I used one for a while, too (as an attempt to tighten the chain on my vertical drop frame, and for laughs)... Sadly, it kept falling out. You need lotsa space within the chainstays so that you can jam in a large enough cog... and you also need a large enough cog, about 24-26t, preferably 1/8 if you run a 1/8 chain.

You have to jam it in real hard, as close to the cog as possible.

BTW, if someone steals it (unlikely) or it falls out and you don't put it back (more likely) you're not stranded. You just risk dropping your chain.


Edit: damn you, guys, now I'm thinking of mounting one again... only now i run a bigger cog so I probably can't (ghost ring would hit the chainstay)