Singlespeed & Fixed Gear - Clunking Drivetrain Skids

I've just recently put some Tiagra square taper cranks on my fixed and have been getting some kinda scary sounds when skidding.

I hear sort of a 'clunk' from the drivetrain (not really a clunk, but the sound is hard to describe) when I'm skidding and the tire grips the road again. Not 'nuts on the stem' skids, but slowing down skids. Other than this the bike feels and rides fine.

I've started using the chainring that came on the cranks too; a 39t inner chainring. It looks a tad flimsy, but it's steel and feels quite solid. The chainring bolts are tight, cog and lockring are tight, and chain tension is fine. The chainline looks maybe a millimeter or so off, but good enough. All gear teeth are fine too.

Does anyone have any clue what the problem could be?

maybe you need to regrease your bottom bracket

maybe you need to regrease your bottom bracket

It's sealed bearing, but I cleaned it and greased the threads when I installed the cranks.

never encountered this, but maybe check the chain to make sure the link you broke to put it on is moving smoothly?

never encountered this, but maybe check the chain to make sure the link you broke to put it on is moving smoothly?
Damn, I thought you might have nailed it with this suggestion. I checked my chain and did find a slightly stiff link, bit fixing this didn't help.

I'll try and make things a bit clearer. Low friction, easy sorta skidding is fine, but the noise occurs at the point where a lot of backwards force is needed on the pedals to fight against the wheel rotating forwards, or exactly when the tire grips and starts spinning forwards again and I fight it to keep slowing down. So it seems to be extra strong backpedal pressure that's doing it.

your cog's unscrewing maybe? if your LBS has real fixie riders like mine does they'll know in seconds, you should too.

There are one or two track riders around the way at my LBS's, but no real street fixed riders, so I dunno how much help they'll be.

The cog seems like the logical thing for it to be, but my lockring is on tight as a nun's and I'm not feeling any slip in the cog; just that scary noise.

I sprinted really hard and stopped with my front brake only on the way home earlier then checked the lockring and it wouldn't budge.

Your cog could be shifted into the lockring so hard that it would seem very hard to undo. If it also happens when you prevent the wheel from spinning and pedal forward it's probably the cog. Otherwise I dunno, any sudden jumps in the chain?

Not sure exactly what ya mean by 'jumps in the chain', but pedalling forwards is fine and smooth as usual.

I'll reinstall my cog when I get a chance later on and give everything there a once over.

"fine and smooth as usual" would be contrary to what I was suspecting, give everything a once over, and if that doesn't help, ask the guys at the LBS. If they give you ****, throw your bike at them scream "[INSERT NAME HERE] CRUSH!" and flying leap through the nearest window to GTFO.

Go back later to recieve bike, lather, rinse, repeat.

Now that's advice you can take to the bank, thanks Astro :-)

One other thing just occurred to me, what if the chainring is flexing under the pressure and throwing my chainline slightly off? Not enough to throw the chain, but enough to make a scary noise. It is just a wimpy roadie inner ring after all. Is this something anyone's ever heard of?

I'm stumped... That doesn't sound very likely.

Not sure, but kind of sounds like a stretched chain. If the chainring is new this is even more likely.

How old (miles) are the chain & cog?

all new unused equipment?

if not maybe it is just your dirvetrain bedding in. for example a new chain can sound like hell for a bit under strain till everything learns to play nice together.

Your cog could be shifted into the lockring so hard that it would seem very hard to undo.
It turned out you were on the money here.

The cog and lockring had pretty much seized together. I couldn't tighten the cog with a whip and I rounded off two of the three lockring indents with my lockring wrench trying to get it off at which point I decided to leave it for the LBS.

I bought a Dura Ace lockring and got them to brute force the old lockring off. The lockring thread on the hub had been stripped a bit, so I'm guessing that the noise was the thread stripping.

Thankfully I could still get the new lockring on the semi-trashed thread and all seems well now, so fingers crossed that the lockring will hold.

The Dura Ace lockring seems a hell of a lot better than my old stock one. My lockring wrench rounded the old one way too easily, but the DA seemed 10x more solid when I was tightening it down.