[V8] Different Tire Sizes

[urq]

... but ... what if the differentials *DO* give?  ... that *is* what they're 
there for after all!

You *could* put 13" wheels with skinny, low profile tires up front and big 
meats on 20" wheels in the back (assuming an open center diff ... which all 
of you automatic pilots have) and drive down the road without worry about 
drive line stress or tire scrubbing.  There's no doubt that the prop shaft 
to the front wheels would be spinning faster than the prop shaft to the rear 
wheels ... but an open diff really could care less ... (here again, 
*assuming* that the tranny CU doesn't decide to try to lock the center diff 
... but, here again, I'm exaggerating to try to prove a point).

It seems to me we've lost track of what I thought we were trying to discuss 
here ...

Michael, try those same calculations with:

    235/55-15 (circumference = 79.1")
    215/60-15 (circumference = 79.0")
    195/65-15 (circumference = 78.5")

... by varying profiles you can very easily put together a set of 
considerably different tires that have close enough to the same 
circumference for practical purposes ... I would argue that you could mix 
and match these tires on your car and no matter what kind of 
open/Torsen/locked differentials you had there'd be very little risk of 
damaging the drive train.  Obviously you could come up with some 
combinations that would very likely create a very real risk in terms of 
dynamic stability.  But what if someone wanted to try to put skinnier tires 
in front for some reason to try to improve the overall handling of the car? 
Perhaps on a quattro it would be better to put the wider tires up front ... 
whatever.  My point is that if someone wanted to do that sort of experiment 
they should not feel that they are risking damage to the drive line of the 
car ... given that they went into the project keeping the overall 
circumferences of the tires the same.

Steve B
San Jose, CA (USA)


> Without getting into what the differentials do, if
> different tire sizes are used at the same time then
> something has to give. When someone says that the
> difference between 65% and 60% of a sidewall is tiny,
> they haven't done the math.
>
> 215 215
> 60 65
> 15 15
>
> 25.16 26.00    diameter in inches
> 79.03 81.69    circumference in inches
> 801 775      revolutions per mile
>
> If the driveline/differentials do not slip then the
> slippage must be between the tires and the road. Even
> if not perceptable to the driver, there is a lot of
> tire scuffing going on.