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Re: [Phys-l] Another tire question



Mmmm, We need a heavy glass block on which to lower one of the auto's tyres. I suspect when lightly inflated the treads close except at the leading and trailing edges. Has anyone heard (seen) the effect of tyre inflation on the coefficient of friction?
bc

Carl Mungan wrote:

I haven't had time to fully read the replies to my post or examine Michael's revised figure, but I did want to add two more things.

First, I had some doubts about Shurcliff's numbers, so I decided to make some measurements on one of my cars (Ford Escort Wagon):

tire pressure = P = 32 psi (all 4 tires)
width of tire = t = 6.5"
diameter of tire = d = 22"

So that gives not 2000 lbs but rather 32*6.5*22 = 4580 lbs for air pressure force on half of the tire. Shurcliff's value seems *way* too low for any real car.

Second, I still have an important question in my mind that I don't think has been answered: If the tire pressure (P) times the contact area (A) of the tire with the ground is *not* equal to the weight (w) of the car divided by 4, then what *does* determine A? In particular, what do you need to know to predict it? For comparison to my car above, note that:

w = 3480 lbs

and I assume A = t*L where I used a ruler to estimate that L happens to be roughly equal to t for my car. (This value of A is probably an overestimate, because I haven't corrected for the treads.) -Carl

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Carl E Mungan, Assoc Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-5002 mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/
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