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Old 06-03-2008, 06:24 AM   #71 (permalink)
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Quote:
Originally Posted by mattW View Post
My question is what is the relationship between contact patch and traction... in physics we are taught that the contact patch is basically irrelevant to friction because as the area increases the force per unit area decreases. Here my a post about it in a different thread:


Is there any reason why what I said isn't true? I don't know anything from my own testing, that was just repeating what I learnt at University.
The old adage that the contact area doesn't affect friction doesn't work for tires.

Go look closely at the pavement outside your house. No, No, I mean "down on your hand s and knees" close. Notice, it has a texture. I'll call this macrotexture to distinguish it from the irregularities on the microscopic level.

This macrotexture allows the relatively soft rubber to penetrate that texture and provides more grip than ordinary friction provides. Under severe situations - such as hard cornering and emergency braking - the rubber gets torn off and that generates more traction. The result of course this that the part of the tread rubber is worn off.

You can see this on a race track as the "marbles" that accumulate outside the racing groove. These"marbles" are bit of tread rubber that have been collected together by the tire rolling over them and then thrown out by the cornering force.

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Old 06-03-2008, 06:50 AM   #72 (permalink)
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Hello,

I think that traction is greatly affected by sidewall flex -- if the pressure is too great, it causes the whole tire to "roll" sideways enough to loose traction; and/or if the tread becomes convex, you loose part of the contact patch.

I get understeer if I go above 40psi, and I loose spin traction in wet conditions.

Also, from my experience with riding a motorcycle for about 12 years (over 100,000 miles!), I know that if the tire pressure is too low, the tire will heat up too much, and if you over inflate it, it won't heat up enough to gain the traction you need. This is similar to any kind of race car; especially F1.

Like anything, it is possible to have too little tire pressure, or too much of it. And both have their own problems. Just because some added tire pressure is good, doesn't mean that a lot is better.

For my commute, I do a 12.4 mile stint on Route 117, and in the morning it takes me up to 55 minutes to "drive" it. A good day, it takes 35-40 minutes... My last tankful included this crawl FIVE times -- and I still got 45.53mpg!
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Old 06-03-2008, 07:44 AM   #73 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
The old adage that the contact area doesn't affect friction doesn't work for tires.

Go look closely at the pavement outside your house. No, No, I mean "down on your hand s and knees" close. Notice, it has a texture. I'll call this macrotexture to distinguish it from the irregularities on the microscopic level.

This macrotexture allows the relatively soft rubber to penetrate that texture and provides more grip than ordinary friction provides. Under severe situations - such as hard cornering and emergency braking - the rubber gets torn off and that generates more traction. The result of course this that the part of the tread rubber is worn off.
This is very true, and often a point missed by many people. This is the reason I can climb Bump Rock at 8 psi, but not 12 psi (slickrock feature on a trail here, surface is much like concrete).

Ever screw around on very smooth concrete (like in a parking garage) vs textured concrete you'd find on a road surface? Much less traction on the smooth surface, even thought the material is the same. It's the "macrotexture".
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Old 06-03-2008, 10:16 AM   #74 (permalink)
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Most of the flexing happens in the sidewalls, up the pressure and the sidewalls will flex less, which leads to less fatigue overall. Remember, fatigue comes from cyclic loads, not constant loads.

As far a grip is concerned, it will be different for every tire and load. You have to check your own contact patch and test your car. From what I read, generally in passenger cars radials on average paved roads underinflation<overinflation<recommended inflation. Hyperinflation would probably rate around underinflation. Note that grip ≠ handling. Sidewall stiffness plays a big role in handling ability.
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Old 06-03-2008, 10:41 AM   #75 (permalink)
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Great so, recommended pressure is just right for everyone. Perfect for fuel economy, stability, safety, ride quality, ect. If any variable changes (i.e. driving conditions, vehicle weight, ect) changes, it doesn't matter. If you hyperinflate, expect to have a blowout immediately and probably rollover, not to mention lose fuel economy. Lemme me just get this all written down somewhere safe so I can read it again.

I was expecting some hard data, not opinions. Perhaps a formula that would describe probability of a blowout vs. pressure. Something to show actual FE with pressure changes.
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Old 06-03-2008, 10:59 AM   #76 (permalink)
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remember the tires pressure changes constantly as it rolls down the road and impacts cracks pot holes etc.. the side wall rating is far from the burst pressure.. I run 5 over and feel perfectly safe... however at higher pressures you get a harsher ride as there is less flex in the tire.. just like a low profile tire is harsher as there is less flex in the sidewall..

Really it depends on your car your driving style etc.. and generally what you feel is best for you... do what you feel is best.. I feel more pressure is better, better FE and control.. less side wall flex.. however i do feel it in the ride quality.
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Old 06-03-2008, 12:58 PM   #77 (permalink)
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At a guess, I'd say that passenger car tyres are designed around the max inflation pressure that can be used at the tyre's rated loading at its rated speed.

i.e. 51psi at 1350lb at 130mph.

These figures will be initially simulated during the design stage, then proven out with correlation testing, then real world testing.
A factor or safety will be designed in (possibly around 1.5 to 2, and I'd guess no less) to account for deviances in manufacturing tolerances and materials specifications between batches.

The testing would include kerb and pot hole strike cycles at specific speeds, with a required number of cycles before failure i.e. 10,000 cycles to failure pass rate. These would be tested on new and aged tyres to determine any age related degredation.

Testing would begin at a baseline predicted by the FEA simulation, then change according to the test results and failure rates. From all of this the result would be a homologated tyre that passes the required tests and would be given a load rating, speed rating and max inflation pressure based on the test results.

Its a bit of a wired science, but more and more tyres are being designed for specific vehicle models these days.

If you fit a tyre rated to 1350lb and 130mph to a car that can only do 110mph and has a corner weight of 600lb, then at a guess you'd be reasonably safe increasing the max pressure over the limit on the sidewall...... by how much I don't know, but a few psi probably wouldn't put it into the danger area?
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Old 06-03-2008, 01:07 PM   #78 (permalink)
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ihatejoefitz, have a look at this post and maybe read the pdfs linked in it.

I have spent a lot of time searching and reading papers and have yet to read anything linked directly to hyperinflation related tire failures. I haven't seen any security warning with explanations or mention of accidents specifically due to hyperinflation anywhere.

I have found 2 studies related to grip, handling and comfort vs pressure and load. They don't go in the hyperinflation zone unfortunately.

The influence of tyre pressure on the comfort and handling of passenger cars

LABORATORY INVESTIGATION OF TYRE SLIDING GRIP COEFFICIENT
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Old 06-03-2008, 01:31 PM   #79 (permalink)
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Hello,

Quote:
Originally Posted by ihatejoefitz View Post
Great so, recommended pressure is just right for everyone.
I didn't say that -- my recommend pressure is just 29psi, and the sidewall max is 44psi. I run them at 38 or 39 psi. There is a strong benefit from this, and I find that going higher causes some understeer and other loss of traction.

What I meant was that while some extra pressure is good, that doesn't necessarily mean that even more pressure is better.
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Old 06-03-2008, 02:14 PM   #80 (permalink)
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The car's recommended pressure is most definitely NOT right. My last set of tires, rated 44 psi, I ran them at 35, while the car called for 32. The wore out on the edges more than in the middle. That's above the recommended pressure, and it was still too low.

The question that remains is this: How much higher than this would be the best compromise between tread wear, traction, tire fatigue, and mileage?

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