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.
