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Old 02-17-2015, 06:23 PM
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4x4 and not matching tires question

So being as my 2011 Wk2 is my first 4x4 vehicle coming from a line of FWD foreign cars, Ive always heard that you must have all 4 matching tires with even wear due to the 4x4 system...I am running 2 different wheel set ups, my summer set up is the oem forteras in 265/50/20 and my winter set up is 275/55/20 Blizzaks,I will be going on 3000 mi trip (1500 one way then back) and want to throw one of the winter set up wheels in the back to use as full spare, if it came down to a situation that I need to use a spare, will I harm the 4x4 system at all running one different tire and if not, then about how far can I drive that way? sorry if dumb question, but need to know.

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Old 02-17-2015, 06:47 PM
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Re: 4x4 and not matching tires question

Rule of thumb... it's generally not a good idea to mix tires of different types, sizes, load or speed ratings on any vehicle. Doing so can affect vehicle handling characteristics, cause excessive tire wear and potentially damage the 4WD components and void your warranty.

Could you do this to get you down the road a few miles to get a flat tire fixed? Probably... but it still is a risk. It's up to you to decide if it's one worth taking.

What's wrong with using your factory spare?
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Old 02-17-2015, 06:54 PM
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Re: 4x4 and not matching tires question

Quote:
Originally Posted by MDBones View Post
Rule of thumb... it's generally not a good idea to mix tires of different types, sizes, load or speed ratings on any vehicle. Doing so can affect vehicle handling characteristics, cause excessive tire wear and potentially damage the 4WD components and void your warranty.

Could you do this to get you down the road a few miles to get a flat tire fixed? Probably... but it still is a risk. It's up to you to decide if it's one worth taking.

What's wrong with using your factory spare?
My spare/jack/tools were all missing when I purchased off of auction, just makes me wonder since the spare is not identical either...whats the big difference.. I think I may need to do some research on the whole idea of negative effects of mismatched tires to 4x4 system and what is actually affected
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Old 02-17-2015, 06:58 PM
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Re: 4x4 and not matching tires question

Quote:
Originally Posted by eugene3524 View Post
My spare/jack/tools were all missing when I purchased off of auction, just makes me wonder since the spare is not identical either...whats the big difference.. I think I may need to do some research on the whole idea of negative effects of mismatched tires to 4x4 system and what is actually affected
You can find your answer pretty quickly and MD already answered it for you.

You will harm your 4x4 system with that big of a difference in tires if you choose to use it as a spare for an extended duration. A few miles probably wouldn't hurt it too bad, but when is that ever how far you need to go on a spare?
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Old 02-17-2015, 07:10 PM
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Re: 4x4 and not matching tires question

I've never been a fan of compact spares, period. I have the OEM Forteras with an oddball size, skinnier Kuhmo compact spare. The only thing it has going for it is that it matches the OD of the Fortera... or at least close enough for limited use.

The owners manual sets speed and distance restrictions when using that spare. And I guarantee the vehicle will have some squirrely handling characteristics when running it.
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Old 02-17-2015, 09:39 PM
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Re: 4x4 and not matching tires question

As others have said, yes, you can damage the system running different size tires. Especially if you do so at highway speeds.

However, the damage comes in from the heat build-up from the circumference difference between the tires. From the perspective of your diffs, front to back looks like you're constantly slipping and left to right looks like you're constantly turning in one direction. That adds wear and tear on your fluids and any slipper clutches involved, depending on type of diff.

You can minimize that heat build-up by driving slowly.

For instance, Subaru includes a small doughnut wheel/tire as their spare. They advise keeping the speed 40MPH or less if you use the doughnut. They also advise moving a good tire from the rear to the front if your front tire is wacked and putting the doughnut on the back only. This improves braking and cornering safety.

