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Crash
05-07-2009, 07:15 AM
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?
Thanks,
Steve

powderpig
05-07-2009, 07:39 AM
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.

Crash
05-07-2009, 07:42 AM
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!

powderpig
05-07-2009, 07:49 AM
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.

Uncle Ben
05-07-2009, 08:07 AM
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.

powderpig
05-07-2009, 08:23 AM
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.

nakman
05-07-2009, 08:51 AM
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.

:lmao: or how about some sidewalk chauk and a sheet of masonite?

treerootCO
05-07-2009, 08:58 AM
10psi is too low for an 80;)

corsair23
05-07-2009, 01:00 PM
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" :hill: - 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 :confused: - I've probably only got a couple hundred lbs of "stuff" in the rear compared to normal when wheeling though...

Inukshuk
12-14-2010, 06:46 PM
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.

Crash
12-14-2010, 08:31 PM
Discount Tire has all sorts of strange ideas regarding tire inflation and lug nut torque which seem to go against Toyota recommendations. I say stick with what Toyota specs. 100 series tire pressure is weighted towards the rear of the truck - front 29 psi and rear 32 for regular loads and 32 front and 35 rear for towing according to the owner manual.

Inukshuk
12-14-2010, 08:43 PM
Discount Tire has all sorts of strange ideas regarding tire inflation and lug nut torque.

I'll check that too. At least I did watch the guy us a torque wrench. Receipt says 115.

Crash
12-14-2010, 09:02 PM
I'll check that too. At least I did watch the guy us a torque wrench. Receipt says 115.

Does your truck have steel wheels?

Inukshuk
12-15-2010, 06:13 AM
Does your truck have steel wheels?

No. 115 too much, aye? I have no idea how tight they really are.

powderpig
12-15-2010, 07:11 AM
Aluminum rims should be around 76(I think I have always torqued to 80 ftlb). 115 is for the steel rim.

Corbet
12-15-2010, 08:14 AM
Every Subaru I've ever dealt with has a different front/rear pressure rating on the OEM door jab decal. They have one of the most sensitive AWD systems when it comes to mismatched tires. What's kind of odd is that our 05 Outback calls for 32 front and only 30 rear. You would think loaded the back would be heavier.:twocents:

Anyway I would not hesitate putting more pressure in the rear when loaded. I do in my 80 and used to in my 40 too.

Inukshuk
12-15-2010, 08:41 AM
Aluminum rims should be around 76(I think I have always torqued to 80 ftlb). 115 is for the steel rim.

Will check.

What about the inflation difference for P metric vs. LT. Nitto says:

"Please note that size-for-size, LT-metric tires require higher air pressures to carry equivalent loads of P-metric tires, and that any failure to adjust air pressures to achieve the vehicle’s load requirement will result in tire fatigue and eventual tire failure due to excessive heat build-up. Due to the higher PSI requirements of LT-metric tires, they may not be suitable for replacing O.E. P-metric tires because of the ride harshness that results from higher PSI requirements.
P265/75R16 114 Max Load = 2,601 lbs @ 35 PSI. In order to carry the equivalent load, a LT265/75R16 Load Range C must be inflated to 50 PSI. Using this
example, even LT265/75R16 Load Range D, or E must be inflated to 50 PSI to carry the P-metric load at
35 PSI. LT tires do not offer any benefits of being "heavy duty" when under-inflated."

http://www.nittotire.com/assets/safety/Replacing%20Tires%20on%20Light%20Trucks.pdf

AxleIke
12-15-2010, 09:08 AM
I can see difference of tire diameter mattering in a mini with a fully geared, non-differentiating t-case.

But with the 80, it shouldn't matter. The minimal tire diameter difference between the front and rear tires due to tire pressure would cause insignificant speed difference front to rear compared to the difference in speeds front to rear in a sharp turn or u turn on pavement. The center diff can compensate for an entire axle not moving, hence the need for a center diff lock.

Jacket
12-15-2010, 10:37 AM
So what you are saying is that I can't convert my 80 into a funny car? Dang!

http://img181.imageshack.us/img181/360/paulblevinsveganf0.jpg

AxleIke
12-15-2010, 11:24 AM
We couldn't keep our 4Runner long enough for me to figure out much about it, though I assumed that a Torsen type limited slip center diff like Toyota uses can obviously deal with it. What I don't know is if constant compensating will harm it long term. It seems like asking it to always be torque sensing would be just like running uneven diameter tires on an axle locking diff. These things are designed to intermittently decouple/lock on pavement, not be held in a state of constant partial decouple. Your point is totally valid, that if it couldn't then Toyota would say use the same inflation on all 4 tires, which they apparently do not.

If the center diff is like the locking axle diffs, then running constantly at different speeds won't hurt it. But, if its like you say, and it is a torsen style, then yeah, I imagine long term it could hurt it. However, I still maintain that the difference in tire diamether in the pressure differences we are talking about, is so small as to be fine even in the torsen style. Yes, if you were running 33's up front, and 35's out back, then yeah, but if you are running a 35 and a 35.2, due to pressure difference, no problems will crop up.

rover67
12-15-2010, 12:17 PM
but if you are running a 35 and a 35.2, due to pressure difference, no problems will crop up.

I think the pressure difference is meant to do exactly the opposite on an 80...

more air in the back to keep the effective diameter the same as the front when loaded.

