AutoX and tire impressions

[Derek Pulvino]

Dave,

Rolling radius will make a difference like you said by effectively reducing
the final drive ratio, but the other thing to take into consideration is the
DISTRIBUTION of the mass of the wheel/tire combo with respect to the center
point of the wheel; this is what effects the rotational inertia.   By my
estimates, a larger wheel will also have a higher moment of inertia as there
is more metal further away from the center point of the wheel.  As the metal
associated with the end of the wheel radius (don't what you call this part
of the wheel, the edge?) is heavier than the tire side wall and air that was
formally at that radius, and this mass is now further away from the wheel
center...

I hadn't even thought about the lowering of the center of gravity that
shorter sidewall tires would give, ie 45/15's vs 60/15's.

Derek P


>From: "Freed, David (Exchange)" <dfreed at bear.com>
>To: "'Derek Pulvino'" <dbpulvino at hotmail.com>, t44tq at mindspring.com,
>dfreed at bear.com, dkpriebe at attbi.com
>CC: 200q20v at audifans.com, S-Car-List at AudiFans.com
>Subject: RE: AutoX and tire impressions
>Date: Mon, 17 Jun 2002 16:26:53 -0400
>
>15" rims are usually lighter than the same width 17" rim.  But as long as
>the actual outside diameter of the tires remains the same, the acceleration
>should be the same.  The trick setup would be a set of low profile 15"
>tires
>so the diameter was greatly reduced.  The same as a faster rear end (or is
>that front end) ratio.  Also it would lower the car with out changing
>springs.  But then as the car rolled it might begin scraping the side of
>the
>body.  So, 215/45x15 would accelerate faster than 215/60x15.  Some AutoXers
>do a version of this but most lower the whole car with stiff springs and
>change the differential gears for faster acceleration.  But I do have a set
>of 15" wheels..... and I saw some used rubber.....heh,heh.
>
>Dave
>
>

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