Stevan,
I'm glad you got to this explaination before I did. Excellent job done illuminating this subject. Although frame geometry/design is not that complex; many aspects of it are misunderstood amongst cyclists. Sometimes important basics are overlooked while focusing on considerably less significant details too much. Thanks for that clear and precise lesson.
Brian Baylis
La Mesa, CA
Stupor Bowl comming to Insane Diego this week! I'm bummed; no fly zone
over the stadium. The only fun I get from these bashes are the blimps!
>
> 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