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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

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  • Have you checked that the rim is true under no-load with you calipers? Did the rim just go out of true when you started riding and needs truing again? Dec 16 '19 at 17:48
  • Yes it could be that. You should really get or borrow a Spoke Tension Meter and get them all set inside the correct range then start fiddling around to true it up.
    – Affe
    Dec 16 '19 at 17:52
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    Is it possible that the wheel isn't mounted squarely in the dropouts?
    – Weiwen Ng
    Dec 16 '19 at 18:56
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    Cracked frame?? Dec 17 '19 at 2:07
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    BINGO - the wobble got worse riding the bike tonight. When I looked more closely, I noticed that the frame was cracked at the end of the chain stay near where the wheel is mounted.
    – JK1
    Dec 17 '19 at 6:28
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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).

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  • With regards to your question: It would be best to ask it as a separate question (ideally including a picture) to make it visible and allow people to really answer it. I am intentionally not answering it in this comment ;-)
    – anderas
    Dec 17 '19 at 7:00
  • I'll respect anderas's point and not talk about frame repair options here except to say it might be worth seeing if Trek will respect their lifetime guarantee. It might only apply if you owned the bike from new and not sure if they had it in the 80s but worth investigating.
    – user68014
    Dec 17 '19 at 8:28
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Unless the spoke tension is so low that spokes are actually going slack as you ride it won't make any difference. You can easily just compare them to another wheel to get them in the ballpark without needing a tension meter.

How much is it moving under load and are you sure it's not an optical illusion caused by the tyre (which can often be much more uneven than the rim especially if yours is within a thousandth of an inch)?

I think the next thing to check is that the rim isn't cracked (the drive side spoke holes is the usual place) and after that check the actual frame isn't cracked.

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Sounds like you need to closely check the spoke tension. Or the rim maybe very slightly cracked, which causes the inconsistency in whether it’s true or not.

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One possible explanation is broken rear axle. This is easy to notice if you remove the rear wheel.

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  • This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review
    – L.Dutch
    Dec 17 '19 at 7:11
  • On the other hand, it is not a critique or request for clarification either, so if the mods do their job, the comment will be deleted. By the way, did you notice that the other comments posted as answers do not answer the question either?
    – ojs
    Dec 17 '19 at 7:21

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