Tch2fly
12-15-2010, 12:36 PM
Toyota definitely uses a Torsen-type and I'm pretty sure that I agree totally with you. Just throwing it out there for discussion.

To be clear the US 80 series (most of the references in this thread) had two t-cases, for the FZJ80 a HF2AV w/Viscous Coupling or in the FJ80 the HF2A which only has the diff lock. The Torsen-type were used much later, i.e. your 4R, and are very different systems.

I have heard many discussions about the operation of the Viscous Coupling but I have only seen one situation where a V/C went bad due to different size tires and it was one a Previa All-trac. The customer had replace the tires on the front axle with a different size than the rear. The engineer said there was enough difference in the output shaft speed that the VC was constantly trying to over come the "slipping" it sensed. Initially the V/C would "lock up" (aka hump) while trying to turn tight turns but got worse unitl any turn would do it ( pretty odd sensation at 55 mph :eek:) it was replaced and no further issues.

Tch2fly
12-15-2010, 01:24 PM
... The 100 is a VC, right?

No, I beleive the 100 series use the HF2A t-case like the early 80.

AxleIke
12-15-2010, 02:29 PM
I think the pressure difference is meant to do exactly the opposite on an 80...

more air in the back to keep the effective diameter the same as the front when loaded.

Sure. I just am saying that say you got to camp and unloaded. Running with the rears over inflated isn't going to hurt anything.

rover67
12-15-2010, 03:01 PM
oh yeah.. no way. I agree.

corsair23
12-15-2010, 04:33 PM
Will check.

What about the inflation difference for P metric vs. LT. Nitto says:

"Please note that size-for-size, LT-metric tires require higher air pressures to carry equivalent loads of P-metric tires, and that any failure to adjust air pressures to achieve the vehicle’s load requirement will result in tire fatigue and eventual tire failure due to excessive heat build-up. Due to the higher PSI requirements of LT-metric tires, they may not be suitable for replacing O.E. P-metric tires because of the ride harshness that results from higher PSI requirements.
P265/75R16 114 Max Load = 2,601 lbs @ 35 PSI. In order to carry the equivalent load, a LT265/75R16 Load Range C must be inflated to 50 PSI. Using this
example, even LT265/75R16 Load Range D, or E must be inflated to 50 PSI to carry the P-metric load at
35 PSI. LT tires do not offer any benefits of being "heavy duty" when under-inflated."

http://www.nittotire.com/assets/safety/Replacing%20Tires%20on%20Light%20Trucks.pdf


Not sure about what others do but I typically run around 6-10 psi more in my LT tires than I did with P tires. Most threads I've read stipulate +10 over stock when running LT tires on a vehicle originally placarded for P tires.

The reality is running 42 psi in tires makes the 80 ride like a brick so I've experimented and with my Toyo MTs found that 38 psi for around town driving works best. When loaded for CM etc. and pulling the trailer, I add more air to the rear tires.

Here is another .pdf file with charts etc. for doing the conversion from P to LT tire size.

Inukshuk
12-15-2010, 05:50 PM
Neat chart. I'll check that out. I typically ran my BFG A/T's at 36-38.

Now we need some Denver snow so I can see how well they work.

Not sure about what others do but I typically run around 6-10 psi more in my LT tires than I did with P tires. Most threads I've read stipulate +10 over stock when running LT tires on a vehicle originally placarded for P tires.

The reality is running 42 psi in tires makes the 80 ride like a brick so I've experimented and with my Toyo MTs found that 38 psi for around town driving works best. When loaded for CM etc. and pulling the trailer, I add more air to the rear tires.

Here is another .pdf file with charts etc. for doing the conversion from P to LT tire size.

Inukshuk
12-16-2010, 10:48 PM
Here is another .pdf file with charts etc. for doing the conversion from P to LT tire size.

So, stock for a 1993 FZJ80 is P275/70 R16 114H (owners manual) at 32 PSI per the door jamb sticker, which, for the 114 Load Index you get 2355 as standard load.

My new Nitto Terra Graplers are LT285/75 R16 122/119Q in a "D" Load Range. (I cannot figure out why there are two numbers here "122/119Q" where I think there should be one. This is the Load Index spot, and the Q is the speed rating. anyone know?)

Going to the Discount Tire tables, for the 122 Load Index there is the closest to stock load rating of 2340 in the 40PSI column and for the 119 Load Index there is the closest to stock load rating of 2330 in the 45PSI column.

I will see what 40 PSI rides like.

jacdaw
12-16-2010, 11:16 PM
<snip>My new Nitto Terra Graplers are LT285/75 R16 122/119Q in a "D" Load Range. (I cannot figure out why there are two numbers here "122/119Q" where I think there should be one. This is the Load Index spot, and the Q is the speed rating. anyone know?)Q = 99mph so I dunno. Maybe load for 2 different rim widths?

corsair23
12-16-2010, 11:59 PM
My new Nitto Terra Graplers are LT285/75 R16 122/119Q in a "D" Load Range. (I cannot figure out why there are two numbers here "122/119Q" where I think there should be one. This is the Load Index spot, and the Q is the speed rating. anyone know?)

Dunno about the dual numbers but looking at Nitto's sight for the 285/75R16 D load range they only list 122Q so I'd go with that. Looks like 119Q is for the 265/75R16 D tire :dunno: