justinschulz9

Im a moderate level bicycle mechanic, and im having some issues with diagnosing symptoms that only occur at high speeds.
I work for a bicycle rental company and wish to have these issues solved on all of our bikes.
here are the issues: "when riding downhill, a loud vibration coming from the rear hub. no loss of control and was only happening while coasting."
I have already pulled the hub apart on this 2016 cannondale synapse and nothing appeared out of ordinary. the hubs were smooth and the wheel spun smooth when up in the stand. the freewheel was also clean so could this problem potentially be the wheel not being in line with the front rim?
2nd issue: " once i hit about 40 kmh there was a harsh vibration that kept me from exceeding that speed. the vibration kept me from maintaining control."
i have been told this could be the resonant frequency. but i dont have too much experience testing this idea.
How can i diagnose these problems efficiently? and what are all of the problems that vibrations at only high speeds be caused by?

Andrew R Stewart 

I think you are dealing with two different issues.

The first I suspect to be a freewheel/freehub body bearing noise. Only when coasting and from the rear wheel. When under little load (as on the stand) this bearing wear would not be noticed much unless you knew what to hear. Of course other reasons might be present or the vibration is actually coming from elsewhere. Without the bike in my hands I can't really say.

#2 This I suspect is what we often call "speed wobble", "Death Shimmy" or "High Speed Bike Shimmy". Try Googling those terms and see if the descriptions match up. There was a recent thread here which had a few links of interest, this topic (shimmy) is a common one here. https://www.bikeforums.net/bicycle-m...ill-speed.html Andy

billridesbikes

The second issue is called “shimmy”. It is caused by the front wheel oscillating around the neutral position, and there can be many causes. If it happens with all riders it could be the the front wheel is out of true but or that the fork is slightly bent and the steering angle is impacted, or tires are worn and tire slip is occurring in a cross-Wind.

It could also be some weight hanging on the handle bars like a water bottle or handle bar bag where the contents are shifting back and forth.

Also, different riders have different center of mass and probably different steering response on the handlebars, so you should find a hill and ride it down to see if you can reproduce the shimmy.

Wilfred Laurier

Weight on the handlebar could be a cause of this problem, but adding weight to the handlebar or fork could also be a solution - more weight will tend to decrease the resonant frequency and so shimmy will not occur at the speed it was happening at. It might become more likely at lower speeds, but 'death shimmy' at 18km/h is unlikely to be even noticed - most would just accelerate through the speed range where shimmy occurs and never notice.

justinschulz9

someone has made mentions of a frame alignment that also could be an issue.
what would symptoms of a misaligned frame cause?

justinschulz9

Its happened again on a new bike.....
client says its a decelerating vibration coming from the rear hub...opened it up and the hub looks just fine and bearings move smoothly....
is the hub compromised?
"vibration is like a phone vibration but 10x worse"
can bearings potentially be the issue at high speed coasting?

mpetry912 

yes, it absolute can be the bearings vibrating, By all means check that first and eliminate it as a potential cause to eliminate further specuation.

Even a small amount of play can account for what your customer is reporting


Mark Petry
Bainbridge Island, WA USA

justinschulz9

wasnt aware that pressed bearings could cause this scenario. has this happened before?