Tuesday, June 8, 2010

Down with Drop!—and, Say Hello to Monty

After the excitement of last Tuesday, when I brazed my nice front triangle, I was brought somewhat down to earth the next day, when I was visited by my old foe: bottom bracket drop.

Niles's one flaw is his unusually low bottom bracket. 83mm for a fixed gear is a whole lot. Although it has not really bothered me in practice, and I've only struck the road one, when I was going over a low curb.

This time, however, the problem was the reverse. When I put my front triangle into Olivier's magnificent rear triangle fixture, I discovered that I had not the 80mm of BB drop I had intended... but 60. Subsequent analysis has revealed three sources for this discrepancy:

  • I measured BB drop in line with the seat tube, not horizontally from the axle line. That cost me about 4mm.
  • I calculated my wheel radius by adding 311 (half of 622, the BSD of a 700c wheel) to 30, the height of my tire. But 311 only takes you half way through the braking surface — to the "bead seat," not to the edge of the rim. So I should have done 316 (the radius to the edge) plus 30. That cost me 5mm.
  • Other peculiarities and errors too dull to go into cost me the remaining 11mm. But I did take note of them all, and won't make the same mistakes next time around.

A bike with 60mm of BB drop would be perfectly rideable. But the resulting bike would really have an absurdly high bottom bracket — you could ride over a log. For a bike I intend to ride often, it just won't do!

Naturally my next step was to determine what to do with the lovely front triangle I had just made. (This step is illustrated in the above image.) I have three bikes planned for myself at the moment: (1) Adam Jr., the randonneur I had been attempting to make; (2) Greg Curnoe bike, a road bike; and (3) an as-yet-unnamed fixed gear bike of utmost simplicity. I would have won back 10mm or so of front end clearance with the Greg Curnoe bike, since it's not going to be made with fenders in mind. But the fixed gear was a better match: for fixed gear bike you want a less bottom bracket drop anyway (and thus a higher bottom bracket); and I'm going to use 700x24 tires and very little clearance. The front triangle ended up matching this design perfectly.

So I decided to use the front triangle I had already brazed for the fixed gear bike. Of course I needed a name. I got it from the rims I was planning to use: Mavic Monthlery Pro tubulars. His name is Monty.

The wheels weren't built yet, so I built them over the weekend. Monthlery Pro rims, Dura Ace low-flange track hubs, DT Swiss double-butted 302mm spokes, and Challenge Strada 700x24 tubulars. These are beautiful and light wheels. I spent quite a bit of time polishing the non-anodized rims. And even though the rims were NOS and came with their labels intact, they had pulled up on the edges, so I safely removed them and put on some replicas in their place. I'm going to have the bike painted to match them: red with gold decals.

Once the wheels were built I started on the fork. I had a flat, Imperial-oval crown of uncertain provenance that seemed like a good fit. I also had some matching Reynolds 531 blades that were pre-raked exactly the right amount (45mm), and a threaded 531 steerer:

Today I did several things. First I brazed the centrepull bosses onto the crown. These are at Mafac spacing, but I happened to have a Universal in my closet that I thought I might use instead. I needed to drill the pivots out, and I'll need to repurpose some bushings from Mafac Racers, but they should work fine. I then brazed the steerer to the crown, and brazed in the dropouts. This is all illustrated here:

It all needs to be filed. And the roughly-cast crown needs quite a bit of work to open it up for the blades. But once that's done, things should move quickly. I'll need to get some track dropouts and do the rear triangle, and then I'll make a stem. But there are very few braze-ons for this frame — probably only water bottle bosses. So it should all go quite quickly...


ford said...

Oi, the frustration that must have caused!
Yet you plow on with a steady hand, nice!
Every one of your posts keeps me salivating to get my hands dirty again. I also got a Fattic fixture, and am just neck-deep in building a backyard workshop. By the end of the summer, there will be fire!

AH said...

Yeah, it was a bit frustrating, but then I got excited about Monty! It's sort of nice to get second chances at things, anyway -- I'll do an even better job next time.

I'm excited to what comes of your fire!

RMHampel said...

That's a tough blow. Still, you've managed to turn a sow's ear into a silk purse, so to speak. Now on to a new front triangle!