Thursday, July 16, 2009

Little Details make a custom frame

One of my current projects is a fast bike for someone who may yet try his hand at racing. I've been working with him to adjust his fit for some time now; and we're starting to see results. He's getting pretty quick on the bike, and can use much more handlebar drop than when we first started. That said, there's no way to get past the fact that he has an extreme body geometry where he needs about a 55CM seat-tube combined with 53CM top tube. And that's using a 9CM stem. You could say he's long in the leg.

He's a strong and heavy rider, so no wimpy frames for him. And it's going to be lugged steel. Now this presents some interesting issues. Generally, lug sets aren't available in multiple angles. So, framebuilders have to 'adjust' lugs to fit their designs.

For this frame, I've chosen Dazza's Slant Six (aka XL Compact) lugs with a Kirk Pacenti l
ugged Bottom Bracket. This combo is designed for use with a 2X Oversize tube set (in this case it'll be Columbus Life), with diameters as follows: DT=34.9mm, ST=31.8mm, TT=31.8mm. These fat tubes should keep everything plenty stiff. The Spirit chainstays will do their part to, running full size (i.e. without a taper) until the last 90mm. Most chainstays taper over the last 250-300mm - so this is a meaningful difference.

The Slant Six l
ugs create a frame with a modest slope (6 degrees) to the top tube, allowing for somewhat more standover clearance. Which is great. They are sized for a 1-1/8" steering tube - unlike most lugs which are sized for 1" steerers. Personally, I don't think that steerer diameter is very important for road bikes - but the rider wants a carbon fork and the larger steerer will leave us with more options.

So, back to the lugs, short top tube, and adjustments. The angles on the stock lower head tube lug, and on the BB between the seat and down tube, are about 3-1/2 degrees away from the plan on BikeCad. This means that it's time to adjust, in a fairly significant

I have some nice bending bars and quickly got the BB in shape. Naturally, the bent lug ports needs some hammering to make sure that they conformed to the shape of the tubes. The lo
wer head tube lug is a bit more difficult. It is somewhat like a bikini lug, in that there isn't much lug on the headtube, especially above the down tube. This means that its hard to clamp this part of the lug in place during the bending, and that this section doesn't have enough material to both fill the gap created by the bend and provide a good surface area on the head tube. That's where this piece comes in. I traced the top of the lug onto a cutoff piece of head tube, and then cut it out.

This will get braze
d into the corresponding section (see the second photo) of the lug, using brass filler. After some file work, it will fill the gap between the adjusted lug and the head tube. Some more filing on the outside of the lug will restore the outer shape (so that the shoreline of the lug doesn't become twice as thick).

The actual joint will be brazed with silver filler, hence it won't heat up enough to weaken the brass filler used to modify the lug. Cool concept but plenty of work.

That's it for this post. Cheers,b

1 comment:

Craig Ryan said...

Neat idea, I hope it works out well.