Would different tyres really ruin AWD?

[subyboy]

'91 Liberty (Legacy) manual AWD 2.2lt

 

Hey all,

 

Been reading a lot on this forum about how it's important to have all 4 tyres the same or you'll stuff up the AWD system.

I had a tyre blow out recently and could only afford to buy 2 new tyres (exact same models as old ones), which were put on the front wheels. Could this really damage the AWD sytem (centre diff) and the viscous coupling? My Subaru manual only says to use tyres of the same size, construction and load range, nothing about new/used.

I don't really understand how the VC could get damaged as it is just fluid spinning in between plates. Is it an overheating issue with the fluid if forced to shear all the time? I drove 500mi on a freeway without issues...

Some people recommend putting the two new tyres on same side of the car to avoid front and rear wheels spinning at different speeds. How would that affect handling?

 

Cheers,

[OB99W]

Here's what Subaru has to say about size difference:

http://www.endwrench.com/pdf/drivetrain/RearAxleBindingInfoF99.pdf

[LeakingOil]

I bought my SVX with a blown front diff due to different tyres. I'm not sure with manuals, but with automatics if you have different sizes you can pop in the FWD fuse and not worry about it until you can buy new tyres.

That's been my experience with the SVX and the Legacy.

[subyboy]

Thanks guys but I think that applies to automatics and mine's manual (and dual range, too)

Also, I've done some more research and it appears the center diff could actually be viscous LOCKING. Found a few articles (inc. this one http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm#VC

) but it doesn't really explain how the viscous coupler actually LOCKS the diff. Anyone know?

[OB99W]

[...]I'm not sure with manuals, but with automatics if you have different sizes you can pop in the FWD fuse and not worry about it until you can buy new tyres.[...]

With automatics, AWD front/rear ratio is determined by on/off cycling of duty solenoid "C", and therefore rear drive can be disabled electrically if the system is functioning properly. Unfortunately, there's no equivalent with a manual's viscous coupling.

[Gnuman]

Yes, it will stuff up the system over time. Yes, it is an overheating fluid issue on both the manual and the automatic transmissions. the tires need to have the same circumfrence within +/- 1/4 inch (Something like 5mm) to prevent it. No, you will likely not notice it when not on pavement. It will also not be real noticable on pavement untill it is real bad. Yes, it is repairable. Actually easier on a manual than on an automatic. On the manual, there is one failure point, and only one way to fix it: replace the VC that serves as a center diff. How it fails is that the Viscious Fluid heats up (normal) causing it to solidify (which locks up the VC to transfer teh power to the other set of wheels). When it is too active for too long, that fluid bakes to a near solid state, and does not return to the free moving condition.

 

What this causes is the rear wheels are locked to the front ones. In that state, tight turns (well all turns to some degree, but it is most noticable on tight turns) the front wheels, which want to turn at a different speed than the rear ones, fight against the turning of the rear wheels. This causes them to act as if the brakes were applied. In bad cases, the wheels will actually break traction and "skip" on tight turns. replacing the VC will return the transmission to normal operation. The problem with this is that the part is not cheap, and you have to take the rear of the transmission apart to replace it. If you are not doing your own work, that is expensive as well. If you are doing yhour own work, it is simply a pain in the arse. . .

 

 

If this sounds like I have gone through this. . .you are right. . .

[subyboy]

Thanks Gnuman, that makes a lot of sense.

Not really good news, I'm afraid, but at least now I know. And since it's better to prevent than to cure, I reckon I might put the new tyres on the same side rather than both at the front.

Lets hope it's not too late...

[porcupine73]

Do the torque bind test, driving in circles slowly in a parking lot and seeing if it feels like the brakes are on or there is any popping or anything. If not you're probably good, at least torque bind hasn't set in yet.

[DerFahrer]

AWD on all cars, not just Subaru, need the tires to be the same type and wear.

 

How many miles were on your set of 4 when the one blew out?

 

If you bought the tires somewhat recently (I'd say within 500 miles), then it is probably okay to buy one new one.

 

And, honestly, it's better to buy just one than two. If you buy just one, then you'll only have the left and right tires spinning at slightly different speeds. That slack will be taken up by the open differentials (wait, is yours turbo? Do you have a rear LSD?). It won't be good on the open diffs either, but they can put up with it for longer than the center diff can by running two new and two old tires.

 

You could even put the new tires on one side of the car and have the old ones on the other side (like new on the left and old on the right). That will be okay also as far as your AWD system is concerned.

 

But yeah, if you had a lot of wear on those tires, then you are slowly doing damage by running two new tires. I'd honestly consider just getting 4 new cheapy tires. Here in the US, we have tire joints that sell 4 new tires for $100 USD.

