Tox

[QUOTE]Originally posted by Nick T:
[B]...22.7 square inche contact patch?!?!? How did you get that? 725lbs divided by 32 psi? Does that mean if I only put in 10psi I have a 72.5 square inch contact patch? A contact patch that is 3 times as large as running 32 psi? You can not measure contact patch with algerbra... If the contact patch stays the same

Luis

... further to what tox said, I do not think this is true. Deformation of the side wall for the same load (weight) will be dependent on width. On a perfectly deformable body, the pressure on the ground is the same, so something got to give. If you decrease the width, you increase the length, and vice versa, thus deformation of the side wall will not be the same.

Let's not get hung up on the tyre stifness issue. We all agree that the tyres are not balloons but they are not made of steel, either.

Tox

It's also interesting to assume the opposite of perfect deformability -- that tires are steel, and perfectly rigid. The area of the contact patch would be zero, with the tire contacting the ground along an infinitely thin line. The line would be longer for a wide tire than a narrow one, but the area for both would still be the same -- nothing.

So interpolating from a perfectly rigid tire to a perfectly deformable one, it appears that whichever tire is more deformable will have the greater contact patch, regardless of shape.

Luis

[QUOTE]Originally posted by Tox:
[B]

Tox

Very likely they do. The problem with simplistic calculations is that it's possible to simplify much of the truth out of them -- the modeler's nightmare.

An empiricist after my own heart. I've already been thinking about doing something similar with the S2000 and one of my other cars that has 185/60s. I don't think the Honda spare is a good idea, though, because at 32 psi the rim might touch the ground. Maybe I'll give that a shot when I get home after Christmas.

[QUOTE]Originally posted by Luis:
[B]...I could do it, but you owe me one...

Nick T

This will be my last post on this thread, I do not want to beat this subject to the ground. But listen to yourselves."The problem with simplistic calculations is that it's possible to simplify much of the truth out of them -- the modeler's nightmare." Simplistic = algerbra. "area tends to zero as stiffness tends to infinite, " That's a CALCULUS problem not ALGERBRA. From how you guys are talking, it does not seem you have taken any courses in Deformable Bodies and Strength of Materials. Two very important courses for any Mechanical Engineer major. While you are experimenting with your different size tires etc etc. Try this experiment. Get two carboard pieces of exactly the same size and material. This will represent the sidewall of your tire. Hold them straight up from the ground 2 inches apart in parallel and put weight X onto each them. Note how much they bend. Now do the same thing only this time have them 20 inches apart. They WILL BEND the same amount. Extrapolating that data you will see it does not matter how WIDE a tire is, with the same amount of force acting against the same size sidewall ( my previous post already postulated we have identical height and material sidewall ) the "bend" or deformation is the same. This is a very simple experiment that does not take in consideration other varibles a real mechanical engineer must include in designing any deformable body construction but the other varibles have little to do with how it will deform but more in things such as how much total force it can withstand, it's heat index and thermal transfer etc etc etc

I have only been on this board a short time and I am sure Luis and Tox have given alot of good advice on other things. Might I suggest you ask around, ask some mechanical engineers and/or college professors. I love to learn new things, but I'd rather not learn anything at all then to learn something that is wrong

Regards

P.S. This was not intended in anyway to be a flame

[This message has been edited by Nick T (edited December 21, 2000).]

Luis

[QUOTE]Originally posted by Nick T:
[B]Get two carboard pieces of exactly the same size and material. This will represent the sidewall of your tire. Hold them straight up from the ground 2 inches apart in parallel and put weight X onto

Tox

[QUOTE]Originally posted by Nick T:
[B]...I have only been on this board a short time and I am sure Luis and Tox have given alot of good advice on other things. Might I suggest you ask around, ask some mechanical engineers

Nick T

I was trying to keep it relatively simple so that you could UNDERSTAND, the same basic principles will work for a "deformable enclosed space". The only differences will be the introduction of extra variables such as air pressure and opposing forces acting upon the tire against the point of contact. As we have already agreed that air pressure is constant, material strength is the same, and sidewall height is identical, the use of an "deformable enclosed space" as a model would only introduce variables that have little or no effect on the amount of deformation of the sidewall. I know enough about Deformable Bodies and Strength of Materials to have passed those classes and gotten my Mechanical Engineering degree from Calpoly Pomona. Although it's been almost ten years, I remeber enough on the matter for the discussion at hand. I therefore take it you have a culinary degree specific to oriental cooking and that "Ant Foreskin" is a major dish that many other more techincal dishes are based on? And that this culinary institute is acreditted? Again I suggest you ask a few mechanical engineers on this subject before you start assuming that I know such and such on the subject. Better yet print out this whole post and show them... just so that they will not misunderstand anything I am saying.

This will be the last post on this thread, I do not have the time nor the inclination to explain every little detail and as it seems my model was too simple, please again I ask... print this page and go ask some mechanical engineers.

Tox

Since we're making things "relatively simple so that you could UNDERSTAND," understand this:

The sidewalls don't hold the car up. Air pressure holds the car up. If this is not clear to you, go let the air out of your tires and watch what happens. (Many people who lack engineering degrees already seem to know this.)

Now get a large airtight plastic bag. Blow it up and seal it. Put it on the floor and lie on it. What's keeping you off the floor? Sidewall stiffness?

Air pressure is not an "extra variable." It is the main variable. Sidewall stiffness is an extra variable.

PS: Taking your illustration of, say, a tire 1 foot wide with a 2-inch sidewall and a tire 6 inches wide with the same sidewall, both holding up the same weight and at the same air pressure -- the narrow tire will deform more than the wide one. I invite you to see for yourself with your car, using your space-saver spare. I'd do it myself, but I already know what will happen.