Pedal stroke slippage
 

Hi folks

One of several problems I'm having converting a new Trek hybrid-style bike to a fixed gear is pedal slip. It happened today after I put on a new 16t Dura-Ace cog and lockring (all properly greased). Hopped on the bike and oops! the pedal slipped a little, both forward and back. The drivetrain's main components are an ENO hub and Sugino XD crank with a 42t ring. The chain seems properly tensioned; chainline good.

Can anyone tell me what accounts for the slippage? Is the chain too tight in places as it rotates? Is this a BB issue. I've ridden the bike on a freewheel for the past week without any slippage. Now this.

Thank you.

Your cog is not screwed on tight enough so it screws in slightly as you apply forward pressure and slips back against the locknut on back pressure.

Use your chainwhip to tighten the cog down then torque down the locknut.

Is that all there is to it? Geez, I feel a little sheepish. Will tighten it down with the old chainwhip. Thanks!

It may also be that the cog is jumping threads. This happens when the lockring isn't tight against the cog. Make sure the lockring is making good contact against the cog, and tighten the lockring after riding around (or tightening the cog with a chainwhip).

Forget the chain whip. Pedal forward nice and hard. Get the cog tight in the threaded position. Then dismount without pedalling backward. Tighten the lockring. You will be set to go.

If it wasn't a loose cog, here's something else to consider.
You mention a new cog, but what about your chain? If you're running an old chain, it may be worn to the point where it isn't meshing correctly with the new cog teeth. You only find out when applying high loads, then it skips.
To know if your chain is "stretched," measure a 12" length. Center or edge of one link pin should line up at the exact same spot on a corresponding pin 12" away. If your chain is more than 1/8" longer over a 12" length, time to replace the chain.
However, is chain skipping that common on fixies? It happens all the time on geared bikes, where there's excess chain length to accommodate the chain lifting off the cog and slipping forward one or more cog teeth.
I'm not a single/fixed gear type, so can't rely on experience like with geared machines.

Good point. If the chain is loose enough and with a lousy cog it may possibly skip, but I doubt that is what is happening with that Dura-Ace cog.

LV2TNDM does remind me that you should replace your chain when you are putting together a new drivetrain, if it's anywhere close to being warn. $10 will keep your cog and chainring investment safe. If the chainring is worn, replace that too, since it'll make the chain wear quickly. Doesn't sound like this is the problem, but you might want to do this anyways to avoid the problems I mentioned.