[(goldfish)]

Here's what Subaru has to say about size difference:

http://www.endwrench.com/pdf/drivetrain/RearAxleBindingInfoF99.pdf

 

Man I reallywant to see more offical documentation on this. The numbers and math just don't make sense.

 

as circ. = 2*radius*pi(3.14 for this)

 

.25=2*r*3.14

 

r=0.04

 

a change of .04 inches in the distance from the hubs center to the road will cause binding? Next time I get a real acurate tape measure I'm going to find out how much those tire defelct with 150 lbs in the seats.

 

 

I'm not trying to attach anyone. Just I think more has to be looked into these numbers.

[OB99W]

Man I reallywant to see more offical documentation on this.

The End Wrench is about as "official" as it gets when it comes to Subaru info. http://endwrench.com/main.php?smPID=HTML::home.html

 

 

The numbers and math just don't make sense.[...]

I'm not trying to attach anyone. Just I think more has to be looked into these numbers.

No problem -- look into it here:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18

[nipper]

Man I reallywant to see more offical documentation on this. The numbers and math just don't make sense.

 

as circ. = 2*radius*pi(3.14 for this)

 

.25=2*r*3.14

 

r=0.04

 

a change of .04 inches in the distance from the hubs center to the road will cause binding? Next time I get a real acurate tape measure I'm going to find out how much those tire defelct with 150 lbs in the seats.

 

 

I'm not trying to attach anyone. Just I think more has to be looked into these numbers.

 

There is always someone who thinks the world is flat

 

 

So endwrench, put out by subaru, is not enough documentation for you?

 

http://rio.mightyautoparts.com/pdf/articles/tt116.pdf

http://www.chevyavalanchefanclub.com/cafcna/index.php?topic=41895.new (bottom)

http://www.transmissiondigest.com/tech/TD200602/UpToStandards/Up%20To%20Stds%20HTML.htm

http://www.4x4abc.com/4WD101/diffautoAWD_AWD.html

 

The front and rear axles turning at differnt speeds cause drag inside the transmission. This drag over enough miles causes heat, and sheers the transmission fluid (or in a viscous coupling the silicon fluid). The viscous fluid heats up and expands. When it expands it applies the clutches to give you power transfer to the slower axle. Once the axles catch up to each other, the silicon cools down and the torque transfer is released. WHen you have a mismatched tire, the fluid never gets to cool down. (its the same thing as rifing a clutch)Even though the front and rear axles are spoinning at differnt speeds, something has to give. Friction creates heat, and the clutch plates end up being wiped out. This is the same as having mismatched tires on a LSD differnential (whic is essentially what it is). You wil cook the LSD unit in a few miles.

 

On the automatic, they are designed (unlike the viscous) to tolerate some plate slippage for curves. This is why it takes them longer to be damaged. The plates are also bathed in tranny fluid, whic is cooled and circulated. Since the differnce is so small between front and rea axles, the TCU does not interceed. the TCU thinks you are going through twisting roads. Normally when this happens eventually everything reaches the same speed and the plates have time to cool down. With mismatched tires equilibrium is never reached. One axle is always traveleing at a differnt speed then the other. The clutches are not made for that kind of wear. The clutches will over heat and either lock up, or become glazed and never engage.

 

Now weather or not you choose to beleive what almost every tire and AWD mfg state is your choice. Its a 1200-1500 gamble on a manual sooby, much more money on other makes.

 

nipper

[johnceggleston]

nipper, a great explanation.

 

1/4 inch difference, over approx. 700 revolutions in a mile (depends on the tire size) equals over 15 feet. now, how many miles will it take to cause damage? if you were buying a car, and the seller said there was just one little thing, "for every mile you drive, one tire will have to drag, unturning, across the pavement the last 15 feet". would you buy the car?

 

obviously not the same, but the wear and tear takes place, just the same, if your tires do not match. it just happens to the internal parts.

[subyboy]

Man, that's all really worrying. Just for info, since I got the two new tires, I drove about 2,000 miles on sealed roads, including stretches of about 200 miles without stopping once. More than enough to cook the fluid to death.

Knowing that the difference in circumference between new and used tires was 3/4" (Subaru recommends no more than 1/4"), looks like my VC should be dead. I haven't felt anyhting as yet, but will do the slow circling test to be sure.

And to think I bought 2 new tires instead of one thinking it'd be better for the car :-\

I guess what they say is true, you learn from your mistakes...

[nipper]

Man, that's all really worrying. Just for info, since I got the two new tires, I drove about 2,000 miles on sealed roads, including stretches of about 200 miles without stopping once. More than enough to cook the fluid to death.

Knowing that the difference in circumference between new and used tires was 3/4" (Subaru recommends no more than 1/4"), looks like my VC should be dead. I haven't felt anyhting as yet, but will do the slow circling test to be sure.

