How worried are YOU about the tread depth of your TIRES?
Applies to All Model Years
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Given that... would all four tires being the same circumference be more important on a Crosstrek with a manual transmission? And do you think the tires would be less or more of a threat to the transmission itself? [Compared to the impact they might have on a CVT]
From what I understand, yes.
Paraphrasing fron another thread, I see no worries at all about the centre differential. The math just don't seem to support any reason it would be subject to excessive strain.
A 3/32" difference in the radius of a tyre (13.8") corresponds to 0.65%. At 75 mph with 730 revs/mile, the normal wheels rotate 15.208 times per second.
With one tire smaller, the difference in the wheel radii will make it rotate 0.65% faster, which adds an extra 0.099 rotations per second... or one full extra turn per 10 seconds (6 rpm).
You now have the rear or front open diff accommodating a 6rpm difference between L and R wheels, which translates to +3rpm at the shaft. That means that the center diff will be subject to a 3 rpm (1 rotation every 20 s) discrepancy between front and rear. A viscous-coupled diff won't even notice it as the resulting friction won't generate enough heat to melt a snowflake. It's harder to analyze the CVT' multi-clutch differential's susceptibility to continous rotational differences, but again.. we're analyzing the impact of 3rpm. Safe to assume it's negligible, because...
...last but not least: tire deformation. Under full cargo load or with a trailer, the rear is squished. Enough to reduce the effective radius of both properly inflated rear tires by a few /32s. Which, in turns, adds an n-times larger rotational speed discrepancy at the center diff.
I just can't see any negative impact on the transmission. Handling, stability, VDC yes.. but not on the mechanical side.
A 3/32" difference in the radius of a tyre (13.8") corresponds to 0.65%. At 75 mph with 730 revs/mile, the normal wheels rotate 15.208 times per second.
With one tire smaller, the difference in the wheel radii will make it rotate 0.65% faster, which adds an extra 0.099 rotations per second... or one full extra turn per 10 seconds (6 rpm).
You now have the rear or front open diff accommodating a 6rpm difference between L and R wheels, which translates to +3rpm at the shaft. That means that the center diff will be subject to a 3 rpm (1 rotation every 20 s) discrepancy between front and rear. A viscous-coupled diff won't even notice it as the resulting friction won't generate enough heat to melt a snowflake. It's harder to analyze the CVT' multi-clutch differential's susceptibility to continous rotational differences, but again.. we're analyzing the impact of 3rpm. Safe to assume it's negligible, because...
...last but not least: tire deformation. Under full cargo load or with a trailer, the rear is squished. Enough to reduce the effective radius of both properly inflated rear tires by a few /32s. Which, in turns, adds an n-times larger rotational speed discrepancy at the center diff.
I just can't see any negative impact on the transmission. Handling, stability, VDC yes.. but not on the mechanical side.
No idea. I'm not an engineer. Honestly, all this reasoning and napkin math is cool, but I'm still not going to risk my drive train on it. It would be nice (and cheaper) if I could just replace a single tire, or even just match axel pairs, but the OEM is very clear about not doing that.
And I don't want to buy a CVT.
2015 XV Crosstrek Premium CVT, 2019 Impreza Sport MT, 2001 Forester S 4EAT, 2013 Outback Premium CVT
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435 Posts
I don't have a CVT but with auto cars Subaru use to have a spot to put a fuse in to put it in FWD mode when you got a flat. I don't think that is the case now but Subaru still requires the temp spare to be put on the rear though, not sure if most people are aware of that.
I've seen first hand what mismatched tires can do, I've mentioned it here before, it was a 4EAT though not a CVT. The transmission developed a shudder when taking off from a stop and on turns, with two new tires in the front and old worn ones in the rear. It was driven almost a week like this and the shuddering got progressively worse. I did not measure the difference but it did stop shuddering when they got tires on the rear that matched. So in this case no immediate damage but I have no idea about long term. Wasn't my car.
From the owners manual :
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From an older Subaru with a 4EAT auto:
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I've seen first hand what mismatched tires can do, I've mentioned it here before, it was a 4EAT though not a CVT. The transmission developed a shudder when taking off from a stop and on turns, with two new tires in the front and old worn ones in the rear. It was driven almost a week like this and the shuddering got progressively worse. I did not measure the difference but it did stop shuddering when they got tires on the rear that matched. So in this case no immediate damage but I have no idea about long term. Wasn't my car.
From the owners manual :
From an older Subaru with a 4EAT auto:
I'm going to keep looking into this. For one thing... I want to find out if the threat is bigger to a Crosstrek with a CVT or with a manual transmission. And I'd like to find more first hand experiences from people who have experienced this. [In addition to the one you gave us peaty] I find it all quite interesting. And a bit puzzling to say the least. There is one story online about a guy with a CVT Impreza. His transmission was destroyed by uneven tires [according to the Subaru dealer he took it to] and the dealer said it was no longer covered by the warranty. But as the story goes... he put two brand new tires on back and the orginal, very well worn rear tires on the front. I imagine he drove it that way for quite awhile.
Was it the transmission that was destroyed or was it the transfer case? I suspect the transfer case.
If you have to buy a new tire because of an unrepairable road hazard, you can have to new tire shaved to match more closely.
