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I’m experiencing vibration somewhere in the rear half of the bicycle when descending at high speed (say more than 30 mph), and I haven’t been able to pinpoint the cause. Everything looks fine, everything I’ve tried wiggling seems solid, and nothing seems to be getting hot except the brakes and rim.

I’d like help figuring what, if anything is wrong.

I could try to describe my observations in more detail, though I hesitate to do so for two reasons. First, they are all fuzzy and potentially misleading, since it’s hard to examine the bicycle while it’s moving 30-40 mph, and it makes me nervous so I slow down and the vibration stops. Second, it seemed like it could be useful to have a checklist of things to look into when experiencing such vibration, that isn’t limited to my particular problem. (If it’s more appropriate for this community, I’ll edit this question to add my observations.)

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First thing I'd suspect is a tire with a bulge, or not properly seated. After that, check whether the rim is true in all directions. Spin the tire with it off the ground and observe the outer margin of the tire from the side. You should not see it moving up and down. –  Daniel R Hicks May 29 '12 at 11:14
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Keep in mind that at that speed the wheel is doing (per my calcs) 373 rpm, or 6.2 rev per second. Very minor irregularities in the wheel will become quite noticeable, especially since the natural resonance of a bike is in the order of maybe 6-10Hz. –  Daniel R Hicks May 31 '12 at 0:54
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It should be noted that if you have a rack or other attachment, it could be contributing to the vibration by resonating. I've experienced high-speed vibrations on a loaded bike when the panniers were not tightly fastened. –  Daniel R Hicks May 31 '12 at 1:05

4 Answers 4

up vote 7 down vote accepted

Note: I've included things not at the rear of the bike as sometimes we think something is coming from one area when it's actually caused by another.

List of things I would check:

  1. Wheels true and round without rubber (check dishing too, but unlikely to cause vibration)
  2. Adding rubber (tire and tube) and inflating does not take wheel assembly out of true (some tires seem to float from side to side a bit and this can make things wobble a bit at speed.
  3. Wheel attachments such as reflectors don't lead to excessive imbalance.
  4. Accessories such as pump, water bottle, lock are firmly attached.
  5. Check entire wheel assembly: spokes, skewers, bearings. Everything should be free of cracks, dents, bends. Parts that move should do so smoothly (bearings, freewheel). Things that should be straight should be straight (spokes, skewers).
  6. Pawls inside freewheel engage and slip smoothly and correctly.
  7. Frame is intact, no cracks or bends. Especially the chainstay and seat stay.
  8. Fork is intact, no cracks or bends.
  9. Headset is installed correctly, no play in steering.
  10. Seat post is intact, no cracks or bends.
  11. Seat post is tight to frame.
  12. Saddle is tight to seat post.
  13. Finally I would check the bottom bracket area. This gets a lot of torque and abuse so give it another look for cracks or fatigue.

Still a problem? Get an experienced rider that is comfortable descending to ride your wheel. Find a nice straight descent with no traffic.

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Thanks! In my case, I'm pretty sure it's #4 — something wrong with the pawls. I was able to reproduce it on the work stand, and the vibration (almost a crunching/grinding on the stand) is present when coasting but not when pedalling at the same speed. –  Reid May 31 '12 at 1:43
    
It turned out that it was #5 -- the bearings in the hub were worn out. The rim was also worn, so replacing the wheel solved the problem. –  Reid Mar 19 '13 at 0:15

I had a similar problem, would a longer chain stay be a problem?

If a bike is specced with a measurement and actually measures another such as the the chain stay is off by 5mm, would that cause it to shake?

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Welcome to Bicycles SE. This site, like other Stack Exchange sites, is not a typical forum. Users post questions and the community attempts to answer them. As this post does not actually answer the question, it would be better suited as a comment. This could also be a a question on its own, since you're asking whether a very specific situation could cause a very specific effect. –  jimirings Mar 12 '13 at 20:12
    
But in answer to your question, the length of the chain stay would have essentially no effect on the propensity of the bike to vibrate. –  Daniel R Hicks Mar 12 '13 at 22:42

Based on recent personal experience, I would suspect the hub bearings. I'm guessing you've checked the wheel for play, but if you did this while the wheel is still on the bike, you may have missed it. Play is often not noticeable when quick-release skewers are used as these provide additional compression on the cups that can artificially tighten the bearings masking the underlying problem. Check for play with the quick-release done up loose, or with the wheel off the bike, for a better idea of hub wear.

This quick release tightening phenomenon is documented at http://sheldonbrown.com/cone-adjustment.html.

My recent experience sounds very similar to your's - vibration only when descending rapidly. Once I fixed the (totally knackered) bearings, the vibration stopped. My bearing play was masked by the quick release tension so it was not immediately obvious that the hubs needed servicing.

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Have you put it on a true stand? I'm assuming you don't see any obvious dents or seam separation, but a wheel can become untrue in such a way that the wheel is further from the hub at one end and closer to the hub at the other. This will be very hard to see because the change is very gradual, but will definitely cause vibrations at higher speed.

Beyond that, the belts in tires sometimes separate, and the rubber will expand between the belt separation forming a 'bubble' in your tire. These are usually noticeable at low speeds, however, so I'd check the truing first.

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