Sunday, December 3, 2017

Tandem repair

Came a phone call from Carl: could I fix his cracked tandem frame. He brought it in his van, and here it is:

Built without a top tube it would have been perfect for a couple of mice provided they had long legs - say, a pair of jerboas - but Carl is heavier than a mouse, and he told me an equally heavy dentist occasionally rode on the back, and I can imagine even his children tugging away on those stoker bars. Anyway, it had, as he said, cracked the captain's seat tube exactly where every engineer in the world would predict there to be a stress concentration.

Carl being mostly composed of leg, I thought it wouldn't be a bad plan to extend the frame's seat tube and then brace it. If I retained the original clamp, there could be two seat post clamps and that would use some of the strength of the seat post itself.

First I pulled the seat tube straight with a sash cramp and welded the crack closed.

Rummage in the bike heap for a seat tube top, add an inch of oversize tube to match the existing seat clamp diameter, drill and plug-weld the two together only at the front. Cut a slot in the back so as not to interfere with the existing seat clamp.

Insert another seat tube to keep everything in line.

Poke into frame, and weld again only to the front.

Clearly this is a bit of a feeble joint, so weld a bandage to the front of the joint, top and bottom.

Now for some bracing. The top tube should be in compression, so the welds ought to be okay without gussets. Lop a couple of top tubes off donor frames, preserving cable clips where possible.

Sit back, admire, and telephone Carl.

With a bit of luck that'll prove adequate, and the jerboas can retire in favour of weightier crew. My only concern is in the 700c wheels, which have but 36 spokes each, none of them especially tight. I suggested he take it off to see Ross (warning: Facebook), who is blessed with both enthusiasm and knowledge for and of old bikes. I think this takes the town's total to five tandems.

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