You state that it "wobbles" and "doesn't spin smoothly" which doesn't lead me to thinking about the true of the rim, but rather about the hub itself.
Questions:
- If you open the brakes up, does the wheel spin smoothly? What I mean is, does it make any grinding sounds, or does it appear to 'catch' at any point during the rotation? When it slows down, is it gradual or does it seem to stop rapidly?
- With the bike upsidedown or in a workstand, grasp the rear wheel by the rim and try to wobble it from side to side in between the stays. Does it wobble on the axle or is it stable?
If it's just catching on the brakes because the wheel is out of true, then a proper true and tension should solve the problem.
If it's wobbling at the axle, the hub cones need adjusted.
If it's grinding/crunchy when it spins with the brake open, the cone race is shot and the hub needs rebuilt. (New cones and bearings, cleaned and regreased.)
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"I feel like my world was classier before I found cyclocross."
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