I don't believe the torsion load to be meaningful and anyway it's shared by the down tube. Phil Brown
On Jun 7, 8:05 pm, "Bill M." <bmenn...@comcast.net> wrote: > The top tube is loaded in more than just compression. Back in the > '80's a Caltrans engineer named Rick Jorgensen did some FIA on frames, > and determined that the top tube is loaded in torsion. Imagine > removing the down tube from a frame. The force of pedaling would make > the seat tube want to deflect sideways, out of plane with the head > tube. That force would be seen as twisting by the top tube. The same > mechanism holds when the down tube is present. > > That twisting mode of the frame is why I tend to think that the > 'diagatube' design is actually a structural improvement over the > parallel undertube. The middle of the seat tube is a great place for > the extra tube to try to resist twisting. The 'marathion' tube in > some tandem designs works the same way which is why Jorgensen included > them in the Tango tandem frames he built. > > A 56 Sam is too big for me, so it's really moot, but I'd regard the > extra tube as overkill for me and the way I would use a Sam. I'm > actually going the other way, with a 650b Rawland Sogn built with > light .8/.5/.8 tubes and a 1" top tube. I've never owned bike that I > thought was too flexible. I also don't generally carry heavy touring > loads. > > Bill > > On Jun 7, 10:55 am, Phil Brown <philcyc...@gmail.com> wrote: > > > > > > > > > The double top tube isn't really necessary. I once made a standard > > tube bike with a very light top tube as an experiment. I'm 6'3" and > > weighed at the time 210 and I could notice no difference from my other > > bikes with the same geometry and heavier top tubes. It's loaded in > > compression and doesn't need to be real heavy. It just locates the > > head and seat tubes the correct distance from each other. > > Phil Brown -- 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-bunch@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.
