Tires & Xmission Damage..Myth or Fact

[tomrose]

My dad has been driving Subaru's for years. His mechanic has been working on Subarus exclusively for decades. His mechanic has always warned that if you get a tire failure and the tire is not repairable, that you must replace all 4 tires so that the tire tread is always maintained as exactly the same between the four. The mechanic claims that driving the vehicle with tire treads unmatched on even one wheel will damage the transmission (automatic) over time. The only option to replacing all four if one has a catastrophic failure would be to find a used tire with exactly (or almost I assume) the same tread depth and profile. If this is fact, then there must be some type of window used to gauge at what point the mismatch occurs. In other words, how much difference in tire depth or profile constitutes a mismatch great enough to cause transmission failure. Can anyone please enlighten me on this subject? Fact or myth and if fact, to what extend does the mismatch have to be in order to cause transmission failure. Thank You.

[brus brother]

Can't help with the science but what I've gathered here and bears repeating is that when you have a flat, place the spare donut on the rear, insert the fuse disabling the AWD, drive less than 45 mph and fix the tire immediately.

It is suggested that you buy a fifth tire and rotate your tires to include this fifth wheel. This would take care of the catastrophic loss of one tire but what happens if you have 2 such failures.

It also begs the question that at the first tire change, the "spare" is already at uneven wear with the other four tires that have been in use, and what of this mismatch?

[The Dude]

Tom, listen to your mechanic, he knows what he is talking about. The tires on an AWD Subaru MUST match within 1/4" in of circumference. It's a pretty tight spec. A fair number of posters have reported damaging their transmission after driving less than 100 miles on a badly mismatched tire. The center diff on the AT is a clutch pack, and it is the component that is damaged by mismatched tires. Replacing then clutchpack isn't the end of the world, but it isn't cheap either. AWD, just like anything else in the world, ain't free.

[ShawnW]

Agree with The dude, trust the mechanic on this one.

[bmfsoprah]

so how about on a mt?

[bjwirth]

1 story doesn't make this a fact, but I've been driving with 1 mismatched tire for about 10,000 miles with no apparent problems. Originally I tried to find a used tire with the same circumference as the other 3 tires. Of course when I had it mounted and filled with air, it was over a half inch bigger. Also the other 3 tires weren't so close either (2 tires within 0.1", the other 0.3").

 

If damage is supposed to occur within 100 miles, then I think this is a myth. but that's not to say I'm not tearing up my center differential as I drive now. (I doubt it though)

 

I just think this 1/4" spec is REALLY tight and we really have a little wiggle room. I'd challenge other folks to go out and find an empty lot and mark your tires and drive until one tire make 40 rotations. if any of the other tires are not aligned within 45 degrees of your reference tire, then you are exceeding the 1/4" spec. (I'm assuming 82" tire diameter- making 1/4" a 0.3% difference. After 40 rotations, difference is 12% or approx 43 degrees. who can really measure that precisely, but if it looks close, than it's ok, but if it's obvious like 72 degrees (angle between 2 lug nuts), then you KNOW you're exceeding the 1/4" spec).

 

EDIT: I have a 00 Forester auto w/ 86000 miles.

[The Dude]

Hi BjWorth,

 

You have brought up some excellent points. First, do you have a manual or an automatic transmission? The automatic seems to be much susceptible to damage from mismatched tires than the manual.

When the AT starts to fail from mismatched tires the car shudders when making sharp low speed turns. When the manual fails from mismatched tires often there are no symptoms, you just silently lose your AWD!!!!

In general, the worse the mismatch, the quicker the damage to the center differential. Admittedly, the cases of damage occuring under 100 miles involved some very badly mismatched tires. Anyway, if you drive an AT and your tires are that mismatched, sooner or later, one day you'll feel a shudder as you pull into a parking space. I can practically guarantee it.

 

BTW, my 1999 Forester AT has 180,000 miles on it, and I live in snow free South Carolina. So, I'm not quite as picky about the AWD as I used to be. If the AWD ever failed, I'd just slap in the FWD fuse and keep on driving.

Anyway, one of the tires on my Forester had a very slow leak. The tires were matched, I still rotated them, and I always filled up the slow leak before it got under 20 psi. The tires were just about worn out, and I was either too lazy, or too busy, to have the tire fixed when I was going to replace the whole set soon anyway. I went about 8,000 miles that way, then one day I'm pulling into a parking space HOLY CRAP!!!, what was that??? The car started shuddering. So, I slipped in the FWD fuse and got a new set of shoes for the car that same day. New tires equalled no problema, the AWD was back on line. Sometimes you get lucky. Even dumb luck counts in life.

