Sunday, October 6, 2013

Duplo fork

The cheery thing about building your own bikes is that when major structural failure happens the only trauma is having to paint it after the repair. I hate painting. Everybody hates painting. It isn't news.

 Quick scan of the collection of old forks for a stout 406 pair; suitable candidate chosen and a thingy welded on for the steerer rod and another thingy for the drum brake arm. I can't be bothered to describe them and I don't suppose anyone could be bothered to read about them.

The steerer tube of this fork was quite short. I had been meaning to cut the head tube of the Duplo bike for ages: the extra length was surplus weight and I've no idea why I left it that long, other, perhaps, than to irritate Mr. English who likes all his bikes to have no extraneous metal whatsoever.

A head tube needs to be an inch-and-a-half shorter than a steerer tube, and not being Mr. Knight who would make an archival quality industrial jig out of tool steel for the purpose on his Myford lathe, I was stuck with how to square it off. Rather more difficult to square the ends of a head tube when it's already welded into the frame: you can't pop it in the three-jaw and do the job in two minutes.
Eventually it occurred to me that I could rotate a rod inside the head tube with an oversize lip fitted with sandpaper.

 In the bling pile I found two candidates. One was a short rusty cylinder. The other was some kind of weird yellow item with a hole and a grub screw to fix something in the hole. No idea what it was. One picks up all sorts of odd things on the roadside and it's always a happy surprise to find a use for them. As the rusty cylinder fitted perfectly I didn't need the grub screw; and to make it fit the head tube I wrapped it in masking tape. For the sake of clarity we will refer to this fantastically sophisticated tool as the Thing.

A Thing, this morning.
A scrap of 120 grit abrasive, and onto a block of wood where the hole in the yellow item was used to mark a circle, which was then cut out with a chisel made out of a short length of Number Eight Wire. - No, it really was. Except that I have recently learnt that Number Eight Wire refers to the old sort, a very pure iron, malleable as copper which is how they used to fix everything with it. Today's fencing wire is galvanised high-tensile stuff so you can't fix things with it, though you can make small wood-carving chisels out of it, and even smaller chisels out of bicycle spokes which are also high-carbon steel. File to shape, heat to cherry red, plunge in Nutella jar full of rusty water, and there's your chisel. A handle, if you must, by drilling a hole in a stick of firewood. It never ceases to amaze me how many things you really don't need to buy in a hardware shop. I watched a neighbour the other day wielding a leaf blower. Are we all stupid? You really need a petroleum-powered motorized anti-vacuum cleaner to sweep the path? - You can do the job quicker and more effectively by cutting a couple of twigs of broom - hence the name - and lashing them to a manuka stick with a strip of rubber inner-tube. It'll take you all of ten minutes to make.

The bike's head tube I marked at the cutting point with more masking tape, deployed a hacksaw, and then marked the sawn edge with a felt pen.

Sawn off head tube, marked with felt pen
In with the Thing, rotated once, and then I could see where the abrasive paper had rubbed the felt pen marks off. Apply file to the high spots; repeat a great many times, stop for lunch.

Using the Thing to mark the high spots

When the bearing cup sat squarely, I assembled everything with the new fork, and went for an uneventful ride. Very slow today. Yesterday had left my legs all wobbly and dead. Tomorrow I have to stop thinking about fragmenting recumbent bicycles and resume the happy contemplation of boat construction.
The Duplo bike, now with its fourth set of front forks, second back wheel, third front wheel, second lot of neck rests, third crank set. But still on its first chain. Because I wax my chains, and they really do last forever.

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