Weight is also a crucial factor in downhill coasting, which is why the
bigger bodies go down the luge and bobsled runs faster.  Also why your kids
can smoke you going up the ridge on bikes, but you can catch up on the way
down!

On 3/3/06, Dennis <guskersthecat@yahoo.com> wrote:
>
> 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.
>

--
Jake
1984 Vanagon GL
1986 Westy Weekender "Dixie"
www.crescentbeachguitar.com