Mid-head tube to mid-seat tube makes more sense to me than the parallel top tubes. Imagine a frame built with only the diagonal tube, no top tube or down tube. If you grabbed the head tube and seat tube and tried to twist them side to side so that they were not parallel, the diagonal tube would be in the best place to resist that force. Moving that tube higher or lower on the seat tube would make it less effective. In a tandem, that is the function of a 'marathon' tube that runs from the head tube to the middle of the rear seat tube - it keeps the stoker's seat tube in plane with the head tube by resisting twist.
Adding the down and top tubes back to that imaginary frame would keep the seat and head tubes from flexing fore-and-aft, and keep the BB stable under pedaling loads. Most modern tandems followed Santana's lead and went away from using the marathon tube. They use a 'direct lateral' tube from the head tube to the stoker's bottom bracket instead. That arrangement is better at keeping the captain's bottom bracket from swaying, and allowed more convenient bottle placement (no telling which factor was more important). An oversized top tube is used to resist the twisting forces. That arrangement has no place in a single IMO, as BB stiffness is not a real issue. Calfee's CF tandems have no middle tubes at all, they use huge top and boom tubes to keep everything stiff enough. The question to me is, how big does a rider or a touring load need to be to need the extra stiffness of the diagonal tube? At what point does the weight of the diagonal tube become less than the extra thickness / diameter of the top and down tubes needed to achieve comparable stiffness in a plain diamond frame? Moot point for me, I'd fit a 54 Hunka so no diagonal tube for me. Bill On Apr 4, 10:03 pm, andrew hill <neurod...@gmail.com> wrote: > parallel with the down tube might look pretty snazzy. > > best, > andrew > > On Apr 4, 2010, at 9:52 PM, cyclotourist wrote: > > > > > No Kevin, don't do it..... > > > On Sun, Apr 4, 2010 at 9:45 PM, Esteban <proto...@gmail.com> wrote: > > Fits the bike. Truly unique. > > > Esteban > > San Diego, Calif. > > > On Apr 4, 9:30 pm, Grant Petersen <gr...@rivbike.com> wrote: > > > Midpoint of headtube to midpoint of seat tube. Sorta mixte-like. (Jim > > > Thrill/Hiawatha said...) > > > > Keven loves the 62 Betty/Yves, and rides it a lot, and rode it with > > > weight, > > > and remarked how well it carried it, not what you'd expect from a mixte. > > > But > > > the design like that works, and that lead to the diagonalization of the > > > 2tt > > > (second top tube, I'm tired of typing it all out), and some friendly > > > turmoil > > > here. It's Keven's call, which way it goes, but I think it's looking > > > diagonal. That's where I'd put my puka-shells. > > > > G > > > > -- > > > Grant > > > Rivendell Bicycle Workswww.rivbike.com > > > 925 933 7304 > > > -- > > 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 > > athttp://groups.google.com/group/rbw-owners-bunch?hl=en. > > > -- > > Cheers, > > David > > Redlands, CA > > > "Bicycling is a big part of the future. It has to be. There is something > > wrong with a society that drives a car to workout in a gym." ~Bill Nye, > > scientist guy > > > -- > > 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 > > athttp://groups.google.com/group/rbw-owners-bunch?hl=en. -- 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.
