February 24th, 2008
Yes, that’s right, a Circle A mountain bike. I know what you’re thinking. Do you people even make mountain bikes? Do you even know how?
Well yes. Yes we do. We just don’t do it all that often.
There are some different challenges and design considerations, but if it’s a bike we like it. Especially a light hardtail like this one, for our great friend Liane.
Among the reasons I’m excited about this bike is that it’s going to be all welded, which doesn’t happen that often. As you may have seen with Carrie’s, often when we do welded road bikes we still use a lugged BB shell for a little fanciness and also for super fast jig setup. There are lugs and lugged BB shells available for mountain-style geometry, but for Liane we’re making a pretty small bike with some serious angles – the top tube is sloping at 16 degrees – so I like the flexibility that welding gives. Also it lets me size the tubing more precisely for her size & weight – in this case I’m using a 35mm Deda Zero down tube, 28.6 Zero top tube, and Zerouno externally butted seat tube:
The chain stays are Columbus Zona, and the seat stays are Zona Road S-bend. I was able to use these and get enough tire clearance thanks to the Paragon disc brake dropouts:
They’re very pretty and svelte and transfer the considerable forces from the disc brake down into the chainstay, and are helpful for tire clearance (clarence) because they’re long and allow the seat stays to start relatively high up.
So doing a fully welded frame (or for that matter a fillet brazed one) means no downtube, seattube, or chainstays extending into the BB shell, so all the miters have to be super tight:
Here’s the frame fully jigged up before tacking:
So first I just tack the front triangle in the jig, and pull it out & put it on the alignment table:
As you can see each joint is just tacked in a couple of spots. The tubes will pull a bit toward wherever you weld first, so I look at it on the table to come up with a welding pattern that will set things straight and true. In this case, the seat tube and top tube were spot on, but the down tube was pulling a little left (go figure). To correct this, on the welding table I finished the beads on the right side of the bb shell/dt joint first. Then I put it back on the table to make sure everything is still jake, and so on. If things go my way, I won’t have to do any cold setting at all.
Then it’s back into the jig to tack on the rear triangle. Here’s a look at those sexy stays:
And here’s the full frame, coming in at 3.5 lbs. Now it’s time for bridges, braze-ons, and paint!
Next bike? A Circle A first – an S&S coupled bike for Kipp! Stay tuned!
This entry was posted on Sunday, February 24th, 2008 at 12:51 pm and is filed under Everything Else. 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.