Contour Enthusiasts Group Archives Forums General General Theoretical Tire Question (EdwardC, I need a physics lesson here)

I was always led to believe that cars with wide tires like Corvettes will more easily hydroplane and slide on ice/snow as well. Wider tires have more room between grooves than narrow tires so there is still area where water is not clearing out. Weight is distrubuted over a wider contact patch meaning less lbs per square inch of contact. While road-hugging weight may be the enemy in dry conditions, it is preferable in wet and snowy, and that weight presses down more when there is less contact patch where it is distributed.

For any who doubt this, think of a pick-up truck that most have us have driven. In dry conditions, when you load up the bed, what happens. The truck gets tail-happy, weight and more specifically, weight distribution hurts the handling. In wet or snowy conditions, however, what do you want. WEIGHT in the bed. Otherwise you spin those rear tires like crazy for lack of traction due to low weight on the tires.

That being said, Tony, I did not seem to have too much trouble in the scenario we discussed at SZ between me and the Cadillac DTS in the rain. And my tires are definitely wider than normal at 215/40. Who says Kumho 712s are no good!!!! Kids, DO NOT try this at home!!!!

The larger the contact patch, the greater the chance for hydroplaning.

Simple application of archimedes principle

FB = density of water * Volume of displaced water*g = bouyancy force.

wide tires possess a larger contact patch which in turn displaced more water when passing through a deep puddle. Being an incompressible fluid, a layer of water builds up underneath the tires with increased tire angular velocity. The effect may be partially mitigated by tread designs which channel water away from the center of the contact path. When the sum of bouyancy forces on all 4 tires exceeds the force of gravity acting on the vehicle's total mass then hydroplaning results.

Lightweight vehicles with wide tires are most prone to hydroplaning.

Don't take this the wrong way.

But how does a wider tire turn into a larger contact patch?

I was under the impression the given two tires of the same construction, one simply wider than the other, and the same air pressure in the tire, that the area of the contact patch is the same. It is only the shape that changes.

(In fact, I learned that tidbit in my freshman Physics course, IIRC.)

I realize there may be some slight difference. But assuming the only change is the width of the tire (the sidewall supports no more or less weight or it's contribution relative to the weight supported by the air in the tire is insignificantly small) doesn't the area of the contact patch remain the same.

This is what I'm struggling with. I believe the contact patch occupies the same area, given the same air pressure in the tire. The only difference is the shape.

Is that wrong, or too simple?

TB

Think of it this way. Cut off the bottom arc of a thick and thin tire and carefully place each tire segment in a kiddie pool so that it floats. Start emptying marbles in both the thick and thin floating tire segments. The thick tire segment will most likely support a greater weight of marbles before sinking. Although the contact patch may not be the best indicator of tire width, the volume of water displaced by a thick and thin tire are different.

You've changed units on me I was using wide and narrow, and you switched to thick and thin.

Again, I'm with you, but I'm not sure how this disproves the authors in my second post of the thread.

Also, generally speaking, is the area of the contact patch the same, given the same tire construction and air pressure.

Or perhaps a better question is how much of the cars mass does the sidewall and the air support, by percentage?

I don't expect you to have the answer to that. Perhaps our member who works for Yokohama can answer that one.

Thanks,

TB

Sorry,

thick=wide
thin=narrow

No change in the originally stated problem.

Tony, maybe you misunderstood your Physic professor. If you take a duck and place him in water, his feet have a certain contact patch. Take another duck of identical weight but bigger feet and put it in water and it has a larger contact patch. Maybe it isn't a good analogy but I don't see how you could think a Corvette with 275/35 tires would have the same contact patch as one with 215/55. Same thing with a building. Wider girders would distribute the same weight over a wider area.

Why not

It is the air in the tires that support the weight of the car. So let's say we have a 3000# Vette (for sake of argument) with perfect 50/50 weight distribution. There is 750# on each corner. If your tires are at 35PSI, then you have a contact patch of 21.4 square inches, assuming the weight supported by the sidewall is insignificant relative to the weight supported by the air in the tire.

So regardless of tire width, your contact patch will be about 21.4 square inches. You can get there with a 10" wide by 2.14 inch long patch or a 5" wide by 4.28 inch long.

I dunno what ducks and there feet have to do with it. I think they float on their belly. Now with my belly, I can hydroplane

Gotta go on a call...

TB

Tony, the pounds per square inch applies to the air INSIDE the tire, not to the ground. Obviously, a 215/55 tire will take more air to achieve 35 psi than a 215/40 tire. That is why the load rating on my tires is dangerously low. The car is riding on less air in the tires even though it is the same psi. There is physically less volume in the tire to put air before the max psi level is reached.

Have a safe trip...not literally, don't want you to hydroplane on your stomach. :p I hydroplane on my ego...and stomach! I'm versatile.

I agree that narrow tires are less likely to hydroplane. Large ship?s hulls are made of 1" thick steel sometimes. You would not think that 1" thick still would float on water. Air has so much buoyancy that I have heard this is why large ships float. I believe compressed air has even more buoyancy but I am not sure.

I purchased narrow Michelin Symmetry All Season 185/70R14 tires just because they are less likely to hydroplane. I hate driving in the rain but the narrow tires seem safer.