Small and mighty, Bionicon's C-Guide is a well-executed compromise for single and double-chainring riders who want quiet, effective chain control. The derailleur cable housing can be routed through the middle of the C-Guide's tubular plastic hinge pin, so it doubles as a cable guide as well.
I use them when if I break my chain on a ride... there's no need to replace the whole chain until it's worn out. I had 3 power links on my chain from 2 years ago. Last year had 2. My new one has one... for now anyway.
bikotic.com/2011/09/13/macca-dangler-mk-2-home-made-chain-device
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