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Old 03-06-2011, 04:41 PM   #1 (permalink)
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tire pressure, when too much?

I know everyone here is happy to run their tires at the max number on the sidewall, and I guess for most vehicles it's not a problem as it is not significantly over the recommended pressure anyways.

I had a jeep wrangler with 31x10.5 bfgoodrich allterrain tires, the jeep called for 23 psi (stock tire size) and the tire could take 80psi max. obviously, that much air in a tire without a full load of weight pressing on it is going to ballon out, where your contac patch might be minimal. i had a chevrolet tracker with 33x12.5 mud tires on it, at 32 psi there was only 2 inches of contac in the middle of the tread. i drove over wet pavement and there was only 2 inches of wetness in the middle of the treads, so I know overpressuring tires has the opposite effect of low air pressure, where the middle of the treads will not ocntact the road and the tires wear out just on the outside.

I'm looking at a 2000 grand cherokee thats shoould remain stock, and my mom has a 2001 Gmc safari that already runs at 36psi all around. My safety is not worth saving 2 mpg, so how do you know if it's too much? is there a scientific way to tell because no two tires are ecer the same

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Old 03-06-2011, 05:20 PM   #2 (permalink)
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You could do the "dust" or "chalk" test as described elsewhere here.
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Old 03-07-2011, 11:36 AM   #3 (permalink)
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Quote:
Originally Posted by bandit86 View Post
I know everyone here is happy to run their tires at the max number on the sidewall, and I guess for most vehicles it's not a problem as it is not significantly over the recommended pressure anyways.

I had a jeep wrangler with 31x10.5 bfgoodrich allterrain tires, the jeep called for 23 psi (stock tire size) and the tire could take 80psi max. obviously, that much air in a tire without a full load of weight pressing on it is going to ballon out, where your contac patch might be minimal. i had a chevrolet tracker with 33x12.5 mud tires on it, at 32 psi there was only 2 inches of contac in the middle of the tread. i drove over wet pavement and there was only 2 inches of wetness in the middle of the treads, so I know overpressuring tires has the opposite effect of low air pressure, where the middle of the treads will not ocntact the road and the tires wear out just on the outside.

I'm looking at a 2000 grand cherokee thats shoould remain stock, and my mom has a 2001 Gmc safari that already runs at 36psi all around. My safety is not worth saving 2 mpg, so how do you know if it's too much? is there a scientific way to tell because no two tires are ecer the same
First, I've looked up Jeep Wranglers in a book called Tire Guides, that summarizes the vehicle tire placards for every vehicle sold in the US - and while you are in Canada, there shouldn't be much difference.

I can NOT find anything that lists 23 psi as the pressure listed on the vehicle tire placard, so let's do 3 things:

1) Find the vehicle tire placard on your Jeep. What tire size and pressure are listed there.

2) Check your tires again because I think the tire will say 50 psi max!

3) Tell us the year make and model of your Jeep and I'll tell you what Tire Guides says In theory, the placard and the book should match.

Now for a bit of theory: The size of the contact patch is a function of the load, the inflation pressure, and the tire characteristics.

No, you can NOT calculate the area of the footprint using the pressure and the load. You have to measure it. The area of the footprint is NOT linearly related to the inflation pressure, nor is it linearly related to load. It is also not the same if you change tire size and use the same inflation pressure.

The vehicle tire placard's size and inflation pressure will result in pretty much the same size footprint regardless of the manufacturer of the tire. Obviously increasing the inflation pressure reduces the size of the footprint and that has implications for traction.

Increased inflation pressure also increases the spring rate of the tire - and that has implications for the suspension following irregularities in the road surface. In extreme cases, the tire will lose contact with bumpy pavement.

In the case of your Jeep, either you have the max inylayion pressure wrong or you are not using the same type of tire that came stock. You have a Load Range E - and it's an LT metric. LT metric tires have the letters "LT" in front of the tire size.

Your Jeep either came with P metric or Flotation tires. P metric tires start with the letter "P", and Flotation tires have a configuration like this: 31X10.50R15LT.

So post back with the info I requested and we'll sort this out.

Oh, and using the "chalk method" is unreliable as many tires will have full width footprints regardless of the load and the inflation pressure.
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Old 03-07-2011, 11:57 AM   #4 (permalink)
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my bad, it was 33 psi recommended, and the tire was aftermarket bfgoodrich A/T, Lt for sure, 31x10.5 equalient, same size as the stock size Goodyear Mt/r but i dont remember the exact number. i rememebr seeing that the max pressure was astrononimical
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Old 03-07-2011, 05:03 PM   #5 (permalink)
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Quote:
Oh, and using the "chalk method" is unreliable as many tires will have full width footprints regardless of the load and the inflation pressure.
Interesting... it sure seemed to work on my Coupe's tires; came up with about a 5 psi differential front to rear too which makes perfect sense considering the forward weight bias.

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