I’ve been slowly work­ing away at mak­ing Rob’s polo bike, it’s tak­ing an infuri­at­ingly long time to get this one together because we’re going for a curved seat tube.

Rob has rid­den a couple of my polo bikes and likes the hand­ling of the short wheel­base, but really can’t stand toe over­lap. By keep­ing the geo­metry sim­ilar in other regards, but mov­ing the BB away from the front wheel, it’s pos­sible to get the best of both worlds.

The first trick with design­ing this was over­com­ing the lim­it­a­tions of my exist­ing frame design soft­ware. The best way to visu­al­ise it was to draw the whole bike up from scratch in 3D CAD. With this model I can work out the clear­ances for such short stays. The Colum­bus S-bend chain stays help provide clear­ance, but work­ing out just where to put the curves can be tricky. Bet­ter still, I now have a lovely para­met­ric CAD model of a bike. This means that I can simply change a few num­bers and gen­er­ate dif­fer­ent bikes in the future. It’s a nice build­ing block to have worked out.

In the above image, the BB shell is clearly very close to the tyre. I plan to scal­lop the BB shell to achieve even clear­ance all around the tyre here.

The trouble is, I’ve never curved a seat-tube before. I’ve only read about how tricky it can be, and I cer­tainly don’t have the tool­ing for it… back to the draw­ing board.

I can use this man­drel with the exist­ing hard­ware I made for my tight radius fork bender, and I had hoped I could get a cab­inet maker friend to make the man­drel for me, but he quit his job. So I bough some hard­wood from a junk-yard, bor­rowed a router from a friend, bought the appro­pri­ate router bits, made a quick router guide and presto..

The first attempt at bend­ing res­ul­ted in a bent bolt and some slight crack­ing to the man­drel. I guess I’ll just have to keep rein­for­cing the thing until it works, or until some­thing breaks permanently.

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