From: goell@nomad.jpl.nasa.gov (Jeremy Goell)
Subject: Re: Time Trial Wheels
Organization: Jet Propulsion Laboratory, NASA/Caltech
Date: Fri, 2 Oct 1992 01:53:23 GMT

In article <30SEP92.16975922.0191@UNBVM1.CSD.UNB.CA>, E5EM <E5EM@UNB.CA> writes:

|>       I'm thinking of buying a set of TT wheels. So far, the
|> trispoke wheels from Cobalt Cycles seems like the best wheel, plus
|> they've lowered their price from $499 to $299. I'd appreciate any
|> info, pro or con, on the trispoke wheels, or on any other good TT
|> wheels. Thanks.
|> 

Each January the USCF does a series of aerodynamic tests of various
commercially available aero products at the low-speed wind tunnel
at Texas A&M University.  The great deal of the time is spent testing
various aero wheels.  I have some hard copy graphs of drag vs. angle
of attack for a bunch of different wheels.  GIVEN THEIR SET-UP, the
CLEAR winner was HED.  THEIR SET-UP, the "algorithm" they used to test
wheels was the following:

All tests are done at a wind speed of 30mph.

1) Wheels were put in the tunnel in a jig by themselves, non-rotating.
   Drag measurement taken.

2) Wheels were put in said jig with a motor that spun the wheel to
   an appropriate rpm that would = 30mph on the road.  Drag measured.

3) Wheels put on a bike, and drag measured w/ and w/o wheel rotation.
   Drag measured.

4) Rider placed on aforementioned bike, drag measured w/ & w/o 
   rotation.

All these tests were varied over an angle of attack from 0 degrees
(straight into the wind) to 30 degrees.

Some interesting results ensued.  Wheels which performed well without
rotation, which included 3- and 4-spoke wheels (Specialized, Zipp,
others), performed the worst under rotational conditions.  Also, many
straight-walled cross-sectional disk wheels (Zipp, Mavic, others) did
well off the bike, but under the most realistic conditions (rotating,
on bike, with rider) their performance dropped quite a bit.  Relevant
to your interest, the Specialized tri-spoke ended up having MORE drag
than a normal 32 hole spoked wheel (MA40 rim).  This was the case for
all the n-spoke wheels.

The clear winner was a Hed disk wheel, which had the least drag at
0 degrees, and past 15 degrees to 30 degrees the drag went NEGATIVE,
indicating that the wheel was generating aerodynamic lift, like a
sail or an airfoil (i.e. it was PUSHING the rider).  This was w/
the disk in the rear.  Second place also went to HED, with the deep-
rim CX wheel.  Never went negative, but never had much drag at any
time.  The next nearest competitor wheel was so far away that I don't
even remember which it was, although I think and it makes sense that
it would be the Zipp that looks like a CX (I forget which model #
this is).

The general consensus seemed to be that a setup with a HED disk in
the rear and a CX in front (not a great idea to use disks in the
front unless you know you are going to ride in a straight line with
no wind) was the fastest.  The CX is somewhat lighter so a more 
versatile setup would be 2 CX's.  This wouldn't be so heavy on hilly
terrain and would be safer in cross-winds (the Hawaii Ironman race
directors don't permit disks because of winds over the lava fields).

The biggest flame on these tests so far--30mph is an unrealistic speed
to test stuff that will be sold to the public and hence used at lower
speeds for the most part.  I think this is potetially valid; I'd like
to see the tests done @25mph, but my gut feeling (I am an aero eng.)
is that the results *probably* won't change much.  However, aero-
dynamics is a non-linear sort of thing, and the data didn't get 
specific enough to test boundary layer thicknesses, speed at which
turbulences and separations occurred, etc...

Have I actually used this stuff?  Yes, I've competed with HED disks
and CX's but I haven't rigorously tested them, nor do I think I could.
I did feel fast when using HED disks, and my times were really good,
but I can't say it was absolutely due to wheels.

Michael Batchelder
mpb@nomad.jpl.nasa.gov
Newsgroups: rec.bicycles.tech