Wednesday, February 24, 2010

Tape! Tiny frames! Couplers!

Yes, you can see all of those things in this picture. Explanations below.

Tape: I use tape for all kinds of things when building a frame. In this case, you can see 2 of the common uses - blocking off water bottle boss holes (to keep the argon inside when backpurging) and holding seatstays in place to tack. Yes, you can get a fancy add-on (though this particular fixture might be too old) for the Anvil Journeyman to hold your seatstays - but if your miters are good, and you've got the taping skills, you don't need it. Easier with road frames than mountain bikes, though. I also use old Cateye computer sensor magnets to hold things in place quite often (bridges and small bosses that will be welded rather than brazed, most often).

Tiny frames/couplers: This is Jennifer's 650c wheel roadie with S&S couplers. It is so small (front center of 535mm!) that while butted top and downtubes exist that are (just barely) short enough, once you add the couplers in, it's a whole different story. The only butted tubing that will work, in fact, is *external butt seat tubes* - in this case 28.6 and 31.8 x 1.2/.6/.9mm ones from Nova. This means that if you look very closely (not in this picture, but when it's powdercoated) you'll be able to see that the front of the toptube and downtube are oversized by just a little bit (about 1mm in both cases). I should come up with a catchy name for this "feature" and charge extra...

And yes, I could have used .028" straightgauge 4130, but the weight savings would only be a couple of grams (literally) and the bike would end up a lot weaker. I'm not a fan of anything thinner than .035" in straightgauge 4130, so this was the route I ended up going.

Long story short, the couplers make the bike quite a bit heavier than you might expect in this case - beyond the additional weight of the couplers themselves, I'm forced to use much thicker tubing than the .7/.4/.7mm I'd probably otherwise use. So this is a tiny frame that still weighs about 4.2# - it would be quite a bit lighter (though less useful) without them.


t-roy said...

Nice! So the top part of the seat tube is 31.8? What size seatpost would be used or do you weld in some sort of permanent shim to run a 27.2?

t-roy said...

Disregard that last question Walt. Make sense now that I had coffee and read that you used a ST for the TT and DT. Duh!