All Wheel Drive experts chime in please.

[mcantar18c]

I have an Associates in Automotive Technology, ASE certs, a decade of experience working in the automotive field and a lifetime of wrenching at home, and extensive experience in offroading as a hobby where axles/diffs/t cases/transmissions/etc. are things you need to know a bit about to build them.

And I have no f*cking clue why your rear diff would kill itself from having different sized tires on the front end.
I would, however, be happy to take a look at it as someone who's not in a position to make a profit off fixing it and see if we can't figure out what happened.

[Irving]

So did I do an okay job explaining everything? I tried to get as much info as possible.

Thanks for the offer, but it's not my car and it has already been repaired. The repair was thousands of dollars cheaper than it could have been ($370 I think). Just trying to figure out why.

I've had all kinds of suggestions thrown out there. Perhaps the factory tires are 215s and the owner got a great deal on some 205s and now the spare doesn't match. Maybe, but it is a space saver spare to begin with, and the two tires on the rear would be the same size, so it wouldn't matter anyway.

I think the best way to look at this is by isolating the rear differential. Looking at nothing else but a rear diff, what could cause it to get so hot that it would melt all three bushings?


Oh yeah, I did a quick google search as well, and found a thread where some kid put a spare on his 2005 Forester in the front, and the front diff caught on fire (white smoke), and was binding. That thread looked promising, but it was the front axle with the odd sized tire and the front diff that failed, so no help. Very strange.

[spyder]

Yes stuving, it will hurt your car. Subaru has that little fuse box for a reason.

[Irving]

But where will it hurt your car?

Oh yeah, I forgot to mention that Subaru America went as far as opening a claim file for me, then took their ball and went home. I was talking to this customer service girl, who was taking a lot of time to ask all her contacts my questions to get as clear of an answer as possible, until she was told to stop talking to me. She said, "They basically told me to stop talking to you and just give you that answer. I'm sorry I couldn't get a better answer for you."

The more I talked to Subaru America, the less info I got. They told me that they would just refer me to a dealer. I responded that I've already talked to 4 dealers, and wanted to go straight to the source. At first I got some feed back, but eventually they said, "Do what the manual says or stuff will break. If you want to bring it into a dealership, we'd be glad to check it out for you."

The mechanic that actually repaired the car was very adamant to not give me his opinion one way or the other. He clearly didn't want to get in the middle. The best answer I got from him was, "I have no idea how this happened. I really can't say one way or the other."


EDIT: I asked the mechanic who did the work about the fluid level. He said it was fine before he drained it. He said he checked the viscous fluid in the center diff and it was fine. He also said there was no damage to the rear diff, just burnt fluid and the bushings. I'm beginning to become concerned that he didn't check the car out all the way and this will turn into a nightmare.

[mcantar18c]

Where will what hurt your car? Having the smaller spare on the front? Third paragraph of my last post...

Talking to dealers generally gets you nowhere. They're all scum with IQ's of a goldfish. Now, if you physically go in to a dealership and ask to talk to one of their senior techs, and ask HIM about it, THAT might get you somewhere.

Did he check the transfer case itself, and not just the fluid? While the fluid of a component can usually give you an indication of what's going on inside of it, it doesn't always tell the whole story.

[Irving]

My question of where it would hurt the car was directed at Spyder, you just snuck a post in.

Every dealer I called, I told them I wanted to talk to whichever tech they thought knew the most about drivelines. Every single service manager said, "Well what's the situation/question?" Listened to me, then put me on hold for two minutes while they went to ask the techs, then returned with various answers. I specifically asked Subaru America if I could just speak directly with their techs (call center people not technically trained, and would just ask someone else), but they said they were internal only and would only talk to dealerships.

Basically, anyone who realized that this damage didn't seem directly related to the tire change, didn't want to get involved one way or another. Thanks everyone for the help so far.

[spyder]

My question of where it would hurt the car was directed at Spyder, you just snuck a post in.

Every dealer I called, I told them I wanted to talk to whichever tech they thought knew the most about drivelines. Every single service manager said, "Well what's the situation/question?" Listened to me, then put me on hold for two minutes while they went to ask the techs, then returned with various answers. I specifically asked Subaru America if I could just speak directly with their techs (call center people not technically trained, and would just ask someone else), but they said they were internal only and would only talk to dealerships.

Basically, anyone who realized that this damage didn't seem directly related to the tire change, didn't want to get involved one way or another. Thanks everyone for the help so far.

Sorry stuving, I tried to make another post, but my wireless was acting up and after the thrid time trying to slightly explain and the damn thing telling me it couldn't connect and erasing what I was trying to write, I gave up. So, on my normal computer here goes.
The problem with the any tires being different sizes, is that the axles are always turning at different speeds even when the car is going straight. So the center differential is always working and that wears it out. There is a clutch assembly or viscous coupling that is really complicated to explain. The tires need to be within a quater of an inch in outer diameter size. The other part is that when the subaru's sensors read the different rotation speed of the tires because of the size, it activates the traction control system thinking that one tire is slipping because of the different rotation speeds. This then activates all the other little things that turn on to try to correct this normally on slippery road surfaces. Basically, you are keeping this system on and wearing it out. There is a timing thing also in the whole TC system that keeps it from turning on during normal turning and everything else, the system is a pain in the ass to explain. So, driving with different size tires messes with all the rotation speeds of everything under the car and the traction control system. Basically.

[mcantar18c]

Yup, I'd say you did a pretty good job explaining the cause and the symptom/effect.

Repairing a toasted axle for a few hundred seems a bit fishy. From the sounds of it I'd bet that the diff was beyond saving (hot enough to melt = heat distortion, pitting/warping of bearing surfaces, fusing of components to housing, etc.) and the entire axle would need to be replaced, which would be at least a few hundred for the part and even more for the labor.

I can kind of see the front axle being smoked from turning two different size tires, although the spider gears should save the diff (they're there so that the two tires can turn at different speeds while turning a corner or when one doesn't have traction) and transfer that stress to the t-case which would fail due to the front and rear shafts turning at different speeds.

Looking at nothing but the rear diff, what could cause it to get so hot that it melts the bushings? Already low fluid would be my first guess, Occam's Razor and all... could have had a leak and this just happened to be its time to go. Hard to say what killed it without seeing it.

[Outlaw1]

The shorter tire on the frontend effectively lowered (numerically raised) the gear ratio. When the VC is in 50-50 it puts the car is in a pretty good bind. Imagine putting 4.10's in the front and 3.73's in the back of a 4x4. Even without a bind, you'd be surprised to see how hot diffs get.

It's common practice to weld the VC and the front diff in a DSM. If you ever want to go for a ride in something that will shake, shutter, buck and generally freak out, just throw one odd size tire on it. Within a mile or two there will be some sort of broken drivetrain part.

[mcantar18c]

Irving:
Yeah, like I said dealerships suck. That's why I suggested actually going into a dealership and talking to them... if you put up enough of a stink the service manager will let you do it, and often times just asking will get you in.
What's wrong with your car?

Outlaw:
Different gear ratios will grenade the t-case, but shouldn't effect the diffs themselves.