Ever since I arrived at Bike Friday I had been thinking about a design for a superlight racing version - that would still pack into a suitcase for travel. Wanting the lightweight was as much for travelling as for the actual riding/racing - in Europe the luggage allowance is often one piece of 20kg. My Pocket Rocket packed into it's case is pretty much at this limit, leaving only hand luggage space for other items. If I could get a bike down to around 14lb, then I would have around 8lb spare for addtional items in the case.
The first step was to buy a full carbon 700C fork and cut it down to fit a 451 wheel. I then glued a reinforcing length of aluminium inside the steerer to allow it to safely extend above the headtube unsupported. A seatpost was crafted from ovalised thin-wall aluminium tubing with a machined clamping head for the saddle (222g for a seatpost over 600mm long!). The seattube of the frame was ovalised to match the post. I decided to use the BB30 standard for the bottom bracket. This uses an oversize shell with pressed in bearings and a 30mm axle - I made some hollow cromoly cranks to fit, which although they turned out pretty well (about 200g lighter than the current Dura-Ace setup, and with a much narrower q-factor), had some teething problems. I will return to the crank project later - in the meantime I machined some spacers to allow regular Dura-Ace cranks to be used with the pressed-in ceramic bearings.
In order to fold, the rear end pivots around the bottom bracket shell, being held in place with a small bolt in the back of the seattube and pinch bolts at the pivots. I left the steerer tube at full length, and then made a custom steel aheadstem (140g with integrated top cap) to get the bars to the correct position. For packing the bike, the fork is removed from the frame - a small collar and expanding wedge fit above the headset to allow for preload. The entire frame is fillet brazed - this is the first bike which I have made all by myself!
To save the need for oversize chainrings I opted to use Shimano's 9-tooth Capreo cassette and hub. However, I wanted to run the new SRAM drivetrain, which is 10spd, whereas Capreo is 9spd. I was able to machine the top sprockets from the Capreo cassette to the narrower spacing, and combined them with sprockets from a SRAM 10spd cassette to create a 9-21 10spd range. But then the Capreo hub freehub body (required as it is special to fit the 9 and 10 tooth cogs) was too short, so I machined the back of the body down to allow everything to fit. This left no room for the spokes on the outside of the driveside flange, so those spokes were laced radially on the inside of the flange (two cross spokes on the non-driveside to transmit the torque). I have since designed a modification for an American Classic freehub body which will enable this custom cassette to be fitted to the lighter hub - when I have time to do this it will save almost half a pound.
The bike is a little lacking in torsional stiffness; however, after initially adjusting to this it doesn't really matter - I have raced it without any problems. The flex comes from a lack of out-of-plane triangulation in the mainframe and the stays not really being stiff enough to be cantilevered. Still, it is just a prototype; I already have plans on how to improve this on the next one, when I have time to build it....