From: jobst.brandt@stanfordalumni.org
Subject: Re: Dum Wheel Aerodynamics Q
Newsgroups: rec.bicycles.tech
Message-ID: <ceGBc.18051$Fo4.236109@typhoon.sonic.net>
Date: Mon, 21 Jun 2004 18:59:20 GMT

Andy Birko writes:

> All else being equal, what is it about fluid dynamics which makes a
> low spoke wheel more aerodynamic than a high spoke count wheel? Is
> this truly the case when rotating?

Each spoke has drag as it moves through air.  Often people claim they
draft each other, which they do ineffectively being more than ten
diameters apart, a distance that in effect makes them NOT draft each
other as bicyclists stretched out in a line spaced about 10m apart.
As has been mentioned, disks have surface drag while spokes create
turbulence for each spoke.  Therefore the more spokes, the more drag.

The reason for deep section rims is to give them a teardrop shape so
that air can close behind with minimal turbulence.  That is to say,
shapes like aircraft wings and rudders allow streamline closure with
low turbulence and therefore low losses to passing air.  The measure
of drag can be seen in what is left behind.  I'm sure drafting another
rider who has the best aerodynamic wheels is imperceptibly different
from the same rider with conventional wheels.  That should be a clue.

The main effect that makes this trivial is that rider air drag is
enormously greater than any of the wheel effects for which riders
spend so much money and concern... especially when they are not
setting world records anyway.

Jobst Brandt
jobst.brandt@stanfordalumni.org