"JS" wrote:
Nope sorry, but that's not what is being depicted in their diagram. The
part that is misleading is the vertical line that is being projected from
the seat tube. It needs to follow the same angle as the seat tube, not
vertically. Simple geometry dictates that the line will be be shorter than
the sloping line below it.
Look closely - although it's confusing, the diagram IS correct.
The points those vertical lines start at are the same points you'd
measure the top tube's actual and effective lengths. This is easly to
imagine with the effective length measurement, since the vertical
lines are perpendicular to the line being measured.
It's not as obvious on the sloping top tube "actual" measurement, but
as long as the vertical lines are the same length (they appear to be),
and they start at the proper points (they do), the end result is the
same as a measurement drawn between the measurement points themselves.
The relationship between the lengths of the two lines isn't as obvious
on a bike with such a shallow slope to the top tube (and that's
probably why they offset the measurements as well, though if it were
me I would have left the actual TT measurement centered on the top
tube itself). Hey, wait... I did...
http://www.habcycles.com/mtb.html
Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame
"Mark Hickey" wrote in message
.. .
"JS" wrote:
According to this website the "horizontal" top tube length "C" is greater
than the top tube length c-c "D" which happens to be the sloping top tube
length on this compact frame. Am I wrong, or is there something wrong
with
these dimensions? If we project two vertical lines from the center of
the
top/head tubes and seat/top tubes and measure the distance of "C', then
that
length has to be less than the sloping top tube length 'D".
http://www.feltracing.com/05_frames/2005_sc1_frame.html
look under 'geometry and specs'
There's nothing wrong with Felt's math.
Think of it this way...
You have two roughly parallel lines (head tube and seat tube).
You want to draw a line between them. The shortest possible line will
be perpendicular to the two original lines. Starting at the same
point from the head tube, and angling the line up to horizontal will
by necessity make that line (or, horizontal top tube) longer.
I'm fairly well acquainted with this concept, having designed a LOT of
frames with sloping top tubes... ;-)
Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame