Re: over/under - steer and tyres

[JustaxPHX@aol.com]

>  Interesting point.  I wonder what the max allowable difference would be.  
>  Since a 3.89 front hooked to a 4.11 rear will lock (btdt), that seems to 
>  indicate 5% is a no.

Well, let's look at it another way.  With a few exceptions, most performance 
street tires these days have a tread depth of between 8/32" to 10/32".  If we 
assume the molded-in wear bars are an accurate indicator of when a tire is 
worn out and they typically appear once the tread depth is worn down to 
2/32", then the difference in diameter between a brand new tire and one 
that's worn out could be as much as 16/32" or .5" ... compared to an overall 
diameter of approx. 25", this works out to roughly 2 percent and I know 
first-hand that the car can be driven with a locked center diff using this 
particular combination of new and used tires.  That said, I don't doubt that 
a 5 percent difference is too much...

>  I'm also suspicious that excessive drivetrain/tire wear occurs even in the 
small differences.

Any time you lock the center diff, you are going to increase driveline wear.  
Even driving down a "smooth" road, you'll alternately drag the wheels at one 
end of the car or the other as their rotational speeds vary slightly each 
time they're forced to go up and down even microscopic bumps.  For normal 
street use, I wouldn't recommend it except under low cf conditions, where 
there isn't enough traction available to feed any significant loads back into 
the drivetrain from the tires; for track use, though, increased drivetrain 
wear is simply the price you'll have to pay if you want to drive the car hard 
enough to have any fun!

>  I'm not game for the experimenting, but those really interested....

Well, like I said, I'm now running equal size tires all around simply because 
it makes rotating them much easier and I was able to buy them cheaply.  On 
the other hand, if you leave the center diff open, you'll still experience 
many of the same benefits and increase wear only minimally by forcing the 
diff to rotate when it wouldn't otherwise be necessary.

Of course, if you have a Torsen center diff, then this isn't a problem as it 
was *designed* to rotate continuously ... in a way, it's too bad my 200q is 
an '89.5 version and doesn't have flared fenders since running different size 
tires front and rear is probably the best after-its-left-the-factory cure for 
"spider bites."  Alas, I can't take advantage of this unless I do some 
bodywork first ... maybe I should consider upgrading to a '91 200q, even if I 
don't want the extra weight and have not yet convinced myself that the 20v 
motor is the way to go...

JG