Mysterious skipping drivetrain
 

As the title says, I have a drivetrain that persistently skips under any significant pedalling force, and I can't work out why. It's a singlespeed MTB with a swing-arm chain tensioner, the chain and sprocket (single cog on a Quando knockoff of a Shimano HG freehub) are brand new and the chainring has very few miles on it. The chain seems to be running fine on both the sprocket and chainring apart from the skips. I've also got the pulley on the tensioner lined up directly under the sprocket.

I don't believe the ratchet in the freehub is the issue, as I marked the spacers on the freehub body and the hub shell in such a way that the two marks were lined up, then rode the bike a few yards, being very careful not to coast. Despite more than one skip happening in that distance, the two marks were still lined up, and the freehub's not skipping far enough to have gone all the way round.

Any ideas?
Thanks in advance

Stiff chain link? Damaged or bent chainring or cog tooth?

I had a similar problem with my singlespeed and it turned out to be a slight misalignment of my sliding dropouts. The bike had never previously skipped and the drivetrain had very few miles on it. Started happening on hard climbs after I switched out the rear cog to ride at a hillier location and I was stumped because there was no slack in the chain. After smashing my knee on the stem a couple of times, I actually measured the distance on both dropouts and the drive side was about 2mm farther back than the non-drive. I evened them up and no more skipping. My theory is that the chain was climbing up onto the teeth of the slightly misaligned cog.

From the use of a tensioner, I assume that you do not have sliding dropouts. Maybe misaligned dropouts or bad chainline could be the culprit. Just an idea...

As far as chain drive is concerned a single speed with a chain tensioner is the same as a derailleur bike.

The most likely suspect is a stiff link or an otherwise damaged link. Chain stretch can also be a factor, especially if either the chain or freewheel was replaced recently.

Backpedal slowly with your hand pushing the idler pulley slacken the chain slightly. Watch as the chain emerges for any link that doesn't straighten immediately, or any popping or uneven movement of any link. If all appears normal, measure your chain for stretch (search chain stretch here or the internet).

I know how to measure chain wear, and I know the chain's not worn, it's literally done about three or four miles, like the rest of the drivetrain. I'll check for stiff links, but I've had it skip twice in one turn of the cranks and then again on the next turn - I'd be surprised if a brand new Shimano chain had enough bad links in it to do that.

Sorry, I didn't notice it was you, and missed the reference about the chain and sprocket being new.

So, a photo might help, but it's possible that the idler is too far back and there's insufficient chain wrap on the sprocket, or is too high and interfering with the chain/sprocket engagement.

Also given that both are new, it could be that friction between the chain and flanks of the teeth causes the chain to not settle fully when it winds on. This causes climb-out when the next tooth comes around. Remove the wheel, wind a piece of fine emery cloth around a file and file both flanks of the sprocket. The freewheel will turn back on the backstroke so the action will automatically file the entire sprocket evenly.

Clean off the grit, and test to see if it improves.