On my ss commuter/rack/rat I have a single cog on the rear with a dmr chain tensioner.
I can run the chain outside the tensioner wheel, putting outwards pressure on the chain. This holds the chain against the rear cog from the top but effectively holds it away from the cog on the underside where the chain is pulled onto the cog. To me this seems like the worse option, but the tension is a bit greater as the tensioner arm has more movement outwards and less inwards due to the chain stays being in the way, and this seems to be how the system is normally set up...
I run the chain on the inside of the tensioner and pull the chain up so it is held closer to the cog both as it joins and exits. This is how I had it but, moving from a bigger rear cog to a smaller rear cog (2 teeth difference) , and taking a few links out, my chain has come off twice now under normal (slightly loaded) pressure.
Do bigger cogs, and therefore more teeth, hold chains better or do smaller cogs, with tighter entrance and exit angles hold better?
*this is probably a terrible explanation. i will try and upload pics!
More tension and more chain wrapped around the cog is good, in my experience. Don't know really about big / small.
After changing a brompton cog from (I think) 12 teeth down to 10, the tension needed to be increased before it was happy.
With a road bike with vertical dropouts, tensioner needed to be reversed from pushing down to pushing up before it was happy.
I hate tensioners. Track ends, horizontal dropouts, offset eno hub, are all better.
How big/small are you thinking?
I think Chris Hoy used to ride Olympic track on 51x14 so I'm pretty sure that's stable.
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