Mismatched small spare tire on an 80. Effects? (1 Viewer)

[BrettinSanAntonio]

I'm planning on using a smaller 30 inch tire as an emergency spare tire and the discussion came up among my cruiserhead friends as to the effects on the viscus diff. My regular tires are 33's. So, if I'm using a 30 to get off the trail and to a tire shop, what are the effects on the center diff?

If the front and rear axles are open and I lock the center diff, will this keep the viscus from damage. Realize the smaller diameter tire is only a temporary and will be removed upon repairing the damaged tire.

Comments?

 

[ERG80]

The owners manual says run the same size tires. You would probably be ok running slow for a short distance. If you had to run mismatched tires it would be best to remove the front drive shaft and lock the center I would think.

Im not sure what the effects would be.

 

[BrettinSanAntonio]

If the front is open, what difference would it make? I understand there would be stress on the locked center diff (in turns and/or dry pavement) but it seems like that would be preferable to stress on the viscus.

Any ideas?

 

[bpassmore]

locking the center will be sufficient for eliminating the viscous coupler issues, but raises the front/rear bias to a very dangerous limit. I would remove the front drive shaft, or install hubs and unlock one during the time (with the locked cdiff) of the limp run.
Running on the odd size spare will do more "damage" to the spider gears if the above precautions are observed and mitigated. Its all subjective. Just take is easy and you will be ok.

 

[96Cruiser13]

wouldn't it be similar to spinning one tire in the mud unlocked? That doesn't seem to harm my truck at all..

 

[bpassmore]

wouldn't it be similar to spinning one tire in the mud unlocked? That doesn't seem to harm my truck at all..

now, do that under pressure for 30 miles. yes, you will damage something.

 

[WarDamnEagle]

I just bought new BFG AT's in 285-75R16 and ponied up for 5 for this very reason. It's really not worth the risk if you ask me for the price of 1 tire. If nothing else, see if you can find a used tire in 33".

 

[MikePL]

You have two options...

1. Run a similar spare wheel and simply change wheels when you have a flat.

2. Run a different size wheel and you either have to remove one of the drive shafts to continue or you will fry the viscous coupler. As simple as that.

When your viscous coupler is fried/seized you will put stress on the driveshafts, differentials and birfs. After some time you will have play in all those components. The front is going to suffer the most as it is pressed down by the engine and the wheels won't be able to spin to relieve stress. You will end up throwing away the viscous coupler, changing bearings in the front diff (or maybe more), changing birfs or maybe you will strip a driveplate. Don't ask me how I know... My f***ing previous owner drove with odd wheels and now I have to repair what his stupidity caused.

So do you still want to have a different spare on board?

 

[jamisobe]

I did some more research on this, but the question of the 80's VC tolerance still remains. An open differential is no problem since they, by design, allow different axle speeds sending power to the path of least resistance. During a turn, the open diff sends power to the axle half turning faster on the outside of the turn. So going in a straight line a smaller tire will turn faster, become the path of least resistance, and thus become the drive wheel where normally both wheels would drive in a straight line.

HowStuffWorks "Open Differentials"



The issue is the center differential which is either a viscous coupler or switched to full lock. The center will see the fastest axle half speeds coming from each differential. That being the case, if the rear has a smaller tire then the rear drive shaft will want to turn faster than the front drive shaft.

I am thinking that if the center is locked either the small tire will constantly slip forward, the front wheels will constantly slip backward, or something will break.

So this leads me to believe that the best practice would be (assuming you cannot repair the tire on the vehicle) to disconnect the front drive shaft when the smaller spare is used in the front or rear until you get to a place for tire repair. No real danger other than obvious handling issues. Would there be any issues whether the center is locked or unlocked with the front shaft removed?



There is still a question of how far one can travel, with input shafts turning at different speeds, without causing damage to the vc. There is some tolerance required for normal operation during turns and, of course, for slippage offroad. How tolerant is the 80's vc to varying shaft speeds?

