have a relatively small volume of material and will therefore appear to have greater wear over any given time'

    Cheers Dave

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