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Unread 11-14-2013, 02:23 PM   #1
wildkardboyz
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I bought mastercraft 33x12.5x15 with 15x8 steel wheels. I was wondering what is a good tire pressure to run these tires at on the trails. I dont want to pop the beads off either.Thanks in advance also sorry for the miss spelling.

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Unread 11-14-2013, 07:12 PM   #2
apaTJ
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I run my 33x12.50 mt/rs at around 25 pounds on the road. You want the tread surface to be flat when in contact with the ground, and if it's too high it will arc up and the middle will wear faster, too low and the outer surface will wear faster. You can perform a test with chalk to see the contact of the tread pattern
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Unread 11-15-2013, 07:39 AM   #3
idiot magnet
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What type of trails? Rocks or mud?
What rated side wall? Makes the wall stiffer or softer. Jeeps are best with a C wall due to their light weight.
I run in the 8-10 psi range with my 33's without ever popping a bead. I can run lower and would feel comfortable doing it. Once you get in the very low psi's (2-6 psi) you should run beadlocks. You also have the chance of spinning your tires on the rims with low psi which will mess up your tire balancing. Another reason for beadlocks.
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Unread 11-15-2013, 07:44 AM   #4
hustler905
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I'd say 8-10 psi as well. Exceptionally muddy ruts may call for slightly higher pressure because I find they tend to push on the sidewalls a bit more.
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Unread 11-16-2013, 05:40 AM   #5
wilson1010
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Sand and mud de-bead tires.

To test this out, simply drop your pressure to 8 psi, find a nice sand hill or mud hill with actual clay based mud and start down the hill, turn your wheels sharply to the right and stand on the brakes. When the tire digs into the mud and it pushes a force of a little more than 8 pounds per square inch of sidewall under the mud, off comes the bead. Its physics. You can't avoid it.

Then you can practice on re-beading the muddy tire on a hillside.

I have re-beaded tires for a dozen guys who lost the bead just like described above and who didn't have a clue as to how to get the tire re-beaded.

The benefits of 8 psi vs. 15 psi (where they won't come off the bead) are ephemeral to say the most.
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Unread 11-16-2013, 06:04 AM   #6
billybooster2
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Quote:
Originally Posted by wilson1010
Sand and mud de-bead tires. To test this out, simply drop your pressure to 8 psi, find a nice sand hill or mud hill with actual clay based mud and start down the hill, turn your wheels sharply to the right and stand on the brakes. When the tire digs into the mud and it pushes a force of a little more than 8 pounds per square inch of sidewall under the mud, off comes the bead. Its physics. You can't avoid it. Then you can practice on re-beading the muddy tire on a hillside. I have re-beaded tires for a dozen guys who lost the bead just like described above and who didn't have a clue as to how to get the tire re-beaded. The benefits of 8 psi vs. 15 psi (where they won't come off the bead) are ephemeral to say the most.
Starter fluid and a lighter?
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Unread 11-16-2013, 06:36 AM   #7
wildkardboyz
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Yep starting fluid and a lighter
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Unread 11-16-2013, 08:13 AM   #8
wilson1010
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Quote:
Originally Posted by billybooster2 View Post
Starter fluid and a lighter?
Starter fluid, a lighter, and a paper taper rolled up.

Clean the mud off the tire, take all the weight off it with a jack or winch, spray a little (just a puff) of ether based starter fluid, light the taper and stick it in the tire between bead and rim and keep your fingers outta there.

If it fails of slips off the bead before you can air it up, you have to blow out the carbon monoxide from inside the tire or the starter fluid will not light the second time.
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Unread 11-16-2013, 06:19 PM   #9
fastfreddie
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You can approach the question somewhat scientifically to make yourself feel better about it but I don't know that the real world will agree with the answer.
I think I read it on here somewhere...

"Max load on tire divided by max load rating of tire multiplied by max psi."

If your wheel puts 1000 lbs to the ground and the tire is rated for 2000 lbs with a max psi of 50...1000/2000x50=25psi
Consider a tire with a heavier rating but the same max pressure...1000/3200x50=15psi

Max ratings should be on the sidewalls.
The weight of your Jeep plus passenger, cargo, gas, etc., would be determined by yourself, then divided by four.

Based on this equation and the load I would anticipate if I owned a Wrangler with Goodyears, I'd have no worries about airing down a 34"(315/70) Duratrac to 16psi.

I don't think it would be necessary for moderate trails but it couldn't hurt to help keep from tearing up the countryside when "treading lightly".
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Unread 11-16-2013, 08:13 PM   #10
wilson1010
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Originally Posted by fastfreddie View Post
You can approach the question somewhat scientifically to make yourself feel better about it but I don't know that the real world will agree with the answer.
I think I read it on here somewhere...

"Max load on tire divided by max load rating of tire multiplied by max psi."

If your wheel puts 1000 lbs to the ground and the tire is rated for 2000 lbs with a max psi of 50...1000/2000x50=25psi
Consider a tire with a heavier rating but the same max pressure...1000/3200x50=15psi

Max ratings should be on the sidewalls.
The weight of your Jeep plus passenger, cargo, gas, etc., would be determined by yourself, then divided by four.

Based on this equation and the load I would anticipate if I owned a Wrangler with Goodyears, I'd have no worries about airing down a 34"(315/70) Duratrac to 16psi.