I am thinking that if one must ravel with a mismatched spare that one should be prepared to remove the front drive shaft. Not sure about whether it matters as far as locking the center.

 

[Darwood]

Brett, what size tires are you running? I have a 295/75/16 BFG AT in my garage doing nothing but that might be to big.

 

[Ebag333]

So this leads me to believe that the best practice would be (assuming you cannot repair the tire on the vehicle) to disconnect the front drive shaft when the smaller spare is used in the front or rear until you get to a place for tire repair. No real danger other than obvious handling issues. Would there be any issues whether the center is locked or unlocked with the front shaft removed?

<snip>

I am thinking that if one must ravel with a mismatched spare that one should be prepared to remove the front drive shaft. Not sure about whether it matters as far as locking the center.

I would think that it would be better to place the mismatched tire in the rear, and remove the rear DS.

If you place it in the front that's a lot of additional strain on the birfs, etc.

Additionally if you run with only one DS you must lock the center diff. You can drive without locking it if you have a VC, but that's only because the VC will think you are slipping really badly on the rear and always be engaged. When it's engaged, you get lots of additional heat. When you get lots of additional heat, bad things happen (like welding itself together).

I guess if you wanted the center diff permanently locked that'd be one way of doing it.....

And of course if you don't have a VC if you don't lock the center diff, you're not going anywhere.

 

[jamisobe]

I would think that it would be better to place the mismatched tire in the rear, and remove the rear DS.

If you place it in the front that's a lot of additional strain on the birfs, etc.

Additionally if you run with only one DS you must lock the center diff. You can drive without locking it if you have a VC, but that's only because the VC will think you are slipping really badly on the rear and always be engaged. When it's engaged, you get lots of additional heat. When you get lots of additional heat, bad things happen (like welding itself together).

I guess if you wanted the center diff permanently locked that'd be one way of doing it.....

And of course if you don't have a VC if you don't lock the center diff, you're not going anywhere.

In theory, it makes sense as far as locking the center when a drive shaft is removed. But why would you prefer to remove the rear DS?

If worried about additional stress on the birfs then put the spare on the rear and remove the front DS. I don't see how axle halfs turning at different speeds with an open diff will stress the birfs.

 

[BrettinSanAntonio]

tire size is the issue

I just bought new BFG AT's in 285-75R16 and ponied up for 5 for this very reason. It's really not worth the risk if you ask me for the price of 1 tire. If nothing else, see if you can find a used tire in 33".

Well, in this case, the issue is not that I don't have a spare that's the same size. I have five, BFG 33's BUT I've gone from having a rear tire carrier to a new Kaymar bumper that doesn't have swing outs. Why is a whole other subject but let's just say, I like the new setup without the swing outs better.

So, where to put the spare? Slee has a really nice internal set up but I don't want to give up space inside my rig for a 33 inch spare.

Why not put it under the truck in the factory location? This is what I planned on doing, HOWEVER, Kaymar bumpers are designed in such a way that there is not enough room for anything larger than a 30 or 31 inch tire in the factory location. Even with the mod to the tire lift (lifting the tire cross member 2.5 inches) the built in hitch receiver comes back to far to allow a larger tire to be stored under the rig.

So, this is why I'm looking to use a smaller tire as a temporary spare.

 

[ThePookieBear]

What if you put two of the normal 33's on the rear, then removed the front DS, and ran the smaller one up there? That would probably cause the least amount of stress on the system. I would just get another 33" tire though.

 

[BrettinSanAntonio]

What if you put two of the normal 33's on the rear, then removed the front DS, and ran the smaller one up there? That would probably cause the least amount of stress on the system. I would just get another 33" tire though.

I've got five 33's so thats not really the issue. It's storage space for the spare 33 that's the problem.

