Jeep Wrangler Forum

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-   JK General Discussion Forum (http://www.wranglerforum.com/f274/)
-   -   PSI question (http://www.wranglerforum.com/f274/psi-question-253231.html)

TNuggs13 07-02-2013 05:13 PM

What would you run the PSI at on 37" BFG T/A's? Recommend seems really high to me.

Super T 07-02-2013 05:15 PM

What size? I originally ran the recommended PSI of 37, but it was too harsh. Now I'm running about 32 and it is much nicer. Stock Rubicon wheels/tires.

Super T 07-02-2013 05:17 PM

Ah, you edited and added the size. Nevermind about what I said then. Others will tell you, but I'm guessing it will be around 30, but I could be WAY wrong here.:hide:

onetraveller 07-03-2013 11:30 AM

You'll want to do a chalk test to fine tune the pressure.

I'd guess somewhere between 25-30 psi, but the chalk test is the way to go.

Mike

ahsumtoy 07-03-2013 01:12 PM

Not exactly the same tire but...
 
I have BFG KM2's. When I got them, they were at 65psi. I now run them at 32psi, what a difference.

gsn 07-03-2013 01:17 PM

Aren't larger tires supposed to hold more weight per psi than smaller ones at the same psi?

BManz 07-03-2013 01:20 PM

Generally the larger the tire in volume, the less the PSI needs to be for proper function. Some tires are sensitive to just a 2 PSI difference and sometimes you can even feel the difference driving.

gsn 07-03-2013 01:23 PM

Quote:

Originally Posted by BManz (Post 3924308)
Generally the larger the tire in volume, the less the PSI needs to be for proper function. Some tires are sensitive to just a 2 PSI difference and sometimes you can even feel the difference driving.

This begs the question, does a 37 inch tire really needs 30+ psi to hold a wrangler? it has more rubber, plus greater air volume.

onetraveller 07-03-2013 01:28 PM

Quote:

Originally Posted by gsn (Post 3924305)
Aren't larger tires supposed to hold more weight per psi than smaller ones at the same psi?

PSI = Pounds per square inch.

The difference isn't in the tire height, but in the size of the contact patch. A 4000 lb jeep requires each tire to hold up 1000 lbs. So a tire at 25 psi has a 40 square inch contact patch. A tire at 35 psi has a 28.5 square inch contact patch (28.5x35=1000).

A 35x12.50 tire has a larger potential contact patch then a 32x10.50 tire. If you run them both at 37psi on the same Jeep, the 35x12.50 won't sit flat on the tread, so it'll wear the center out and the tire won't be able to perform the way it's designed to.

This is where the chalk test comes in. It allows you to identify the appropriate pressure so the tire uses all of the available tread cross-section.

Mike

HK_Runner 07-03-2013 01:29 PM

I run my 35" MTRs at 25-27 now. I'm guessing 28 will be a really good starting point. The chalk test will help you fine-tune it.

Reece 07-03-2013 02:33 PM

SuperT - at 32psi on your stock Rubi wheels/tires, is your TPS light on? I'm at 35psi with no light but I'd like to bring it down some more. I know the light means little but it's annoying to see it burning. At 30psi it is on for me.

gsn 07-03-2013 02:35 PM

Quote:

Originally Posted by onetraveller (Post 3924339)
PSI = Pounds per square inch.

The difference isn't in the tire height, but in the size of the contact patch. A 4000 lb jeep requires each tire to hold up 1000 lbs. So a tire at 25 psi has a 40 square inch contact patch. A tire at 35 psi has a 28.5 square inch contact patch (28.5x35=1000).

A 35x12.50 tire has a larger potential contact patch then a 32x10.50 tire. If you run them both at 37psi on the same Jeep, the 35x12.50 won't sit flat on the tread, so it'll wear the center out and the tire won't be able to perform the way it's designed to.

This is where the chalk test comes in. It allows you to identify the appropriate pressure so the tire uses all of the available tread cross-section.

Mike

I agree, an overly inflated tire will produce an uneven wear pattern. I believe we are going in different directions.

I'm talking about as to why people go with the manufacturer's suggestion of 35 PSI for larger tires. For example, this is my main point, even if the 35" tire does not sit flat on the ground, it still has a greater load capability, you just need to apply it. Hence an over inflated tire will cause a uneven pattern, because the wrangler is not heavy enough to properly squeeze the tire at that PSI and engage all the tread. Thus it provides a rough ride as it does not comply with road imperfections and is resisting the vehicle's weight more.

This also goes with load range. A higher load range of the same size tire means it can hold more weight due to stronger inner structure of the tire. Many larger tires usually carry a greater load range. Thus, the same would apply as above, the wrangler is too light of a vehicle for such load ranges, thus, 35 PSI on them is too much, "technically" speaking.

I don't know if I'm making myself clear. I'm not a tire guru, just applying logic, but I admit logic is not always linear as I'm assuming right now.

Looking at weight ratings of the stock goodyear 255 75 R17, is 2535 lbs at 35 PSI. That makes up for a combined load of 10,140 lbs. Am I right with math? What is the maximum weight of a JK stock? 4500~5000 lbs? Even the stock tire can be aired down a bit to leave room for the force of the car bouncing, or am I wrong?

All this is obviously moot if we are discussing the best PSI for best gas mileage. But I'm talking from a comfort and wear angle.

onetraveller 07-03-2013 02:49 PM

I'm no tire guru either, just applying some basic physics.

I think we are on the same path. An E rated tire can hold a higher load rating because the tire structure is strong enough to withstand the higher tire pressures and heat that occur with long term use at heavier loads.

The stock JKU has a GVW of 5,400 pounds. I've seen several modified ones that would push 6,000 pounds. But even at that weight, the stock tires are more than adequate.

I think most start out with 35+psi on their larger tires because they don't know any better. It's the path of least resistance. The door plate says put the tires at xx PSI, so that's what they do.

Mike

Up Hill Bill 07-03-2013 03:37 PM

Quote:

Originally Posted by Reece (Post 3924489)
SuperT - at 32psi on your stock Rubi wheels/tires, is your TPS light on? I'm at 35psi with no light but I'd like to bring it down some more. I know the light means little but it's annoying to see it burning. At 30psi it is on for me.

Your TPMS should be set at 37 psi nominal pressure and -8 psi "trigger point" for the TPMS light from the factory.

So, you SHOULD be able to run 33 psi cold inflation. But, when it gets cold, your light will trip (pressure drops 0.8 psi per 10F,) and not reset until you go back over 37 psi. IIRC*


*Caveat..... this is just what I have picked up from my service writer and tech after having TPMS problems. (Turned out, the dealership forgot to reprogram the TPMS after replacing the WCM/SCREEM.)

HK_Runner 07-03-2013 05:14 PM

The dealer can reset your TPMS. Alternatively, get an AEV Procal or other competing device and do it yourself. Mine is set to 26 now.


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