Tire Size and AWD [Merged 5-7] diameter radius rolling different sizes size

[King Salami]

Gurus - I've got an 92 TSi AWD that I recently had to replace a tire on. All other tires are about 20% worn. Will the difference in tire diameters (due to wear; tire brand & size for the new tire is the same) cause any harm to my transfer case/AWD system?
I know wheel/tire size differences are very important on an AWD, but would 20% wear on 3 tires vs. 1 new make a difference? Since I rotated my tires, I'm experiencing an odd low-pitched "hum" or "grumble", especially when downshifting or raising off the throttle.
I've asked the guys at Discount Tire and my local Audi dealer, but they didn't seem too concerned.

 

[tkelly27]

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

I was all ready to say drive it... Although I would take into account the fact that Tire Rack makes money by selling tires.

Part of me says "they have a differential for a reason" and the other part says "why make that diff work harder than it has to".

Why not call up a mitsu dealer and see what they say is an acceptable tread difference?

Is it the rainy season in FLA? If it's wet all the time you might be able to get away with it

 

[DGajre777]

I drove with mismatched tires for a year - make only, same size, the fronts were falkens, max psi of 51 and the rears were Goodyears, max psi of 44. I ran 36 psi on all four. I have new tires now, bought them a few months ago.

Will it hurt the VC? - It will if you run tires with different sizes, like 225/45-17s in the front and 225/35-17s in the rear.

 

[VanIsleDSM]

There are always people who will tell you the world will end when it comes to things like this, but the fact of the matter is the car is perfectly capable of handing the tiny difference in 2 tires the same size with slightly different tread wear.

Think of every time you take a turn, every time you're not driving straight the outside wheels are going faster than inside.. especially in 90 degree turns, the difference is far more vast than some petty tread wear differences.. where I live a straight road is a rare thing, my VC would have been pooched long ago if it couldn't handle discrepancies that small..

 

[lccynmbr13]

If you want the opinion of someone who works in the tire industry I work at Les Schwab. It's Subarus and Fords that are really sensitive to mismatched tires but for our liabilities sake we won't install mismatched brands/ models on AWD vehicles period and if the tires are the same make and model they need to be within 2/32 of an inch tread depth on all 4 to be acceptable for application. Hope this helps

 

[spinz4]

Ya I've heard Subaru's don't like it and have never ran into any problems. Just made sure they were the same size..

 

[nightspeed87]

Well go throw your better tires on a fwd or rear wheel drive car and do some nice burn outs and put them back on the awd car once they look about even with your more worn tires .

 

[THE REAL DEAL]

paranoia will destroy ya. you'll be fine with 2 old ones. the only way it may make a difference is running something like 55 series in the back and 35 in the front or vise versa. new or old doesn't make a difference.

 

[SanTa_]

You're better doing everything new now then later. It's best to be safe than sorry.

As for me, I got a FWD. So yeah, I only need the front two.

 

[polen]

ya i also suggest new tires

 

[gy2005]

The difference in rolling resistance that you get from having two same sized, different sets of tires is probably equivalent to the difference in rolling resistance that a small variation in tire pressures will cause. Since people don't check their tire pressure every time they go out, there's usually a few pounds difference between tires, but that's never seemed to cause any damage, so I wouldn't worry about it.

The size difference caused by the wear on one of the sets shouldn't matter too much as long as it's not drastic.

 

[Defiant]

It's not "rolling resistance", but tire/while radius from the hub to the contact patch, and the number of revolutions per distance. I don't know what the viscous couplers will tolerate nor for how long. You want to have one size tire all around on the DSM's AWD setup. Differing radii, regardless the amount, will cause more stress than matching ones.
I know of no other AWD setup that is like the DSM's. Most have some kind of mechanical or electronic fudge between the ends, or split the torque output between them. DSMs are built to go 50/50 until something slips and shows a difference in the front and rear axles, and then the driveline tries to go solid through the VCs; this is why they can't be towed with one end on the ground.

 

[nates6969]

hey so long story short, after doing my clutch I must have shifted some of the suspension geometry and caused some serious toe-out on the front end. However the car didn't really pull so I never thought anything of it...well 4 months later my front tires are shot! the back tires have about 50% left (all 4 tires have been on car for about 2 years). My question is how bad is it to throw new two new front tires on without replacing the backs?

the tires are kuhmo 712s and say have 10/32" tread new, then the backs now have 5/32", thats a difference of .3125" tire height from new. that means when the new are moving 60mph, the old are moving ~59.27mph.

The question is: is this severe enough of a variance to cause damage to the viscous coupler?

I'm sure people have done it countless times before, I just want to make sure the VC can handle it without killing it...

 

[project_tsi]

You'll be fine bro.

 

[DGajre777]

You'll be fine, just make sure you use the same brand, model and tire size.

 

[nates6969]

ya I like the 712s and want to stick with them, just didnt want to have to buy 4 new ones when I already have 2 decent ones.

 

[Defiant]

That's probably less difference than most people have just in air pressure variation.

