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Old 05-07-2009, 08:15 AM
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Default Robbie Powderpig Tire Pressure Q

In reading your column in the most recent Trails, Robbie, you mentioned that you run 40 psi in the rear tires and 35 in the front when the truck is heavily loaded. I know that you have removed the front driveshaft in the past and I'm wondering if that is the case when you run different pressures front and rear now. I've always thought that with an AWD vehicle that tire pressure should be the same on both axles or is that not necessary?
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Old 05-07-2009, 08:39 AM
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With a normal situation with no real load in the rear, tire pressure can and should be run equal as per the book. But if we look at when we load the Cruiser for the trip and we load the rear more than the front. Tire pressure should increase to keep contact patch similar in size. This increase in Tire pressure handles the load better with less heat.

This increase in tire pressure will also improve handling. How so? Well by stiffing up the rear wall of the tires of the truck, supporting the load better.
This does not have anything to do with me removing my front drive line.
I have also run up to 45 psi if I think I need more in the rear for the load I have.
So currently I have a rear fuel tank(with fuel can weigh 430 or so), Drawer system(120 with out my junk, maybe another 60 or so for tools and such), rear tire carrier with tire(over 200 with tire). So I carry well over 1000 extra lbs when I leave for a trip. So adding more pressure to the tires will get my contact patches closer to equal.
Hope this help to hear my thought process.
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Old 05-07-2009, 08:42 AM
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Makes perfectly good sense to me Robbie and thank you for your response. I've always run higher pressure in the rear tires of the 62 when the back of the truck was heavily loaded but just wasn't sure that was a good idea or not in the full time four wheel drive 80. Now I know!
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Old 05-07-2009, 08:49 AM
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The way the VC has worked for me and how I see it explained in the book is different than I see on the boards.
The small rotational differences between the front and the rear if one tire is low or if the fronts are not that far off in tire diameter seem not to effect the VC that much. I have even run a 33 inch(with 35 the rest way around, from breck to louisivlle) tire up front to get home with no ill effects on the VC.
One article on the subject from 93 or 94 UK press, indicated that the VC was for more drive line differential speeds(meaning big differences between the two drive shafts). So minor differences should not create enough heat to activate the VC.
If you want the experence of how it feel to ingage the VC, Pull the front drive line, leave the center Open and see how long it takes the VC to engage full to more the truck forward. Fun little experiment. I had this happen accidently one time when I had the front drive line off. It took a bit for me to figure out want was wrong with the cruiser. I was happy when I did find out I had hit the switch accidently to turn off the CDL.
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Old 05-07-2009, 09:07 AM
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I also have run 40-45 in the rear tires of the 80 when the truck is heavily loaded on a long trip (ie Rubithon, Moab, Black Hills...etc) I think Dave is spot on about the contact patch too as I will inflate the rears until the bulge is comparable to the front tires.
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Old 05-07-2009, 09:23 AM
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It would be a good experiment to see when the contact patches are equal that the tire diameter would be the same.
My guess is that the diameter would be almost equal.
May need to try this some time when I am on Payment and I have the time.
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Old 05-07-2009, 09:51 AM
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Quote:
Originally Posted by DaveInDenver View Post
Problem is those are far less convenient to measure on the side of the trail while your buddies are getting upset waiting for the jackass who pulls out a plumb, T-square and tape measure instead of just a tire gauge.
or how about some sidewalk chauk and a sheet of masonite?
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Old 05-07-2009, 09:58 AM
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10psi is too low for an 80
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Old 05-07-2009, 02:00 PM
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Is there a "guideline" on how much to increase pressure in the rear tires based on "x" amount of weight?

I've been running the 285 Toyos at 38psi all around based (IIRC) on Toyo's load chart and extrapolating out the numbers based on going from OEM size P tires to Toyos LT tires. The last couple years I had been running about 42psi (before finding Toyo's load chart) based on what I read on MUD about adding 10psi to the recommended OEM tire pressure when going from a P tire to a LT tire. This year, like previous years, I didn't add any air to the rear for the trip to Moab despite the additional weight in the rear and the weight of the trailer on the hitch.

There were a few times this year during the trip out where the LX didn't feel as "stable" towing the popup, especially on corners, and the nose of the rig seemed higher which I attributed to my rear springs loosing some of their "spring" - Now I'm wondering if it was something as simple as a lack of the right pressure in the rear tires? Maybe I should have bumped the pressure back up to 42psi, or more, in the rears...

I've never even contemplated running different psi front/rear on the trails - I've probably only got a couple hundred lbs of "stuff" in the rear compared to normal when wheeling though...
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Old 12-14-2010, 07:46 PM
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Today at Discount Tire they inflated my new 285/75 16's to 33 all around, but the invoice says "OEM Inflation F:29 R:41". If true, that means Toyota has no issue with different pressures. Door sticker says: 32 front and rear. Hmm.

1993 Manual says: Conventional tire, normal: 32 front and rear.
"Tailer" (he, he) towing: 32 front, 35 rear.

Supports Robbie's concept on loading.

I have normally run 36ish, and placed a priority on equal loading.
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