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.

 

[14.5 drift]

audi
bugatti

theres two for ya. BTW what is the difference between the rear or mid engined cars? If you'll notice they both have the same wieght distributions so the drive trains would likely be setup the same, no?

 

[Eagle 5]

Originally posted by 14.5 drift
audi
bugatti

theres two for ya. BTW what is the difference between the rear or mid engined cars? If you'll notice they both have the same wieght distributions so the drive trains would likely be setup the same, no?

mid engine means the engine sits behind the passenger compartment but in front of the rear axles. rear engine means the engine site behind the rear axles.

mid engine cars can probably do the diff size because they would have one shaft going to teh front and one to the rear and coule probaly have diff ratios so larger tires in the rear would be needed. but thats only a guess

 

[ACM]

The centre diff can be geared any way the engineer wants it to be, and similarly the wheels/tyres can be different ODs, which produces a similar effect, that of altering the static torque distribution.

The limitation with DSMs is purely the use of a Viscous Coupling. If the VC is required to constantly rotate differentially it will overheat and die, so with a 50/50 geared centre diff the tyre ODs must be the same to avoid this. However, if the centre diff internal gearing was changed, one could fit larger OD tyres to one end (the output shaft with the slower rotational speed) and that would neutralise the gearing difference, and the VC would be happy again. Or simply get rid of the VC and employ a different slip limiting mechanism.

Charles

 

[Eagle 5]

Originally posted by ACM
The centre diff can be geared any way the engineer wants it to be, and similarly the wheels/tyres can be different ODs, which produces a similar effect, that of altering the static torque distribution.

The limitation with DSMs is purely the use of a Viscous Coupling. If the VC is required to constantly rotate differentially it will overheat and die, so with a 50/50 geared centre diff the tyre ODs must be the same to avoid this. However, if the centre diff internal gearing was changed, one could fit larger OD tyres to one end (the output shaft with the slower rotational speed) and that would neutralise the gearing difference, and the VC would be happy again. Or simply get rid of the VC and employ a different slip limiting mechanism.

Charles

I figured this is is the reason why you have to have same size tires, however, what if you have a clutch type slip or say the quiafe? With the clutch would it simply burn out the clutches much quicker? or say with the quiafe, would it just cause the car to "jump" or jerk occasionally?

 

[pneumo]

It would jump and jerk all the time. A regular LSD in the center diff would try to stop the wheels from turning at different speeds and it would lock up. So it would be like driving a car with a welded center diff and different size wheels.

Like the guy up there ^ said, you have to change the gearing inside the diff to match the different size tires.

 

[DSM4DSM]

Hey Eagle, decided to make my first post on here aimed at a fellow Natian. I can't recall where I read it, but on awd DSMS (1gs at least) power distribtuion is 70/30 rear/front. I find that believable too as my talon tends to have a more rwd handling bias. You may be able to get away with different sizes, but I honestly have yet to see a dsm to try it and would make things more expensive anyway, as you wouldn't be able to rotate your tires at all. Just some food for though

 

[Defiant]

Originally posted by DSM4DSM
Hey Eagle, decided to make my first post on here aimed at a fellow Natian. I can't recall where I read it, but on awd DSMS (1gs at least) power distribtuion is 70/30 rear/front.

DSMs are 50/50.

http://www.dsmtuners.com/forums/showthread.php?s=&threadid=86527&highlight=50/50

 

[DSM4DSM]

Thanks for the clarification

 

[Eagle 5]

Originally posted by DSM4DSM
Hey Eagle, decided to make my first post on here aimed at a fellow Natian. I can't recall where I read it, but on awd DSMS (1gs at least) power distribtuion is 70/30 rear/front. I find that believable too as my talon tends to have a more rwd handling bias. You may be able to get away with different sizes, but I honestly have yet to see a dsm to try it and would make things more expensive anyway, as you wouldn't be able to rotate your tires at all. Just some food for though

Hey, I think I have seen your name on the local board.

Would it be possible to change the gearing in the differential? Or would that be way too much work or cost too much for something that would not be very beneficial? Seems like if you were hardcore or something it could help out. Maybe make a 40/60 or 30/70 power split, throw a little wider tires in back, maybe like 215 up front and 235 or 245 in there rear and it could help with the understeer.

