After a crappy repair job by a garage to replace the rear differential, we were told that the were now officially frowarned that by having 2 new and 2 worn tires (new on front, worn on back but worn meaning about 2-3 years of wear left - certainly not bald), this creates excessive wear on the rear differential (full-time 4 wheel drive large SUV).
Sounds like bullshit to excuse them using a poorly rebuilt rearend or screwing up the replacement. Is there any rationality to their contention that the problem is caused because of the tires?
Sounds like bull to me. I think they are trying to say that because two of the tires are worn, they have a (slightly) smaller diameter than the others. This would cause the two axles to turn at different rates. The difference would be very slight, and I would expect any vehicle with all time four wheel drive to have an way to compensate for this. The rear tires will travel a shorter distance than the front when turning under normal conditions anyway.
The problem would be at the transfer case, not the rear differential. And if it is a full time 4 wheel drive, the transfer case would be able to handle this with no problems. It sounds like BS to me too.
If the front is going faster than the rear because of tire difference, there will be wear. Also, the antilock brakes will be affected because it will sense slippage in the faster turning tires.
That’s where the problem is. They say the differential is getting uneven wear but the repair was a couple weeks ago and the difference in tread is maybe 1/2 inch. You’d have more difference in tire radius simply be not keeping tire pressure consistant.
The rear differential itself won’t experience “uneven” wear due to tire size issues. If it’s equipped with a limited-slip feature (which prevents one wheel from spinning wildly when it encounters a very-low-traction condition), then the limited-slip device (depending on type) may experience undue wear, but only if the left and right tires on the rear axle are of different diameter.
The front differential has the same situation: its limit-slip mechanism (if any) could wear only if there is a left-to-right difference in tire diameter on the front axle.
In the case of your full-time 4WD vehicle, the transfer case splits power between the front and rear differential units. It is this transfer case and its limited-slip mechanism (if any) that could experience undue wear if there is a substantial front-to-rear difference in tire diameters. “Substantial” in my mind means using the wrong size of tire, not just a mismatch in wear.
Bottom line: If both rear tires are the same diameter, then the left and right outputs of the rear differential are spinning at the same speed (when driving straight down the road), and the rear differential should be operating normally. It receives power from the transfer case, and it has no idea what is going on up at the front axle; all it knows is the torque and RPM at its own input yoke. Hell, you could lock up the front wheels with wheel clamps, and the rear differential wouldn’t know a thing about it (but the transfer case might not last forever…:D).
They aren’t yanking you around. The center differential* on AWD vehicles can be extra sensitive to tire size differences because they will be forced to operate 100% of the time instead of just when the tires slip or in turns. Read the owners manual. Some manufactures require the tires to be as close as 1/16th of an inch in tread difference.
This is a differential that AWD vehicles have inside the transfer case, not the rear differential in the middle of the axle.
One might note that the difference in rotation rate between the front and rear drive axles with different diameter tyres front to back will be the difference you would see if the left and right tires on the same axle were different, multiplied by the final drive ratio. So the centre differential will see maybe four times the rotation rate difference. The type of transfer differential will matter. A viscous coupled centre diff will probably not see enough slip to start to grab, although it might heat up a little. A Torsen diff, or any diff with a friction element may become much less happy with the constant difference in rotation rates.
It’s unclear. Later someone mentioned the transfer case and the OP replied that’s where the problem is. Regardless, having it not be a problem in the rear doesn’t mean it’ll be ok. I imagine the center diff is a more expensive fix than the rear.
I’m not so worried whether the mechanics said rear or center, it can cause problems in the center no matter which one that are trying to cover their butts on. Think of this; your tires have a tread difference of 1/2 inch, which sounds small but is the same as an inch difference in tire size.
I saw a page last night that said manufacture recommendations vary from “have to be the same” to “1/2 inch max difference in tread” so yours are at the upper end of most recommendations. If you’re lucky, your car will be one of the more tolerable ones and your manufacture is in the 1/2 camp. I would think it would be in the owners manual somewhere.
Depends on the year make model etc.
I can name one car where one year model a 1/2" difference would cause damage at freeway speed ( but not around town) the following year a 1" difference would have no effect.
Reason? Different systems.
You can’t make blanket statements about this, there are two many different systems out there.
Why I think it is BS is that the mechanics never mentioned it when repaired the first time and the grinding started immediately after we picked it up and got worse over 2 weeks. THEN they dicide to tell us the problem was not with the rebuilt differential but one set of tires being a couple years older than the other.
According to my info it has a Quadra-Trac I system, with a transfer case which allows the front and rear driveshafts to rotate at different speeds without causing damage to the drivetrain. Likewise, differentials are specifically designed to allow their left and rear axle to rotate at different speeds, hence the name. For argument’s sake, I’d allow that possibly some limited-slip differentials might wear faster if they’re constantly dealing with different speeds such as different size tires from side to side might cause (though I question that), but that’s not going to cause noise immediately, or even after two weeks, and that’s not the situation here anyway. I agree that it sounds like they’re trying to BS their way out of a faulty repair.
In a nutshell, different tire sizes front to rear cannot cause the rear (or front) differential to fail prematurely, and on this vehicle shouldn’t be an issue relative to any part of the drivetrain.
I agree that it probably wouldn’t start grinding immediately after picking it up and the rear differential wouldn’t be related to any difference in tire size.
But for mismatched tire sizes (which is now somewhat unrelated to what’s happening with the shop), the 2006 Commander can have three different systems and only one will tolerate tire differences.
Quadra-Trac I is safe because it uses an NV140 transfer case with an open differential. It also uses a brake control system to react to slip too, which would make me worry except for one thing, Quadra-Trac I isn’t really AWD. Well, who knows what the definition of AWD is, I guess it’s AWD if they call it so. But the NV140 transfer case delivers a 48/52 split between the axles. The front axle (48%) is not normally getting enough power to actually pull the vehicle, it’s only getting enough to react quickly. If the rear slips, it lags enough for the front to take over and start pulling. It’s very basic and fairly passive. Any tire difference will be far less that %2 torque difference.
**
The Quadra-Trac II**'s NV245 transfer case uses brake control and a 48/52 split, but it has a lot of active sensors and an electronically controlled clutch pack to respond to any difference in torque. The clutch packs will experience problems with different tire sizes.
The owner’s manual just says “Proper operation of four wheel drive vehicles depends on tires of equal size, type, and circumference on each wheel. Any difference will adversely affect shifting and cause damage to the transfer case” and doesn’t list any tolerances. The NV245 will actually throw error codes if it detects a difference in the tires. I don’t know the amount of difference it will tolerate before throwing codes.
The Quadra-Drive II uses the same NV245. The only real difference I’m aware of between Quadra Drive and Quadra-Trac II are the differentials and some electronic wizadry that constantly varies the torque split between front and rear axles. I don’t know a lot about it but Quadra Drive should be the same- if not less- tolerant of tire differences than the Quadra-Trac II.
So, regardless of the shop and the rear differential, the tire sizes may still be a separate concern.
I didn’t mean to imply a blanket statement on it. I think 0-1/2 inch is where most manufactures fall, but that’s just “most.” I was going to add that it wasn’t a rigorous survey of all manufactures and some can surely pass with higher differences, but my phone kept autocorrecting “rigorous” and I gave up thinking it was unnecessary.