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.

• Run the chain through a tube. See 'Some idle thoughts on recumbent chains'

• See this pic at Earth Cycles using the bottom part of a derailuer.

• Richard Hoad's idea Pic 1 & Pic2

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  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