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
jpw24@cam.ac.uk
and for my next trick, a recumbent...