Can rear diff bushings cause both rear tires to wear excessively fast ?

[idosubaru]

Very loud roar.  - That’s a bad rear bearing.

Fish tailing rear end at 40+ mph.  Massive rear tire wear. Can’t tell if fishtailing is due to tires being bald or something else.

Can bad rear diff bushings cause that or it’s probably alignment/suspension?

Edited August 17, 2020 by idosubaru

[el_freddo]

I’d say suspension or rear alignment issues. 

I doubt the diff would do it unless it’s welded. 

Cheers 

Bennie

[Rampage]

Kind of sounds like carrier or pinion bearings in the rear diff. If the carrier bearing (on the pinion side) wears it will allow the ring gear to move into the pinion gear. That could cause a roaring sound. If the ring and pinion are loose, you get a whining noise. If really tight, the drag would cause tread wear.

With the rear wheels on the ground or locked, how much play is there in turning the driveshaft and does it turn freely?

Get all four wheels in the air and run it in gear. Applying the brakes will put a load on the rear diff. Does it make that noise?

I used to run cheater slicks on the street with the cars I drag raced. No sway or fishtailing on dry roads. They were nasty on wet roads. Rear end was all over the place.

Excessive toe in or out will cause swaying and tread wear, but it did not cause a roaring sound. Been there before.

[idosubaru]

Thanks guys.  I’ll get it up in the air later today.

what suspension item(s) would cause both tires to wear?  Came from 1,000 miles south, very little rust so rear subframe rusting seems unlikely.

210,000 mile Tribeca I’ve had for years. I think these have open rear diffs. 

I drove a college friend 1,400 miles to NH for grad school.

The noise was present the entire time.

The fish tailing didn’t start until I drove in rain the last 50 miles, so that was lucky.  That’s when I noticed the bald tires which were above the wear bars when I left.

Edited August 16, 2020 by idosubaru

[idosubaru]

List to check:

Rear suspension bushing play

Rear diff noise/play

Rear diff fluid drain and fill

Alignment - I’ll need to find the Tribeca front and rear track width first. 

[idosubaru]

Thanks guys - there’s a bad wheel bearing.  I missed it at first because, as normal, it didn’t show itself with normal tests.

That doesn’t explain the fishtailing and excessively fast tire wear on both sides.

Alignment?

The two rear diff hanger bushings have cracks all the way through - could this cause tire wear issues?  Seems unlikely.

Bearing tested fine, even stethoscope was silent. I felt lumpiness with hands on lugs at a very specific point of rotation once I took the rotor off. It’s like the weight of the rotor helps conceal it or something. I’ve seen that happen multiple times before. 

Edited August 17, 2020 by idosubaru

[idosubaru]

Bushing cracks.  At least one crack per bushing goes all the way through. I doubt this has anything to do with tire wear.  But I know the old gen guys have seen this before so I’ll search there. 

[Rampage]

The rear tires should be very close to 0 degrees toe in. With the tires on the ground, get someone to help with a measuring tape and measure between the front and rear of the rear tires. Keep the tape as high and straight as possible at the front of the tire, then use the same height at the rear of the tire. Hold the end of the tape in the outer groove of one tire and read the measurement to the outer groove on the other tire. The two measurements should be the same.

If the measurements are different, one side or both sides have worn lateral link bushings.

Wear or softness in the front or rear lateral link bushings will cause sway. It lets the tire toe in or toe out on its own. The front lateral link inner bushing has an adjustment washer for toe adjustment. The outer connection looks like a tie rod end. Check it for looseness.

[el_freddo]

Diff issues are a red herring in this issue. 

Those cracked bushes will allow the diff to rotate under load, this will be added slack in the drivetrain. 

The wheels and tyres are held in place by the suspension arms, struts and bushes in the system. Something in there must be amiss for the tyre wear issue you’re seeing. 

Going by those diff bushes, there will be other very well worn bushes throughout the whole suspension system on your tribeca. Pretty common for older vehicles to have issues like this. 

Cheers 

Bennie

[idosubaru]

14 hours ago, el_freddo said:

Diff issues are a red herring in this issue. 

Those cracked bushes will allow the diff to rotate under load, this will be added slack in the drivetrain. 

The wheels and tyres are held in place by the suspension arms, struts and bushes in the system. Something in there must be amiss for the tyre wear issue you’re seeing. 

Going by those diff bushes, there will be other very well worn bushes throughout the whole suspension system on your tribeca. Pretty common for older vehicles to have issues like this. 

Cheers 

Bennie

Thanks, that makes more sense than the diff causing tire issues. 

Good call - Replaced two rear bushings a couple months ago, I didn't see any others yesterday that looked problematic.  I'll check alignment first and see where it's at. 

[idosubaru]

16 hours ago, Rampage said:

The rear tires should be very close to 0 degrees toe in. With the tires on the ground, get someone to help with a measuring tape and measure between the front and rear of the rear tires. Keep the tape as high and straight as possible at the front of the tire, then use the same height at the rear of the tire. Hold the end of the tape in the outer groove of one tire and read the measurement to the outer groove on the other tire. The two measurements should be the same.

If the measurements are different, one side or both sides have worn lateral link bushings.

Wear or softness in the front or rear lateral link bushings will cause sway. It lets the tire toe in or toe out on its own. The front lateral link inner bushing has an adjustment washer for toe adjustment. The outer connection looks like a tie rod end. Check it for looseness.

Got it, thanks.  I replaced both of those lateral links you mentioned (i think subaru calls them suspension control arm) due to bad bushings earlier this summer.  I probably marked and retained the eccentric adjustments, but I haven't aligned it since then so this is the likely culprit and now that the tires are bald they're hydroplaning with bad alignment.  New tires are on the way, new wheel bearing goes on today.

For rear suspension/linkage reference:

https://parts.competitionsubaru.com/p/Subaru__Tribeca/Suspension-Control-Arm-Front--Rear/49229241/20250XA060.html

https://parts.competitionsubaru.com/a/Subaru__Tribeca/49229241__6029069/REAR-SUSPENSION-06MY-06MY/W10-201-01.html

 

 

[CNY_Dave]

If your tires are now worn differently front-rear you are inviting AWD problems.

[idosubaru]

2 hours ago, CNY_Dave said:

If your tires are now worn differently front-rear you are inviting AWD problems.

Thanks. I’m well aware of Subaru AWD specs and this vehicle. Zero concern. 

[Rampage]

Yep, the FSM calls them lateral links, but when you go shopping they are called suspension control arms. At least we know they are the same.

[idosubaru]

12 hours ago, Rampage said:

Yep, the FSM calls them lateral links, but when you go shopping they are called suspension control arms. At least we know they are the same.

Okay thanks.  I’m not well versed in all the rear suspension part names so im likely to get confused!

[Rampage]

Part names. I find it amusing when looking through some of the FSMs and see what "they" name some of the parts. Completely different from what I grew up with.

[idosubaru]

Drove fine on dry roads today, so an alignment and new tires should take care of this.   I'm going to measure alignment when I get home, wanted to drive it some before taking alignment measurements after having it off the ground. Tires are warn evenly across the surface so it's all toe and not a camber issue. 

Hopefully the mis-alignment and bald tires couldn't handle wet roads. 

Thanks guys.