I have a single-speeded Voodoo Wanga which has an issue. When pedalling hard, I get a fraction of a revolution of no resistance, and a clunking sound.

I thought that this was a freehub issue (I use a 9 speed rear hub, with freehub and conversion kit), so I swapped the wheel but the issue continues.

It sounds like the noise comes from the bottom bracket, but why would I get any free movement (the fraction of a revolution of no resistance) from a BB problem?

Can anyone enlighten me before I but and install a new BB?

check chain ring bolts too.

chainslip ?

Worn sprocket or maybe chainring (more likely sprocket)
does your tensioner pull down ?

I was wondering about chanslip, but there’s no tensioner, as it’s got sliding dropouts, so I don’t think that could happen.

What’s the story with the chainring bolts, would it be that they’re not tight enough and the chainring slips against the spider?

They’d have to be pretty loose to be honest to generate slip, just another thing to check. If it’s not freehub, and chain / drive train is all fine, then BB might be loose and clunky. Stiff link in chain, chain wear? (chain no longer sitting/engaging with teeth..)

How can a knackered BB result in no resistance? Not sure I understand that – surely the cranks are either turning and pulling the chain round, or they’re not. No amount of BB knackeredness is going to allow the crank to turn without pulling the chain round.

It’s got to be the freehub or the chain slipping

How can a knackered BB result in no resistance?

Well quite. But how can I get chain slip on a single-speed with sliding dropouts and a reasonably tight chain, and how can it be the freehub when using a different wheel, that is fine on another bike, doesn’t solve the problem.

Just had a thought: The chain chainring and cog have a lot of miles on. Could the chainring be worn enough (shark toothing)) to hang onto the chain on the underside of the chainring, and then release it with a snap. Not home until late this evening, I’ll have a look then.