Take it this is the one that stops the chain from damaging the frame when it drops off the small ring. I did try one many years ago and although it did hold the chain away from the frame above the bottom bracket- the chain still wore the paint away underneath the BB.
I am particular about setting the stops on the F Derailler so it cannot happen- it still does occasionally. So to protect the frame- I have put some clear plastic- like thick sellotape- around the area to be damaged.
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How long was I in the army? Five foot seven.
Spike Milligan