In a message dated 1/20/03 5:45:23 AM Pacific Standard Time, sachs@erols.com writes:
> what fascinates me is that this is another large frame with greatly
> undersquare design. My Cinelli SC is 64 cm (c to top) x 57 cm top
> tube. From this I'm beginning to believe that these designers, back then,
> (still?) thought that the thin-air breathers were all leg and no
> torso. Are most "oversize" frames that undersquare?
>
Hi Harvey,
This takes us back to the idea that top tubes are two dimensions. The length
behind the bb will change with the seat angle without affecting the reach in
front of the bb. 57 seems short for a 64cm frame. 58 is in the ball park for
a 62.5 ctt, depending on what the seat angle actually is and how far back or
forward you adjust your seat. I design frames around the necessary seat angle
to get a proper position, and derive the tt length by backing into it. I
first determine the forward reach to be used and the desired front end
geometry (head angle, rake, front center and toeclip clearance). All this is
forward of the vertical line through the bb. Then I put in the seat angle and
see what the top tube length turns out to be. It doesn't HAVE to be done in
this order, this is just an example. You can also start with the seat angle
and design forward from there. The point is, if you don't know what the seat
angle really is, the top tube length doesn't mean much, unless you bisect it
and treat it as reach (forward) and set back. The "fit stick" sizing system
uses this principle.
Stevan Thomas
Alameda, CA