Interesting Find 2017 Subaru Outback Differential Service

[GSCJR]

Now I'm really confused Not the first time and sure won't be the last time

That makes two of us. I had to replace one tire last week due to a flat, am I putting any components at risk? I have the CVT in my '12 Legacy.

 

[SubieRubyRoo]

Does that fuse still exist? My 97 Legacy had one for emergencies only - pull the fuse and some indicator comes on (“FWD” or something).

I thought that all recent WRX’s have an an open front and rear diff. The only Subarus with an LSD are the STI and BRZ.

I also believe that the real difference between Subaru AWS and other makes, besides the symmetrical thing, is that the center diff in a Subaru mechanically connects the front to the rear, has a front bias, and uses a clutch pack to transfer more power to the rear. Many other basic AWD systems are only connected when the clutch pack engages, otherwise it’s an open connection. The difference is that the rear wheels need to slip before ANY power is sent rear.

Yes, the STI is the only AWD left that has an LSD to my knowledge, that’s why I was saying the older Legacy/Outback were really the best for offroad (I believe the STI is adjustable from 50/50 to a 35/65 rear bias).

The older 4EATs had electronically controlled clutches that had progressive solenoid engagement; they would vary the power transfer to the rear wheels based on throttle input, vehicle speed, and the difference in speed between front and rear axles. Basically, full acceleration from a stop OR front wheels spinning would result in 100% clutch engagement, which delivered a 50/50 split. At higher speeds, lower throttle inputs the clutch would not be fully engaged, resulting in as much as a 90/10 front wheel bias. In the Subaru scan software (the name escapes me) you can actually watch the center diff solenoid engagement on the live data.

On the newer CVTs I’d have to check on my mom’s ‘18 Forester to see if the fuse is still there.

 

[gathermewool]

That makes two of us. I had to replace one tire last week due to a flat, am I putting any components at risk? I have the CVT in my '12 Legacy.

Consult your owner’s manual for the max difference in tread between all tires. I believe it’s something like 1/4”. Use a tread depth gage, which reads out in 32nd of an inch. New tires are usually 10-11/32”. Worn out is technically 2/32 for non-winter tires and 5/32” for winter tires, but traction/safety suffers greatly when you get to the limits…usually. My previous Michelins were purported to have much better than usual worn out traction. I didn‘t find that to be true - I was able to do a bit of a power slide in a light rain (on purpose) with mine nearly worn out.

It‘s not a super easy to find someone who will do it for you, but if your other three tires are pretty new but just over the limit compared to the new tire, try to get the new tire shaved down.

 

[GSCJR]

Consult your owner’s manual for the max difference in tread between all tires

No actual specs were found in the OM, just general warnings regarding size, tread, etc.

 

[SubieRubyRoo]

Now that all of the CVT Subarus do not have LSDs, I can’t see it being hyper-critical to have exactly the same tread life left. The wording likely isn’t in the manual because it’s not essential- every tire can easily spin at its own rate, and short of having one bald tire and 3 brand new ones IMO it’s a nothingburger. This pretty much matches up with the 1/4” (8/32”) of tread depth tolerance; most new car tires are 11-12/32” and if you subtract a 1/4” that leaves 3/32”, which is down to the wear bars.

If you were to ask me, I’d say all along that this was just a scheme to sell more tires, by telling owners their system was “so advanced” that the tires need to be the same diameter. What about manufacturing tolerances on brand new tires??

 

[gathermewool]

No actual specs were found in the OM, just general warnings regarding size, tread, etc.

I don’t think that is true.

 

[gathermewool]

Now that all of the CVT Subarus do not have LSDs, I can’t see it being hyper-critical to have exactly the same tread life left. The wording likely isn’t in the manual because it’s not essential- every tire can easily spin at its own rate, and short of having one bald tire and 3 brand new ones IMO it’s a nothingburger. This pretty much matches up with the 1/4” (8/32”) of tread depth tolerance; most new car tires are 11-12/32” and if you subtract a 1/4” that leaves 3/32”, which is down to the wear bars.

If you were to ask me, I’d say all along that this was just a scheme to sell more tires, by telling owners their system was “so advanced” that the tires need to be the same diameter. What about manufacturing tolerances on brand new tires??

Correct me if I’m wrong, but Subarus still have a mechanically connected center diff. If that is the case, tread depth still matters.

 

[GSCJR]

I don’t think that is true.

Are you saying that I need to look again? Or are you saying something else?

 

[gathermewool]

Yes, the STI is the only AWD left that has an LSD to my knowledge, that’s why I was saying the older Legacy/Outback were really the best for offroad (I believe the STI is adjustable from 50/50 to a 35/65 rear bias).

The older 4EATs had electronically controlled clutches that had progressive solenoid engagement; they would vary the power transfer to the rear wheels based on throttle input, vehicle speed, and the difference in speed between front and rear axles. Basically, full acceleration from a stop OR front wheels spinning would result in 100% clutch engagement, which delivered a 50/50 split. At higher speeds, lower throttle inputs the clutch would not be fully engaged, resulting in as much as a 90/10 front wheel bias. In the Subaru scan software (the name escapes me) you can actually watch the center diff solenoid engagement on the live data.

