[V8] Differing Tires & Sizes (revisited) iii

[Etdmail@cs.com]

Hi all:
Another .02:
Direct measurement when tire (or rim) shopping is always best.
Even the exact same 'tire size' from different 'tire manufacturers' 
can vary a great deal - in rolling diameters and cross sections - etc.

Makes the purchase of some of those tire 'close-out sales' a potential 
liability and/or false economy - if needing to replace one tire later on. 

Best,
Ed 

In a message dated 10/27/05 urq at pacbell.net writes:
> Message: 6
>  Date: Thu, 27 Oct 2005 00:37:12 -0700
>  From: "urq" <urq at pacbell.net>
>  Subject: Re: [V8] Different Tire Sizes
>  To: <v8 at audifans.com>
>  
>  ... 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)
>  
>  Michael Thomas writes:
>
>  > 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.
>  ---