> 3. The effective geometry changes should balance out. The seat post
> angle will rotate to about 75-76 degrees, from the 73 degrees it is
> at now. The reduction in the head tube angle will help to reduce the rake
> of the front fork, making it more stable for riding in aero position.
I'm not sure I follow...switching to a 650 C front wheel and fork will increase both the
seat tube angle AND the head tube angle, the latter of which (by itself) will tend to make the
bike LESS stable (i.e., more responsive). The way to counter this is to get a fork with less
offset ("rake"), so as to keep the trail within the normal range.
The reason I'm up to speed on this (pardon the pun!) is that I am in the process of
making such a conversion myself. Fortunately, at the time I was considering this, Craig Calfee
of Carbonframes posted the following useful formula on rec.bicycles.tech (thanks, Craig!):
trail = (wheel radius * cos of the head tube angle - fork offset)/sin of the head tube angle
Craig went on to suggest that 5.5 cm of trail is appropriate for the average road racing
bike. I did these calculations for my bike (head tube angle 72 deg, fork offset 4 cm), and came
up with a trail of 6.7 cm - which is consistent with the way the bike feels (slower than your
typical criterium bike, but perfect for long rides or the use of aero bars). Switching to a 650
C wheel and fork will increase the head tube angle to almost 75 deg (I did a scale drawing to
work this out), which BY ITSELF would reduce the trail to only 4.9 cm - more like a track bike
than a nice, stable TT bike! However, the smaller radius of the 650 C wheel and the lesser
offset (2.5 cm) of the 650 C fork means that the trail will in fact be 5.8 cm - still less than
it was, but more within the range of most road bikes.
Another factor to consider in making such a conversion is that the bottom bracket height
will decrease by about 2 cm (more or less depending on the wheelbase). This may not be an issue
if you brake and coast through turns, but the first few times watch out trying to pedal through
those sharp bends, especially is your using long cranks! Fortunately, my bike has a
higher-than-average BB height to begin with, so I don't anticipate any problems there.
I hope to have the new wheel and fork installed in the next couple of weeks. If anybody
is interested, I'd be happy to report on how the changeover "feels".