[scoobdude]

isn't the MT center diff a viscous coupling? this would mean that you would over heat the fluid, but no advese affects. clutches and fluid wear out but in a MT i do not see any perm damage that could not be fixed via new fluid.

 

Please correct me if i am wrong

 

also in a MT i would recommend thowing the "different sized tire" on the front. this is cause in the WRX you have an open diff. this means the transmission will use the path of least resistance to push torque though. on the rear you have the LSD which is usually a cluth type which will wear out if the tire is placed in the rear. this will affect the center diff more than the the if the tire was placed in the front.

 

Joe

[richierich]

If your vehicle still has a Duty Solenoid C, and you ruin it because of the tires being mismatched, even putting in the fuse may not help it, it may still bind. Saw this with a 91 Legacy AWD Auto the other day.

 

I probably not as strigent as the 1/4 inch rule, but I have seen mismatch tires cause damage to the AWD system. The ones that get ruined within a 100 miles are usually stories of people driving the donut spare at freeway speeds for a couple of day. These people are DUMB because they should read what is written on the spare as well as the owner's manual.

 

bjwirth: It CAN cause damage. Not it WILL cause damage. You drive with mismatch brand of tires (same manufacter's size ex. 185/70/15) with different tread depth, it CAN cause damage over time. Not a risk I want to take knowing that a rebuilt transmission costs over $1500. You drive for days with your donut spare on, or for month with tires with different manufacter's size ( 185/80/15 vs 185/70/15) and you WILL cause damage.

[The Dude]

Scoobdude, the viscous differential on the MT is a SEALED unit, you aren't adding a thing to it. When the viscous center differerential is toast, it's strictly remove and replace. The VCD appears to be tougher than the clutch pack in the AT, but I still wouldn't abuse it. Mismatched tires cause the fluid in the VCD to heat up and lock the axles together like a 4 wheel drive system with locking hubs. This a not a normal state for this car. Turns are accomplished by the tires "scuffing" on the pavement to compensate for the locked up VCD. Just what you want, right? The Subaru MT is a tough little car, but it was not designed to be in VCD lock up 100% of the time. Also, excessive heat is a bad thing for almost all things mechanical.

[WoodsWagon]

I once saw a jeep grand cherokee, mid 90's, with quadra-track or whatever AWD they use with mismatched tires. Actually, I heard it first. It was howling like a banshee and screamed its way alll the way to the gas station I was fillin up at. The noise was obviously eminating from the transfer case. I approached the female driver and mentioned the issue. She said that "Its been doing that ever since I bought new tires for the back." She had matching tires, just the back ones were brand new, and the front ones were bald. Needless to say, the LSD in the transfer case must have been smoking wreckage, but the noise was really amazing. If you have to buy only a pair of tires, put them on opposite corners of the car. That way, the rotational speed of the front axel and the rear axel are equal, saving the center diff. each end diff gets more of a working, so the only way is to get all 4 and be safe.

[bjwirth]

The ones that get ruined within a 100 miles are usually stories of people driving the donut spare at freeway speeds for a couple of day.

 

bjwirth: It CAN cause damage. Not it WILL cause damage. You drive with mismatch brand of tires (same manufacter's size ex. 185/70/15) with different tread depth, it CAN cause damage over time. Not a risk I want to take knowing that a rebuilt transmission costs over $1500. You drive for days with your donut spare on, or for month with tires with different manufacter's size ( 185/80/15 vs 185/70/15) and you WILL cause damage.

 

I can believe that tires of different SIZES (ie 185/70 vs 185/75) will cause damage. But I think the original poster was getting at (or at least there are others who wonder) what will happen if one replaces only one tire with the correct size. My suggestion was for people to go out and actually measure their tire diameter. I'm willing to bet very few people have all 4 tires within 1/4" of each other.

 

Now if you're in a situation where most of the tires are worn when you get a flat, you'll probably replace all 4. and if you're in a posiotion where the 4 tires are practically new, then you're probably safe just replacing the 1 tire. But what if you have about 2/3 your tread remaining.... what do you do- bite the bullet are replace all 4 or just take your chances and replace only 1. I'm saying if 1/2" difference is OK, then getting 1 new tire is probably ok.

