I replaced one tire on my awd car

[SeaJay]

I think most AWD systems, even the notoriously picky Subaru systems, can be fine with replacing 2. This is assuming the typical scenario of two 10/32" tires on one axle and 5-6/32" on the other.

I remember working on a Subaru where the owner had not rotated the tires in a several years; the two front tires had 2/32" more wear than the rears. System had no issues.

Replacing one may be a different situation....that is unchartered territory for me.

I had a Subaru and at least twice I had one tire that had to be replaced. Wouldn't surprise me if I had 15 or twenty thousand miles on the set. My trusted tire shop that had been in the same family for a number of decades recommended just putting on a new one.

Either I was lucky, or as I believe it really doesn't matter as I never had any problems and drove that car for many tens of thousands beyond.

YMMV

 

[The Critic]

I had a Subaru and at least twice I had one tire that had to be replaced. Wouldn't surprise me if I had 15 or twenty thousand miles on the set. My trusted tire shop that had been in the same family for a number of decades recommended just putting on a new one.

Either I was lucky, or as I believe it really doesn't matter as I never had any problems and drove that car for many tens of thousands beyond.

YMMV

Just for giggles:

https://static.nhtsa.gov/odi/tsbs/2019/MC-10160485-9999.pdf

Rough calculations (.25/pi) shows 1/4" circumference to be roughly equivalent to a 2.5/32" change in diameter (or tread depth). This probably explains why the Subaru from earlier had no issues; the different in tread depth was right at the limit.

 

[MRC01]

Rough calculations (.25/pi) shows 1/4" circumference to be roughly equivalent to a 2.5/32" change in diameter (or tread depth). This probably explains why the Subaru from earlier had no issues; the different in tread depth was right at the limit.

The diameter of the tire includes the tread depth on both sides, so cut that in half.

That is: circumference is pi*diameter, so a .25 difference in circumference is a .25/pi = .0796 difference in diameter, or about the 2.5/32 that you mention. Since tread depth on both sides of the diameter sums to 0.0796, it's 0.0398 on each side. That is so small it seems like Subaru is just trying to develop specs that enable them to deny warranty claims.

 

[TiGeo]

Crap. Deleted. Stand by for logs/video!

 

[Pew]

Sounds like a good excuse for some Star Specs.

 

[TiGeo]

Also, the ABS and traction control, and ESP may be compromised if a wheel is out of spec by more than 1.3% or whatever the common tolerance for the tire diameter is. I remember 1.3% from my dad's old Phaeton but I may be mistaken.

Phaeton was a mechanical AWD system plus is now over 20 years old. Very similar to my W8 Passat with Torsen (read "Quattro") awd.

 

[The Critic]

The diameter of the tire includes the tread depth on both sides, so cut that in half.

That is: circumference is pi*diameter, so a .25 difference in circumference is a .25/pi = .0796 difference in diameter, or about the 2.5/32 that you mention. Since tread depth on both sides of the diameter sums to 0.0796, it's 0.0398 on each side. That is so small it seems like Subaru is just trying to develop specs that enable them to deny warranty claims.

I stand corrected. Thanks.

 

[vavavroom]

Phaeton was a mechanical AWD system plus is now over 20 years old. Very similar to my W8 Passat with Torsen (read "Quattro") awd.

Yes, but Haldex is a hydraulic-mechanical system with gears, ring gear, pinions, shafts, a clutch pack with friction discs, and slip differentials. It just doesn't use a Torsen diff. The system is still subject to wear and tear.

 

[TiGeo]

Yes, but Haldex is a hydraulic-mechanical system with gears, ring gear, pinions, shafts, a clutch pack with friction discs, and slip differentials. It just doesn't use a Torsen diff. The system is still subject to wear and tear.

The diff itself yes, the Haldex unit itself is just a clutch pack and pump, any slip front to rear is handled by the clutches and should see no additional wear due to slight tire size differences. The issue you are pointing out is a r. differential issue of side to side variance in wheel speed creating additional wear on the diff bits you have listed.

 

[TiGeo]

Here is some logging with the 4 ABS wheel speed sensors with some various driving situations with commentary. I'll post the logs next. I did this earlier/posted but saw I goofed and had the left ABS sensor twice...no wonder it looked perfect! DELETED! I can only hope my YT production quality meets the discerning opinions of the BITOG-osphere/kollective. I need the watch hours so make sure you give me all 5 min I can only hope to compete with the likes of Project Farm and The Motor Oil Geek's channels.

 

[TiGeo]

Ok, here are the two data logs, unfortunately my logging tool (OBDEleven) can crap out sometimes/lose connection so it's broken up into two logs. I calc'd the delta left to right for the front and rear and really that's what to look at here, from a quick go through, the variance left to right on the straight portions of the drive (the flatter/tighter squiggles vs. the big peaks which are turns) look to be about the same for the fronts and rears. Next I'll do a little bit of analysis of the raw data in the .csvs. Also note I was stopped for a bit and that's easy to see b/c the sensors are all 0.

https://datazap.me/u/karstgeo72/testing-1-new-tire-wabs-speed-sensors-0?log=0&data=5-6

https://datazap.me/u/karstgeo72/testing-1-new-tire-wabs-speed-sensors?log=0&data=5-6

 

[krzyss]

I had a Subaru and at least twice I had one tire that had to be replaced. Wouldn't surprise me if I had 15 or twenty thousand miles on the set. My trusted tire shop that had been in the same family for a number of decades recommended just putting on a new one.

Either I was lucky, or as I believe it really doesn't matter as I never had any problems and drove that car for many tens of thousands beyond.

YMMV

Which AWD system did you have?
Manuals have center diff that hates to constantly spin at different rates.
Automatic trans have system similar to Haldex, AFAIK. But it seems that CVT may be different.

Krzyś

 

[TiGeo]

My quick and dirty analysis? The small variations while driving straight between left and right on the rear with the ~6/32" and ~10/32" look about the same as the variation you see between the fronts that are both nearly the same at ~4.5/32". I've attached the raw data as txt files b/c the forum doesn't support .csv format. It's not like anything here jumps out as being wildly different between front/rear etc.

 

[TiGeo]

Which AWD system did you have?
Manuals have center diff that hates to constantly spin at different rates.
Automatic trans have system similar to Haldex, AFAIK.

Krzyś

PS CVT apparently too.

It's Gen 5 Haldex.

 

[krzyss]

Haldex is more forgving than Subary Center diff with manual cars.
But as far as I know you do put more strain on the system. Is it enough to break it?

Keep us posted,

Krzyś

 

[TiGeo]

Haldex is more forgving than Subary Center diff with manual cars.
But as far as I know you do put more strain on the system. Is it enough to break it?

Keep us posted,

Krzyś

Also, no manual here, just dual-clutch (DSG) not that it matters in this instance. Data looks completely normal to me w/r to nothing really different left to right/odd with the wheel speed sensor data. Car behaves normally. I actually engaged ABS doing this on accident so it works haha. I'm quite sure the ECS/ASR (stability control, traction control, respectively) are I'm sure working fine as well.

 

[eljefino]

I'd have the new tire shaved down to 6/32".

If you're throwing out 4/32" of tread, replace one of the 4.5/32 front tires.

 

[samven]

Just run the new tire a couple PSI low.

 

[TiGeo]

Just run the new tire a couple PSI low.

Brilliant! Have to remember to reset the indirect TPMS when I do it

 

[TiGeo]

I found some forum posts suggesting 3% variance in diameter is within the window for Haldex. But of course no good sources stated.