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Unread 07-19-2013, 03:20 PM   #1
rambo3489
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Let's talk tire pressure

Alright guys,

Long story short I caused a lot of trouble saying I ran my MTR/Ks at 14 psi. I just got them and that's what the chalk test said to run them at. Gave me full tread contact across the tires. I was then informed the chalk test is not the best tool to use with big tires on proportionally small wheels. OK. So I ran my tires at the PSI range that was suggested to me.

Well I just got back from a little light wheeling got some mud on the tires and made an interesting observation. The tires are only contacting on the center as I feared form the start. See:



There is clearly still mud on the outer lugs which were untouched by the asphalt.

I am not trying to stir up the pot here. I am looking for an informative discussion as to why this type of wear will work out in the long haul. So let's keep it on track here.

Now I have noticed that the outer lugs get beat up quite a bit more than the rest of the tire. Does that have something to do with it?

Thanks

Tyler

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Unread 07-21-2013, 10:12 PM   #2
Charley3
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Chalk test, or classic formula for calculating pressure, works good when tires are 75 aspect ratio (or at least 70 to 80 aspect ratio) and the tire's section width is approx 2" to 2.5" wider than the wheel width.

This is one reason I like my tire/wheel size relationship to conform to what I said in first paragraph. For example, 225/75R15 on 15 x 7, 235/75R15 on 15 x 7, 30x9.5R15 on 15 x 7, 31x10.5R15 on 15 x 8, 245/75R16C (31x9.5R16C) on 16 x 7, or 265/75R16C (32x10.5R16C) on 16 x 8. There are larger examples too. All these have approx 75 aspect ratio and a tire section width 2" to 2.5" wider than wheel width. This works really good on and off road and gives a flat contact patch at street pressure. This is approx the formula used by engineers who set up stock SUVs, at least back in the day when SUVs were still made to go off road.

If you get outside that relationship, like a wide tire on a narrow wheel, or a tall skinny tire with aspect ratio larger than 80, you can't get a flat contact patch at a reasonable street pressure. So then you have to always air down for off road, then air back up for on road, and your on road pressure will not give a flat contact patch (until center treads wear down).

Guys set up for hard core rock crawling like to use wide tires on narrow wheels so they can use very low tire pressures without losing a bead. For other off road terrains it works fine if you air down. The wide tire, narrow wheel, combination is not ideal on road (because you can't get a flat contact patch at a decent street tire pressure.

If you had a traditional tire/wheel relationship (like I prefer), you wouldn't be setup for rock crawling, but you'd be set up fine for all other off road terrains and setup ideally for on road. Also, you wouldn't need to air down for off road, unless conditions were extreme. For most off road conditions I don't need to air down. My contact patch is flat at street pressure.

Since your tire/wheel relationship is non traditional, I assume you have a rock crawler set up (wide tire, narrow wheel). You won't be able to get a flat contact patch at a reasonable street pressure. You'll have to find the best compromise psi. When your center treads eventually wear down, you'll finally have a flat contact patch. It sucks, but that's what happens when you put a wide tire on a narrow wheel.

Other fellows with a wide tire, narrow wheel setup can help you figure out the best possible compromise tire pressure for your tire/wheel combo. I suggest follow their suggestions, and realize any psi you run is a compromise (when wide tire, narrow wheel).

Tires with aspect ratio larger than 75 (tall skinny tires on small diameter wheels) have similar contact patch problems at street presssures, especially when aspect ratio is larger than 80. An 85 aspect ratio tire like a 33 x 10.5 R15 or 255/85R16 (33 x 10 R16) will have similar contact patch problems at street pressures.

This is why I prefer a 75 aspect ratio tire that is 2" to 2.5" wider than wheel. It works good and has no contact patch issues. This set up corners good, gets decent gas mileage, and has good traction on mild to moderate off road conditions without needing to air down. For extreme off road conditions, air them down a little.
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Unread 07-21-2013, 11:18 PM   #3
Charley3
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That probably wasn't the answer you were looking for, but it's reality.

You're just going to have to use the pressure(s) recommended by guys with a similar setup to what you have, and realize there is no perfect psi for your setup, and you aren't going to get a flat contact patch at street pressure (until center treads eventually wear down).

That is the nature of your tire/wheel width combination.
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Unread 07-22-2013, 06:59 AM   #4
rambo3489
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Thank you. That's exactly what I was looking for. Great explanation.

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Unread 07-22-2013, 07:21 AM   #5
WXman
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Another thing about the "chalk test" is that it ASSUMES that the tire is flat. And you know what they say about assuming....

There are lots of tires that are not built with a perfectly flat tread surface. One that really sticks into my mind is the General Grabber AT2. The tread contact patch is purposefully designed with a little bit of a crown. You'd have to run really low pressures to get all the tread blocks to touch the pavement equally. And low pressure generates heat and heat is what causes tire failures.

I usually just go by what percentage of the tire's capacity I'm using. If my 4 tires together can hold 10,000 lbs. of weight, and my vehicle only weighs 5,000 lbs... then I'm only using 50% of the tires' capacity. So, if the max PSI is 50 I'd run them at 25.
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Unread 07-23-2013, 11:56 AM   #6
Charley3
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I too base my pressure on tire capacity and vehicle weight formula you describe.

It gives a good street pressure AND a flat contact patch if you tire has 75 aspect ratio and a section width 2.5" wider than wheel width. With those tire/wheel relationship specs the formula and the chalk test are in agreement.

When your tire/wheel relationship specs are outside those parameters, the formula and the chalke test disagree. i.e.- you can't get a flat contact patch at the reasonable street the formula indicates. So then you must use formula to figure out a reasonable street pressure, and use that psi, even though contact pacth isn't flat.

