November 3rd, 2010
Allow me to introduce you to the Incomparable Thundercraft, Sara’s 650B mixte commuter:
We’ve got a Nexus 8 speed internal hub, dual monostays, Honjo hand-hammered fenders, a generator hub (lights to come!), Velo Orange stainless steel front rack, and a custom head lug. But more on that later…
OK, so when I went to make a bike for Sara, my girlfriend, co-habitator and the mother of my son, I knew I wanted to do some sort of skirt-friendly step through frame; but what kind? There are 3 common types: using a curved top tube, a parallel top tube, and a mixte, where two skinny seatstay-like tubes go all the way from the head tube to the dropouts, around the seat tube;
Although I like the curved-tube look a lot (this lovely one is courtesy of Mike Flanigan at ANT), I wanted to use light tubing and therefore wanted the extra strength of a mixte-style design. In a head-on collision, the force applied to the front of the bike is transferred along the top tube into the middle of the seat tube, so often you see old step-through frames with a bent seat tube (warning to yard-sale shoppers!). But twin-top-tube mixtes are everywhere, so I was particularly inspired by this by our friend and co-conspirator JP Weigle:
And I’m a big fan of the slender look of monostay (or wishbone) rear ends; and if one monostay is good, obviously two is better, right? Clearly.
Now, in order to make the top tube & intermediate seat stays form a straight line back to the dropouts (for strength and aesthetics), the position and angle of the top tube had to be very precise. Not a stock placement, and typically when we’re using unusual geometries, as with mountain bikes or really anything without a horizontal top tube, we will fillet braze or weld the frame. Lugs are cast pieces that we buy, and are available in a handful of conventional geometries. But Sara had her heart set on lugs, so what’s a boy to do? Well, make a lug, I guess.
We had some extra Rene Singer lugs from our friend Richard Sachs, so I decided to use two down tube lugs: one as-is, for the down tube, and the other I would modify for use with my top tube. This involved grinding off the socket for the down tube, and mitering a sleeve that would fit over the 28.6mm top tube.
Then I trimmed the Sachs lug to fit the new smaller TT socket, and welded it all together. It starts off pretty ugly, but a little grinding and filing and the next thing you know, it’s just pretty.
I didn’t have to make a custom seat tube lug, as the geometry allowed me to use a modified Henry James seat lug, upside down. The other issue is, how do you connect 3 sets of stays to dropouts that are designed for 2? Well, here’s one way, as well as some shots of the 50’s sci-fi inspired graphics.
I also painted a vintage Raleigh, a Riordan, and started Holly’s 26″ tourer:
Next: Holly! More bikes! More paint! Watch the skies!
This entry was posted on Wednesday, November 3rd, 2010 at 6:36 am and is filed under Bike Building. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.