> 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)
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
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...
> in towards rider, resulting in better balance for a forward rider, than
> 700c wheels.
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 ***.
> me to be the views of most posters.
people's views on the subject have been shaped almost entirely by
advertising, not by any careful consideration of the physics involved.
> 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
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.