The tyres are often out of round. This can be because they aren't exactly centred on the rime, because the tyre has been stretched when put on the rim or over-inflated, because the tyre has picked up excess rubber, because the tyre has been unevenly heated in use, because the tyre is not balanced and has worn out of round, or because you have flat spotted the tyre.
The rim itself may be bent, or the centre plate is bent with respect to the rest of the rim,
the tyre may not be fully mounted on the wheel hub (all it takes is a bit of grit to cant the wheel...do you wipe the mating surfaces before doing the wheels up)
The spider may be bent or out of true.
AS Itpro has pointed out, you need to do all the tests, because the sprocket carrier may be bent, worn, out of true all of which can produce an fore and aft motion on the sprocket.
If you think the axle may be bent, then set up a fixed point reference (itpro's wire pointer) and point it close to the axle itself. If the bearings are in good condition and tight in the hanger, then there should be very little movement. The further out, towards the end of the axle you take the measurement, the easier it will be to see 'not straight'.
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