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Re: Going to look [now tire contact patch]

To: "" <>
Subject: Re: Going to look [now tire contact patch]
From: Max Heim <>
Date: Mon, 20 Dec 1999 12:41:24 -0800
Paul Hunt had this to say:

>What do you say happens to the patch then?  Take two tyres, identical except
>that one is twice the width of the other.  Forget different pressures,
>different sidewall construction etc that's just chaff.  For the patch to
>stay the same size the wider tyre would have a patch that was half the
>length of the narrower tyre.  

That is precisely what I am saying. And Road & Track in its tire testing 
articles makes that exact point. I was just leafing through a friend's 
copy of "The MGB Survival Guide", and I noticed that the author also made 
this point in the section on Performance Modifications. I admit it is 
counterintuitive, but it makes sense if you consider the physics of gas 

>Are you saying that this is accounted for by
>the different squish of each tyre?  No way.  

Well, think of a spherical inflated object, like a soccer ball. Press it 
down with your hand against a glass tabletop, and observe the contact 
patch area (which, of course, will be circular). Press harder, and the 
patch area increases. Inflate the ball to a higher pressure, and it takes 
more force to achieve the same patch area. Let air out of the ball, and 
it takes less force to achieve the same patch area.

Now use an oblong ball, like an American football (or rugby football). 
Everything works exactly the same, except that the contact patch is a 
different shape. But given the same inflation pressure, and the same hand 
pressure pressing on the ball, the area of the contact patch will be 
exactly the same.

Well, replace hand pressure with car weight (a constant). Keep inflation 
pressure a constant. That leaves you with the same area tire contact 
patch *regardless of the shape of the tire*. Width, circumference, and 
tire aspect ratio are non-factors.

>Why else do high-performance
>cars have wider tyres?  You could say it is just for show, but single-seat
>competition cars like Grand Prix cars don't do anything for show and they
>have massively wide rear tyres - wider tyre = more grip because it has a
>wider patch.  In fact recent Grand Prix rule changes have reduced the
>maximum allowable tyre width in an attempt to reduce cornering speeds -
>narrower tyre = less grip.

Now you're getting closer. Obviously, it isn't "just for show", as the 
evolution of tire profiles over the last fifty years in relentless 
competition has demonstrated. But we have just demonstrated that tire 
width or aspect ratio has *no* effect on contact patch area. What does 
this mean? It means that *contact patch area is NOT the determining 
factor in grip*. You are correct in saying, "wider tyre = more grip". The 
misconception is that more grip is a result of a larger contact patch. It 

>For further confirmation of 'narrower tyre gives more grip in snow' (if any
>were needed) see also film of World Rally cars on dry tarmac and snow - dry
>tarmac = wide tyres, snow = narrow tyres.
Of course, I was never arguing this point. I was merely pointing out that 
contact patch area is not the determining factor. Otherwise the logical 
extreme for snow tires would be close to *zero* contact patch, using 
razor-thin tires, and I think those would function more like ice skates!



Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.

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