On the newer CVTs I’d have to check on my mom’s ‘18 Forester to see if the fuse is still there.

That split for the STI DCCD sounds about right. I think they changed it once, maybe for the redesign in ‘08, but I don’t recall. I’ve stopped visiting IWSTI and keeping track of STIs since I got rid of mKe five years ago. They also haven’t changed much at all, so keeping up didn’t seem necessary...and now they’re going to go away, so…

Anyway, I’ve made the mistake of accidentally leaving the center diff locked 50/50 in snow and then experienced binding during turns on clear road. Whoops!

 

[gathermewool]

Are you saying that I need to look again? Or are you saying something else?

I was wrong. The 1/4” (2/32”) info was in a Tirerack article. It’s not in either of my owner’s manuals. I also checked my previous ‘08 manual and it wasn’t in there either. Sorry about that.

 

[SubieRubyRoo]

Correct me if I’m wrong, but Subarus still have a mechanically connected center diff. If that is the case, tread depth still matters.

The only ones that have ever been mechanically connected AFAIK were the stick shift ones, and again the Forester/Impreza never had rear LSD, only Legacy/Outback (and then, when optioned so). All of the 4EAT and I believe the 2012-newer CVT are solenoid-controlled center clutches, not a mechanical connection; but even then with open diffs I fail to see a real issue. I still have to doubt the “true” impact of even 4/32” difference in tread depth, considering that running a tire 5-10 psi low will have more impact than that in OD while the tire is loaded… JMO

 

[gathermewool]

The only ones that have ever been mechanically connected AFAIK were the stick shift ones, and again the Forester/Impreza never had rear LSD, only Legacy/Outback (and then, when optioned so). All of the 4EAT and I believe the 2012-newer CVT are solenoid-controlled center clutches, not a mechanical connection; but even then with open diffs I fail to see a real issue. I still have to doubt the “true” impact of even 4/32” difference in tread depth, considering that running a tire 5-10 psi low will have more impact than that in OD while the tire is loaded… JMO

How is the default split 90/10 achieved? Is there some friction in the center diff, like a mini version of a clutch-type LSD?

IIRC, the 2000 and/or 2002 Impreza RS had a rear LSD

Finally, AWD is still beneficial with open diffs in vehicles with vehicle dynamics control, where the brakes are used to control the brakes. I‘d say it’s possible to be in a situation where the traction differs front and rear, so allowing the rear to engage increase stability and traction. In all of my previous FWD cars I’ve spun tire(s) in the rain.

Even without inclement weather, it’s much harder to do a one wheel burnout in two locations than one. This obviously isn’t much of a concern with less power, such as with my Legacy, but it does rain here, so it’s not just a northern concern.

 

[SubieRubyRoo]

How is the default split 90/10 achieved? Is there some friction in the center diff, like a mini version of a clutch-type LSD?

IIRC, the 2000 and/or 2002 Impreza RS had a rear LSD

Finally, AWD is still beneficial with open diffs in vehicles with vehicle dynamics control, where the brakes are used to control the brakes. I‘d say it’s possible to be in a situation where the traction differs front and rear, so allowing the rear to engage increase stability and traction. In all of my previous FWD cars I’ve spun tire(s) in the rain.

Even without inclement weather, it’s much harder to do a one wheel burnout in two locations than one. This obviously isn’t much of a concern with less power, such as with my Legacy, but it does rain here, so it’s not just a northern concern.

Yes, the RS’s likely did have LSD, but I meant the garden-variety ones 😂

The center clutch pack has a solenoid engagement. Like I said earlier, if you have the SubaruSSM (think that’s it? Been years since I used it) software you can monitor the center solenoid real-time. It’s been many years since I was doing this so it’s a bit fuzzy, but 100% center engagement splits 50/50, and when the center solenoid is at minimum, it sends a mere 10% to the rear axle. The real magic is when the vehicle is stopped/slow speeds and large throttle inputs are given; the 50/50 split is great for traction, and is transparent. I honestly believe this is why Subaru seems to get better mpg than other “AWD” cars- the % true AWD is both speed and throttle dependent, but when you’re most likely to need all 4 wheels producing traction, it’s there in spades.

 

[GSCJR]

I was wrong. The 1/4” (2/32”) info was in a Tirerack article. It’s not in either of my owner’s manuals. I also checked my previous ‘08 manual and it wasn’t in there either. Sorry about that.

It's all good brother; thanks for checking.

 

[Wheel]

I had a great document that explained the development of Subaru's 4EAT, which I can no longer put my hands on. This site offered the best explanation of their various systems http://youwheel.com/home/2016/08/07/subaru-awd-system-fully-explained/ - as well as which vehicles ended up with limited slip diffs.

Found it, but can't attach the pdf - it can be found at https://www.scoobymods.com/showthread.php/some-interesting-awd-vtd-and-3047.html - it's about 1/3 the way down called VDC Development.pdf