Picked up a cheap but worn dirt jumper. The chain tension keeps changing.
It has an 8 spline BB with 3 piece cranks. I’ve set the chain tension perfectly on the stand but on the ground it changes. Pedal backwards a few times and it goes really tight, pedal forwards again and it goes really loose.
Immediate thoughts were that the wheel isn’t tight in the drop outs, but the nuts are as tight as I can get them.

The chainring is very worn as is the chain, but I wouldn’t expect this to have much effect on chain tension.
Before I start swapping bits out can anyone suggest where the issue is?
Many thanks

chainring isn’t concentric to the bottom bracket axle.
bottom bracket axle is bent
Sprocket isn’t concentric to the rear wheel axle, freewheel mechanism isn’t concentric.

In fact, if it takes a few turns, it might be wrong at both ends, only goes tight when you get the maximum out of round at both ends. (and loose when you get the minimum Out Of Round)

chairing is not centred. All SSs will have it to some degree.

i dont know how you would solve it on a 3 piece crank. If the chainrings are attached to a spider with bolts as on a Hollow tech or something, you loosen all the bolts a little. Not completely. enough so that the chainring is staying still, but will shift when tugged on.
Then rotate the crank to a tight point, and squeeze the upper and lower chains together lightly to pull the chain ring towards the rear wheel. Rotate to find a tight spot, and repeat till its as uniform as you can get it.
Tighten the chainring bolts
retension the wheel.

as i say, dont know how your chainring is attached to the crank, but presumably there must be some for of adjustment there.

Tight spots on chain?

If it’s taking a few backward rotations to loosen and then a few forward rotations to tighten, is there something somewhere that’s not tight and is unscrewing a bit and causing the problem? Sprocket, freehub, that sort of thing?

I’ve found this’ll happen on my SS MTB with drivetrain bits that are worn, but not worn in together. My theory is that the chain can ride up on the teeth of the sprocket or chainring and sit with a different number of poorly engaged links. Only properly effective remedy is to replace the whole lot, but luckily that’s not ruinously expensive with a single speed. If you keep that set together, it’ll run until completely knackered.

As above, chainring not centred will cause tight spots. A badly worn chain will also contribute to it.

Also the chain tension doesn’t need to be as tight as you think.

Thanks all. Looking at it again it does seem to be the chainring. The round hole for the bottom bracket axle is a mangled mess of chewed up aluminium. Even with the spacer fitted it’s still moving about.

I’ll see if I can get a Shimano crank on which I suspect will fix the problem.

hang on. track ends?

Are you rolling the wheel forward and back i the slots under load?

Are they still tight after you get off? or do they need tightened again?

if its getting tighter and tighter rather than tight loose tight tight loose I fail t see how its any concentricity issue.

As tthew said, it could be the chain riding up on the worn teeth when backpedaling. Trying to imagine it, if it happens when backpedaling I’d guess that the chain ring or sprocket is on backwards to try and get some more use out of it.