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Thread: Troubleshooting a stubborn vibration on bike

              
   
   
  1. #1
    Senior Member Hugues's Avatar
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    Unhappy Troubleshooting a stubborn vibration on bike

    Hi guys,

    Been troubleshooting a very stubborn vibration problem on the bike, pretty much since i started to ride it this spring.

    The problem:

    Starting at 70 km/h, there is a strong vibration on the bike, strong enough that i cannot really go above that speed. The bike has always been like this since i started to ride it this spring. I've recorded the vibration using my smartphone attached to the front fork, the picture shows it below (unit is m/s2). The shorter wave corresponds to the rpm of the wheel, then you can see a longer wave, this is the one that can be felt above 70 km/h, it has a period of around 1.3 seconds, looks like there is a resonance effect at that speed. I've also recorded on the rear wheel but i do not see such pattern. When recording on the front frame, i see the vibration but with a smaller amplitude. When recording on the rear frame i do not see that vibration.

    What I've done so far:

    Initially i thought the vibration was coming from the back because the rear tire was not completely round, about 2 mm difference from one side to the other. I even changed the tire twice, but not difference in the vibration. I had the rear wheel balanced as well, statically. No difference in vibration. I changed both rear bearings, no difference.

    After i discovered it was coming from the front, i had the wheel balanced, once, no change, then another time this week, no change. I changed the front bearings, no change in vibration.

    I measured the alignment and roundness of the tire, rims and disc brakes, i cannot measure any difference when spinning the wheel.

    I also put the bike on the lift and managed to spin the wheel while recording with my phone, i can record the same vibration, the fork is also shaking up and down if i spin fast enough.

    I even tried to put weight inside the tire (vegetable oil) to see if it changes anything ( i saw people trying this on youtube), i've added progressively from 60 g up to 500g of oil, but no difference, maybe a bit more amplitude with 500g. I removed the oil.

    I had it balanced statically only, i could not find someone who can balance a motorbike wheel dynamically. I'm told that statically should be good enough, even for racing.

    I'm seriously running out of ideas here, any help would be immensely welcomed !

    vibration.jpg
    Last edited by Hugues; 13 September 2014 at 0901.
    Regards from Switzerland
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  2. #2
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    Your frame with its long curvy top and front tubes is not going to be as stiff and more prone to harmonic flexing as a frame with shorter, triangulated tubes. I don't know what the wall thicknesses of your tubes are, but I've seen Harley frames of a style similar to yours with tubes with 1/4" to 3/8"(6-10mm) wall thicknesses! This seems to what's required to have a stiff enough frame of this minimalist style, not to mention the huge weight penalty.

    Talk to your frame builder. Maybe they'll have some suggestions

  3. #3
    Senior Member Hugues's Avatar
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    Quote Originally Posted by Electro Flyers View Post
    Your frame with its long curvy top and front tubes is not going to be as stiff and more prone to harmonic flexing as a frame with shorter, triangulated tubes. I don't know what the wall thicknesses of your tubes are, but I've seen Harley frames of a style similar to yours with tubes with 1/4" to 3/8"(6-10mm) wall thicknesses! This seems to what's required to have a stiff enough frame of this minimalist style, not to mention the huge weight penalty.

    Talk to your frame builder. Maybe they'll have some suggestions
    Why not, but: what makes the frame oscillate in the first place ? remember i put the bike on the lift in the garage and spin the front wheel, then the fork starts to oscillate and the handlebar. The frame is a static part of the bike, something must make it oscillate. What is it ?
    Regards from Switzerland
    My 2.5 Upgrade Thread

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    Electric Warrior CaptainKlapton's Avatar
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    Quote Originally Posted by Hugues View Post
    Hi guys,
    I measured the alignment and roundness of the tire, rims and disc brakes, i cannot measure any difference when spinning the wheel.

    I also put the bike on the lift and managed to spin the wheel while recording with my phone, i can record the same vibration, the fork is also shaking up and down if i spin fast enough.
    vibration.jpg
    By roundness of the brake rotors do you mean trueness? aka wobble side to side.
    Only other thing I can think of is that something is loose inside the fork and is allowing undamped movement.
    "Never let schooling interfere with your education."
    -Mark Twain

  5. #5
    Senior Member Hugues's Avatar
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    Quote Originally Posted by CaptainKlapton View Post
    By roundness of the brake rotors do you mean trueness? aka wobble side to side.
    Only other thing I can think of is that something is loose inside the fork and is allowing undamped movement.
    yes, i meant trueness. I cannot measure any defects on discs, rims or tire. From side to side, or up and down.

    That gives me an idea, i should find a way to immobilize the fork (on the lift) and spin the wheel and record again, to see if it makes any difference.
    Regards from Switzerland
    My 2.5 Upgrade Thread

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    Senior Member EV_Scoot's Avatar
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    Captain mentioned what I was thinking, forks. How are the head stock bearings?

