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Discussion Starter · #1 ·
Last night I made up a tire gauge / airdown adapter, with a 30 psi industrial gauge, a valve, and a tire chuck. Went out into the driveway to try it out, ans surprise--- it doesn't agree with my "stick" tire gauge (a good quality professional one, not a cheapie) at all. Now I don't expect super accuracy at under 20 psi from a gauge that reads to 160, but I decided to check it, as the gague on the new setup was an old used leftover thing.

At work I borrowed a calibrated (0.25% accuracy gague) and hooked it up. Surprise, surprise! the "new" used gauge was OK and the stick gauge was off-- when the stick gauge read 22 psi I was actually at 16. I couldn't test much higher pressures as I didn't have fittings for the compressor, maybe the stick gauge is OK at higher pressures..... but not at the pressures I run! A real cheapie ($.25 in the closeout bin) was actually better than the expensive one, but still not good.

Anyway, the point is, don't believe gauge readings at the low end of the gauge's scale. If you want accurate readings under 20psi the gauge's max reading should be no more than 30 psi (careful, you could destroy it you run higher pressures on the street than the gauge's max reading!) If you use your own gauge all the time and know where it indicates (correctly or otherwise, as long as it's consistent) for the pressures you find work, fine, but don't expect the same readings with a borrowed gauge if you don't have your own with you.

-Dana

Space isn't remote at all. It's only an hours drive away if your car could go straight upwards.

 

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/wwwthreads_images/icons/wink.gif Uh....Dana,.....are you IN there? What are you DOING? Normally, I look forward to your posts for the clever footnotes. This post, however, left me gasping for air./wwwthreads_images/icons/shocked.gif Here is my question for you. What is the difference between your Jeep and a Geo Metro? Yep, it has FOUR WHEEL DRIVE. Now Dana....what does that mean? Yes, it means that it has each wheel hooked to THE CRANK! Yeah, yeah, you say.....so what....big deal. Uh...Dana..how is that actually done...how does the crank drive all four? Oh, you say.....a TRANSFER CASE....hey, that's pretty good.....is it a FULL TIME case? Oh, you don't really know? Well, it doesn't matter....the rule is the same, and the rule is(now pay attention) thus: FORGET PRESSURE.....IT'S ROLLING DIAMETER THAT MATTERS! Sheesh! On the same Jeep, at the same pressure, with the same tires and rims, you can easily have an inch of difference! Now, Dana...how about giving us a GUESS as to what that does to that FOUR WHEEL DRIVE that you mentioned? Hmmm...?? Yes, Dana, it SCREWS UP the differentials; wrecks the air lockers; expands the bearing fits in the transfer case; SUCKS UP FUEL; scuffs tires; wrecks u-joints; and did I mention that it makes it hard....if not impossible...to get OUT of FWD once you are in? Now I wouldn't get on your tail like this without giving you the solution that will get you instantly out of trouble, and I know this is like telling someone how to build a clock when they ask what time it is, but HERE is what you must do: First...take a 2-1/2" X 30" trim board from the local lumber yard....get a nice one...and bolt a little tin "foot" on one end such that the tin foot will just tuck under the tire when the trim board is vertical. Then make an adjustable "pecker" which can slide up and down. Set the pecker so it just rests atop the edge of the rim. Do that for all four tires, adjusting the air to get them PERFECT! Chose the biggest or the smallest as a "get to" point, but GET THEM ALL THE SAME. Do this after a run, with the Jeep on real smooth concrete./wwwthreads_images/icons/crazy.gif/wwwthreads_images/icons/wink.gif/wwwthreads_images/icons/laugh.gif I can send some moon guys over to help if needed. They are almost recovered after rolling their CJ5 at Farmington a few weeks ago./wwwthreads_images/icons/crazy.gif

CJDave
 
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OK Dave, you've said that before and it makes perfect sense. But, just for grins /wwwthreads_images/icons/smile.gif, what effect does this have on handling, the
different tire pressures I mean. I know Jeep is not a Ferrari /wwwthreads_images/icons/shocked.gif, but I would think different pressures would have an adverse
affect on handling. Just a thought./wwwthreads_images/icons/wink.gif

