VC and undersized spare tire

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Sep 23, 2004
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MTNRAT said:
BTW you will need 5 tires. You cannot run an undersized spare with the 97's viscous coupling.
Cheers,
Sean
Oh and congrats and welcome.
Noticed this in another tread about tires. I have 305 tires and only carry the stock 275 as a spare. I figured it wouldn't be ideal on the drive train but better than nothing and wouldn't affect things too much for a limp home. Why will an undersized spare not work with the VC and is it a 97 thing? And what if I just lock the center diff if I have to run the smaller spare?
 

Doc

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It's not just a "97 thing". Any of the full time 4wd's need 4 identically sized tires to operate correctly.

As for a 'limp home' I don't think anyone has ever tested how long you can run differing sized tires on our rigs, the x-fer case is too expensive to burn up though.... so exercise your best judgement and caution.

As for locking the center diff.. it's an intersting idea, but I don't know if the strain on the gears for force your tires to 'chirp' will be less than the strain on the x-fer from constantly slipping.

Maybe time to look into a rear carrier? ($$$ I know.... but it's a good excuse...)
 
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It is not just a 97 thing it is all 80's with a viscous coupler witch would be 93 to 97.

When the front and rear drive shats turn at diffrent speeds the viscous coupler resists this movement, this resistance tunrs into heat, the VC can handle tuns just fine but constant rotation will overheat the VC and the rest of the transfer case shortening both of thier lives, Robby has seaid he has found several T-cases gummed up with sludge, heat would be a major cause here.

would be nice to be able to tell you "as long as the tires are within x' incehes, and you travel no more than y' miles at no more than n speed then you will be OK" but no one that I know of has this info.

get a same size spare.
 
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I don't follow. Are you all saying that the rotational difference from having one different tire size, is taken out in the TC and not the open diff at the axle? Now if both front tire were smaller, then yes I see where the rotational difference would have to be at the TC.
 

CreeperSleeper

Cascade Cruisers
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It would be taken out on both, depending on what tire the axle "desided" to put power to at a given time. I see what you are saying, but it would take a toll on both. It's just that the axle would be more durable in that situation.
 
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The center diff/TC will see a F/R difference that is the average of the two different size tires on one axle vs the other axle that has same size. Toyota knew that, but it doesn't change that there will be a difference they don't want you to run at. I'd get off a trail with the spare (w/o lockers), and I'd drive home slowly with the flashers on. But I wouldn't drive it normally.

DougM
 
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LR_RESQ said:
It would be taken out on both, depending on what tire the axle "desided" to put power to at a given time. I see what you are saying, but it would take a toll on both. It's just that the axle would be more durable in that situation.
I guess I don't have a clear enough understanding of the system then. I thought the TC took out differences between the front and rear axle, and the diffs took out the side to side differences.
 
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Doc said:
I don't think anyone has ever tested how long you can run differing sized tires on our rigs, the x-fer case is too expensive to burn up though.... so exercise your best judgement and caution.
Kurt has replaced 2 center diffs due to (he speculates) VC overheating. The last that I remember, he was pretty sure the cause was running different sized tyres for extended periods.

There was another 'Mud member that was running 1 or 2 different sized tyres from a 4-Runner or something. He bought the vehicle used and the PO worked for a Toyota dealer; service manager or something like that. He was experiencing severe center-diff "lock-up" to the point that he could barely drive the vehicle after the VC heated up. I can't remember all the details but the thread is in the archives. I will try to find it.

[ Later... ]

Can't find the post. Anyone else recall more details that would help in a search?

-B-
 
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CreeperSleeper

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That's true, but if the power is going to the smaller tire in the axle, it will change the overall drive ratio and transfer that to the TC.
 
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In the normal context of going down the road, the power is going to all four of the wheels. It is full time four wheel drive. To the extent that the tires are of differing diameters, they are rotating at different speeds, which will result in differences in driveshaft rotation rates between front and rear.
 
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Boston Mangler said:
You can fit a 305/70-16 in the stock spare tire location!
yeah and you could probably fit a 30570-16 in my wallet because its so empty.
 

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