koala

Then we'd be talking about rolling resistance @ 10 PSi.

magician

The contact patch doesn't widen or narrow (much); it lengthens or shortens.

If the tire is bearing, say, 750 lb. of weight, and the tire pressure is 35 psi, the area of the contact patch will be 750 / 35 = 21.4 sq. in. (Actually, it will be slightly smaller than that because the tire isn't perfectly elastic.) This is true whether the it's a 205 or a 225. If you raise the pressure to 40 psi, the area drops to 18.75 sq. in.; if you drop the pressure to 30 psi the area increases to 25 sq. in.

PLYRS 3

ok, i'm not the smartest guy on the block....but not the dumbest either.

but, i don't get it?

if i understand correctly, the size of contact patch has no bearing on the size of the tire (specifically width)??

furthermore, your formula does not take into consideration size AT ALL.

i can apply your weight/pressure formula to a typical adult bicycle tire...

if the bike tire is bearing 750 pounds (that would be one strong bike ) and is inflated to 35psi, i'm pretty damn sure that the contact patch of your typical adult bicycle tire isn't a constant 21.42 sq. in.

on the flip side, i'm pretty sure the rear tire of a viper is putting down a contact patch greater than 21.42 sq. in per the above criteria.

is this what you're telling us?

chrissa

The size (or width) of a tire does not effect the size of the contact patch. What the width of the tire will do is change the shape of the contact patch. A wider tire will result is a contact patch that is longer and narrower.

It would be 21.42 sq.in. if the bike tires had enough volume. What'll happen is that the tire contact area will grow until the rim hits the ground. So some of the weight will be supported by the air pressure in the tire, and the rest of the force will be supported by the rim. I hope the wheel builder knew what he was doing.

It just might be. It's probably 10 inches wide and 2 inches long. That's a nice shape that doesn't offer a lot of rolling resistance moving forward but offers a lot of resistance slipping sideways.

Chris.

magician

Chrissa got it right. I'll add some details.

I think you meant it the other way round: that the size of the tire has no bearing on the size of the contact patch.

This is true. As long as the tire isn't collapsed (i.e., there's air between the bottom of the tire and the bottom of the rim), the size of the contact patch is determined by the amount of weight the tire bears and the tire pressure. Pounds per square inch times square inches equals pounds. Simple physics.

True. Because tire size doesn't matter AT ALL in determining the size of the contact patch. The tire pressure matters. To a very limited extent the strength of the rubber matters, but it's a very small effect. (I mentioned this in my original post.)

Chrissa covered this one well. As long as the air is keeping the tire from collapsing, my formula is correct. Once the tire collapses - which it would do in the example you cited - then the rim is bearing the weight, and the air pressure is irrelevant.

You're wrong.

If the rear tire is supporting 750 lbs. (What are the curb weight and weight distribution of a Viper?) and the tire is inflated to 35 psi, the contact patch will be 21.4 sq. in.

Yes.

PLYRS 3

ok, i got you guys.....i was trying to visualize it and couldn't.....until last
night.

the viper example of 10x2 makes sense.

thanks....i learned something.

(still not a hundred percent convinced in my "heart", but you guys are the science/math majors....i'm just an accountant )