The Carbon Fibre Unicycle

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 mandel 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.

Jez and for my next trick, a recumbent…

Re: The Carbon Fibre Unicycle

Your carbon fibre unicycle sounds fascinating! Be careful loading it into and
out of the car; that’s probably when it’s most vulnerable if you pack it with
other unicycles and equipment. I would love to see a picture (or pictures) of
this cycle. If you send them, I’ll make sure they get into the hands of our IUF
newsletter editor Mark Schecter, and/or USA editor Constance Cotter, so the
world can see what you’ve made. I have a large collection of unicycle photos
myself, and if you’d like some kind of trade, just name your subject!

Stay on top! John Foss, President International Unicycling Federation