Need to get to the bottom of tire circumference and diffs.

[musketeerracing]

I know there is a thread on tire mixing, and I agree that mixing sizes is foolish when you have even one LSD (including viscous) in the system. Even with three open diffs I don't see it as a good idea.

My question is more technical: what are the tolerance parameters for tire circumference differences? I had a tire shop tell me that I needed four tires of the same tread design, spec, and wear level in order to not stress the diffs. Obviously that's the ideal, but what's the reality (when they're not trying to sell you two more tires)?

I presume there can be a not insignificant difference in a worn 185/70-15 vs. a new one, and that different constructions might cause them to oval to different degrees at high speeds. But the bottom line is this: is the center viscous in a 1990 5MT LegacyLS able to handle new 185/70-15 in the rear and somewhat used 185/70-15 of a different design in the front?

And how about at 110mph, where it was smooth and level when tested yesterday under controlled circumstances ?

ACP

[vrg3]

Subaru's position, I believe, is that you should always replace all four tires at a time on an AWD Subaru model. It's also important to rotate the tires regularly for the same reasons.

In this little article, they say "differences of as little as 1/4 inch can have an effect."

http://www.endwrench.com/pdf/drivetrain ... foS000.pdf

I've heard the "1/4 inch" figure many times in reference to Subarus... In any case:

A 185/70-15 tire is about (2*0.70*185/25.4+15) = 25.20 inches in diameter.

That makes it 25.20 *Pi = 79.16 inches in circumference.

A reduction of 1/4 inch would be 78.91 inches, so the diameter would be about 78.91/Pi = 25.12 inches.

That makes a diameter difference of about 0.08 inches, or a radius difference of about 0.04 inches.

So that means that even with otherwise identical tires, if one has 1/32" more tread wear than the other (which is roughly a tenth of its expected lifespan), you're already getting close to the limits.

[rallysam]

I have some related quesitons for people:
QUESTION -
What would be the symptoms of an unnacceptable mismatch?
What would be the worst possible consequences?
Isn't the extra wear and tear mostly affecting the diff oil as opposed to affecting mechanical components? (easily replacable vs. expensive repairs)


musketeerracing - I don't have a definitive answer for you, but I can at least put some numbers on it.

Official Subaru story:
"On All-Wheel Drive (AWD) vehicles, it is extremely important that the rolling or outer circumferences of the tires be within 1/4 inch of each other"

Now, that's 1/4inch on the circumference. If I do my math right, that equates to about 1/32 inch of tread depth (that's practically zero!). I have trouble believing that new tires are even so closely matched, let alone tires that have undergone typical wear and pressure variations. But, Tire Rack has a similar story, saying that 2/32 inch variation is the limit for Subaru's (I don't know how they come up with slightly different numbers).

Practical Experience?
On my station wagon, one tire was of a completely different model than the other 3 tires, and I think the tread depth was way off too. It still drove fine - but I can't confirm whether the center LSD was still acting like a limited slip after that supposed wear and tear.

On the RS, I just put a new tire on and it's tread depth is off by 2/32 of an inch from the others. I've put a thousand miles on now. I noticed some strange behavior (binding when the drive train is really cold, smells when things are hot). But, these could be symptoms of the really freaking cold weather we've had. Dunno.

If you looked at the old threads where we've hashed through this before, I remember people saying that things were really overheating after a few laps around the block.

[vrg3]

What would be the symptoms of an unnacceptable mismatch?
What would be the worst possible consequences?
Isn't the extra wear and tear mostly affecting the diff oil as opposed to affecting mechanical components? (easily replacable vs. expensive repairs)

I don't have a good understanding of the answer to your question when you're talking about automatics.

With manuals, though, the trouble indeed in the fluid in the viscous coupling. You'll shear it apart. That basically degrades its ability to limit slip but otherwise leaves everything functional. The fluid is not replaceable; you have to replace the whole center differential.

