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.