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Old 05-25-2008, 04:34 PM   #31 (permalink)
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Quote:
Originally Posted by trikkonceptz View Post
I think we were just taken for a ride folks .. funny how our "tire Engineer" hasn't been back on since the post and really didn't give us any information with his first post.

I think he wanted to spark some argument and was dissapointed when we began a rational conversation ...
Give him time it is a long holiday weekend.

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Old 05-25-2008, 05:46 PM   #32 (permalink)
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Anyone can be a self proclaimed whatever engineer on the Internet...

This has been a hot and recurent topic for a while. I'll try to dig as much stuff as I can tomorrow when I get some time. Hopefully I'll show up with more than the "trust me, I know" and "it happenned to me" we see way too often.
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Old 05-25-2008, 07:34 PM   #33 (permalink)
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Well, I been a mechanic all my life and about 40% of my work involves tires. Overinflation does not cause any safety or driveability issues unless you go far beyond the suggested pressures. I am currently running 46PSI in my tires that suggest 44PSI and it's been over a month.
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Old 05-25-2008, 09:54 PM   #34 (permalink)
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tires really are very complex especially modern tires. from the compound to the belts, and other materials and layers within the tires. modern tires are radials so they bulge a bit at the bottom the part of the tire that hits the ground is flat for several inches front to back, so your contact patch on a tire thats 5 inches wide that has 4 inches front to back would be like 20 square inches per tire.


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trikkonceptz ;If the underlying argument here is contact patch why not test it. Take YOUR tire down to 36 psi or whatever, lift the car, roll some water based paint over the tread and drop the car on some heavy duty paper. Then re-inflate to YOUR max psi and repeat, then measure. If the patches are the same, case closed right? This should even show weight distribution by how much of the treads are printed on the paper, to see which treads are making contact and how much.
the static contact patch is different from the dynamic contact patch ie when you go around turns and accelerate and brake the load changes on the tire and the sidewalls flex quite a bit in a hard corner with the right pressure your tire will actually use some of its sidewall thats why the treads curve over the edge a bit and are not just on the bottom of the tire. if your over inflated you might get scalloping or the center might wear down etc... I am not saying i know exactly the type of negative tire wear that will happen.

and its all relative 5psi over max maybe theres no difference 20psi over max i bet you see something.

also thats why I brought up taking your tires temperatures before I do it for racing its a very valuable tool for figuring out how your using your tires.

also as your car leans into a turn your suspension flexes the inside wheel gets positive camber and the outside wheel negative caber so the car leans out and the tires rubber actually flexes the whole tire sort of moves left and right on the actual wheel, if your tires are too hard I don't know what that might do to cornering ability I will try to contribute actual numbers on tire temperatures vs pressure when I have a free day. I can take some readings under inflated , inflated properly, and over inflated. that will at least provide some data to back up what im trying to express however poorly in text.
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Old 05-26-2008, 03:57 PM   #35 (permalink)
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Rolling resistance coefficient (RRC) literature review:

At moderate speed, 90%-95% of the tire RRC is due to material hysteresis, 2%-10% is due to surface friction or micro-slip and 1%-3% is caused by air friction.

Tire inflation pressure is the main factor affecting hysteresis.

Warm tires have lower RRC than cold tires not only due to increased pressure, but also because the internal damping of rubber decreases when its temperature increases.

Given equal pressure, the initial RRC of a tire at -20 C can be more than twice as at 40 C.

The RRC is almost the same from 0 up to 55 mph, when it starts to increase rapidly. At 80 mph, it is roughly twice as much as 55 mph.

Worn out tires can have a RRC up to 20% lower than new tires as a significant part of the hysteresis happen in the thread which is not so much affected by inflation pressure.

The relation between tire pressure and RRC is almost linear, and is still significant in excess of 100 psi.

As a side note, it is interesting to note that fuel consumption can vary by more than 10% depending on the smoothness of the pavement.

Effect of width, aspect ratio and diameter on RRC:

Note: This applies to RRC only, not rolling resistance in terms of force, which is more important than RRC in stop and go driving.

This aspect of RR is very hard to tackle as there are multiple variables working in opposite directions. Nonetheless, by looking at the variation in RRC of the same tire when one variable is modified at a time, and by looking at average RRC for each tire size, we can conclude that generally, all other things being equal:

- Larger diameter tires have a lower RRC than smaller ones.
- The lowest RRC tire width is the closest one to the tire diameter times 14 (ignoring units of measure). (At max sidewall pressure and max load)
- The lowest RRC aspect ratio is lower as tire diameter gets larger. It appears as the the lowest RRC Diameter/Aspect ratio are: 13/80, 14/75, 15/70. (At max sidewall pressure and max load).

Overinflated tires will wear more in the middle than optimally inflated tires. However, optimal inflation varies with load and tire size.

