Wheel tru, bearing & cones tight but still WOBBLES I have a 35 year old Trek Mountain bike that I have refurbished.
- Rear wheel is true within +/-5 thousandth (it is true as mounted on the bike using a calipers clamped to the frame and perpendicular to the rim)
- Bearings (9 x 1/4") have been replaced and cone is tight (no perceptible rocking)
- All spokes are intact
- The wheel is set all the way back in the frame slot

When I spin the wheel under no load, as expected, it spins true without any perceptible wobble.

BUT when I ride, I feel and hear it rubbing back-and forth on the brakes -- and when I look down, I can see the wheel (and the rim itself) very clearly wobbling back and forth between the caliper pads.

Also, if I put lateral pressure on the rim with my fingers, I can feel it giving laterally, though this may be normal given that it is all under tension anyway.

Note bike was working fine until last week.

What could be causing a wobble under load that doesn't exist under no-load?

Could it be that the spokes aren't under enough tension? (i.e. would it be helpful to say tighten each spoke by say 1/2 turn?)

Thanks
As one commentator pointed out, the frame was actually cracked (severed even) at the end of the chain stay (on the gear side) near where the wheel is mounted.

For some reason I only noticed it tonight after riding home from work and experiencing an even worse wobble. Perhaps the crack got worse and became through-and-through on my long commute today.

While frustrating, I guess I am lucky I didn't get hurt...

Thanks for all the helpful suggestions...

One question, is it possible (and worthwhile) to have someone weld it back together -- while the bike is almost 40 years old, the frame is solid, heavy and steel (it's a Trek Mountain Bike from the early 1980's).