Not a bad idea really, the conversion to a tensioner & the toothed cog would also help to reduce felt vibration... my only concern is if I were to buy one, for the money I'd want one that addresses the major issue w/these kit idlers & that's it's mount stability.
While the lateral force from a taught chain is reduced due to the spring & travel, it's still mounted to the lower stay, in that example it's prevented from twisting into the spokes w/a self tapping sheet metal screw. Folks have done this w/success even with the kit idler alone - but I'm not a huge fan of drilling holes in structural tubing if I can avoid it.
Again tho, I'm not sayin' it's "bad" - just that it's such a comparatively simple & low cost fabrication project *shrug* The minimum tools would be jus' a hacksaw & drill and the materials a lrg derailleur cog or similar, a coupla conduit clamps, a chunk of aluminum (an old yardstick would be perfect) a spring & some fasteners to make something like this (narrower if wanted) w/the same swing arm & spring;
There's a bunch of discussion regarding alternate mounting techniques & DIY solutions like the above here BTW:
http://motorbicycling.com/f11/chain-tensioner-suggestions-11815.html
Jus' some thunks
