Guiding the lower part of the chain
I've found that the lower part of the chain does come
considerably close to the ground when using a normal long arm
derailleur on a 20" bike frame. A short arm derailleur is
much better but may not be able to be used with the range of
gearing. This is most prominent over bumps and when changing gear
and this can be rectified in one of these ways:
- See the
Greenspeed page of Michael
Painter.
- Mount a bracket on the rear that is connected to the
derailleur by a spring to tension the chain. These are
also commercially available, often used on mountain
bikes.
- A jockey wheel can be made. I did this by assembling the
existing bottom bracket with a pedal arm. Screwing on a
pedal axle then cut most of it off. I then drilled a hole
to mount a small derailleur cog through a bolt with a
nylock nut. It is best to make a small guide out of scrap
tin (see diagram for
similar) to keep the chain on the wheel. The arm pivots
forward and is held up by mounting a spring or passing a
plastic tie through the guide to raise the chain. see picture
- A lower portion of a derailleur can be mounted 1/2 way
along the trike. See Don Boose's article.
or alternatively mounting the lower portion of a
derailleur at the 'old' bottom bracket point. See picture 1 & picture2. This is
the lightest and most successful for me to date. I held
the derailleur into the BB bearing retainer nut using a
spacer and the original sir-clip. It can then be screwed
into the BB and held tight with the normal lock nut.
- Many home built HPV's
have an old bottom bracket left on the main frame. (main
pedal axle) Recently what I've found is the cone nut that
holds a pedal axle in place is the same thread and size
as a rear cluster. (set of cogs mounted on the rear hub)
The idea then is to use a rear cluster as a chain guide.
The ratchet part inside the cluster needs to be removed
so that it will freewheel both ways with all the bearings
etc left in place. The BB then has only the right cone
nut screwed in half way and the cluster screwed onto that
nut. It would look a lot better and be lighter if you
took off all but one cog.
Now there's one problem with all of this. The right hand
thread on a BB is usually on the left which means the
cassette won't screw on so you will need to improvise.
You could either re-fit the BB the other way round,
re-thread the tube or weld a right hand thread cone to
the outside of the right one.
-
Recently what I've
found is the cone nut that holds a pedal axle in place is
the same thread and size as on a rear wheel that accepts
a cluster. (set of cogs mounted on the rear hub) The idea
then is to use one of these ratchet's as a guide. The
ratchet part inside the cassette needs to be removed so
that it will freewheel both ways with all the bearings
etc left in place. The BB then has only the right cone
nut screwed in half way and the ratchet cog screwed onto
that nut.
Now there's one problem with all of this. The right hand
thread on a BB is usually on the left which means the
ratchet cog won't screw on so you will need to improvise.
You could either re-fit the BB the other way round,
re-thread the tube or weld a right hand thread cone to
the outside of the left-hand thread one.
- Recumbent bicycles - Bob Earl's project
Charles
Gaffen's Chain idler project
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Thursday, 29 January 2009