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post #1 of 11 Old 03-10-2015, 05:42 PM Thread Starter
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Tire Pressure

I can't seem to get the proper tire pressure for a full contact patch on my new tires. I'm currently running 37x13.5x17 Cooper STT's on 17x8.5 Trailready beadlocks. I started them out around 25psi and dropped the rears to 16psi where they currently sit. Fronts are good at 23psi, but I'm not running a tire carrier and have limited weight in the back. I'm still having the very outside/inside edges of the tread not touching the road. I haven't really seen any improvement in contact below 20psi. So do I keep letting air out of these or do I call it good enough and put em back to 20 in the rear?
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post #2 of 11 Old 03-10-2015, 06:38 PM
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Aw... come on, man.

Really... 16lbs or even 20lbs is too low to be running on pavement imo. Dunno about any contact patch issue with these tires... or any tires for that matter. I would think that either you are not really seeing accurately the true contact ( regarding where the sidewall ends and the tread surface begins) or that there may be an issue with the way that the tires are mounted on the bead-locks. You might consider trying to re-seat the tires on the wheels...idk.

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post #3 of 11 Old 03-10-2015, 07:07 PM Thread Starter
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Ran my old E rated 37x13.5 toyo's at about 26/24psi on the street, with no issues and even wear. Tires are mounted correctly, and were balanced with less than 10oz per tire. I'm just wondering why I can't get all the shoulder lugs to make contact. I've put maybe 1000 miles on these so far and still have the little nubs on the outside edges. I would chalk them but good luck finding a parking lot that isn't currently covered in salt.
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post #4 of 11 Old 03-10-2015, 07:28 PM
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I run my 33x12.5 x 15 Duratracs, on 15x9 Enduro rims at 20 PSI for every day driving.
I have perfectly even tire wear across the entire contact patch.

So once you don't see any improvement by dropping the pressure stop because you can go to far. Also you will reach the limit in part due to the section width VS. the rim width. There comes a point that the wall won't collapse any more due to the rim width.

Lower pressure is a price we pay to run larger than stock tires.

I am glad to see that someone else was smart enough to pay attention to the contact patch and I often feel bad for the guys that run 35 PSI in a 35x12.5 tire. I'll bet everyone of them burn out the center of their tires at 30000 miles.

Also they are not getting the traction offered by a wider tire and are really missing out on the cornering and braking benefits.

Another BIG benefit is how much 'softer' the ride is.
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post #5 of 11 Old 03-10-2015, 07:29 PM
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Pour a line of water on the ground from a cup. Roll tire over water, look at water stripe on tread.
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post #6 of 11 Old 03-10-2015, 08:21 PM
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I use this formula max tire rating x 4 / by max psi = / by vehicle weight = actual psi

3525x4 / 65psi = 217lb per psi / 5000lb = 23psi

For good measure you can add 10% for safety. I'm not sure what the max psi is for your tires or what you rig weighs. If you weigh each axle separately you can get exact psi on each axle. Instead just divide by 2 and use the axle weight. For my 37"km2's and my unlimited I run 19psi on the front and 17 in rear.

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post #7 of 11 Old 03-11-2015, 03:11 AM Thread Starter
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Quote:
Originally Posted by 8000ft View Post
I run my 33x12.5 x 15 Duratracs, on 15x9 Enduro rims at 20 PSI for every day driving.
I have perfectly even tire wear across the entire contact patch.

So once you don't see any improvement by dropping the pressure stop because you can go to far. Also you will reach the limit in part due to the section width VS. the rim width. There comes a point that the wall won't collapse any more due to the rim width.

Lower pressure is a price we pay to run larger than stock tires.

I am glad to see that someone else was smart enough to pay attention to the contact patch and I often feel bad for the guys that run 35 PSI in a 35x12.5 tire. I'll bet everyone of them burn out the center of their tires at 30000 miles.

Also they are not getting the traction offered by a wider tire and are really missing out on the cornering and braking benefits.

