From: R Biegler <biegler@castle.edinburgh.ac.uk>
Date: Tue, 29 Sep 92 18:54:51 WET DST
Subject: Homebuilt

In April I mentioned plans to build a LWB recumbent with intermediate
idler and rear wheel suspension.  After many delays, none of them
David's or John's fault (they built the bike) I got it three weeks ago. 
I went on holiday a few days later.

Before I tell how it all worked out I give a better description of the
bike and some of the problems in building it.  The Bumblebee looks
similar to the Peer Gynt.  Long wheel base, underseat steering, rear
wheel suspension, hydraulic brakes.  The differences are that the Bumblebee
has 20" BMX wheels front and back, the suspension is probably less 
sophisticated and the rear derailleur is not at the rear hub, but at the
idler, so that I can have a rear wheel without dish (I twice had spokes
breaking on the freewheel side and I never want that to happen again). 
The seat attachment is also different, but difficult to describe.  Wait
for the photos in HPV News and RCM, provided I ever get around to
writing an article they are willing to accept.  The backrest folds
forward, which is necessary to carry the bike up the stairs into the
second floor flat in which I live (third floor for the Americans).  The
bike is quite heavy, around 18kg, similar to the Peer Gynt.  I notice it
when going up hills and when carrying it.  The frame is built of
aluminium tubes, about 4 cm thick with 3 - 4 mm wall thickness (I don't
have the exact figures).  The thick walls were chosen so that the
welding would be less difficult.  I forgot: The suspension consists of a
pivoted rear triangle with a piece of rubber in compression.  I think
the rubber was intended to support a car engine.

So how does it work?  The suspension has difficulty coping with high
frequency disturbances as created by cobblestones.  It works quite well
cycling off the kerb and over small to medium potholes.  The wide tyres
(1.75") coped well even with a muddy footpath.  I am not so sure about
their rolling resistance.  I have to do a coast down comparison with my
old bike.  The most unusual feature is the placement of the rear
derailleur.  The following is a schematic diagram of the placement of
front chainset, rear cluster and freewheel (I hope those are the right terms),
with the two chains omitted and not to scale.

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    |     |
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    ||||| |    Rear cluster and transfer gear
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          |      Freewheel
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The size difference between transfer gear and freewheel compensates for
the smaller rear wheel.  Actually it more than compensates and I could
do with lower gears.  Due to the size of the transfer gear the
derailleur was initially too far below the sprockets and the gears did
not change properly.  As the distanc could not be decreased sufficiently
David took out the upper wheel of the derailleur cage, extended the cage
upwards and reinserted the wheel there.  The attachment of the
derailleur to the rear triangle is too flexible and the position is
never quite right in indexing mode.  Unfortunately friction mode doesn't
work.  It indexes too.  Ground clearance is rather small; only about 10
cm, depending on the load.  That is an unavoidable consequence of the
suspension design and a small rear wheel.  The other bugs could probably
be worked out with a bit of fiddling.  Nevertheless I think that it was
not one of my brightest ideas.  For my purposes a 5-speed hub gear would
have been perfectly adequate, but when we designed the bike I still
thought they would have a lot more friction and that I would need a
wider range of gears than a single hub gear can offer me.  To put in a
hub gear now David would have to rebuild the rear triangle because the
spacing is currently too small (only 108 mm).  I will probably ask him
to do that next year and if I then think I need a wider range we may try
to put two hub gears in tandem, one in the wheel, one in the idler. 
After thinking about the joys of a completely enclosed drivetrain (no
messing up my clothes and, most importantly, no chain cleaning) I have
come to dislike derailleurs.  Getting rid of the derailleur would also
allow me to fold the rear wheel under the bike, as you can do with the
Linear, to shorten it for train transport.  There is enough space on 
Intercity trains (but wait for privatisation), but not on local trains. 
The idler would prevent the chain from dropping to the ground as happens
with the Linear.

If you have more questions you are welcome, though you will need
patience.  I may take several weeks till I find time to answer.

Robert Biegler