quilkin

I saw those wheels in some photos somewhwere on the web and wondered what they were - thanks for defining them - but they're $25 each at Bricklink! I think mine built from parts were cheaper and could be built to a range of sizes. I've built more large wheels since then (not bike wheels), I'll post some pictures in a new thread soon.

Yes, it's a 'fixie' (fixed-wheel bike); there's no freewheel so you can back-pedal to stop it. Rear brakes aren't required (or even legally required!) on fixed-wheel bikes. Also a rear brake would need some very long flex cables; I tried joining some together but wasn't happy with the results. A freewheel won't be impossible, maybe that can be a future project, but I think I'll go for 1/2 scale rather than 1/3 scale if I build another.

It's pretty well finished (for now at least) - you can see it here - including working brake, dropout wheels, water bottle, and pump (!). I started with the frame, (I have mostly yellow beams) then found the cheapest 1x2 thin liftarms (for the rims/tyres) on Bricklink were red or gray. Yellow 2x1 liftarms were not ever made! Maybe I'll do a red frame one day to match. About the wheels, is it off-limits to strain Lego parts in this way? Real bike wheels are under a lot of stress, they have to be, so I maybe it's OK. Thanks again for help from contributors.

The part I most often need is simple: a combination of this pin and this axle pin so you can join three liftarms, only one of which has an axle hole.

Thanks again, allanp. Your ideas have put me on the right track; the set of 6 x 4019 gears is too big (and the liftarms make it too wide as well) but the same idea works fine with 8-tooth gears, and I can even fit the spoke retainers underneath! Some nice mathematical patterns emerging here...

Many thanks for all the suggestions! Yes, my forks were too thin, after some experimenting I found that a double liftarm 32271 held together with some 4x2 studded plates makes the perfect slot for an axle. And I didn't realise that the 2695 wheels were free-spinning, that's good. I know about the threaded axle but the little nuts are too fiddly, I think I can use some discs 2958 on normal axles to clamp them in place. So, next stage is to somehow get the rear sprocket driving the 2695 hub without driving the axle. (It's a single-speed 'fixie' by the way, maybe designing a derailliuer and freewheel in Technic is going a bit far for this stage.)