The special fuse to make your car FWD was pre-2010, before CVT's and before the AWD system became electronic. It was only on 4EAT cars.
My guess the reason they say not to use the temp spare on the front is that it can affect steering and make the handling unsafe. Interestingly, there is nothing there that states how long a temp spare should be used other than the life of the temp spare. As I stated before, the difference in circumference between a normal and compact spare is WAAAAAAAY more than the difference between a new and worn normal tire. One has to wonder.
let's say you have 10k miles on your tires and you get a sidewall puncture. you agonize and theorize but eventually you buy a full set of tires -- the same tires you have on your car because you like them so much
in 10k miles you will then have seven tires! you could then rotate seven tires instead of four and recoup your "loss"
brilliant, huh?
Here's more of "napkin math". The spare is 145/80R17. That's a whooping 5.4% in radius/circumference decrease w.r.t. stock size. As a reminder, a 3/32" tread depth difference is equivalent to 0.68%. Running the spare is basically like running a tire with 24/32 (!) less tread depth.
The owner's manual allows continuous driving up with 1 spare wheel at speeds up to 50mph. At 50 mph with 730 revs/mile, normal wheels rotate 10.1 times per second. The spare will rotate 5.4% faster, namely +0.55 rotations/second or +33rpm - and it's within allowed specs
So, how can a 33 rpm rotational speed difference be perfectly acceptable for the center diff, autorized by the owner's manual, but a 6 rpm difference (corresponding to a 3/32" tread depth difference at 75mph) would be detrimental ?
Note that the owner's manual does not include any warning about avoiding driving for too long with a spare. Just a limitation on the top speed (which does drive the wheel RPM difference, but could very well be justified by degraded braking and handling abilities). If that's not a pretty rock solid argument that the drivetrain is immune to tread depth mismatches, then nothing else is..
The owner's manual allows continuous driving up with 1 spare wheel at speeds up to 50mph. At 50 mph with 730 revs/mile, normal wheels rotate 10.1 times per second. The spare will rotate 5.4% faster, namely +0.55 rotations/second or +33rpm - and it's within allowed specs
So, how can a 33 rpm rotational speed difference be perfectly acceptable for the center diff, autorized by the owner's manual, but a 6 rpm difference (corresponding to a 3/32" tread depth difference at 75mph) would be detrimental ?
Note that the owner's manual does not include any warning about avoiding driving for too long with a spare. Just a limitation on the top speed (which does drive the wheel RPM difference, but could very well be justified by degraded braking and handling abilities). If that's not a pretty rock solid argument that the drivetrain is immune to tread depth mismatches, then nothing else is..
I think the difference here is that you are never running a compact spare for more than its life which is only 200-300 miles.
Consider a set of tires with a life of about 40K miles. You trash one of them at 20K miles and replace with a new one. So you are then running the mismatched set for another 20K miles. Note that you won't wreck your AWD system in a NY minute, but you may shorten its life by..................who knows? Maybe 20K miles? So instead of wearing out at 200K miles, you may wear it out at 180K miles? Maybe 50K miles? So then it would wear out at 150K miles instead of 200K miles?
All of this is just speculation. My point is the term "premature failure" is relative. To be clear, most of my cars die of rust before much else. That may change though now that I have a two car garage.
Agreed. We just don't know whether potential differential damage is instantaneous (i.e. due to heat/strain induced by real-time slippage or rpm difference) or cumulative (included by fatigue or wear, accelerated by a rotational mismatch). With a manual and the viscous-coupled differential, I'm quite certain there is no cumulative wear due to its construction - there no friction or contact in between solid plates at all. With the multi clutch pack in the CVT, I'm less certain (friction-induced rotational abrasion is cumulative over the total number of turns). Interesting discussion... and it sucks that the Crosstrek doesn't have a full-sized spare.
I'm inclined to think it's cumulative rather than instantaneous. Sort of like not changing your oil in a timely manner.
I believe the cumulative damage is worse with the viscous coupler than with the system with the CVT - at least that's what the service manager at my old dealership told me.
For those of us who don't drive the average annual mileage, remember that rubber ages and while you may have a lot of tread depth the tire could still be dangerous at highway speeds. And while writing this I realized I haven't checked the age of the tires on my new to me 25yo car...
2015 XV Crosstrek Premium CVT, 2019 Impreza Sport MT, 2001 Forester S 4EAT, 2013 Outback Premium CVT
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435 Posts
The Subaru Automatic AWD is electronic even back then, the manual isn't and uses viscous coupling. You are right about it only being on the 4EAT, depending on the year it was a separate place along the firewall with FWD on the cover others it was in the fuse box like the pic above.
Dry rot can indeed be an issue when time far exceeds miles. Though if you have an attached garage, that happens much more slowly.
Examine carefully for cracking.
Back in 2001, I bought my first Subaru, a used 97 Outback with 61K miles on it. My dealership had a special clinic for "new Subaru owners". Mine wasn't new, but it was my first Subie, so I was considered a "new owner". It was there that the service manager explained this thing with the fuse to make the car FWD when using the compact spare. Of course this didn't apply to me because my car was an MT.
Can you even tell just by a visual exam? I wasn't able to find a good source but I recall that any rubber tire over seven years from manufacture could be dangerous.
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