January 3, 2009

…the rest of the story

This will be my last long post before going on vacation (Jan. 6th-14th, guys) so enjoy!

I finished up Marcus’ 36er frame today. It was an interesting process for a number of reasons – the wheel size demanded quite a lot of thinking on both of our parts about how to do the geometry, and fabricating some of the frame members was fairly unusual (and hence difficult) as well. I think it came out very nice (it’s only 6 pounds, which is amazing to me) and I hope it’ll ride really well too. Big thanks to Ben Witt for geometry ideas, too.

First off: check out the mitering drawing. I have a set of Excel macros that I use to figure out miter lengths and angles normally, but that program is set up to have the seat tube meet the BB shell – this frame has the seat tube offset 6″ forward of that.

I thought about doing some new macros, but decided I trusted good old pencil/paper more.

With that out of the way, I started building the front triangle. Here’s an early shot of the main structural members all in place.

Downtube was welded to the BB shell, and then partially welded to the head tube, and then I compound mitered the TT at the head tube end. The downtube is a 1.5″x.035″ 4130 chunk (there are no commercially made butted tubes long enough) and the toptube is a 1.375″x1/.7/1mm butted Supertherm jobbie that is arguably overkill. But the only way to use a butted tube (the toptube is very short due to the weird seat tube placement) and weld in a brace safely was to do it this way. So while it’s way stronger than it needs to be, it’s also lighter than the other option, which would be a .035″ straightgauge tube.

Here’s the compound HT/TT/DT miter. The DT is partially welded in (where it’ll be covered by the toptube) before the toptube is put in to ensure maximum joint strength.

You might note that there’s a scribed line/cut in the head tube – that’s a solution to a problem I ran into with my Anvil frame fixture. Basically, the head tube portion of the fixture has to be adjusted to a different height depending on what length fork and bottom bracket height you’re building. In this case, that height was 30mm taller than the fixture can be adjusted to, so I had to leave the head tube too long and cut it most of the way with the lathe, so that I could easily cut it off with a hacksaw (and then face) afterwards. I also had to flip the dropout portion of the frame and chainstay fixtures backwards to make them long enough (this baby has 490mm chainstays!) To be fair to my pal Don who makes these excellent fixtures, I’m pretty sure this isn’t something that comes up too often.

The seat tube uses 2 sleeves (one at the TT, one for the seatstays and TT/ST brace) which I made, as usual, from .058″x1.25″ 4130 cromoly. I’m getting much better at this process at this point and it went quickly and smoothly. Marcus and I discussed how to orient them for maximum cool looks, too. Here’s a picture of the toptube sleeve and toptube before I tacked them.

I welded everything up, of course, and here are some random shots of that.

Seat tube/toptube:

Seat tube/downtube:

Almost finished HT/DT/TT:

Finally, I tacked in the chainstays, and did some bending/dimpling of the seatstays. Tire clearance on this bike is *tight* – it’s got 65mm of space for the 2.2″ tire, but I had to go absolutely nuts with the bending and dimpling and tweaking to get everything to fit. It uses an 83mm BB shell, which helps a ton – if I did the equivalent with a 29er, it would have 400mm chainstays!

Here’s a shot of the frame before finish welding the rear end:

That’s all for tonight. I did do the bracing, but that’s not exciting enough for me to spend any time on posting pictures. I will do an update when the bike is together and Marcus has some feedback on how she actually rides.