It seems to me (this is just conjecture) that automatics should be more resilient. Unless the tires are actually binding and scrubbing, I can't see why having slightly different diameters would matter. After all, the MPT clutch is designed to slip, right? And it's strong enough to overcome the friction between the tires and the road, since that causes torque bind problems, right?

[musketeerracing]

Hmmm. Interesting. thanks for the intelligent replies.

But I still think we need to investigate the tolerance more closely. Only a novice engineer would design a diff that couldn't tolerate 1/32 inch of tread-depth difference on a real-world car without failing. There has to be a tolerance. Further, if you are to drive on the supplied spare for even tens of miles I think you'll see a diameter difference - you have to - the regular tires wear over the life of the car while the spare does not. Additionally, if you damage a tire they can't expect you to replace all three others only because the new one has 2/32 more tread on it. If so, this would be very bad policy - your average owner would trash the diff in his car at some point, as most of us replace a single tire from time to time.

It must come down to shear speed in the differential - obviously the problem is worse at 70mph than at 20 mph, and the shear speed in the diff will be higher at higher speeds for a given difference in tire diameters. But is the data available anywhere? One of you seems to have access to a dealer service manual site....

A.

[scottzg]

the one that gets me is tire inflation. A couple psi more in 1 tire will give you plenty of difference. Americans suck at checking tire pressures. I would think that they would factor that in. My guess is that really uneven tire pressures + turning + a worn out tire + some leeway for mechanical failure= the spec it is engineered to.

With that in mind, i bought a used tire that had ~ the same tread left when i popped one. That was a long long time ago.

[rallysam]

With manuals, though, the trouble indeed in the fluid in the viscous coupling. You'll shear it apart. That basically degrades its ability to limit slip but otherwise leaves everything functional. The fluid is not replaceable; you have to replace the whole center differential.

I was talking about manuals.

The center diff fluid is not the same as the gear oil???? That surprises me because a rear LSD's can use plain old gear oil and you can replace it at intervals. I would think, for simplicity's sake, they would engineer the center LSD to work with the same resevoir of fluid as the gears.

PS Everything else you said makes sense to me.

[vrg3]

You can replace the differential lubricant, but the viscous coupling itself is completely sealed. The viscous goo inside can't be replaced.

[ciper]

"rear LSD's can use plain old gear oil and you can replace it at intervals."

Not correct.
http://forums.nasioc.com/forums/showthr ... did=423584

[evolutionmovement]

I still don't buy the low tolerances. Just basic driving would would wear the center differential - braking, turning, acceleration. All have an effect on overall tire diameter. If this was true, I doubt there'd be any functioning center diffs more than a couple months old. If it is true, I'll also either stick to fwd or convert to rwd only. I've seen plenty of old Subarus plow through snow banks like nothing and I know they've been beat on, neglected, and never had the transmissions serviced.

Steve

[legacy92ej22t]

I know that if you drive around with one tires pressure lower then the others long enough it can make the center diff lock causing binding because it happened to my old 4EAT Forester. It caused a full-time 50-50 split which sucked in low speed turns but kicked butt in the snow.

I was talking to a guy at a local tire shop here awhile back and he was telling me a story about a time that a women came in with a blown out tire on a new Outback wagon or something and they replaced just the one tire. I can't remember exactly how worn he said the other three tires were but I remember it wasn't much. Well about a month later they get a call from a Subaru dealership telling them that by doing that they caused some 1k in damage to this womens car. So now it is their company policy to replace all 4 tires on all AWD Subaru's, no exceptions.

[rallysam]

You can replace the differential lubricant, but the viscous coupling itself is completely sealed. The viscous goo inside can't be replaced.

OIC. You're telling me that the oil I replace in my rear diff is completely seperate from the sealed viscous fluid in the core which causes the LSD action. I guess that makes sense to me.

So, when we replace our rear diff fluid, we don't have to worry about getting the special variety of gear oil for LSD's because that probably only applies to other types of LSD (clutch type? torsen?) in which the limited slip mechanicals are actually immersed in the gear oil.

