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One bad week for diamond-frames, one good week for recumbent builders. I have been to the Dump, and there must have been forty bike frames and their concomitant bits and pieces cast aside by the unworthy souls who once possessed them. I confiscated a tiny BMX frame which appeared to have relatively sound front forks, an Australian racing bike of ghastly mien with 27 inch wheels which I was delighted subsequently to find weighed in at 22.5 lbs, and eight wheels. The racing bike was branded Ricardo, of which I have never heard, but it has a frame sporting a seamless chromoly sticker and anyway with a name like that my ego was unable to resist it. Among the wheels was a crippled Campag 700c with an aero rim which reverence forced me to save, and which, duly straightened and fitted with a widened rear hub has the potential to become a stiff rear wheel for a trike or something that I haven't yet thought of. And I also found what I was really looking for, which was a rubbishy old MTB front wheel with a fat alloy hub.
 

If alloy hubs are big enough, and this sometimes applies to MTB hubs, you can unbuild them and knock the cups out and then machine 28mm recesses into the sides and fit 6001 bearings. If there's sufficient meat to bore at least a 16mm hole through the middle for a spacer it allows for 12mm stub axles, which are useful in so many applications from trikes to trailers that I need not rehearse them. Had I done my sums and measured everything beforehand I would have chickened out because when you've machined these recesses you find there's precious little metal inside of the flange, but one of my trikes, which has just such front wheels, has been around for 3,000 miles and this gives me confidence.

Elderly wheels from the Dump have to be dismantled with care in New Zealand. You'd be amazed at how many 700c 4-cross wheels there are until you sneak onto the WISIL site http://www.wisil.recumbents.com/wisil/spokes!.asp and discover that the spoke length is that of redundant 27 inch 3-cross wheels. At tenpence each (we cheapskates always used galvanized) and with  - erm - several machines then a stash of spare spokes is a sound investment.

Unfortunately it sometimes rains here - 'tis doing right now - and rain + wheels = chemical welding. When the spoke key really won't turn, a single drop of diesel oil on the outer end and then ten seconds of small propane flame applied to the nipple, is generally enough to make the oil bubble and seep into the thread whereupon the spoke concedes defeat.
 
 
Removing the axle and cups is straightforward but locating the hub in the 3-jaw isn't. It is an axiom of Clive Sleath and W.C.O. Pettingill and everyone else with a lathe that if you take something out of a chuck and reverse it, the other end is guaranteed to be out of true. Guaranteed. And if you just nip the flanges, which is all there is to nip in hubbery, then even the 4-jaw is useless because the bit you can clock won't resemble the bit that you're about to machine. The problem is alleviated with a steady, but even so the trick is to machine the other end's recess slightly over-sized, and glue the bearings in with Loctite 660. When nipped with the bolt, the theory is the bearings find one another's parallelism.

Oh, and Mr and Mrs Loctite, since I've just very generously given you a gratuitous advert, don't let me discourage you from sending any free samples you care to pass my way.

Thursday, July 23, 2009 11:37:30 AM Categories: engineering problems
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