Twin top tubes have been popular on working bikes in Mexico and other developing countries. I suspect that there is a simple engineering reason for this: Many riders use the bike as a cart, by loading huge loads onto the top tube and then wheeling the bike, since the load prevents the rider from actually riding it. The heavy load in the middle of the top tube puts a bending load on the tube. A single top tube could buckle. Adding a second top tube, which doesn't have the load sitting directly on top, and thus is stressed in compression only, keeps the seat and head tube apart, and prevents the frame from collapsing.
Of course, the second top tube isn't properly triangulated* - ideally, you'd have a second set of seat stays to the lower top tube, or even run just one set to the lower TT - but it's better than nothing. And alternative design is the Labor "Tour de France," which has a girder structure for the top tube. See http://www.bikequarterly.com/books_comp_bicycle.html (it comes up in the rotating sample images from the book) * The load of the second top tube is transferred to the seat tube, which tends to bow toward the back... Jan Heine Editor Bicycle Quarterly www.bikequarterly.com Follow our blog at http://janheine.wordpress.com/ On Feb 2, 2:56 pm, Michael Williams <mkernanwilli...@gmail.com> wrote: > Wait a second here. Grant didnt invent the 2TT frame design. IIRC, > there are older model bikes Id say from the early 20th century and some > 80s era MTBs with 2TT. -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bunch+unsubscr...@googlegroups.com. To post to this group, send email to rbw-owners-bunch@googlegroups.com. Visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en. For more options, visit https://groups.google.com/groups/opt_out.
