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Quick Question on tire pressure

Senior_XJ

NAXJA Forum User
Location
NY
Im running BRAND new,, NEVER wheeled set of 35x12.5x15 baja claw radials & the guys that mounted & balanced them put 35 psi in. I kinda thought that was too high, too stiff, so i dropped it a few psi to 33ish.
I just checked them earlier today & she was sittin for close to an hour when i took the readings. They were all spot on 30 psi.

So should i keep em at 30 since the psi builds up as they heat up,, BUT this XJ doesnt see alotta highway speeds NOR is any one way trip over 30 minutes & thats MAX!

SO,, should i stick with a cold reading of 30 or bump it up to 32, 33, 34 psi????
 
The tire company reps at the race track shoot for a 10% increase in pressure from cold to hot lap (in the sportbike world). If your 30psi cold turns into to 33psi after you drive it, I'd say you are there.
 
30psi in 35s is going to be rough, and you won't have as good traction, my 32x11.5 BFG MTs were scary in the rain at 30psi. They're almost like stock tires at 25psi though - I'd try 25 or even 22 or so for 35s.
 
My tire guy said to always start with the stock factory pressures and work from there.

When running stock tires, that is a good starting point.

With large aftermarket tire sizes, the factory has no idea what they are going to be mounted on.

Bit of a weight difference on say a lightly modified XJ and a 1/2 ton pickup.

When you don't have a starting point you need to test for the right pressure. The chalk test will give you the right pressure for the tires on a particular vehicle. The result you get on an XJ wouldn't be the same if you put the same tires on a larger or for that matter smaller vehicle (Tracker).
 
Ok,, im gonna address some replies on here....

-- Joe_Peters, *quote: "With large aftermarket tire sizes, the factory has no idea what they are going to be mounted on!!" GOOD REPLY & VERY TRUE!!

-- KASTAN, My 35" claws wont break traction 4 squat! (I'll give ya an ex.) FIRST i tried a break stand 4 the VERY FIRST TIME in my XJ & axle wrap started b4 the tires broke traction!!(wont do that again!) Second is takin a 90% turn in the rain with a HEAVY PEDAL & HIGHER RPM's,, done several times & STILL didnt break traction!!

-- TRIPLEDUCK, you sound like im thinking! BUT w/most trips being 20 min & little highway speeds,, is that enuff to heat them up ESPECIALLY in the dead of winter??

Thanks Guys, its deffinately helping!
 
I fill mine til I think the footprint is good. and the ride is good. I think it's somewhere between 15 and 20 psi with the 31's, not sure about the 32's I had, it was a while ago.
 
Not bad! I'd probably still worry about uneven tread wear, but if you're cool with it, :thumbup:

As for tire temps - I'd give it a try and see what the results are. If you don't like em, change as required! Assuming the volume changes relatively little (a pretty decent assumption for the pressure range we're talking about), the ideal gas law indicates that pressure and temperature (ABSOLUTE temp, as in Kelvin, not Celsius or Fahrenheit) are inversely related, so given say a 50K difference in temp from room temp to 75C (348K) the pressure will increase by about 17%, so from 30psi you will end up at approx 35psi operating pressure. Measure the pressure, then go for a drive and measure the pressure afterwards, that'll give you a pretty good approximation of the temp increase.

Equation (making the assumption that tire volume stays the same):
Final Pressure = Starting Pressure * (Final Temperature + 460) / (Starting Temperature + 460)
Final Temperature = Final Pressure * (Starting Temperature + 460) / Starting Pressure - 460
As for measuring starting temp, look at the thermometer. All temps for that equation should be in °F.
 
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Well I do look at tire wear and inflate or deflate as needed, I'm not ignorant to that issue. But with as little as I drive mine the wear is fine.
 
Well Said!! I understand all that u say cause i ran 33" baja claws BIAS 6 PLY's for nearly a year..... they saw some highway to get to the trail head & stuff. Took a while but i finally settled at 27 psi for on road use with NO drive going over 30 minutes! ((i was very picky)). When i sold em they still looked new!! BUT as everyone said 2 me,, every trail i hit it became harder & harder to balance them out even with the airsoft pellet idea.

My XJ wont be seeing any trails for maybe a full year,, so thats why i switched to there radial baja claw!! I believe i may run them at a precise 31 to 32 max. This is for 20 min tripps with top speeds of 45mph at most. She doesnt see much highway but if im gonna go 60mph for longer the a half,, then i'll let a few pounds out,, & refill when back home!

THANKS GUYS!!


Not bad! I'd probably still worry about uneven tread wear, but if you're cool with it, :thumbup:

As for tire temps - I'd give it a try and see what the results are. If you don't like em, change as required! Assuming the volume changes relatively little (a pretty decent assumption for the pressure range we're talking about), the ideal gas law indicates that pressure and temperature (ABSOLUTE temp, as in Kelvin, not Celsius or Fahrenheit) are inversely related, so given say a 50K difference in temp from room temp to 75C (348K) the pressure will increase by about 17%, so from 30psi you will end up at approx 35psi operating pressure. Measure the pressure, then go for a drive and measure the pressure afterwards, that'll give you a pretty good approximation of the temp increase.

Equation (making the assumption that tire volume stays the same):
As for measuring starting temp, look at the thermometer. All temps for that equation should be in °F.
 
For 35" tires, I would probably start at 26psi front, 24 rear and adjust from there.
 
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