 

But how can I prove this? I don't think I can. but if people start measuring their tires and say, "oh, looks like some of my tires are not so close and i've been driving like this for a while with no problems." Maybe, just maybe, the 1/4" spec is too tight. I think there are people on this board who are religious about proper tire pressure and rotating tires every 5000 miles. But the mass public (including myself) is not. Try measuring your parent's car or a friend you know who is not diligent.

[The Dude]

Hi Bjwirth,

 

Once again, you have raised some valid issues. Do I think 1/4" is an excessively tight spec? Yes, I do. Do I suspect that Subaru made the spec tight to deal with clutch pack claims made under warranty? Maybe. I'll say this, if I had a warranty clutch pack claim I sure as hell would measure my tires BEFORE I took the car to the dealership. Because you know they're going to measure your tires at the dealership, and if they exceed the 1/4" spec, you just might have problems getting a warranty repair.

 

Subaru is probably trying to keep their customers out of trouble with a spec that is on the conservative side. But know this, mismatched tires are one of the real weak spots in the AWD system. It is a real matter for concern. If you wanna be Capt. James T. Kirk and boldly explore the unknown, make up your own tire spec and go for it. Drop us guys on Earth a line, and let us know how it goes.

[Johngenx]

Let's look at the math...

 

Suppose we have driven our four tires down to the wear bars, meaning we have 2/32" of tread remaining. (Since the majority of posters are in the US, I'll use non-metric units not to tax you...)

 

We have a flat. The tire is shot. We buy a new one, and it has 11/32 of tread. The height difference is about 9/32 of an inch. That is 0.28125". That is barely more than a quarter inch. That is an extreme example where the tires are worn completely and we replace one with a brand new tire. More likely is that the tires are worn less than to that point, and we'll be under the 1/4" tolerance limit, if that limit is indeed true.

 

Not sure about different models and markets, but both our Subies have full-size spare tires on OE wheels, standard equipment.

[The Dude]

HEY YOU. Ya, you, Mr. Metric from the frozen North. It's 1/4" in CIRCUMFERENCE, not tread height. I hope you're not feeling too taxed, ya hoser.

[Setright]

As Loyale has pointed out: One tyre alone can't do the damage. The open diffs on the axles will absorb the difference. (Yes, yes, some models have LSD in the rear.)

 

IF you have different sizes front to rear, you will run a high risk of damage. Even the manual center diff will be destroyed. Heating the oil causes it to expand and press the plates together, giving more or less torque transfer. OVERheating the oil with different axle speeds will shear the oil molecules and thin it out. In the end it won't expand as much, or be able to transfer the torque.

[bjwirth]

If you wanna be Capt. James T. Kirk and boldly explore the unknown, make up your own tire spec and go for it. Drop us guys on Earth a line, and let us know how it goes.

 

CAPTAIN'S LOG: Stardate 2005

 

I'm trying to convince my coleagues on the USMB that I, with little to no knowledge in the field of automotive engineering, know more than the engineers at Subaru. So far, for whatever reason, it's not working.

 

I guess I'm just naturally a risk taker (or just cheap). To each their own. If I do get transmission problems, I'll report back here with my tail between my legs. But for now, everything SEEMS to be running fine.

 

So to all those that are considering changing only 1 unrepairable tire, do so at your own risk. I'll just say I did it, and 10,000 miles later, no problems. I don't know what the magic number is for the tire circumference difference. I suspect the higher the difference, the greater the probability of problems. I tried to minimize the risk by trying to match the circumference with a used tire (of the same specifications).

[Scottbaru]

My (smallish) tires are 24.3"dia, 76.34" circumference. At 60mph they turn at approx 830 rpm. If one tire were .25" smaller in circumference (slightly over 1/32" tread depth), it'd turn at 832.7 rpm. The 2.7 rpm difference would have to be taken up by the axle differential, while the center diff would take up half that speed. Those are pretty slow speeds, I don't think much heat will be generated. I don't know how the clutch packs or viscous fluid shed heat, so I can't even guess how much would be too much. I'm on my eighth AWD vehicle, and have always spent a little extra on tires. I like traction.

[Commuter]

We have a flat. The tire is shot. We buy a new one, and it has 11/32 of tread. The height difference is about 9/32 of an inch. That is 0.28125". That is barely more than a quarter inch. That is an extreme example where the tires are worn completely and we replace one with a brand new tire. More likely is that the tires are worn less than to that point, and we'll be under the 1/4" tolerance limit, if that limit is indeed true.

As The Dude pointed out, Subaru's spec is circumferance. You failed to multiply by 2pi (2 x 3.14 = 6.28).

 

1/32" tread depth variation and you are still ok (~0.2" on circumference). But 2/32" tread depth variation and one is over Subaru's spec. Reliably measuring the tread to this accuracy is difficult IMHO. I was faced with this exact situation a few years ago when I had a sidewall puncture on a fairly new tire. The wear was "around" 1 or 2 32nds of an inch. I ran the fuse until the weekend. I then jacked up the car and wrapped a tape measure around all 4 tires. The new tire was right at the 1/4" allowance. I pulled the fuse and did not experience any trouble.

 

Commuter

[Setright]

"Captains log: I've lost my toupee and girdle. And I can't leave my room!"

 

Courtesy of MST3K

[scoobdude]

As The Dude pointed out, Subaru's spec is circumferance. You failed to multiply by 2pi (2 x 3.14 = 6.28).

 

1/32" tread depth variation and you are still ok (~0.2" on circumference). But 2/32" tread depth variation and one is over Subaru's spec. Reliably measuring the tread to this accuracy is difficult IMHO. I was faced with this exact situation a few years ago when I had a sidewall puncture on a fairly new tire. The wear was "around" 1 or 2 32nds of an inch. I ran the fuse until the weekend. I then jacked up the car and wrapped a tape measure around all 4 tires. The new tire was right at the 1/4" allowance. I pulled the fuse and did not experience any trouble.

 

Commuter

 

Ok i just want to point out a few things... drive line slop, tire differances (even air pressure effects the total circumfrence or revs per mile) will affect this. my spare is not even teh same size as my stock tires ( off the car where pressure has less of an affect on circumfrence). so subaru is not the know all say all.

 

now i am no engineer, but physics still play their role in this equation, and due to the lack of proof of either side, i suggest we just make our own oppinon and only express scientific facts.

 

and in regards to the center diff... its not a "sealed unit". in an auto the transfer case and final drive assembly are seperated due to differnt needs. auto tranny has auto tranny fluid...while the diff/ transfer case has gear oil.

 

if i am incorrect in any of these please let me know...link would be great

 

that is all

 

Joe

[The Dude]

Yo Cowboy, the viscous center differential is a Subaru MT IS a sealed unit. And no, I'm not going to provide a weblink for you just to prove my point. Tell ya what, why don't you just pry the viscous center differential on your car open with a crowbar or something. I'm sure them fellers down at the Sube dealer will be able to refill that puppy with new silicone fluid in less time than it takes a country boy to milk a sunburned armadillo. Whatever.

[scoobdude]

Yo Cowboy, the viscous center differential is a Subaru MT IS a sealed unit. And no, I'm not going to provide a weblink for you just to prove my point. Tell ya what, why don't you just pry the viscous center differential on your car open with a crowbar or something. I'm sure them fellers done at the Sube dealer will be able to refill that puppy with new silicone fluid in less time than it takes a country boy to milk a sunburned armadillo. Whatever.

 

My bad the VCU is a sealed unit. my apologies as i am used to the clutch type units.

 

however your rude comments are not needed on this board

 

thanks

that is all

 

Joe

[The Dude]

My bad the VCU is a sealed unit. my apologies as i am used to the clutch type units.

 

however your rude comments are not needed on this board

 

thanks

that is all

 

Joe

 

 

Joe, what can I say? I already replied politely to your question on the viscous differential in this thread. If you thought that my information was incorrect, you should have at least done a search to check out my info. Instead you just reposted the same incorrect info, and asked that someone else do the research and provide you with a website. I'm sure you can do a website or Google search as well as the next guy.

And I can be as wrong on something as the next guy. The whole idea is to make sure that the information passed around on this board is as accurate as possible. And that's not easy because even SOA doesn't have the straight poop a lot of the time. No hard feelings, I'm not the most patient guy on the planet. I tried praying for patience for a while, but it just didn't come quick enough.

[scoobdude]

Joe, what can I say? I already replied politely to your question on the viscous differential in this thread. If you thought that my information was incorrect, you should have at least done a search to check out my info. Instead you just reposted the same incorrect info, and asked that someone else do the research and provide you with a website. I'm sure you can do a website or Google search as well as the next guy.

And I can be as wrong on something as the next guy. The whole idea is to make sure that the information passed around on this board is as accurate as possible. And that's not easy because even SOA doesn't have the straight poop a lot of the time. No hard feelings, I'm not the most patient guy on the planet. I tried praying for patience for a while, but it just didn't come quick enough.

 

no hard feelings.

 

include procrastination in your next prayers. takes patients to a whole new level.

 

Joe