Date:         Sat, 4 Mar 2006 00:46:00 -0500
Reply-To:     Dennis <guskersthecat@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Dennis <guskersthecat@YAHOO.COM>
Subject:      Re: Power loss with 15" wheels?

http://gerry.vanagon.com/cgi-bin/wa.exe?A2=ind0409D&L=vanagon&P=R6602

I did some math on this a while ago.  The reason folks often have varying
results is pretty much because there are a lot of variables.  Rolling
diameter is somewhat of a red herring in this discussion, because that's
really not the issue.  Rotational mass is everything.  It is entirely
possible to have two wheel sets, same rolling diameter, with significantly
different rotation mass.  How far the mass is from the hub is also critical.

A great example is my winter tire/rim combination vs my summer setup on my
daily driver.  Both have identical rolling diameters, but the winters are
on 13" rims and the summers 15"  There is an immediate difference on
switching over to winters in that the car goes to redline much quicker, and
is noticeably faster.  The 13" alloy/winter tire combination is also
significanly lighter.  It's a bit hard to measure on the G-tech (0-60
times) as the starting traction is far less with the skinny winters....but
it feels like swapping a lightened flywheel in there.  Like a lightened
flywheel, there is no difference in horsepower, but the vehicle is quicker
due to less work being required to spin up the various rotating systems.
Remember in these tests, that the wheel systems are within .25" in rolling
diameter!  Another great example is switching over to a lightweight
rim/tire system on a mountain bike.  .5kg per wheel (you can do this with
tires alone) is absolutely noticeable in your legs when accelerating, or
ascending.  Sports Car Driver did a dyno test on an Acura RSX before after
a larger disc brake swap.  The swap cost the equivalent of 11HP in
acceleration times.

Once you've accelerated to a steady state, rotation mass is less of an
issue.  As far as you out coasting your buddy, that's all about rolling
resistance...your skinny tires have less.  Think 10 speed vs mountain bike
tires.