Well, in this case, the issue is not that I don't have a spare that's the same size. I have five, BFG 33's BUT I've gone from having a rear tire carrier to a new Kaymar bumper that doesn't have swing outs. Why is a whole other subject but let's just say, I like the new setup without the swing outs better.

So, where to put the spare? Slee has a really nice internal set up but I don't want to give up space inside my rig for a 33 inch spare.

Why not put it under the truck in the factory location? This is what I planned on doing, HOWEVER, Kaymar bumpers are designed in such a way that there is not enough room for anything larger than a 30 or 31 inch tire in the factory location. Even with the mod to the tire lift (lifting the tire cross member 2.5 inches) the built in hitch receiver comes back to far to allow a larger tire to be stored under the rig.

So, this is why I'm looking to use a smaller tire as a temporary spare.

 

[bpassmore]

I'm planning on using a smaller 30 inch tire as an emergency spare tire and the discussion came up among my cruiserhead friends as to the effects on the viscus diff. My regular tires are 33's. So, if I'm using a 30 to get off the trail and to a tire shop, what are the effects on the center diff?

If the front and rear axles are open and I lock the center diff, will this keep the viscus from damage. Realize the smaller diameter tire is only a temporary and will be removed upon repairing the damaged tire.

Comments?

I think we are really over complicating this. (myself included)

I don't see a problem with running a smaller tire as a temporary fix. The biggest difference is that the rear end (assuming that the spare is mounted there) will have more spider gear rotation activity while you limp back to the shop.

The VC can cope with this, the front and rear DS will be spinning at or near the same RPMs, just the spiders will be spinning faster on that rear axle while the one tire is smaller.

This is a temporary fix to get home, right? not a long term patch.

 

[Chaldaean]

Yeah, I agree with everyone else that another 33" tire is the best solution and is the route I would take.

However, if you found yourself with three 33s and one 31 with no other options, could you Mickey Mouse it and let some air out of the 33s until the bottom-of-rim to ground distance is the same on all four tires? Although less safe, it seems easier than removing the drive shaft.

 

[Grench]

I've got five 33's so thats not really the issue. It's storage space for the spare 33 that's the problem.

I ran with my 285/75R16 MTR spare tire under my truck for close to a year. Ground clearance wasn't great, but it fit up there and snugged up fine for me. I did have to be careful to line it up right while cranking it up. It was not ideal, and the spare was very visible from behind.

I've since gone to a Slee rear swing out and put a 24 gallon aux tank in where the spare used to be.

YMMV

 

[BrettinSanAntonio]

I have to agree and this is what I was thinking. We're talking a very temporary situation. In addition, the place I'm most concerned about tire wise, is primarily loose, dirt roads that will permit slip.

I really don't intend on removing drive shafts. If I can't fix the tire in the field with a plug, I'll limp back to the gas station or nearest tire shop.

I think we are really over complicating this. (myself included)

I don't see a problem with running a smaller tire as a temporary fix. The biggest difference is that the rear end (assuming that the spare is mounted there) will have more spider gear rotation activity while you limp back to the shop.

The VC can cope with this, the front and rear DS will be spinning at or near the same RPMs, just the spiders will be spinning faster on that rear axle while the one tire is smaller.

This is a temporary fix to get home, right? not a long term patch.

 

[BrettinSanAntonio]

I ran with my 285/75R16 MTR spare tire under my truck for close to a year. Ground clearance wasn't great, but it fit up there and snugged up fine for me. I did have to be careful to line it up right while cranking it up. It was not ideal, and the spare was very visible from behind.

I've since gone to a Slee rear swing out and put a 24 gallon aux tank in where the spare used to be.

YMMV

Yes, I'd do this but as per the previous posts, the Kaymar bumper doesn't provide enough room for a 33 in the factory location. I did the tire cross member spacing modification, and that really allows the spare to tuck up nicely under the truck.

Swing outs are OK but I prefer to have the rear hatch clear. I just removed my swing out carrier.