  7. #7
    Seņor Member podolefsky's Avatar
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    I agree, check head stock bearings. And that the forks aren't loose, either in the triples or the seals (seems unlikely though).

    Other things to check are the brake calipers and pads. It's possible the pads are catching and vibrating. Does the vibration go away if you touch the front brake lightly? Could mean they need more break in. Or could mean there is some incompatibility between your rotors and pads - unlikely, unless you have composite discs or some other exotic thing.

    You might also check that the forks are correctly installed so that the axle is straight. If the forks are off it can put uneven pressure on the bearings.
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  8. #8
    Senior Member robo's Avatar
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    Hi Hugues. If the head bearings are ok, I agree with you- look at the forks. Particularly, does the resonance change if you change the stiffness of the forks (and/or damping).

    If something periodic in the front wheel (even slight) is tuned close to the resonant frequency of the forks, changing the fork stiffness and/or damping would have a good chance of tuning those two frequencies farther away from each other.

    Your graph kind of looks like a beat frequency. A low frequency vibe like what you see could come from that (two frequencies just out of phase enough to cause that low beat frequency) . Otherwise, I can't think of much on a bike at that speed that would produce a frequency that low (maybe frame resaonant frequency?).

    I think you're smart not to push it until you find it. Adding more energy could make the resonance destructively worse.

    If the bearings are OK (in the front wheel too...later edit: I see you already checked wheel bearings), see if changing the resonant frequency of the forks changes anything (even if the frame frequency interacts with the forks, it should still have an effect if the forks are involved).

    Keep us posted on what you find. Sorry you have to work through this. These can be hard.
    Last edited by robo; 13 September 2014 at 2129.

  9. #9
    Seņor Member podolefsky's Avatar
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    Quote Originally Posted by robo View Post
    Hi Hugues. If the head bearings are ok, I agree with you- look at the forks. Particularly, does the resonance change if you change the stiffness of the forks (and/or damping).

    If something periodic in the front wheel (even slight) is tuned close to the resonant frequency of the forks, changing the fork stiffness and/or damping would have a good chance of tuning those two frequencies farther away from each other.

    Your graph kind of looks like a beat frequency. A low frequency vibe like what you see could come from that (two frequencies just out of phase enough to cause that low beat frequency) . Otherwise, I can't think of much on a bike at that speed that would produce a frequency that low (maybe frame resaonant frequency?).

    Interesting ideas. In order to excite a resonance, you usually have to be close to the resonant frequency. So you'd need to find something that is oscillating at about 0.75 Hz (1.3 s period). Since the wheel itself is about 9 Hz, it doesn't seem likely it is exciting the 0.75 Hz vibration. Not impossible though.

    To get a beat, you need two frequencies. The wheel is 9 Hz, so you need something else at about 8.25 or 9.75 Hz to create a 0.75 Hz beat. I'm not sure where that would come from, since the only thing turning is the wheel. If it was a beat, you would hear it change in frequency as the wheel changed speed. When the two frequencies got close to each other, the beating would go away. So if they were 9 Hz and 9.75 Hz, you would hear the beat at 70 km/h, and it would go away at 76 km/h.
    - Noah Podolefsky -
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  10. #10
    Senior Member Hugues's Avatar
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    thanks for the input guys, much appreciated,

    The only time i managed to reduce the amplitude of the vibration (to about 80% of original) was by setting the fork , playing with rebound and dampening. I set it as stiff as i could, but the vibration persisted, although with a reduced amplitude.

    The garage has pushed the bike to 120 km/h and he said the vibration disappears around 95 km/h. I haven't tried that myself.

    Pads and calipers are probably not to blame because i can reproduce the same vibration on the lift with calipers away.

    What are head stock bearings ? They are in the fork ? I have this fork, it's new:
    http://moto.marzocchi.com/en/front-f...11231&LN=en-US

    ABout the beat, i just have a thought, the perimeter of the front tire is 194 cm, and the rear tire is 212 cm , so a ratio of 0.915 . The rear tire is not completely true, about 2 mm difference from highest point to lowest point. But all my vibration measures show the strongest amplitude in front. Could this create the resonance (front wheel, rear wheel ) ? Although i'm able to reproduce a similar beat with the bike on the lift, spinning front wheel only, although i did not reproduce the same amplitude, because i cannot spin it close to 70 km/h.

    Today i noticed the balance of the rear wheel was not all that great, and the weight (about 50g) was all on one side of the wheel. So i removed them all and carefully balanced it on the bike, placing half the weight on each side of the rim (i've got a 12 inches rim). I managed to reduce the amplitude of the vibration, maybe 70% of original, with a slightly faster frequency of the beat. I will bring the rear wheel to another garage to have it balanced properly.
    Regards from Switzerland
    My 2.5 Upgrade Thread

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