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Unread 02-26-2012, 11:42 AM   #1
PredatorNC
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Chalk test?? I'm not driving at 16 PSI!!!

So its a nice sunny day here in NC and since i've had the BFG's for about 2 weeks I decided it was time to do the chalk test since the little rubber whiskers are still on the outsides of the tire. When I got them back from the tire shop they had 35 psi in them, which knowing I run 30 PSI in the JK and it being heavier I knew i would have to drop them. I dropped them to 30 and ran them for a week and it rides ok. Then last week i dropped them to 28 psi.

So here I was...on the flat, straight section of road by my house (dead end street). I get lined up and chalked the tires. No suprise that the chalk did not wear on the outsides of the tire. I dropped them down to 24 psi since the entire outside lugs showed no wear.

So I tryed 22, then 20, then 18....it was getting closer!

Then it hit....DING DING DING! 16 psi and the outsides were showing worn off chalk!

16 psi!?!?!?! That's about 2 psi less then I was planning to run off road! lol

So for me...the chalk test is a sham. I did not even bother pulling the JK out to try it, and I'll be putting mine back up to 28 psi, maybe 26.

Just for a FYI...it was flat, dry and about 48 degrees using sidewalk chalk on 33" BFG A/T's and a stock TJ

If anyone is using the "chalk test" it should be taken with a grain of salt...I can't see how that would be safe, and from the searches I've done on it here most are running 24-30 psi on the street.

As one poster said it..."common sense must prevail"

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Unread 02-26-2012, 11:51 AM   #2
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It's not uncommon to run under 10 psi while off road.
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Unread 02-26-2012, 11:52 AM   #3
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I did the chalk test on my TJ when I had the MTZs and got 12-14 psi but I ran 24-26 instead. Gotta agree with the common sense approach.
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Unread 02-26-2012, 11:52 AM   #4
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Quote:
Originally Posted by PurpleTJ View Post
It's not uncommon to run under 10 psi while off road.
How is this helpful given that OP talking about onroad air pressure? Chalk test not the most accurate for large, wide tires. I'd run 26 lbs. area for those tire.
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Unread 02-26-2012, 11:55 AM   #5
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Quote:
Originally Posted by biffgnar View Post
How is this helpful given that OP talking about onroad air pressure? Chalk test not the most accurate for large, wide tires. I'd run 26 lbs. area for those tire.
Because the OP said this:
Quote:
Originally Posted by PredatorNC View Post
... Then it hit....DING DING DING! 16 psi and the outsides were showing worn off chalk!

16 psi!?!?!?! That's about 2 psi less then I was planning to run off road! lol ...
18 PSI is waaaaaaay too high for offroad.
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Unread 02-26-2012, 12:26 PM   #6
AKGeo
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How wide are your wheels? Assuming the tires are 33x12.50s.

Wider tires will crown on narrower wheels. The chalk test is designed for tires mounted on wheels that are just about as wide as the section width of the rubber, so the sidewall is practically flat and vertical instead of bulging. So you'd need 10-12" wide wheels to get an accurate chalk test on 12.5s.

Common sense is the way to go. and you'll always have increased center tread wear on offroad tires using a useful wheel width.

Another thing to consider is that taller or heavier duty tires require more vehicle weight to get a proper contact patch. A friend ran 40" MTRs on his YJ, and he barely saw any sidewall bulge when the tires were at atmospheric pressure. Valve stem out, just sitting with the vehicle's weight on them. They did break in eventually, but he was able to wheel on 1-2 psi without beadlocks until then. Taller tires have more sidewall to flex and will offer more resistance to the settling of the vehicle with weight on them. So they can be ran with less PSI than a shorter tire would need.
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Unread 02-26-2012, 12:26 PM   #7
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I run 22 on the street with my 35s I drop to about 12 offroad, sometimes lower. I did the chalk test when I first got my bfgs and the place that mounted them had like 40psi in them. 22 was the magic number for me and I have no issues with extreme wear or cupping at all after 23,000 miles and I'm lazy with rotations doing them about every 10-12k
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Unread 02-26-2012, 12:27 PM   #8
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The chalk test, when done by an inexperienced individual with no idea what a reasonable tire pressure is, is probably as responsible for underinflated tires as simple neglect is.
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Unread 02-26-2012, 12:30 PM   #9
greg_kulosa
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I had a similar experience when I did the chalk test. I ended up with a flat pattern at 13 PSI. In my case, it's because I am running a 12.5 inch wide tire on an 8 inch wide rim. The skinnier rim pulls the sidewalls in, and makes the center of the tread bulge more.

I ended up running 23-25 PSI on the street.

The chalk test is not bunk, but it may give different results in some cases.

There is a thread here where several of us discussed this phenomenon, but I can't find it on my phone right now.
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Unread 02-26-2012, 12:39 PM   #10
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I always run 26 psi for 33x12.50's and 24 psi for 35x12.50's.
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Unread 02-26-2012, 12:58 PM   #11
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24 sounds low for street pressure, but I'm going to check my 315's when they get installed. So I guess I'll find out.
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Unread 02-26-2012, 01:11 PM   #12
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I run my 35's at 26 psi and my staun deflators are set to 10. I have gone lower than that but 10 psi seems to work well for now.
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Unread 02-26-2012, 01:57 PM   #13
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Hint: Call the regional representative for the tire manufacturer and ask a few questions about your specific setup and vehicle. Those guys see hundreds and hundreds of problems and they can be very, very helpful. In my case, I didn;t have enough experience with my specific type of tire, but the regional rep had seen people do just about everything with them. They're to tires, what JF.com is to Jeeps. A great repository of experience/knowledge.
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Unread 02-26-2012, 02:19 PM   #14
PredatorNC
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Good points made by all, and being a noob to the jeep world, there was a healthy amount of fear thinking that 16 psi was a sweet spot...and that 16 psi was optimistic...there was still chalk there so another 2-4 lbs would have probably made it even.

TYVM! bringing the info on the wheel width! Yes, I am running 15x8's. So literally I would have to run a 12" width for it to lay flatter with more psi correct?

Thanks for being noob kind! lol
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Unread 02-26-2012, 02:21 PM   #15
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I like the spray bottle test better than the chalk test, but I agree that narrow rims will skew the results. I perfer a 10" wide rim for 12.5" wide tires, but I know many folks perfer the 8" wide rim instead.
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