"Funny" bike conversion (700->650C)

"Funny" bike conversion (700->650C)

Post by Andrew R. Coggan, Ph.D » Thu, 01 Aug 1996 04:00:00

A while back, there was a thread concerning the feasibility/effects of replacing a 700C
front wheel/fork with a 650C setup. Having recently completed the conversion, I thought I'd
share my experience.

The bike in question is a 54 cm ***, with a 56 cm top tube. Original geometry with dual 700C
wheels was 72 deg head tube, 74 deg seat tube, 102.2 cm wheelbase, 29.2 cm b.b. height, and 4 cm
of fork offset ("rake"). The trail, calculated as (wheel radius*cos head angle - offset)/sin
head angle, was 6.7 cm. After replacing the front wheel/fork with a 650C combo, the dimensions
are now: 75 deg head angle, 77 deg seat tube angle, 100.7 cm wheelbase, 27.3 cm b.b. height, and
5.8 cm of trail (offset of the 650C fork is 2.5 cm). Of course, the point of this whole exercise
was to lower the elbow pad height, which it did by 5 cm (to 20 cm below the saddle).

The biggest concern in such a conversion, of course, is the effect on handling. I am happy to
report that, while the bike feels like it steers slightly "quicker" than before, it is not at
all unmanageable, even with the narrow *** aero bars. (In fact, now that I have the option of
using the new training wheel - which has a shallow clincher aero rim - in strong crosswind
conditions, I may be better off than before.) Of course, this is a function of the fact that
although the conversion reduced the trail, it is still somewhat on the high side as racing bikes
go - not necessarily ideal for criterium racing, but better for riding a straight line while
TT'ing on the aero bars.

If anyone else is considering going the same route, I recommend that you recall your trig and
drafting skills, and carefully draw out to scale the old and new dimensions on some graph paper
before proceeding. This will give you a pretty good idea of the issues involved before
possbily wasting some hard-earned $$. Also note that not all bikes are likely to be amenable to
such a conversion - with a very high b.b., long top tube, and relatively "slack" front end to
begin with, the *** was an ideal candidate. With other bikes, however, you may find that
switching to a 650C front may result in too low of b.b. or too little trail to be workable.
Also, since most people want to move the seat forward when they lower the aero bars, you may
find that top tube length is too short, necessitating a long stem and too much weight on the
front wheel. (If all else fails, you might have to break down and splurge for that QR bike
you've been wanting all along!)

In theory, my new "Boardman/Pearce" position should yield some significant time savings. I'll
report again after I've had a chance to do a race or two to see if this proves to be true in the
real world...