Would it adversly affect my handling to put 215s on the rear and 205s on the front of a MY2000 RS sedan? Maybe even put a lower profile on the front.

bad idea
the center dif will not like this one bit.
it will be turning at different speeds all the time and overworking it till it goes BANG!
tom@kartboy

but it might look cool

Well if they are REAL close in height, I don't see why it wouldn't be ok. The entire reason we have a viscious center diff. is so that there is slack in it instead of totally screwing up like locked center diff's like in 4x4 trucks that have to shift into it.
http://www.miata.net/garage/tirecalc.html

Using that tool, I think the closest match I can see to stock (205-55/16) is 225-50/16. If you were to go with 205-50/16, 225-45/16 would be the closest.

*Disclaimer : I am not responsible if I am wrong and the center diff blows up. This is in my own theory and you should consult a professional before attempting this.

The problem is, I wanted 225-55/16s and 205-45/16s or so.

Ummmmm... that's a 10% difference. You will definetly blow up your center diff with that combination.

What are you trying to do, anyway?

Explain to me why the center diff wouldn't like it. I think it would look cool.

[This message has been edited by JC (edited August 01, 2000).]

JC, I think you meant 225/50R16 and 205/55R16. You can't rotate your tires http://www.i-club.com/ubb-files/wink.gif.

No, I want bigger tires in the back and smaller in the front. Mostly just bigger in the back though. You can still rotate your tires, just side to side.

Why? If you use 225's in the front and 205's in the rear then the car will not push as much because you have more grip in the front relative to the back. No larger rear sway bar would be needed. I have considered doing this with the BFG G-Force R1's because they are the same diameter and would not have the rubbing problems in the back. If you go to the bigger autox's like Divisionals or National Tours, you will see where a lot of people will put wider tires in the front than the back for this reason.

Larger tires in the back? What are you trying to look like a Camaro boy? http://www.i-club.com/ubb-files/wink.gif

Honestly, the front has more clearance for wider tires then the rear does anyway...

Read your manual kidd. You're gonna blow your center diff. It states specifically in the manual that the sizes of your tires do matter. Even too much of a difference in tire pressure matters because the tire's size changes with pressure. If you really need to know why your diff will blow, head to your nearest Subeshop and ask the techs.

I'm not saying this idea is dumb. Just very unwise.

[This message has been edited by Pinochle (edited August 02, 2000).]

You can definitely run wider tires on front than rear because of the fender shape, but the rolling diameter needs to be EXACTLY the same.

Everyone has told me that it will mess with the center diff, but no one has said why yet. This is the tech forum right?

The basic theory behind the viscous center diff (as far as I know) is that any speed differential between the front and rear causes the two inputs to the diff to "stir up" the viscous fluid. As the fluid gets stirred up, it begins to heat up. As it heats up, it gets thicker, and begins to lock up the two input shafts (it won't ever actually lock them together, of course). That's how the power gets transferred from one end of the car to the other.

Well, if your front and rear driveshafts spend the entire time spinning at speeds that are 10% off of each other, the viscous fluid is just going to heat up and heat up and heat up and heat up until you overheat the fluid. Now the fluid doesn't operate the way it is supposed to, so you have a hunk of crap rather than a torque transfer device. On a Good/Bad scale, this is BAD!

Pat Olsen
'99 Legacy 2.5GT 5speed sedan

It will screw up your center diff because your Subaru is an AWD!! That means that all 4 tires drive the car forward at the same time.

Now think about that for a second.

eg. Say your front tires have an outside diameter of 25 inches and your back tires have an ouside diameter if 26 inches. For every complete turn of the center diff, your back tires want to move forward 26" and your fronts only 25". Can you see the problem now? Your back tires will driving forward, while your front tires are literaly being draged along.

This is a bad thing. Your center diff can't take that sort of constant abuse.

JC,
If you take two wheels of different sizes and roll them one full revolution of their respective circumferences, you will find that the larger of the two will cover more ground in one revolution than the smaller one.
So in a system that drives both sets of wheels (your car) if you put larger tires on the rear your fronts will have spin faster just cover the same amount of ground. Since your center diff is in charge of assigning traction it is my guess that it would let the front wheels spin at their designated speed and bleed off the "extra" power going to the rear.
At the tire sizes you are thinking of this could be in the neighborhood of a 100rpm difference that the diff would have to correct every moment the car is moving. All of this is in addition to the normal work the diff has to do!
What you need to understand is that whenever the diff has to compesate for front to rear speed differences it produces heat. If you make it work constantly what do you think might happen?
Yeah, POW! is right.

*Disclaimer* the above ramblings are the product of some guy who doesn't actually own an RS (yet) and may be totally off base but being of somewhat sound mechanical mind he's pretty sure that it's a bad idea

Patrick's comments are spot-on except this one small thing:
That's how the power gets transferred from one end of the car to the other.

With a manual transmission's viscous coupler, power isn't getting transferred anywhere-- it's just that slip is being limited. You start off with 50% torque going to each driveshaft. When one end slips, the goo tightens which LIMITS the speed difference allowed between the two driveshafts.

[This message has been edited by ColinL (edited August 02, 2000).]

Alright, thats what I was looking for, thanks.

to answer your question in a word... No.

No, it wouldn't adversly affect handling?

The official word I got from my service dept. is that if you measure the circumference (read: outside diameter around) of your tires, they cannot be off more than 1/2" or it will ruin the AWD system

But Gary, he was talking about putting wider tires in the REAR. Aside from center diff problems, the car would have massive understeer, and probably be uncontrollable on launch as the front tires light up before the diff locks.

Of course it makes more sense to put more grip up front, but then that doesn't look as "phat" as your average Vette. Or does it -- is that silly import trend of putting steel wheels in front still "in"?

Here in San Diego they've pretty much switched to putting steel wheels all around. I think it's so they can save money for their body kits (that they leave flat white on a glossy painted car).