And to think I bought 2 new tires instead of one thinking it'd be better for the car :-\

I guess what they say is true, you learn from your mistakes...

 

I dont like to recomend this, but you can put the two on the same side of the car.

 

Also keep in mind that a tired of one size from Goodyear is not exactly the same size from Firestone.

 

Go and measure your tires and see what they really are. You MAY get away with it, but remeber, no one has an unlisted number when it comes time for murphy to call.

 

nipper

[subyboy]

Thanks Nipper.

I have actually measured the tires circumference, and the differnce was indeed 3/4 inch

I've now put one new tyres at the front left, and one on back right. Is that OK or is it important to have them on the same SIDE?

[nipper]

Thanks Nipper.

I have actually measured the tires circumference, and the differnce was indeed 3/4 inch

I've now put one new tyres at the front left, and one on back right. Is that OK or is it important to have them on the same SIDE?

 

Thats ok.

 

The differnential is a mechanical computer. It always splits the rotational speed 50/50 between wheels. It doesnt care which one is which.

 

Now with the newer computer controlled AWD with the integrated traction control, you cant even do this.

 

But DO get another pair of tires when you can.

 

nipper

[subyboy]

I sure will!

[subyboy]

OK, I did the slow circle test and didn't notice anything wrong. This is kind of a relief, although I realize it doesn't mean I won't have torque bind problems in the future...

Silly question: I use Redline MT-90 in the transmission, could it make any difference as to wether or not you get torque bind problems???

Can't see how it would since the problem is the CV, which is a sealed unit and doesn't 'use' trans oil, but anyway...

[nipper]

Can't see how it would since the problem is the VC (edit), which is a sealed unit and doesn't 'use' trans oil, but anyway...

 

Answered your own question.

 

 

 

nipper

[pinister]

hi guys

for the last 3 years my forester wears YOKOHAMA GEOLANDAR AT+II tires.

about 4 months ago, after my left rear tire was damaged by a drunk driver (together with other parts...), i replaced the 2 rear tires by similar new tires.

these 2 tires were half of the only set left in isreal, when new shippment was delayed for months, and i had to share this set with another guy (that knew about that set and needed only 2 tire).

after 3 years of wear, the difference between the new tires and the old tires was 6 mm in diameter only.

recenetely i've found out that this kind of tire is no longer being manufactured, and has been replaced by a new AT model, probebly A\T-S.

so, i am confused. using 2 new and 2 old tires may damage the viscose coupling. using 2 new on one side and 2 old on the other is not a very good idea, because i have a LSD in my rear diff and i am going to "plant" the same LSD in the front. i really don't want to buy 4 new tires, and it seems that the best idea is to use the new 2 tire of the old model together with 2 of the new model.

i guess these 2 models are different, but they are both AT, manufactured by the same company.

and after 4 monthes ride these new 2 are still new. 6 mm difference after 3 years prove it.

what do you suggest?

[subyboy]

hi guys

and after 4 monthes ride these new 2 are still new. 6 mm difference after 3 years prove it.

what do you suggest?

 

I don't really get it, but it doesn't matter. What matters is that the difference between the almost new tyres, and the brand new ones is no more than 6mm (1/4inch) circumference. They also need to be same construction and load range. Have you tried calling the manufacturer to check with them directly?

[pinister]

hi subyboy

i called the importer of yokohama tires, and he said that there isn't any problem with using 2 pairs of different AT tires (both yokohama).

on the other hand the subaru man said that the 4 tires must be the same. there might be some difference in diameter between them, but too much might be bad, and he doesn't know what is the difference that above it there is a certain damage.

he also said that in "normal" ride the rear wheels get only a small part of the power, so the problem can appear when the car works "hard".

meanwile the larger tires are in the front, so, because turns make the front wheels turn faster, there are already bigger...

so, do i have to throw the new tires?

[calebz]

hi subyboy

i called the importer of yokohama tires, and he said that there isn't any problem with using 2 pairs of different AT tires (both yokohama).

on the other hand the subaru man said that the 4 tires must be the same. there might be some difference in diameter between them, but too much might be bad, and he doesn't know what is the difference that above it there is a certain damage.

he also said that in "normal" ride the rear wheels get only a small part of the power, so the problem can appear when the car works "hard".

meanwile the larger tires are in the front, so, because turns make the front wheels turn faster, there are already bigger...

so, do i have to throw the new tires?

 

I would trust the yokohama guy to know tires.

 

But I would trust the subaru guy (plus all the resources here) to know subarus.

[subyboy]

Like I said, difference in circumference should be less than 6mm. If you're at all worried, buy 4 new tyres, it's a lot cheaper than replacing a differential or Viscous Coupler.

You can always keep the 'old' tyres as spares, or sell them to another Subaru owner in the same situation as you...