---

I had General Grabber AT2 30 x 9.5 R15 on 15 x 7. It had a flat patch at 27 psi front and 25 psi rear.

It had a very flat patch at street pressure.

However, that was because it was a 75 aspect ratio tire with section width 2.5" wider than wheel width.

If Wxman had General Grabber AT2 that didn't want to sit with a flat patch, or crowned as gou called it, it was probably cause by aspect ratio or by wide tire on narrow wheel.
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Unread 07-23-2013, 12:01 PM   #7
Charley3
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For another example:

I had 30 x 9.5 R15 BFG AT on 15 x 7 wheels and a flat contact patch at street pressure.

I had 33 x 10.5 R15 (86 aspect ratio) on 15 x 8 wheels and could NOT get a flat contact patch at street pressure.

I had 33 x 11.5 R16 on 16 x 8 and could NOT get a flat contact patch at street pressure.
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Unread 07-23-2013, 12:37 PM   #8
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The sidewall of tires, even heavy duty truck tires does not support anything. It only maintains the profile of the tire filled with air. Just have a look at any flat tire and you will see a sidewall completely collapsed to the rim, often folded back and forth under the weight of the rig. Pressure inside the tire is the only thing supporting weight.

However, the rule of taking max load and max pressure proportionate to the actual weight of the rig and actual pressure, is not too bad of an approach. It will get you a tire patch that is more like the one the manufacturer had in mind when they rated the tire.

Still, at the end of the day, the tire patch size is always equal to the weight on the tire divided by the inside tire pressure. 1000 pounds on each tire. 25psi. 40 square inches of tire patch.

PS: Every time I have this argument about sidewall support, I point out that when you jack up a car with a flat tire, the sidewall does not spring back. It just raises slowly off the ground, still flat as a pancake.
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Unread 07-23-2013, 12:58 PM   #9
Charley3
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OP, if you doubt what I said vs what Wilson said, go ask a tire store.

Or try the different aspect ratios and tire/wheel width relationships yourself. That's what I did.
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Unread 07-23-2013, 04:39 PM   #10
wilson1010
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Quote:
Originally Posted by Charley3 View Post
OP, if you doubt what I said vs what Wilson said, go ask a tire store.

Or try the different aspect ratios and tire/wheel width relationships yourself. That's what I did.
Yes, Charley, great idea. Most tire stores employ physicists and mechanical engineers at the tire counter. No, wait a minute, tire stores employ racing consultants and tire designers. No, wait a minute, the dweebs that stand behind the counter in most tire stores are usually not smart enough to find the mens room on their own.
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Unread 07-23-2013, 06:09 PM   #11
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True, most tire shop employees know less about tires than we do.

And as far as my Grabber AT2s go.. they were 255/70-16 on a factory 16x7 wheel. Perfect combination in terms of width matching. Those tires are just built with a crown in them. They aren't meant to be perfectly flat. So a chalk test would have been a waste of time.
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Unread 07-23-2013, 07:33 PM   #12
rambo3489
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Thank you guys. Great discussion. Hopefully someone will use the information here to their benefit. I know i have.

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Unread 07-23-2013, 10:08 PM   #13
Charley3
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Quote:
Originally Posted by WXman View Post
True, most tire shop employees know less about tires than we do.

And as far as my Grabber AT2s go.. they were 255/70-16 on a factory 16x7 wheel. Perfect combination in terms of width matching. Those tires are just built with a crown in them. They aren't meant to be perfectly flat. So a chalk test would have been a waste of time.
Huh. OK. I believe you. Though that wasn't my experience with that tire in 30 x 9.5 R15.

I don't know why we had different experiences with that tire.
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Unread 07-23-2013, 10:14 PM   #14
Charley3
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Wilson is correct that sidewalls don't support much weight. It's the air pressure that supports most of the weight.

However, sidewalls do affect shape and foot print. The sidewall (section width) relationship to wheel width is key to that.

When a wide tire on narrow wheel, the bead is pulling the sidewalls in toward wheel, the sidewalls are pulling up the edges of the tread, which crowns the tread, which results in a non-flat contact patch at street pressure.

Why exactly a crowned tread results with a tall skinny overly high aspect ratio tire on a small diameter wheel, I'm not sure. I think it's because the sidewalls are so long/tall that they curve/bulge to much, which either pulls up the edges of the tread, or fails to hold it down flat. I'm not exactly sure why an 80, and especially an 85 aspect ratio tire on a small diameter, narrow wheel doesn't give a flat contact patch at a reasonably high street pressure, but it doesn't (unless you use a wider wheel relative to tire width).
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Unread 07-31-2013, 07:01 AM   #15
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If you read what the tire manufacturers say about this (Yokohama and Nitto) the don't do much concern themselves with how flat the contact patch is, but rather how well the tire supports the weight and forces put on it at various inflation pressures. They studied using LT rated tires in place of OE P rated tires. What they found was that if you put LT rated tires on a vehicle specified originally with P rated tires and don't increase inflation pressures, the tires start to run very hot at higher speeds. This is due to the tire carcass flexing from side to side. That eventually causes failure. I run 235/75-15 LT General Grabber AT 2s on my ZJ and run them at higher than stock pressure( 38-40) . The run cool when checked with an IR temp gun and are wearing evenly. The chalk test cannot tell you this.

Two things:
An LT tire of similar size compared to a P tire requires more air pressure to support the same load. You will see this if you look at their tire charts.
Tire temperature is very important. If you are trying different pressured get an IR temp gun and leave it in the Jeep. Check pressure after driving for a while at highway speeds. If the temp of the tread/sidewall exceeds 150*F you are to low in pressure no matter what the chalk test says and eventually you could have a catastrophic tread separation. In the boating world we see this all the time with trailer tires. The better tire pressure monitoring systems (aftermarket) monitor both pressure and temperature.
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