I have a Diamondback one-piece crank and the chainwheel seems to be not centered on the crank axle, much worse than I usually see on such bikes. The result is that it's impossible to get the chain tension right: chain is very loose when the right pedal is forward and too tight when it's back.
How tight is the tolerance of the chainwheel/axle supposed to be, or is this just the curse that comes with cheap bike parts?
I expect I can dismantle and look, but is there any adjustment that can be made or maybe a shim?
If the actual chainwheel bit can be removed from the crank it's usually possible to tweak the alignment a little bit. Loosen bolts with the pedals in the tight position, then give the chainwheel a tap with a hammer. Use a piece of wood to avoid damage to the teeth/chain. Spin crank, look for improvement, repeat if you think it'll help, then tighten and ride.
Otherwise it's the curse of cheap parts that's on you.
Subject line says one-piece crank. No chainring bolts. The chainwheel is a single disk with a hole in the center for the axle of the crank. If that hole is not actually in the center of the disk you'd get the problem I am having.
the way the sproket is on 1p cranks is with lock ring. so before you go taking stuff apart make sure you have the tool for that. if you cant tighten it down properly youll be taking the whole thing apart again to do it all over again.
Its hard to say, your bottom bracket might not be adjusted properly giving your cranks play and getting the wrong chain tension.
ASSUMING your BB is adjusted correctly. id assume your chainring is bent. Just get a new one and make sure you got the right spacer on there and adjust it all down
I already ajusted the BB. There is no play in the BB, and there's nothing to adjust other than the bearings themselves. The chainwheel is a solid disk, and it's not bent but behaves as if it's out of round, or never had the hole in the center exactly in the center to begin with.
But now that I think of it, if the BB drive-side cone is out of round (the cone that built into the locknut that holds the chainwheel on) that would cause the same problem.
My understanding is that most chainwheels are constructed in two steps: one to cut the teeth and one to cut the middle. Cheaper ones have less quality control meaning you're more likely to get the teeth and the centre hole out of alignment.
Measure it against something flat (like a straight-edge, glass table top, etc) to confirm it's not just bent. If it's not bent, get a new one, cause it's not really fixable
This is common with one piece cranks and is usually not a biggie, but like you said when you like the chain tension to be just perfect on the tight side (for doing no-footers or whatever), then it becomes a problem. One of the culprits for this is the "bore" or hole in the middle of the chainring is usually machined with a tolerance slightly bigger than the machined axle shank that's supposed to locate it dead-center.
To "minimize" this (I've had good results, but haven't been able to take it off completely), dismantle the one piece crank and unscrew the bearing race that clamps-down the chainring. In all likelihood, your chainring will have a wobble against the axle shank. What you want here is a snug-fit or better yet - a slight press-fit. If the wobble is very slight, you may have to go to an auto-engineering shop that sells extra thin sheets of brass. You want to cut a collar-spacer to wrap around the exact circumference of the shank, to get rid of the "gap wobble" when you locate the chainring back in it's place. In some instances when the wobble is pretty big, I've had success in cutting this collar-spacer from a tin can, or some steel or aluminum sheet I've got lying around. You have to try various thickness until you find the right sheet metal that gives you a snug fit.
Then screw the bearing race back on, but make sure it's really tight. Reassemble the rest of the guff and hope for the best!