Tyre stuff

[QSHIPQ@aol.com]

sbigelow@sprint.ca writes:
>A 2800 lbs car with say, 60% of its weight on the front axle, has 1680 lbs
>on that axle, or 840 lbs per tire.
>@ 32 lbs per square inch in the tire, how many square inches of patch does
>it have?840/32 = 26.25 square inches.
>Wanna change its size? Change vehicle weight, or tire pressure.

Or change wheel width.  There's a point at which 840lbs on a wide tire on a 
narrow rim, isn't enough to give you 26.25 sq in.  Why?  Because you have 
effectively done the same thing as increasing pressure, you changed the 
diameter of a given tyre.  Not by pressure, by rim width. Statically, we can 
change wheel width to change contact patch @ the same tire pressure.  
Dynamically, a small change in wheel width, has a great affect on contact 
patch.  

Look at it this way, put 40psi into the tire you had 32 in above.  What 
happens?  The rolling resistance decreases due to an effective larger 
diameter tyre which gives a smaller contact patch.  Take the same tyre, put 
it on a narrow rim. Rolling resistance decreases due to an effective larger 
diameter tyre and a smaller contact patch.  We can change overall diameter 
with tyre pressure OR rim width. Both effectively do the same thing:  
Decrease rolling reistance due to a reduced contact patch

>Wanna change its shape? Change tire size.

What happens to a 255/50 15 on a 15X6in rim at 32psi?  By definition you 
changed it's shape, it's contact patch AND it's tyre size.

>Wanna change its quality? Change tire brands/models.

A 255/50 on a 6in rim will change 'quality' of an identical tyre.  In 
function, contact patch, and performance.  Turbo quattros are famous for 
this.  Anyone that has gone from a 6in turbo wheel to a 7in aftermarket with 
the exact same tyre (205/60 15) knows exactly the point here of comparing 
"quality" of the same tyre.

The argument is missing some caveats, see the former posts.  At what point 
can you make your claim without them?

Scott Justusson
QSHIPQ@aol.com