"rear LSD's can use plain old gear oil and you can replace it at intervals."

Not correct.
http://forums.nasioc.com/forums/showthr ... did=423584

Well, I was right about being able to change the diff fluid, I just didn't realize that was completely separate from the "sealed viscous goo."

I still don't buy the low tolerances. Just basic driving would would wear the center differential - braking, turning, acceleration. All have an effect on overall tire diameter.

I feel the same way - it's just hard to believe.

[ciper]

" basic driving would would wear the center differential - braking, turning, acceleration. All have an effect on overall tire diameter. If this was true, I doubt there'd be any functioning center diffs "

Yep. The differentials dont completely fail, they just become weak. It would be the difference of silly putty thickness when the car is new and honey thickness when you have worn it out. Why is this so hard to accept?

[evolutionmovement]

Subaru has a good reputation for their AWD systems and I would figure this wouldn't have happened if the diffs went to hell so easily. So what the hell's the point of AWD?! It would seem stupid (against Subaru's usual engineering thoroughness) to design the system with so little slack that it would fail so easily.

I'll suspend my disbelief for a second (I'll pretend you've shown me the holes in his hands) and ask this then, what alternatives are there? I will destroy my center diff very quickly if I convert to AWD. I drift the car a lot and beat the bag out of tires. If I wanted high maintenance and expense I would've bought that Lamborghini Espada five years ago - rotating the tires and replacing all 4 tires any time 1 goes with my history of frequent blow outs is out of the question.

Converting to RWD would probably be fun once in a while, but I don't want a drift car for my daily driver, and I especially don't want to drive it in the snow. There's also the uncharted territory of tuning the suspension for RWD with this chassis. Not that I think the rear end could take it alone, even if I go R180.

I hate fwd and the tires spin too easily as it is with the 3.70, never mind with over double the power and a 3.90, so the other alternative would be another type of center diff - something more sophisticated and tolerant, or maybe an open diff? If there is one, could it be modified to send most power to the rear?

Steve

[ciper]

Our generation has a 8kg unit right (or 6?). Using a lower mileage, a 10KG (from wrx) or either of the STI versions would help.

Its not that the differential becomes useless in the 5 speed, just not helpful if you drive it in any way agressive. The automatic is where it gets hosed, but this is usually after 130k+ miles.

[rallysam]

So to get closure on one point, these are the possible consequences:
overstressed Viscous LSD (5MT) acts like OPEN diff
overstressed clutch/solonoid type LSD (4EAT) acts like LOCKED diff

Is that what I'm hearing from you guys?

[ciper]

overstressed Viscous LSD (5MT) acts like an OPEN diff
overstressed clutch/solonoid type LSD (4EAT) acts like locked diff until it becomes an open diff (if that makes sense)

[musketeerracing]

But, but, but:

The owner's manual for my Legacy says "if you use chains, use them on the front only." OK most usages will see slippery conditions and may not stress the diff that much, but surely that's a significant diffence in rolling diameter?

Second, how can I diagnose the condition of the center diff in my 5MT? Rolling one whole axle by hand won't create enough shear speed to tighten it I suspect. Do I need to drive off a 2wheel dyno or something?

This is so funny to me: I can tell when my mechanical center and front diffs in the rally car have been changed to different ramps by the steering character under braking, but I can't diagnose a simple viscous center!

ACP
www.musketeerracing.com

[ciper]

If you dont have a rear LSD you can use a needle torque wrench and compare it to what others have seen.

Jack up one side of the car and let it idle in gear. The front and rears will try to spin and the torque wrench will show a number.

If you have a rear LSD then it really messes you up. You can put the car up on all fours and use the parking brake to stop the rears. See how easy it is to stop (subjective of course).

[musketeerracing]

[quote="ciper"]If you dont have a rear LSD you can use a needle torque wrench and compare it to what others have seen.

Okay - I geddit. So what have others seen?

ACP
www.musketeerracing.com