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Old 02-24-2016, 11:41 PM   #1
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Tire pressure on the asphalt

So I was in Les Schwab today in PDX, Oregon to get tires rotated. I'm on an extended road trip. I had just been in Bend and running my tires at about 40psi both on and off road, mostly on cinder and logging roads. In PDX the tech asked me what I wanted my pressure at... I asked what do you recommend out of curiosity, he said 65psi. I said let try it I can always deflate if I don't like it. So my question is what are most people running the 4x4 Quigleys or sportsmobile at? I have 17" alloy weld racing rims with Toyo M/Ts 295/70/17. Needless to say 65psi was way too much. I'm try 55psi right now. Thoughts from members would be awesome.
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Old 02-25-2016, 08:42 AM   #2
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What's your axle weight front and rear?
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Old 02-25-2016, 08:48 AM   #3
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Quote:
Originally Posted by MSD View Post
So I was in Les Schwab today in PDX, Oregon to get tires rotated. I'm on an extended road trip. I had just been in Bend and running my tires at about 40psi both on and off road, mostly on cinder and logging roads. In PDX the tech asked me what I wanted my pressure at... I asked what do you recommend out of curiosity, he said 65psi. I said let try it I can always deflate if I don't like it. So my question is what are most people running the 4x4 Quigleys or sportsmobile at? I have 17" alloy weld racing rims with Toyo M/Ts 295/70/17. Needless to say 65psi was way too much. I'm try 55psi right now. Thoughts from members would be awesome.
Not a Quigley here, but with my current Mickey Thompson MTZ P3 tires in 265/70R17 I am down to 40 psi on the street for an acceptable ride quality. They seem very stiff, even when aired down to 25psi offroad. I haven't noticed any concerning wear characteristics yet so it seems ok. My rig is quite light compared to a SMB, but that may give some insight on the low end of the pressure scale.

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Old 02-25-2016, 11:23 AM   #4
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I'm running 32in Dura Trac's. The front weighs 3580lbs and the rear is 4520lbs. The steering has always been nervous and twitchy. I have replaced the steering box, shocks, and damper and got some improvement. Tierod ends and ball joints are fine. I've been running about 55 to 60lbs, but when I increased the pressure to 70 front and rear, the steering got a lot better, no more constant corrections needed. Max recomended pressure is 80psi, so i'm not over inflating them. The ride however is now pretty harsh, but I'll take that over the need to constantly make steering corrections, it's far more relaxing to drive now.
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Old 02-25-2016, 12:48 PM   #5
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Thanks for the info guys, this helps a lot. I don't know my actual axel weights but Quigley lists front axel at 4,400 lbs and rear axel at 6,100 lbs, inside my door. The Toyo MT's are rated to 80psi max, which seems crazy high and based on the ride quality at 65 I would never run that... I do really like the ride quality at 40psi on the road. Guess I'm trying to find that happy medium for pressure. In my industry (cycling) we are finding that wider tires and lower pressure is better for wear, handling and even rolling resistance. So it got me thinking, I know we are talking a complete different beast when it comes to contact patch and ratings from a bike tire and car tire, but I've learn a lot from F1 and MotoGP and used it in my bike building process, so that's why the thought process of tire pressure came up.
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Old 02-25-2016, 03:16 PM   #6
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MSD, I also have a 6.0 Quigley on 17 inch Weld Racing wheels but slight smaller 285/70 tires. I usually run mine at 65 and I like how it handles there, but honestly I've never run it at 45 on the road. I think I'll give it a try and see how I like it.
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Old 02-25-2016, 03:43 PM   #7
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The only really accurate way to determine air pressure is to weigh the van, determine the load on each axle (preferably each wheel) and finding the proper pressure from a load inflation table for the tire size you are running. My weights and pressure may vary significantly from yours.

If you can't weigh it, the really old school method was to inflate to a cold pressure and drive on the highway ~20 miles and check the pressure. If the pressure goes up less than 3 psi your cold pressure was too high; if the pressure goes up more than 6 psi your cold pressure was too low. That should get you in the ballpark until you can weigh it.

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Old 02-28-2016, 03:56 PM   #8
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DEFINITELY weigh it. Easy to do on a trip; there are tons of scales and it takes just a sec.

I have a SK, about 3600 front, 5000 rear if crammed with junk. I run just over 40 in front, just over 50 in back, sometimes a teeny bit more if I want mileage and I'm on smooth roads (which means no freeways in CA...sigh BUMP sigh). Handles quite well, and is reasonable off road.

It depends on the max pressure/weight rating of your tires though; I went recently from 65 max to 80, so had to raise the pressure a hair (I can't remember if the max weight capacity went up much; probably some).
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