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Discussion Starter · #1 ·
More than 1.2 million Subaru Crossteks have been sold in the US and Canada since 2012. And yet - I can't find anything online that talks about how unevenly worn tires caused problems with the AWD. I find that a bit baffling given the warnings regarding tire wear. "It clearly says so in the owner's manual!" Yes. I get it. I read it. I know it. And I'll always keep that warning in mind. STILL.... it's a bit baffling. Don't you think?

Do YOU know of anyone who has experienced the consequence of uneven tire wear on their Crosstrek AWD?

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Being a Subaru Owner since 1979, a Subaru mechanic for many years, and owning 13 Subaru's since 1979, here are my experiences.

Subaru's first 4WD was a direct drive with open differentials both front and rear. There was a lever next to the shifter you would pull to engage front to rear. If you left it engaged when driving slow and parking, it definately had wheel hop. Was not meant for slow speed manuvering.

Starting in 1990 ( USDM ) , AWD was offered. 2 completely different systems. Auto was 4EAT with a front and rear speed sensors in the trans to 'lock up' the coupling. This was a FWD preferred and rear 50/50 lockup when slip was detected. This was not as prone to problems with slight changes in tire sizes. Most times these would lock up all the time, referred to as Torque Bind, due to either dirty trans fluid ( blocking the very small passages ) or a duty solenoid failure.

The 5 and 6 speed manuals had no electronics. Mechanical only. They have a center differential with a 50/50 split all the time. And when there was the smallest difference between front and rear, the fluid in the center diff would heat up and bind the front and rear. If this overheated for any reason, it would stay 'bound' and you are locked front to rear. I have seen this many times when a customer would come in and cold start, car drove fine, after 30 minutes or more, it was bound up and torque bind was evident. Only fix is to replace the center diff. Tire circumference differences definately cause the fluid to heat all the time causing this to fail.

Now comes AWD/Traction control. Used on CVT. This uses the ABS sensoros on each wheel to lockup that caliper to allow the torque to go to the other wheels. This system SEEMS not as picky about smaller tire differences, but larger ones will cause the system to work more frequently.

Seems Subaru wrote it's owner manuals to cover both systems as one and recommended very tight specs on tire circumference. Subaru manual trans to this day are still built the same as those in 1990 when they introduces these to the US.

Once again, my experiences.
 

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2022 Canadian Sport trim (6MT // 2.0 )
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Don't forget that 90% of warnings and doomsday messages in the user's manual is only caused by the litigation culture of our friendly southern neighbours.

For a reality check, maths can help:
1. Our tire size is 225/60R17 = 35.05 cm radius with a rotational frequency of 454 revs/km
2. Let's presume a tire wear difference of 5/32 = 4 mm.
3. The worn tire will then have a radius of 34.65 mm, with a rotational frequency of 459 revs/km. That's a 1.1% increase (smaller tire, more rotations required to cover the same distance).
4. Now, the rotational speed (in revs/min) as a function of linear speed: The "good" wheel spins at 454 rotations/km = 454 revs per hour per km/h = 7.567 revs per minute per km/h.
The bad wheel spins 1.1% times faster, so for each km/h, the bad wheel will undergo 0.08324 more rotations

  • At 10 km/h, it will spin 0.832 revs/min faster
  • At 10 mph, it will spin 1.34 revs/min faster
  • At 50 km/h, it will spin 4.16 revs/min faster
  • At 50 mph, it will spin 6.70 revs/min faste
  • At 100 km/h, it will spin 8.32 revs/min faster
  • At 100 mph, it will spin 13.4 revs/min faster

Now, can a sustained rotational difference of, say 10 rotations per minute (one every 6 seconds) at 120 km/h / 75mph can in any way degrade the centre differential/clutch pack?
 

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Discussion Starter · #4 ·
Being a Subaru Owner since 1979, a Subaru mechanic for many years, and owning 13 Subaru's since 1979, here are my experiences.

The 5 and 6 speed manuals had no electronics.................. Tire circumference differences definately cause the fluid to heat all the time causing this to fail.
I appreciate the reply. The Crosstrek I'm getting is the 6 speed. With that said... could you SEE with your eyes the difference in tire circumference or did you have measure the circumference of each to have your answer? Thanks.
 

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I appreciate the reply. The Crosstrek I'm getting is the 6 speed. With that said... could you SEE with your eyes the difference in tire circumference or did you have measure the circumference of each to have your answer? Thanks.
I would measure it ... If you can SEE it, then it's probably TOO much.
 

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2022 Canadian Sport trim (6MT // 2.0 )
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Can't speak for the CVT clutch pack - but the LSD in the MT is viscous-coupling, reactive to heat. IMHO (this is intuitive, not backed by evidence), a sustained 10 rpm difference will not generate nearly enough friction /heat to have any impact long term. In low-traction conditions (snow, mud), one axel can easily spin at 20 km/h with the other being fixed, which results in a 150rpm difference at the differential. THAT causes it to heat and lock.

The other thing we don't consider is the effective tire radius change under load. Put 200 lbs in the cargo, and the rear tires will squish, I just don't know by how much the rolling radius will decrease. Again, without any data to corroborate my hypothesis, I would argue that a 5/32" variation in the sidewall height when loaded is definitely possible. Which, if true, would indicate that the differentials can easily take such a rotational difference.

Edit: found a reference https://www.sciencedirect.com/topics/engineering/effective-rolling-radius Have fun!
 

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Being a Subaru Owner since 1979, a Subaru mechanic for many years, and owning 13 Subaru's since 1979, here are my experiences.

Subaru's first 4WD was a direct drive with open differentials both front and rear. There was a lever next to the shifter you would pull to engage front to rear. If you left it engaged when driving slow and parking, it definately had wheel hop. Was not meant for slow speed manuvering.

Starting in 1990 ( USDM ) , AWD was offered. 2 completely different systems. Auto was 4EAT with a front and rear speed sensors in the trans to 'lock up' the coupling. This was a FWD preferred and rear 50/50 lockup when slip was detected. This was not as prone to problems with slight changes in tire sizes. Most times these would lock up all the time, referred to as Torque Bind, due to either dirty trans fluid ( blocking the very small passages ) or a duty solenoid failure.

The 5 and 6 speed manuals had no electronics. Mechanical only. They have a center differential with a 50/50 split all the time. And when there was the smallest difference between front and rear, the fluid in the center diff would heat up and bind the front and rear. If this overheated for any reason, it would stay 'bound' and you are locked front to rear. I have seen this many times when a customer would come in and cold start, car drove fine, after 30 minutes or more, it was bound up and torque bind was evident. Only fix is to replace the center diff. Tire circumference differences definately cause the fluid to heat all the time causing this to fail.

Now comes AWD/Traction control. Used on CVT. This uses the ABS sensoros on each wheel to lockup that caliper to allow the torque to go to the other wheels. This system SEEMS not as picky about smaller tire differences, but larger ones will cause the system to work more frequently.

Seems Subaru wrote it's owner manuals to cover both systems as one and recommended very tight specs on tire circumference. Subaru manual trans to this day are still built the same as those in 1990 when they introduces these to the US.

Once again, my experiences.
Everything that I have read about Subaru's AWD (CVT version) is that it applies power to all 4 wheels all the time
 
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