tire pressure question

[surfbum]

Just a quick question. Just had an issue with the 35" BFG M/T tires developing "torque cracks" where the sidewall meets the tread on my 1 ton p-u. Apparently I was running the pressure too low. My question is; what pressure should I be running in my new 33 x 12.5 x 15 Dunlop Mud Rovers on my '79 CJ7 on the street. Obviously I know the Jeep is a lot lighter than my 1 ton, but after seeing what happened to my 35's, I don't want to have the same problem on my Jeep. Thanks all

 

[Old Dog]

I run 20# in my 33x12.5 MTZ's. They are 5 years old now with no problems.

 

[surfbum]

Thanks Old Dog, I never even heard of "torque cracks" until yesterday, but after my buddy at the tire shop told me about them, I went on the web and looked it up. It seems that BFG has had an issue with it for awhile. When running their tires with too low air pressure, the tires develop cracks between the small lugs where the sidewall meets the tread and after awhile, the mud,sand, and road salt(here in Jersey) wick their way into the tire and cause slow leaks. Since they crack in different places around the tire; by the time you notice the tire leaking, it is too late . Luckily my bro at the tire shop had a couple BFG M/T take offs that I was able to pick up. Didn't like the price of 4 new 35's. Thanks again for your input.

 

[dalefan88]

i run 20 in mine to.

 

[CJ]

Right around 19 in my 33's...

 

[surfbum]

Thank you both too !! Heading out to check them now.

 

[BusaDave9]

It's hard to say an exact air pressure. It depends on the weight of your jeep. Those 35 inch tires can hold up a very large truck.
Here is how you decide on the pressure. Use chalk to make a line from the inner edge of the tire to the outer edge of the tire. Drive 100 feet. If the line is rubbed off the edges of the tire first then your pressure is too low. If the chalk gets rubbed off the middle of the tire before the edges then the pressure is too high. Do this a few times until the chalk gets worn off your tire evenly. That's the correct pressure for your weight of vehicle.

 

[BajaEdition]

well Busa beat me to it, that was going be my answer.

 

[CJ]

well Busa beat me to it, that was going be my answer.

And that is how I got 29 on my 33 Pro Comps... Best way!

 

[surfbum]

Thanks for all the input. I'll give that a try when I get home today... unless its already raining, in which case I'm thinking the chalk will wear off in about ...... ohhhh 1 rotation. lol

 

[1960WillysCJ5]

What does the tire manufacturer say the pressure should be?
That's what I would go with.

 

[Sasquach]

What does the tire manufacturer say the pressure should be?
That's what I would go with.

They give max. psi for a max rated load. Your Jeep is no where near the tires max load rating so therefore no need to run max pressure.

 

[1960WillysCJ5]

Not the stuff on the sidewall, the stuff they tell you when you buy them and install them.

I personally like to run 32 psi in mine.

 

[sapulpa cj]

i am running 33-14.50-15 swampers on my jeep and after putting a line across them came up with 18 in the rear and 20 in the front.


drives fine and you do not even notice the low air pressure running around town.

 

[BusaDave9]

When I had a local tire shop mount my 35 inch BFG tires they put so much air in the tires I could put my finger under the outer edge of my tire. The tire was running on the center of the tread.
A passenger car tire can run at 32 psi because most passenger cars weigh roughly the same, or at least compared to trucks. How much does a truck weigh? 3000 pounds or 13,000 pounds? They vary widely.
Big tires are made to handle very big trucks. If you buy a 35 inch or larger tire, you will find a load capacity that is so high that one tire could hold up an entire CJ. The manufacturer always states the load capacity at a certain pressure. Here are some examples:
BFGoodrich 35x12.50R16.5 has 3415 lb capacity at 50 psi http://www.bfgoodrichtires.com/assets/pdf/all_terrain_ta_kd.pdf
PitBull Rocker 39.5x16.50-17LT has 4330 lb capacity at 45 PSI http://pitbulltires.com/downloads/PBSPEC2.pdf
If you dont have that much weight on the tires you will run much lower pressure. The test of putting masking tape or chalk marks on a tire can help you find the correct pressure for your tires mounted to your vehicle.
This holds true only up to a point. If you have huge tires rated for a big truck and install them on a CJ you may not get much sidewall flex even at 15 psi. For that reason you may want to avoid the larger wheel sizes for off road. A 38 inch tire made for a 17 inch rim will be designed for such a large truck you may not get a good contact patch until you go down to single digits with the air pressure.

 

[Sasquach]

Nice explanataion Busa!