> Yes, after much hassle and sorting out of all the little niggly bits, my
> carbon fibre unicycle is finished and I'm rather chuffed. Originally (3
> months ago) it was a Pashley. Talk about heavy. The biggest improvement
> was chucking the steel rim and putting on a Mavic 231 mtb rim with a
> skinny Specialised slick tyre. But it was still an excessively chunky
> steel frame and those horrid, horrid Pashley bearing mounts. So it was
> time to make a decent frame.
>
> The frame is three carbon epoxy tubes held together by a Pace mtb fork
> crown. The tubes are T300 carbon fibre wrapped 0,90 made by mandrel
> wrapping. They get a big metal rod, 25.4mm dia, soak the carbon cloth is
> epoxy, wrap it around the mandrel, cover in heatshrink tape and heat. As
> I only wanted a metre length of each of 1.6mm and 3.2mm wall thicknesses
> it was tricky to get a wrapping company to deal in such a small order but
> Custom Composites in Rochdale managed it nicely. The fork legs are
> 28.6 mm o.d., the seat tube 31.4 mm o.d. Carbon is a devil to cut
> cleanly - a hacksaw will go through it easily enough but make a mess of
> the tube. Luckily the departement I'm in had a diamond saw in the
> basement. Did it make my day to find that out? Just a tad. Anyway,
> perfectly clean square cuts to the ends of the tube and a 50 mm slit
> down the back of the seat tube to allow a bit of flex when clamping the
> seat post.
>
> The fork crown (sold by Stif in Leeds) is a beautifull piece of machined
> and polished aluminium. The tubes just slide in and clamp up. Now carbon
> tubes tend to crush if you clamp them so BERTs were needed (BERT - Bar
> End Reinforcement Thingy). These were 25.4 mm o.d. bits of aluminium
> that go inside the tubes where you are clamping them. Clamping bolt
> torque was determined by acoustic emission - I did the bolts up untill I
> heard the carbon begin to crack, then I stopped.
>
> The seat post is clamped by two seat collars, bought from the local bike
> shop. My Pashley saddle lasted ~2 months so I'm now running a DM. Weighs
> a veritable ton so the seat tube of the uni was extended to allow the seat
> post of the saddle to be cut down to 150 mm.
>
> The bearing holders were the tricky part. I replaced the Pashley bearings
> with four sealed SKF ball bearing units. Talk about a tight fit! The axle
> is slightly oversize. Consequently getting the bearings off nearly
> destroyed the bearing puller; putting the new ones on nearly bent the
> beefy pillar drill I was using as a press. The bearing holders themselves
> I machined down from a big chunk of aluminium magnesium I found lying
> around the lab - oh I do love working in materials science. Each piece
> started at ~600 grams, finished at 100 grams. Lots of machining.(I've a
> DWG if anyones desperate to see what they look like.) These
> holders just push fit onto the bearings and have 50 mm long cylindrical
> spigots that just push up into the ends of the fork legs. Epoxy adhesive
> holds them in and provides load transfer. I know this bit sounds a bit
> shaky, I mean its held together with glue! ferchrissake, but so far they're
> solid. As for long term durability, ask me in a years time.
>
> Well, I haven't weighed the whole thing but the frame and bearing
> holders feel at least half the weight of what they replace. The
> difference in ridability is immense and that's what counts, not the
> polishability of it all. I came back after xmas and sussed idling in a
> couple of hours. Its so much easier to move it about under your self. All
> that's left of the Pashley is the hub and cranks and, hey, I'm not sure I
> like the cranks either. The pedals, by the way, are now Odessey SharkBite
> BMX ones (well, I couldn't get any DX's in the right threading). Like
> having a pair of pitbulls grabbing at your feet. Mind you, get it wrong
> and its like having a pitbull chewing on your shins so I'm off to the BMX
> shop for some pads shortly. The first time my foot came off and the pedal
> grabbed it back I was so surprised I slammed. Oops. But i love the
> machine now. Masses of grip, seriously responsive, now all it needs is a
> decent rider.
I've now been riding it for 4 weeks, it's still stupidly light and hasn't
broken yet. I cut down some old Campy aluminium cranks, less than half
the weight of those steel ones, put them on, picked it and broke into a
huge grin that lasted most of the week. Rode Paul Makepeace's steel 24"
machine today and was like: hey, major exercise, whats this lead weight
on a wheel? Should hopefully be at the Brussels con. in Feb so any of you
europeans reading this may well see me there falling off it.
Jez
jpw24@cam.ac.uk
and for my next trick, a recumbent...