650c wheels versus 24" wheels: Benefits

650c wheels versus 24" wheels: Benefits

Post by Andrew R. Cogga » Tue, 20 Jan 1998 04:00:00


Quote:

> There's been a debate over at RST for some time now concerning the
> relative benefits of 650c wheels (as appear on many triathlon specific
> bikes) versus 700c wheels (which appear on most road bikes).

> I thought I'd also post my question here. The concensus seems to be that
> in general (ie same spoke count, same hub, same rim type, etc) 650c
> wheels are: (1) lighter (easier to accelerate)

Almost certainly - but how many times do you accelerate/how important is
acceleration in a TT or triathlon, compared to, say, a criterium or a
match sprint? I'll take aerodynamics over weight almost any day.

, (2) more aerodynamic

Quote:
> (less frontal profile),

In theory, yes; in practice, maybe not. That is, I have not seen any
published wind tunnel data showing this - the data that I have seen
shows no consistent difference in a pure headwind. In fact, the larger
wheels may have less drag in a crosswind (which is almost always the
case in real life), since for the same rim depth they have greater side
surface area and thus may "sail" better.

 (3) permit the rider a lower position,

A 650C front wheel *may* be necessary for smaller riders using an
average position, or for average-sized riders using a very agressive
position. However, the majority of triathletes I've seen riding 650C
wheels are higher when on their aero bars than when I'm on the drops of
my road bike (and I'm 6'0"), so I don't see this as a major advantage
for most. In fact, the overall combination of high position/650C wheel
may be worse than the same position with a 700C wheel, because the
longer head tube and/or taller stem required to make up the extra height
is less aero than the taller 700C wheel.

Steve Hed, who has been a proponent of smaller wheels, is apparently now
rethinking his position, based on #2 and #3 above...

 and (4)

Quote:
> permit the frame builder to shorten the chainstays, tucking rear wheel
> in towards rider, resulting in better balance for a forward rider, than
> 700c wheels.

Moving the rear wheel forward to achieve better weight distribution when
in the aero position does seem like a good idea. However, even with a
straight, round seat tube you can build a 700C-wheeled bike with
chainstays close to/just under 16", i.e., so short that shifting
suffers. If you want an even tighter rear end w/o going to a smaller
wheel, you can a) curve the seat tube (ala the KHS ZH2B, or a couple of
the Lightspeed models), and/or b) put a cut-out in the back of the seat
tube (ala the Cervelo P2). The Cervelo folks are of the opinion that
such a cut-out really aids aerodynamics.

Another way of getting more weight on the rear wheel is to move the
front wheel forward, by using a long top tube. This has the additional
benefit of stretching out the rider more, thus lowering drag (how many
records were broken using the "Superman" position before it was
banned?). To my knowledge, though, the only aero bike following this
philosophy is (or was) the ***.

Quote:
> Not that these are universely agreed upon, but they seem to
> me to be the views of most posters.

At the risk of offending many in this newsgroup (R.S.T.), I think many
people's views on the subject have been shaped almost entirely by
advertising, not by any careful consideration of the physics involved.

Quote:
> It seems to me that if these four benefits can be realized by moving to
> a smaller sized wheel; why not go further down to a 24" wheel?  Would
> not all four (1,2,3) continue to be realized?  Why not 20", or even
> 16"?

You left out several potential disadvantages to smaller wheels, which
are a) greater rolling resistance, b) harsher ride, and c) limited
availability of quality rims, tires, etc. (case in point: over on
rec.bicycles.marketplace, I just saw a "WTB" ad by somebody searching
for good 24" tubulars). For a 650C wheel, these may not be all that
important; however, at some size these become the overriding issues. Be
that as it may, 24" front wheel "funny bikes" were a dime-a-dozen in the
early to mid-80's, and I've seen several riders on custom dual 24"
bikes. Such bikes are becoming rarer and rarer, though, as people
realize that they don't provide any particular edge over the more common
700C/700C or 650C/700C set-ups.
 
 
 

650c wheels versus 24" wheels: Benefits

Post by Adam Ri » Tue, 20 Jan 1998 04:00:00



Quote:
> > It seems to me that if these four benefits can be realized by moving to
> > a smaller sized wheel; why not go further down to a 24" wheel?  Would
> > not all four (1,2,3) continue to be realized?  Why not 20", or even
> > 16"?

> You left out several potential disadvantages to smaller wheels, which
> are a) greater rolling resistance, b) harsher ride, and c) limited
> availability of quality rims, tires, etc. (case in point: over on
> rec.bicycles.marketplace, I just saw a "WTB" ad by somebody searching
> for good 24" tubulars). For a 650C wheel, these may not be all that
> important; however, at some size these become the overriding issues. Be
> that as it may, 24" front wheel "funny bikes" were a dime-a-dozen in the
> early to mid-80's, and I've seen several riders on custom dual 24"
> bikes. Such bikes are becoming rarer and rarer, though, as people
> realize that they don't provide any particular edge over the more common
> 700C/700C or 650C/700C set-ups.

Not to mention that wheels smaller than 650c are now illegal according to
the UCI!

One advantage of smaller wheels that was not mentioned is that they are
stronger. However, a good 700c wheel can be pretty strong, so this may not
be much of a factor for most road riding. All the objections raised above
are important points; while there seems to be an increasing range of
high-quality options for 20" wheels (thanks to recumbents, bike fridays,
etc), you still have a hard time finding race-quality parts in small
diameters.

Whoops, just noticed this is also going to R.S.T, which doesn't care about
the UCI! Although the UCI ban will only narrow the market for odd-sized
wheel parts even further.


Austin TX USA | http://www.crossroads.net

 
 
 

650c wheels versus 24" wheels: Benefits

Post by chen1.. » Tue, 20 Jan 1998 04:00:00

The advantage of a 24in wheel are of lighter weight and that's about it.
It may be that 24in wheels "may" be have less aerodyanmic resistance in
certain situations, but the lack of quality racing wheels in that
particular size puts 650c and 700c wheels ahead.

Trust me...I still own dual 24" wheeled bike.  I gave up on it and went
to a 700c Slingshot after getting beat up ALL the time.  

Dave

==============================================================================
David Chen
Department of Exercise Science
Arizona State University

Speed is Life...are you alive?
==============================================================================