Thursday, May 22, 2014

Long Wheelbase Recumbent

So, gentle reader, you thought I was clever. Try again.

You thought my wife was clever too. Try that one again.

My wife says some very strange things. The other day she said to me
"You need to clear out that junk. D'you really need a potato on your workbench? Why have you got a lipstick?"
For a wife to posit that junk needs to be cleared out shows astonishing feeble-mindedness; for anyone to fail to perceive the use of a potato or a lipstick on the workbench is remarkable. Nevertheless she is in my Good Books at present because she found the 14mm spanner that I put on the registrars' bed when changing my saddle last week. (There are several registrars. They have to share the bed. - Sequentially, not all together.) I had thought it was Kairn Peter again, erroneously conceiving that anything left within the confines of the shed but not actually secured with nail or padlock was merely evidence of my generosity. - He called again yesterday, and I had to pop out of the workshop hurriedly because he was popping right into it, and since he asked (again) if I had any work for him, and since he knows full well I don't, I think it's his standard ruse to escape suspicion when casing any particular joint. I am quite fond of my tools and don't mind at all if Kairn Peter leaves them where they are.

Anyway, the old boot's clear-out hint has stimulated me to turn a lot of unrequired bike frames into useful steel tubing, and having a fondness for long wheelbase bikes I thought I'd use some of it at the same time. It is factually correct to say that I don't need another bike; it is however irrelevant. Ever since I learnt that Mr. Knight once owned Mr. Ballantine's Avatar 2000, the one on page 24 of Richard's Bicycle Book which first made me fall in love with the idea of recumbent bicycles, I have liked to have the odd LWB kicking around. And old tubing is the perfect excuse, along with Anna Joz's broken Peugeot, a mixte frame which snapped at the stress concentration caused by the Frenchmen drilling through the downtube to accommodate the rear light's wiring. (Frenchmen should stick to growing onions. Or being shot by Englishmen with longbows. I don't mind which. I'm not fussy.)

To design a simple LWB you automatically assume a 20" and a 27" wheel. The front wheel is lightly loaded, and lacking suspension the back wheel needs to be big. A LWB with no suspension to clutter the design ought to be a quick weekend's project. Wrong.

I start with the chainline. This needs to run in a straight line from 4 5/16" above the bottom bracket centre to 2 5/16" above the rear axle. Neither handlebar nor seat may occlude this line or you need a pulley wheel as a chain deflector. The BB is made as high as possible relative to the seat, the only mistake of the Avatar 2000, where the BB is lots lower. Comfy, but slow, and we don't make recumbents for slowness's sake.

The centre of the BB must lie outside an arc 18" from the front wheel axle to prevent warfare twixt steering and feet. The seat angle has to be 34 ½ inches from the centre of the BB, or I can't ride it. (I'm not interested in whether you can.)

Along with the Known Fact that you'll need 1 ¾" trail, and the other Known Fact that kids' bikes' front forks always have 1 ½" rake, all you have to do is measure the various bits and the bike will design itself. (Oh, and the Kn. Fact that a 20" wheel is only 19 inches in diameter.)

Except for one thing. Which is that when you convert a one-tenth pencil drawing into half-a-dozen bits of steel, all sorts of mysterious inches appear and disappear and a great deal of head-scratching takes place. I know not why this is. Which is how we establish that you need to Try Again if you ever thought that I was clever.

Anyway, here's the drawing.


And here's the Much Better Idea, which is to make a very simple wooden jig and fix the front and rear forks to it and work from there. Simple. Wish I'd thought of that ages ago.


And I am making this in great haste because civilization is doomed and going to crash horribly. I had proof yesterday. I was going past Brooklyn School and the caretaker was using a leaf blower to blow leaves from the pavement onto the road. When leaves can't be untidied from one public space to another public space without recourse to a petrol motor, you know civilization has lost the actual plot. So it's a good job that Mr. Knight has finished his bicycle because he can now ride without fear of punctures.

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1 Comments:

Blogger Mr Larrington said...

I've fallen off the late Mr Ballantine's Avatar 2000. Camden Town, 1983.

June 21, 2014 at 3:45 PM  

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