I don't think it would be necessary for moderate trails but it couldn't hurt to help keep from tearing up the countryside when "treading lightly".
OK, I don't want to get something started here. No one likes to see an argument about physics, but . . . .

The pressure in the tire, be it 15 psi or 8 psi is all that holds the tire on the rim. Oh, there is a little friction between the bead and the rim, but that is inconsequential. Most beads break on their own at 0 psi.

So, it makes absolutely no difference if a tire is rated for 50 psi or 80 psi. If there is 8 psi in the tire and the sidewall presents itself to a muddy hill and there are 40 square inches of sidewall being compressed, it will take exactly 320 pounds of force to de-bead the tire.

Maybe we are saying the same thing.
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Unread 11-17-2013, 03:19 AM   #11
fastfreddie
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Not really. Tire construction has a lot to do with how well it stays in place. Jack up a tire with a stiff carcass and let all the air out. Now take that tire off the bead with your hands, if you can. A soft carcass, yeah, you might do it.

As to your math, it makes as much sense as mine, I guess.
The variable is the tire's rating to hold a load. My math takes that into account. The heavier built tire can get away with less pressure before being unseated.
That's all I was suggesting. Given enough side force, a fully inflated tire can have its bead unseated.

If we were to argue physics, I would say, yeah, 320 lbs sounds right, but that 320 spread over 40sq" being 8psi...well, I'd like to see 8psi knock a tire off its bead.
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Unread 11-17-2013, 05:24 AM   #12
wilson1010
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Quote:
Originally Posted by fastfreddie View Post
Not really. Tire construction has a lot to do with how well it stays in place. Jack up a tire with a stiff carcass and let all the air out. Now take that tire off the bead with your hands, if you can. A soft carcass, yeah, you might do it.

As to your math, it makes as much sense as mine, I guess.
The variable is the tire's rating to hold a load. My math takes that into account. The heavier built tire can get away with less pressure before being unseated.
That's all I was suggesting. Given enough side force, a fully inflated tire can have its bead unseated.

If we were to argue physics, I would say, yeah, 320 lbs sounds right, but that 320 spread over 40sq" being 8psi...well, I'd like to see 8psi knock a tire off its bead.
Consider "dunnage bags."

They put a kraft paper bag, sometimes coated with a thin layer of plastic, in the vertical space between two pallets of drums (weighing a ton each pallet) in a semi trailer to keep them from moving and banging into each other. Internal pressure= 3psi. Its all about the area. a 4x8' dunnage bag supports 13,000 pounds at 3psi.

As far as the stiffness of the sidewall affecting the force required to de-bead a tire, the force being applied to the sidewall is a lateral force, perpendicular to the sidewall, trying to overcome only the friction between the bead and rim. If anything, a stiffer sidewall requires less force to de-bead because more of the force is transmitted to the bead itself instead of warping the sidewall.

I changed a lot of tires back in the day when we used a manual tire machine, With a spindle you laid the tire and wheel on, a hand tool that locked into the spindle with a levered handle you pushed down to de-bead the tire. Same as now only no pneumatic cylinder assisting. A tire with no Shrader valve could sometimes be de-beaded with the palm of the hand, sometimes a bit of pressure on the tool was required. Mostly it depended on how long the tire had been on the rim. The steel cords inside the bead itself apply pressure to the rim which increases the friction needed to be overcome de-bead the tire. But, the sidewall doesn't really enter into it.

A couple of years ago, I had to re-bead a tire for a guy twice at Haspin Acres only to find that he had used soapy water to help him get his tires around the rim. Note to self - no soapy water to mount off road tires.
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Unread 11-17-2013, 07:31 AM   #13
fastfreddie
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I understand a sidewall's make-up isn't the reason a bead "breaks" but it's been my experience the softer carcasses break easier. We may not be having an apples-to-apples conversation about it tho'. On the trail, with air in the tires, the softer carcass allows more deflection giving the bead a smaller area to have a force exerted upon it. The harder carcass spreads that same force over a wider area diminishing its ability to unseat the bead.

With no air in the tire, yeah, more leverage can be applied with a stiffer carcass to break a bead. I don't think it makes 'em any easier, you just don't have to lever them as far.
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Unread 11-17-2013, 08:10 AM   #14
wildkardboyz
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Lol ok ok I didnt asked for a hypothesis and theorys of the universe's capacity and stuff like that. I just ask what would be a good trail tire pressure to start with. lol. I love you guys but way to in depth. I could of skip college and learned everything here. Not knocking the argument just knocking the depth.
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Unread 11-17-2013, 08:40 AM   #15
wilson1010
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Originally Posted by wildkardboyz View Post
Lol ok ok I didnt asked for a hypothesis and theorys of the universe's capacity and stuff like that. I just ask what would be a good trail tire pressure to start with. lol. I love you guys but way to in depth. I could of skip college and learned everything here. Not knocking the argument just knocking the depth.
Relax. You got your answer a dozen posts ago. 15psi, if you don't want your tires de-beaded on a muddy hill, 8psi of you don't care. We have just been having fun since then. Physics is fun? Who knew.

In the final analysis, you can only benefit so much from putting more tire area on the dirt. You get double the area of a normal tire at 15psi. Isn't that enough?
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