Full size spare is best, but a doughnut is better than being stranded.
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Old 02-17-2015, 09:58 PM
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Re: 4x4 and not matching tires question

That makes more sense now, I did some additional research on the subject and how the transfer case works with the differentials... Not the best idea for long term but can do as a short term spare, thanks guys
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Old 02-18-2015, 01:33 AM
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Re: 4x4 and not matching tires question

Can anyone confirm what actually gets damaged? There appears to be a lot of hearsay. I would expect the following possible issues with the WK2
-Diff output shafts rotating at different speeds causing spider gears to rotate (I think this is what may cause damage at high speeds) WK2 however does not have LSD so not too big of an issue.
-Transfer case detecting wheel slip and applying torque fowards or backwards.
-Electronics activating traction control, it should be possible to disable this via the ESC button or pulling a fuse.
- Uneven tyre wear, negligible over short distance.
- Rear diff locking on QD2 causing axle wind up.

I dont see how any of these issues would cause damage over a short term (eg. 100km), even at decent speeds. Please point out any issues with my theory or points I have missed.
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Old 02-18-2015, 06:17 AM
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Quote:
Originally Posted by indydof View Post
- Rear diff locking on QD2 causing axle wind up.
Note: I'm only speaking about WK2's equipped with QuadraDrive II here.

As you pointed out, the possibility of the rear diff locking on QD2 caused by the system detecting "slippage" in the case of different sized tires was something that came to mind. That seemed like it could cause damage if the odd sized spare were used in the rear. However, the front diff on QD2 is open and any left vs. right slippage there is controlled via differential "brake traction control", at least the way I understand it.

In general, someone else already pointed out that it is conceptually similar to constantly turning the steering wheel in one direction and that's not something that would cause damage to the drivetrain.
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Old 02-18-2015, 07:41 AM
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Re: 4x4 and not matching tires question

If it's any difference, I have the Quadra_Track2 that is on my limited, not QD2
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Old 02-18-2015, 11:50 AM
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Re: 4x4 and not matching tires question

Subaru AWD on my manual 2011 Outback is different than Jeep, but they use a center diff with fluid that binds as it warms, locking the front and rear together. If your front and rear are on pavement and spinning at different speeds that will cause major problems since the tires won't slip to release those stresses.

Since the JGC is full time 4WD, I would imagine a similar mechanism is in play.

If your diff or transfer case fluids overheat, they break down. Less lubrication causes more heat and so on. If that cycle goes runaway because heat can't be dissipated fast enough, the gears seize. Expensive.

Hearsay or not, it isn't an experiment I'm willing to try on my vehicle, especially given much less expensive easy alternatives.
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Old 02-18-2015, 01:35 PM
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Re: 4x4 and not matching tires question

Quote:
Originally Posted by akinney View Post
Subaru AWD on my manual 2011 Outback is different than Jeep, but they use a center diff with fluid that binds as it warms, locking the front and rear together.
Yeah, that would be bad. You seem to be describing a viscous coupling. Neither QuadraDrive II nor QuadraTrack II use that approach, opting instead for "real" 4x4 rather than AWD.

Jeep Grand Cherokee WK2 - Transfer cases

Here's a white paper about our t-cases.

Quote:
If your front and rear are on pavement and spinning at different speeds that will cause major problems since the tires won't slip to release those stresses.
Only if you have a locked center differential, such as in AWD once the viscous coupling locks the diff as you described, or if you lack a center diff altogether such as in "part-time 4WD" (as in Toyota vehicles), or if you have a "real" 4x4 and lock your center diff.

That's why those part-time 4WD vehicles are only allowed to engage 4WD on surfaces with slip (gravel/snow/whatever), and they can grenade their t-case if driven in 4WD on dry pavement and turn the steering wheel (driveline binding occurs due to the lack of center diff and inability to let one wheel slip to compensate).

This illustrates the problem: Part time 4WD should not be used on pavement

We can lock our center diff by engaging 4WD Low. This is also the reason why you never want to drive in 4WD Low on pavement that isn't slippery.

Because our QT2/QD2 vehicles have a center diff, we can negotiate turns on pavement in 4WD High (aka "Auto") without stressing our t-case. Furthermore, the front diff is open in QD2 meaning there is no way to lock it left/right (that's what the selective braking attempts to mimic). In QD2 the rear axle *can* be locked left/right (which is what the ELSD does)... that seems like a bad scenario if the tires were mismatched left/right in the rear in case the system decides to lock the rear diff to compensate for perceived "slip" due to the mismatched tire OD.
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