chain/crankset jammed??

I was just finishing my turbo session so no sprinting and all of a sudden the chain jammed and I couldnt pedal.

If you give it a bit of effort it will pedal all the way around but it seems to get stuck every 1/2 and a bit pedals. Its not always in the same place.

Ive taken it out of the turbo and same issue. if you pedal backwards the chain just goes totally slack and falls off.

Any ideas??

is it possible youve damaged a chain link?, check if one side of a link is starting to come off. is this fixed gear or geared bike? if geared it may be catching in your mechs or it could be as simple as it not sitting on a jockey wheel right?. Id also check that your chainring bots are tight, that your chainring/s are straight. The way you describe backwards pedalling it sounds like a buggered freewheel.

swapped the wheel onto another bike and its not that (thankfully!) will have to look at the chain and crankset. So odd that it happened all of a sudden doing about 100w

Damaged link, if your riding brakeless fixed is really not worth any risk that a chain is damaged, only cos £6-10 for a quality new one

it's a road bike

Sounds like a combination of chainsuck and a sticky freehub

https://youtu.be/2lvOjZqm4eE

wheel/freewheel seems perfectly ok on another bike. Took the cranks off and the bottom bracket feels smooth. Chainring bolts are tight.

Happens in any gear, even if youre not shifting..

Does the chainset get stuck like that without the chain mounted on the chainring? I bet it does.
Fucked BB bearings?

BB faced?

it is completely smooth when there isnt a wheel fitted (with chain going round on the front).

Maybe I need to look at the rear derailleur?

Fucked freehub bearings?

Nah scratch that, seems to be happening at th same point in the pedal circle? That to me would say it's related to something at the front.

Be methodical FFS.

Remove the chain: do the cranks spin freely?

This will rule out either the front (cranks & BB) or the rear (wheel & derailleur).

Report back plz.

not sure how the crankset would not spin freely with no chain given that it spins freely with chain (no wheel)?

But now I have access to the bike I checked and yes the cranks spin freely with no chain..

Bike herpes. Definitely bike herpes.

so, put a old 10speed wheel I had knocking around on and its fine.

So basically it must be the wheel, but the wheel works perfectly fine on another bike.