Brad (from the 4 Wheeling center of the universe, 4 corners USA)
 

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/wwwthreads_images/icons/tongue.gif If the tires are not the same diameter at the same pressure, then weight distribution is not right, and if you notice, new Jeeps have different presures front/back to compensate for just that very thing. The tire's reaction to the weight has a lot to do with handling, and the auto guys have somehow determined that pressure adjustment is the answer./wwwthreads_images/icons/crazy.gif When are you going to begin lowering your Jeep, Brad?/wwwthreads_images/icons/crazy.gif

CJDave
 
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The vehicle can actually handle better this way. Ever watch a Nascar race? If you watch close you will see them talk about air pressure and stagger. Yes they use different tire sizes to change stagger but air pressure changes stagger as well. So if you think about it this way taking into account that no two tires are identicle that if you run the same pressure in all of them you have four different staggers on your jeep.

 
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Uh Dave, I know tire tolerances aren't perfect, but as one who's spent several years MEASURING tires on competition mud racers, I've yet to see two new tires on a single truck that were off by more than fractions of an inch - even Swampers, Hawgs, Mudders, etc. And I've measured as many as 121 complete sets in a single afternoon. And anyway if you had tires that mismatched, you'd move 'em around so your rears & fronts matched. But, let me assure you that pressure does matter. Try watching a competition mud racer leave with 8 pounds in one rear tire & 6 pounds in the other. A perfect recipe for "last place".

TEX

/wwwthreads_images/icons/wink.gif Got Mud?
G.U.M.B.O. Mud Racing
 
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Like I said Dave, it makes perfect sense to me. I was just being the "Devil's Advocate" and throwing that out there for anyone who
did happen to be wondering. To many people adjusting tire pressures is a truly novel idea.

Brad (from the 4 Wheeling center of the universe, 4 corners USA)
 

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/wwwthreads_images/icons/tongue.gif Remember....it isn't the PRESSURE that we are concerned with as much as the ROLLING RADIUS. We want the turns per mile to be exact for each tire. My CJ was off almost 3/4" tire to tire the first time I measured it. With Quadra-Trac drive, that's a BIG problem. Of course it was also STUCK in "lock" when I got it, so the issue was moot. When I popped the QT out of "lock" with a vacuum pump, the whole Jeep "shrugged"./wwwthreads_images/icons/frown.gif/wwwthreads_images/icons/crazy.gif

CJDave
 

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Discussion Starter · #10 ·
I agree that rolling radius must be closely matched to avoid excessive stress on the drivetrain. However, if I found my tires to be 3/4" off at the same pressure, then I'd think I had a defective tire. The fact that I can easily get out of 4WD after a run indicates that my rolling radius is pretty well matched. Off road, on rough ground, the variations in rolling distance due to uneven ground will be more than any reasonable tire diameter variation. And on a dry road, you shouldn't be in 4WD anyway. Sure, check your radius with all four tires at the same pressure..... that's a good idea! But once you've verified that the tires are well matched, checking air pressure (with an accurate gauge, which was my whole point) is a lot easier. And after all, it's the "squishier" tire we're trying to achieve when we air down, not a reduced radius.

-Dana

End rush hour traffic now! Legalize vehicular weaponry!
 

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/wwwthreads_images/icons/smile.gif Right on, Dana......squishy IS better off road, and thinking about your requirements(we are strictly snow) to need "airing up" I would think you would want to pump them up to a pre-determined psi, then verify rolling radius with a fixture; then adjust as needed. 3/4" is not all that unusual due to uneven weight distr., and with the advent of all-wheel drive like our Select-Trac 242 case and our Quadra Trac CJ7, we have to watch them like a hawk. A local tire shop installed new rubber in front of a little Subaru or some such car, and the customer drove to Sacramento, about 100 miles. It SMOKED the power divider. The thing is, the tire could be CIRCUMFERENTIALLY equal, but the ROLLING RADIUS due to differences in chassis weight distribution could be off by quite a bit./wwwthreads_images/icons/crazy.gif Four wheel drive is a terrible thing to waste.

CJDave
 
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