Blowout risks related to sudden pressure rises in overinflated tires are non existent as radial tires will not unbed under 150 psi.

Overinflated tires have higher risks of cuts, but reduced risks of rim damage.

The temperature increase delta decreases as initial cold pressure increases.

References:

TYRE ROLLING RESISTANCE ON ROAD

Passenger Tire Rolling Resistance

Rolling Resistance Testing: California Energy Commission’s Fuel Efficient Tire Program
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Old 05-27-2008, 04:42 AM   #36 (permalink)
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My first post here other than my intro and i'm in the thick of it!! I have pics of my tires with 105,000km on them at sidewall pressure(44psi, recently upped to 50) but don't know how to upload them here. When i do, the bulge in the middle theory should bugger off! On the other hand, i am still, like everyone here, looking for DEFINITIVE data on braking distance versus tire pressure. I know cars weigh differently and therefore have differing loads on the ground but you get the point. Every time i used to go for a new tire, (which was a long time ago) it was pumped to 32 psi. A month ago i had to get a new tire for my 4 month old kia. The guy put on a new tire and pumped it to 32psi without even asking was it going on the front or back, what kind of car was it (i was in my civic); heavy or light. Nothing, just put 32psi in there, no questions. When i got the car first, it had 32 front and rear and i was afraid to corner in it. It's a diesel and is heavy on the front and i can tell you this, it looked like it was flat on the front and that the tires might pull off the rim at any moment. I suppose my main question is this: If the tire pressure is so bloody important then why dont the tire manufacturers supply us with a grip v pressure graph with every tire. I've seen a LITTLE bit but the data only covered one or two tires and only up to this magical figure of 32psi!! I reckon grip goes up as pressure goes up for an awful lot more than they would have us believe. And of course tire life is extended dramatically. Ok, rant over. Now i want DATA from somebody before i go crazy!! I'm "tired" of this argument. It will stop when i get my data. I promise. But if anybody brings it up after that, i'll give them a serious dose of "copy and paste" Ok, rant over this time. Sorry, i'm sleep deprived, need to go to bed. I work nights.....zzzzzzzz....zzzzzz

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Old 05-27-2008, 05:50 AM   #37 (permalink)
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105,000km at 44psi. Actually i only went up from 44 to 50psi a month ago.
No uneven wear and hit the odd pothole too. No tire damage. So, as far as i'm concerned, the only argument left here is braking distance. Does anyone here have datalogging equipment? Microwave ground speed sensors? Thats what we need to finish this argument off. Personally i've braked hard at 50psi and noticed no difference, wet or dry. But personal feeling doesn't count does it!! Later lads.

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Old 05-27-2008, 08:21 AM   #38 (permalink)
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Sorry, guys. Holiday weekend.

Plus I didn't know where to start - but I certainly don't think the "Introduction" forum was the place. I'm glad - and surprised - that this generated such a lively discussion.

But I suggest we move it to some place more appropriate. How about this:

http://www.ecomodder.com/forum/showt...8793#post28793
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Old 08-09-2008, 08:43 PM   #39 (permalink)
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Quote:
Originally Posted by vtec-e View Post

105,000km at 44psi. Actually i only went up from 44 to 50psi a month ago.
No uneven wear and hit the odd pothole too. No tire damage. So, as far as i'm concerned, the only argument left here is braking distance. Does anyone here have datalogging equipment? Microwave ground speed sensors? Thats what we need to finish this argument off. Personally i've braked hard at 50psi and noticed no difference, wet or dry. But personal feeling doesn't count does it!! Later lads.

ollie
Great pic, Ollie!
Those "wear in the middle with anything over sidewall recommended P." people just can't see for looking. There is a (steel?) belt in todays radial tires...it wraps around the circumference of the carcass, very much like a tank tread. Higher P. won't bulge through this belt. Bias plies, yes ; radial plies, no. Bias ply tires are gone (except for some trailers and specialty tires).Good riddance!
There has not been a word about "tread squirm" on these posts. This "squirm" is the wear mechanism for the tire tread; as the tread squirms it acts like a pencil eraser...wearing away against the pavement and heating the tread.. IMSA race cars had their street tire ( rules) treads ground half-way down...to reduce this additional mass from pulling the tire apart at speed and to give the tire better lateral response.
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Old 08-10-2008, 07:55 AM   #40 (permalink)
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Whitevette,

As I indicated in the post right above yours, this is the "Introduction" forum, and not the place to have a technical discussion - so we've moved this discussion to:

Edit: The link doesn't take you to the followup thread. Here is a new link:


http://ecomodder.com/forum/showthrea...ires-2582.html

Nevertheless, your post has a couple of fallacies in it, so I think you would benefit from the discussion.

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