Some of the statements here (not just this post) lead me to wonder if folks aren't confusing some basic issues.
so I tried the 175mm
> crank in order to have a
> "virtual gain" in the seat tube.
The effective seat tube length depends on the distance from the BB center to the top of the saddle (accounts for variations in frame designs, seat designs etc.) This is simply the distance from your butt to the center of the circle you will pedal. The crank length determines the size of the circle that you will pedal. We could go around and around about what size circle is ideal and what factors determine this size. Clearly it depends on both the rider's anatomy (lengths of bones, ratios of thigh, shin, foot) and the cadences that suit the rider (which depends on a huge variety of pyhsiological variables and rider preference.)
Do I notice a
> difference in how this bike
> pedals from others with shorter crank arm? No! The
> Bianchi goes up hills
> well, but I think that is due much more to the
> wheels (Mavic GL-330's) and
> tires than crankarm length.
>
> I also ride a Bianchi Cyclecross bike, with 170mm
> crankarms. This also goes
> up hills quite well, but I'm quite certain it's
> because of the gearing :-)!!!
It's pretty obvious that changing gears substantially
changes the effort at the pedal that is necessary to
move the crank. Gears allow the mechanical advantage
of the whole bike/rider system to be varied to meet
the requirements of different terrain, speeds, wind
conditions etc. Crank length is varied to optimize
the relationship between the rider and the bike. The
2.5 mm differences between sucessive crank sizes
amount to 1.5% differences in leverage. This is
certainly noticeable at the pedal, but I don't think
it is strictly a leverage vs. speed issue. It is a
matter of what feels right to your individual body.
At some point, a shorter crank doesn't allow for
faster cadance, it is just too short and confounds
your spin.
> There is more leverage with a longer arm, but in
> reality, it is a very small
> amount, and as Sheldon (and others) has pointed out,
> it is much easier to
> gain mechanical advantage by changing the gearing
> ("that's what gears are
> for...") than the crankarm length. To "measure" the
> difference(s) in output
> with crankarms of different length, would require
> very elaborate laboratory
> conditions, and to what purpose?
>
> As far as modern "trends" are concerned, I don't
> know what motivates
> (marketing?) manufacturers to equip current road
> bikes with 172.5mm & longer
> cranks. Perhaps (as Hilary stated) it's simply the
> current "trend" and will
> also change in the years to come?
>
> For those of you who are Rivendell members, there
> was an article in RR 5
> ("Cats, Cranks, and Moments") by Nicholas Jasper
> which might be of
> interest....?
>
> I'm certain "correct" position over the pedal(s) is
> far more important than
> the length of the crankarm!
I agree that seat effective seat tube length and effective seattube angle are of greatest importance, but you really do want to have the your butt at one end and your "best" size circle at the other.
Tom Dalton
Chuck Brooks
> Malta (A Rainy Day - it's needed!), NY
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