I don't think top tube length is anywhere near the most important measure of a frame. Reach from the ischeal support points on the saddle to the points of grip on the handlebars is important, as is the position of the ischeal supports relative to the BB axis. The horizontal distance from the ischeal supports to the BB, or to a vertical that intersects it, is a function of seat tube angle, its length, seatpost setback, and saddle selection. The remainder of reach (making the simplifying assumption of horizontal reach) is from the BB intersecting vertical to the chosen handlebar grips, let's say the hoods. That distance is made up of the remainder of the top tube, the stem and it's extension, the handlebar curvature, and the design of the brake lever or brifter. There's a lot more involved than the top tube.
By making some assumptions about handlebar, brifter, and stem reaches one may come up with a criterion for top tube length, but if you don't specify the seat tube angle, seatpost setback, saddle design, and saddle positioning, it is not possible to compare frames based on TT length. The difference in this rear half of the reach among different frame arrangements can be in the 4 cm range. How can one say "I need 53 cm, and 54 is too long" if there could be a 4 cm difference in hand gripping position? On Mon, Aug 9, 2010 at 8:48 PM, Lisa <ukulele.l...@mac.com> wrote: > I think the point is, that reach isn't the same as top tube length. > Reach, which for me is a key component of fit, does depend on top tube > length, but also on seat tube angle, handlebar height, etc. > > I have an old Peugeot and a new Atlantis that have exactly the same > effective top tube length. But the seat angle on the Peugeot is 76 > degrees (so I have to shove my saddle all the way back), and on the > Atlantis it's 72.5 degrees (and my saddle is centered on the seat > tube). In the end, my reach is quite a bit longer on the Peugeot. > > On Aug 9, 5:14 pm, William <tapebu...@gmail.com> wrote: > > In the most recent step on the frame-drawing tutorial, Grant stated > > more directly than I can recall his feeling about top tube length: > > > > "It's not the key, or even a key dimension. It matters, but not nearly > > as much as other things, or as much as people think it does. Bar > > height affects how far you have to reach to the bar more than top tube > > length does. Seat tube angle affects reach, too. But at some point > > yep, you gotta pick a top tube length." > > > > On every bicycle forum on the planet, including this one, there will > > be numerous posters who emphatically state that top tube length is the > > single most important dimension on a bike, and that frame size itself > > should be stated as a top tube length dimension rather than a seat > > tube length dimension. I know I used to be convinced of that thinking > > and am only beginning to accept the possibility of an alternative. > > The fact is, everybody wants to know what the top tube length is, so > > Grant lists it. If handlebar height is way more important, then I > > wonder why Grant/Riv don't propose a way to quantify that > > characteristic on a frame or a bike. I can't think of an easy way to > > do it, either. Is it just the altitude of the headset locknut with a > > particular normal tire? Is it the x,y coordinates of the headset > > locknut relative to (0,0) placed at the center of the BB? > > -- > You received this message because you are subscribed to the Google Groups > "RBW Owners Bunch" group. > To post to this group, send email to rbw-owners-bu...@googlegroups.com. > To unsubscribe from this group, send email to > rbw-owners-bunch+unsubscr...@googlegroups.com<rbw-owners-bunch%2bunsubscr...@googlegroups.com> > . > For more options, visit this group at > http://groups.google.com/group/rbw-owners-bunch?hl=en. > > -- Ken Freeman Ann Arbor, MI USA -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To post to this group, send email to rbw-owners-bu...@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