 

[nates6969]

well possibly true however its roughly the same difference in tire diameter as running a 205 40 17 on the back as opposed to running a 215 40 17 on the front. Many people claim that this is a big no no. I've seen posts where people completely shut down the idea of running that combination where in reality is pretty much what I'm doing by running a new front set with a 50% remaining back set.

for example: http://www.dsmtuners.com/forums/showthread.php?t=230390&highlight=205+215+different+tires

when you look at it this way, then really running two different tire sizes this close is not really that big of a deal, getto maybe, but not a major issue

 

[asexton6969]

Manfuacturers SUGGEST that your tires have no more than a 2/32 differece all the way around. Otherwise you have the POSSIBILITY to cause damage to major components.

Thats how I tell all my customers when they only want to replace two. I just let them know, it could cause damage, but it's not for sure.

BTW if you had rotated your tires every 6,000 miles, you would have noticed the wear. Also, the alignment can be out and not pull. Always look at your tires for wear, remember to check air pressure and rotate. Plus it is suggested that you check your alignment atleast once a year. With roads up north, its suggested to check it every 6 months.

 

[a73_4g67]

I have 195 x 60 x 15 up front and 195 x 65 x 15 at the back in my Galant VR-4. Is this bad?
Please explain why would the VC suffer? The front tire speed is independent from the rear because of the center diff.
When youre turning or even on a straight uneven road, tire speed up front are always different in the rear. Even left and right tires dont travel the same.

 

[asexton6969]

Manfuacturers SUGGEST that your tires have no more than a 2/32 differece all the way around. Otherwise you have the POSSIBILITY to cause damage to major components.

Thats how I tell all my customers when they only want to replace two. I just let them know, it could cause damage, but it's not for sure.

Some companies WILL NOT install tires that are more than 2/32 difference, different brands, styles, etc. It is a huge liability, it is all manufacturer suggested, but guys they suggest this stuff for a reason. Hell, studies have shown if you follow your maintenance schedule in your owners manual, your car should last well over 200k miles. (Read that a few weeks ago on Msn.com/autos and also in an automotive magazine at work)

 

[a73_4g67]

Ok, I did some searching. Some certified mechanics says is its ok.

Isnt the center VC allow the front wheels to turn at different rates from the rear wheels. This is generally necessary for normal street use.

Heres a quote from some ASE mechanic
"Ask yourself this: How exactly is having one H speed rated tire and three Z speed rated tires going to muck with your drive train? Same thing for the load index--how will one tire rated at 1680 lbs @ 40 PSI and three others rated at 1750 lbs @ 40 PSI affect your drive train?

Basically, they can't. Subaru is just trying to avoid lawsuits.

On their FAQ site, they do recommend that all four tires be with 1/4" circumference because of the drive train; however, unlike the direct coupling found in most 4WDs, the viscuous coupling used in Subarus and other AWDs specifically allows the front wheels and rear wheels to spin at different rates. The left and right sides always have to be able to spin at different rates or else going around corners would be difficult.

As an ASE mechanic and an owner of AWDs of several makes and models for 20 years, I'm telling you, there is no need to replace all four tires at the same time. Several times I have replaced one or two tires at a time and NEVER had a problem.

Again, Subaru is just being overly cautious"

I'll replace the rear with the same size on the front anyway. But I doubt it if I can get the same brand. It will be surplus/2nd hand anyway so it should have the same treadwear.

Manfuacturers SUGGEST that your tires have no more than a 2/32 differece all the way around. Otherwise you have the POSSIBILITY to cause damage to major components.

Thats how I tell all my customers when they only want to replace two. I just let them know, it could cause damage, but it's not for sure.

Some companies WILL NOT install tires that are more than 2/32 difference, different brands, styles, etc. It is a huge liability, it is all manufacturer suggested, but guys they suggest this stuff for a reason. Hell, studies have shown if you follow your maintenance schedule in your owners manual, your car should last well over 200k miles. (Read that a few weeks ago on Msn.com/autos and also in an automotive magazine at work)

 

[asexton6969]

How many people have actually taken their drivetrain apart to see if there is any damage? Just a thought.

Like I said though, it is SUGGESTED not REQUIRED. However, I myself would prefer to have four tires that all have the same traction capability, treadwise. (Speaking about my fiancee's 98 GSX as well as my GST)

 

[a73_4g67]

I guess not that many.

If my thinking is correct, the VC will just take care of the torque split. less tire spins for the bigger diamter tire. It will jerk if its a normal 4wd. I dont feel it.
Now why would the VC get damaged? Isnt it like changing the ratio in the rear diff?
What Im after is the myth surrounding VC gone bad if not all tires are the same.

How many people have actually taken their drivetrain apart to see if there is any damage? Just a thought.

Like I said though, it is SUGGESTED not REQUIRED. However, I myself would prefer to have four tires that all have the same traction capability, treadwise. (Speaking about my fiancee's 98 GSX as well as my GST)

 

[Defiant]

Subaru does not use the same purely-mechanical split that DSMs do. In fact, no other vehicle that I've found does.
Do not mix tires on an AWD DSM.

 

[asexton6969]

Subaru does not use the same purely-mechanical split that DSMs do. In fact, no other vehicle that I've found does.
Do not mix tires on an AWD DSM.

I couldn't agree more.