Spare tire vs operating tire size?

fishguy1123

Original poster
Member
Dec 5, 2011
310
So, I'm need new tires and I'd like to go larger on stock 17" rims without any mods. My question is if I go to a 30.5 ish size and don't switch my stock spare will it do any damage for a temp (20-30 mile) run to get a new/repair tire to match? I know it's important to keep the same size, just want your advice on if it will screw something up in a short time if it came down to it. Thanks
 

Short Bus

Member
Dec 2, 2011
1,906
Just put the spare on the front
 

fishguy1123

Original poster
Member
Dec 5, 2011
310
That was my thought too, just want to make sure it won't damage anything even in 2wd.
 

ElAviator72

Member
Jan 11, 2012
118
Probably not such a big deal...I drove my Sunfire GT 40 miles on the donut spare on the front axle (drive axle in that car). Quite a bit smaller in diameter than the normal wheels. No damage, but the differential went "Whoop, whoop, whoop" all the way down the Interstate. I would only do that to get to the tire shop, though.
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
On the front in 2WD mode (not one of the AWD-only trucks), there will be absolutely zero issues with different size tires. Only the spider gears are spinning in the differential, not the pinion gear or the carrier, so the diff doesn't even know if you have a tire on the right front. All due to the splined disconnect.
 

Sparky

Member
Dec 4, 2011
12,847
As mentioned there will be no problem with having the smaller tire on the front, except that your truck will look funny :tongue:
 

ElAviator72

Member
Jan 11, 2012
118
For short distances, I don't think it's even that big a deal on the rear axle (at least, with the open differential). Not sure how the locking diff would handle different tire sizes, though... I've seen plenty of Jeeps, pickups, and other SUV's going down the road with an oversize tire and a stock spare on the drive axle. Wouldn't drive it that way for an extended period of time, though.
 

Sparky

Member
Dec 4, 2011
12,847
Maybe, but I'm sure we all see a lot of things going down the road that shouldn't be :tongue:
 

fishguy1123

Original poster
Member
Dec 5, 2011
310
Thank you everyone! I do have a g80 so I'd stick to putting the stock one on the front if I needed to. Now to decide on which to get... :undecided:
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
7,464
Brighton, CO
Or... You could get a hitch mounted spare tire carrier, and mount up 5 same sized tires..

I personally hate taking a good front tire off, putting on a undersized spare, to put the good front tire, on the rear blown tire.. just sounds like a lot of work when its 120 degrees out in Texas/kansas/Oklahoma.
 

fishguy1123

Original poster
Member
Dec 5, 2011
310
I use my hatch all the time since it's my work truck too. Probly be more hassle in my case. I don't blame ya on the 120 day thing! Worst case I call AAA and have them do it if it's really nasty cold/hot out. lol
 

leehype

Member
Mar 20, 2012
17
I have run smaller spare tires (within reason) on drive axles with open carriers with no problems. The biggest issue that comes up is with a limited slip carrier that uses clutch packs. I don't remember the math too well, but if you have a 32 tire on one side, and a 31 on the other and drive 10 miles, that is enough to completely burn out the clutch packs in a limited slip. Running a smaller tire on one side on the drive axle will also cause your speedometer to be slightly off, although not enough to matter much to most people. On my Jeep, I ran 32's and 33's, and ran a 31 inch spare, and used it often enough. Never made any difference. The Awd setting on the 4x4's might have an issue though, but I'm sure most people would not try AWD or 4WD with the spare tire.
 

ElAviator72

Member
Jan 11, 2012
118
leehype said:
I have run smaller spare tires (within reason) on drive axles with open carriers with no problems. The biggest issue that comes up is with a limited slip carrier that uses clutch packs. I don't remember the math too well, but if you have a 32 tire on one side, and a 31 on the other and drive 10 miles, that is enough to completely burn out the clutch packs in a limited slip.

So, does anyone here have the down and dirty as to how the locking differential (G80 option) in our trucks actually works? I'm sure someone somewhere has managed to burn one up...
:yes:
 

fishguy1123

Original poster
Member
Dec 5, 2011
310
I'm not good with links, but use the search function and use "how a g80 works" and that may help. Good luck.
 

ElAviator72

Member
Jan 11, 2012
118
fishguy1123 said:
I'm not good with links, but use the search function and use "how a g80 works" and that may help. Good luck.

What I gathered from googling it:

[video=youtube;tTGZOJQQBeE]http://www.youtube.com/watch?v=tTGZOJQQBeE[/video]

So, the locker, if working properly, won't engage above 20 MPH (there is a spring that sees to that). Doesn't look like it has a clutch pack in it, and much to my suprise, it's 100% mechanical (I thought it used cool electronics!). It becomes a standard diff at that point. Ergo, driving with different sized tires, you're just as safe as long as you're driving above 20 MPH. Now, how it reacts to different sized tires when you're getting going is a matter of debate...the vehicle might even handle in an unsafe manner (don't know, but it certainly seems possible). If the locker engages, you will have two tires of different diameter forced to turn at the same RPM, which will in turn cause the vehicle to turn towards the smaller tire. However, that shouldn't happen on dry pavement...
 

Sparky

Member
Dec 4, 2011
12,847
I can see the weakness in the G80 just from that video.
 

ElAviator72

Member
Jan 11, 2012
118
Sparky said:
I can see the weakness in the G80 just from that video.

How so? Other than the fact that I wouldn't want to disassemble/reassemble the unit unless I was an Eaton engineer, nothing looks overtly flimsy about it... :confused: Although since it's 100% mechanical, you're stuck with what the factory deems necessary for the set values on engaging the lockup :wink: No adjustment possible on that...
 

Sparky

Member
Dec 4, 2011
12,847
The mechanism it uses to lock is really pretty small. If you're on the gas hard enough when it locks it looks like it can snap those little parts right off. Those little parts then end up in places they don't belong and crunch goes the rest of the diff. That's usually when they break from what I understand, when power is applied right when it spins and locks.

Roadie and others I'm sure are more familiar with exactly what goes wrong in a G80 when it grenades so I'm sure they'll correct me if I'm wrong on the weak point :biggrin:
 

essaycho348

Member
Mar 7, 2012
58
ElAviator72 said:
Doesn't look like it has a clutch pack in it, and much to my suprise, it's 100% mechanical (I thought it used cool electronics!).
in the video when it gets to 1:40 what are the disk packs? wouldn't those be clutches? they look like disk packs/clutchs like on the inside of an automatic transmission.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
What Eaton calls "friction disks" are indeed clutch material, but the design is such that they engage with enough ramp-generated force that they share torque totally and do not slip. Limited slip diffs allow slippage.
 

essaycho348

Member
Mar 7, 2012
58
If one was to have different tire sizes on the rear and the fluid was old could that keep them from not engaging properly? before i changed the diff fluids i was having problems with lockup when i would get in on a muddy road or wet conditions, after i changed the fluid it locks up great now. so has their ever been a case of the clutches burning out?
 

leehype

Member
Mar 20, 2012
17
Most clutch pack based differentials require a additive compound to help the clutches stick and to help with wear.

About the comment about clutch packs, keep in mind, you have an axle shaft/tire spinning are a great speed, then slam the other axle shaft into it, that shock load will destroy parts. Think of a manual transmission. It has a clutch, but it doesn't slip when fully engaged, and you have to disengage it to get into gear when not moving. You can't simply slam it into gear from a stand still at a stop light (well, you could, but how long would it last?) You need slip somewhere to cut down on shock loads therefore prolonging the life of parts.
 

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