Febuary 11, 2004
MnHPVA Meeting
Meeting Report: Tim Dunsworth
Photos & HTML: Mark Stonich
Meeting Notes Below
Show-N-Tell Pages
Click thumbnails to read details and see more and larger images.

With cold, snowy, dark weather outside and the dog training class taking over the big room of the community center, we were reduced to schmoozing in the hall until we finally gathered in the little pottery studio room. We had a decent turnout, though, and more show-and-tell stuff than we have had for the last few months. For some reason (the already long and snowy winter, perhaps?), this meeting was also strong on joking around, including a lot of double entendre lines about nipples, double butts, and limp noodles (we're talking about spokes and V brakes here, people!).
After looking at two kinds of backbone frame bikes, Carl Gulbronson got to wondering if it might be lighter to do a space frame with several smaller tubes. He cited the Lightning P-38, which is lighter than the Thunderbolt model from the same company (though you would have to check bare frame weights in the same size to be sure how much was due to frame design and how much to component choices). He suggested two smaller frame tubes, one above the other. But Mark felt that while that would be stiff vertically (not always a good thing if you want a comfortable riding bent), it would be significantly less strong resisting pedalling forces, compared to either a larger backbone tube or a space frame with multiple tubes laterally like the P-38.

A well triangulated space frame (like the newer Moultons have) is stiff in every direction but is also trickier and more labor intensive to build than a backbone frame. Mark says he would consider a two tube frame design for his next attempt at a weight weenie bike, especially if it could have the bottom bracket right behind the head tube and tucked between the two main frame tubes.

Mark's Comments. Most top and bottom tube frames are plenty strong in all planes. However they are not all that light. My current thinking is to have a fairly large top tube, to resist pedalling forces, and a much smaller bottom tube to triangulate the frame against vertical loading. Curving the bottom tube could allow some vertical compliance.

Space frames can give you the highest rigidity for a given weight. However, due to the need for tubes to be strong enough for incidental side loads, they must be much stronger in tension and compression than needed for a bicycle. Therefore you get frames that aren't all that light, are difficult to build but much more rigidity than needed. Space frames could be the optimum design for an upright tandem or when working in Titanium.

John (Reese?) is teaching at a new school that has given the OK for a student-built ice racer project for next year's event. Just what we need – more strong young legs to make the rest of us look bad!

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