Of course, I dont know and changing the gearing would probably be quite hard or just cost a lot... but hey, its only an idea.

 

[Doug99RS]

Regardless of the engine location you need to keep the rolling diameter of the tires all the same. If you're good with numbers and can figure out the right width to go without changing the diameter then you'll be ok. If you put taller tires on one end and smaller tires on the other you will destroy the transfer case and viscous coupler. Not to mention you will then lose the 50/50 split by simply putting different tires on.
Refer to your owners manual for tire replacement requirements.
Doug

 

[14.5 drift]

so are you saying that gsx has a center vlsd? I was told last week that there was no lsd any where in a gsx. Which is it?

 

[jtmcinder]

All 2G AWDs have a VC LSD in the center. Some have a VC LSD in the rear, too, and some don't. (Someone correct me, but I think that the non-LSD rears don't have heat fins on them; I know the LSD version does have fins.) The front on all 2Gs is open.

- Jtoby

 

[14.5 drift]

What is a heat fin, how can I identify if I have a rear lsd?

 

[jtmcinder]

Heat fins are metal fins on the diff to dissapate heat. I don't know any other way to say it. Take a look at a subwoofer amp. Those are heat fins all over it.

I also think that you can see whether or not you have a rear LSD by reading the info on the door jamb, but I'm not sure about that.

In any event, why not just test the car? Jack the entire car up and spin one rear wheel. If the other one turns in the same direction, you have a LSD; if it turns in the opposite direction, you don't.

- Jtoby

 

[14.5 drift]

thanks, i'll give that a try.

 

[n0va91TSI]

Popped a tire from a nail. All tires are about halfway thru their tread. Will putting one new tire on the front left be bad for the AWD or differentials? Got a quote for like $485 for all 4 and im poor. I can buy them if i need to but i will be almost broke.

Any info is greatly appreciated

 

[ckozlgod]

You can get them a lot cheaper online ya know...probabaly around $315-ish

 

[anduril]

I've heard that you generally want to get atleast a pair changed on an AWD car if one of the tires goes, that way you keep everything similar throughout either the front or back. It might be an urban legend but...it actually makes some sense.

Check out www.tirerack.com. They have good prices on tires and are quite reliable.

 

[anduril]

Btw, you live in Frenchtown? Thats funny...my g/f's best friend lives there and is obsessed with Talon's She's already wrecked two hehe

 

[diviniti]

I've heard that you generally want to get atleast a pair changed on an AWD car if one of the tires goes, that way you keep everything similar throughout either the front or back. It might be an urban legend but...it actually makes some sense.

Check out www.tirerack.com. They have good prices on tires and are quite reliable.

This is half true.

No matter what you want to keep the exact same size tire on all 4 wheels.
Having some with 50% tred left and 1 with 100% tred, I really dont think will matter at all.
If you can get 2 new tires, it would probably be better, but not neccesary.
Reason you must have all of the same tires/wheels, is because if not, it would strain the VC in the trans, and eventually wear down. This = Not good.

 

[n0va91TSI]

if her name is Erica i was in the car for one of the crashes :laugh:

 

[midnght]

The proper way to replace tires on an AWD car is to replace ALL 4. Just ask your transfer case.

 

[steve]

As long as you get exactly the same tire as the others the difference between a new tire and one worn 50% isn't going to cause a problem. The difference in diameter is less that the difference between a underinflated tire and a correctly inflated one.

If you can't get the same tire then you should replace them.

What your stressing when the diameters are off is the VCU not the transfer case, either the rear or the center depending on where the different sized tires are.

Steve

 

[n0va91TSI]

well im getting a different model of tire but both are the same size 205/55/16 kumhos

It sounds like getting two new fronts wouldnt mess things up that bad


but i will wait till more ppl post cuz i dont wanna wreck the VCU or difs

 

[red23]

i would check out http://www.discounttiredirect.com

they have some pretty good prices. you can easily get 4 brand new tires between $250-$300 and they have free shipping.