Another BIG benefit is how much 'softer' the ride is.
I figured it might be due in part to the rim width
Quote:
Originally Posted by X50ARM View Post
Pour a line of water on the ground from a cup. Roll tire over water, look at water stripe on tread.
That's actually how I first noticed I was still too high when in the low 20s
Quote:
Originally Posted by Rubiconsnow View Post
I use this formula max tire rating x 4 / by max psi = / by vehicle weight = actual psi

3525x4 / 65psi = 217lb per psi / 5000lb = 23psi

For good measure you can add 10% for safety. I'm not sure what the max psi is for your tires or what you rig weighs. If you weigh each axle separately you can get exact psi on each axle. Instead just divide by 2 and use the axle weight. For my 37"km2's and my unlimited I run 19psi on the front and 17 in rear.
Using this I come up with about 21psi. So I'll put them back to 20 and call it a day.
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post #8 of 11 Old 04-17-2015, 11:08 AM
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Bump

Did your quest to get the correct patch end well??

I'm running 35' Goodyear MTR/K. Using the formula I get 15-19 psi. But this is close to what I generally air-down to for off road work(actually 14) and seems way low. I run 28-29 day to day on road with good patch using chalk test. Is my math flawed?

for mtr/k 35x12.5x17:

2405 max load
35 max inflation
Vehicle exact weight unknown. Factory spec curb weight=4104 stock, but I have mods in wheels, tires, skids, lift, etc. plus gas, occupants and misc stuff.
@4104 I compute approx 15 psi.
@4700 I compute approx 17 psi
@5104 18-19 psi

Just saying doesn't sound right. Thoughts?

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post #9 of 11 Old 04-17-2015, 07:30 PM Thread Starter
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I ended up at 25 in the front, 22 in the rear. Now that it's warmed up the I seem to be getting a full contact patch.
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post #10 of 11 Old 04-20-2015, 03:57 AM
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Quote:
Originally Posted by ratchettt View Post
Bump

Did your quest to get the correct patch end well??

I'm running 35' Goodyear MTR/K. Using the formula I get 15-19 psi. But this is close to what I generally air-down to for off road work(actually 14) and seems way low. I run 28-29 day to day on road with good patch using chalk test. Is my math flawed?

for mtr/k 35x12.5x17:

2405 max load
35 max inflation
Vehicle exact weight unknown. Factory spec curb weight=4104 stock, but I have mods in wheels, tires, skids, lift, etc. plus gas, occupants and misc stuff.
@4104 I compute approx 15 psi.
@4700 I compute approx 17 psi
@5104 18-19 psi

Just saying doesn't sound right. Thoughts?
Where are you getting the 35 PSI max inflation number? Is that the max inflation listed in the sidewall of the tires?

For what it's worth, I come up with pretty much the same numbers you get. I doubt that ideal tire pressure is as simple as a single formula for all tires. But it seems like opinions on ideal tire pressure are as varied as just about any other opinions.

Last edited by BigTB; 04-20-2015 at 04:13 AM.
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post #11 of 11 Old 04-20-2015, 01:42 PM
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Quote:
Originally Posted by BigTB View Post
Where are you getting the 35 PSI max inflation number? Is that the max inflation listed in the sidewall of the tires?

For what it's worth, I come up with pretty much the same numbers you get. I doubt that ideal tire pressure is as simple as a single formula for all tires. But it seems like opinions on ideal tire pressure are as varied as just about any other opinions.
Goodyear web site and on sidewall, 35lbs. max. I was surprised it was so low. Guessing the formula may not work for such a low max pressure and max load. Just trying to reconcile my real world results with the physics of this formula so my obsessive self can rest easy.

2013 JKR, Auto, Yukon 4.56, Goodyear MTR/K 35/12.5/17 on XD222 KMC Enduro Beadlocks,Teraflex Big Brakes & Spare Carrier, Warn RC Winch, Metalcloak Front & Rear Bumpers, Metalcloak Overliner Flares & Front Inner Fenders w/ Corner Guards/Tail Lamps, Metalcloak Game Changer 2.5" w/6-Pac's, ARB Diff Covers & Onboard Air, AFE High Tuck Muffler, Front Lower Control Arm Skids, Artec Aluminum Skid System, Poison Spyder Hood Vents, Ripp SC.

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