Slight play in cranks

Hi everyone, first post.

I changed the rear cog on my bike the other day to a slightly larger one for the hills of my commute. I’ve noticed now there is some slight play in the cranks i.e. there is some free back and forth movement of the cranks before the chain is engaged. Any idea what this is/ how to fix it?

I thought it might be a chain tension issue, but the chain is pretty tight. Could it be the tooth width of the new cog is narrower than before?

The bike is otherwise quiet, the chain feels secure on the cogs and once the cranks are moving the play is not really noticeable, just when at standstill/ going very slow.

Any tips gratefully received.


Sounds like chain tension to me. Remember chain rings aren’t perfectly circular so the chain could be tight in certain areas of the crank rotation but looser in others.

Could also be loose chain ring bolts but I doubt that.

Your rear sprocket is loose, stop riding the bike, walk it to a bike mechanic and get them to fix it before you destroy your hub, if you haven’t already.

There’s been heaps of threads on this. Do a search to find out more.

Not convinced its a loose rear sprocket from the description but as Dan says, best to get someone to look at it before you stuff anything up.

Hey thanks all for the quick responses!

I need some other gear so I’ll drop into the cycle shop tomorrow and ask their advice. I don’t really think it’s a loose rear sprocket as I tightened that up pretty well, but I’ve been wrong before!!! I’ll have a look at the chain tension again tonight.


Just had another look. I’ve previously trashed my track nuts by using the wrong spanner, therefore I can’t tighten them as much as I would like. The rear hub has consequently shifted forward on the fork ends (dropouts) and the chain is now waaay loose. So that’s where the play is coming from. Ooops. New nuts tomorrow.

Thanks for the responses.