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  1. #1
    Gear Combo Guru Chris_W's Avatar
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    Carbon timing belt - buzz / whir / vibration coming from ???

    We've got 3,000 km on our carbon belt drive, and it has recently now developed an odd problem. It first started on a 200 km mountainous ride a couple of weeks ago. We could both feel an odd vibration coming through the pedals when climbing, particularly when climbing at below 10 kph (6 mph). It came and went, and once actually became a barely audible buzz / whir. We couldn't localize the sound beyond the fact that it was coming from the drivetrain. Since it was a noise that we'd never heard coming from any other bike, we were reasonably sure that it was due to the belt drive. It went away again, and we didn't hear it for the last 80km of the ride.

    I've since broken down the bike and we've brought it on vacation to Canada. On our first ride of 30+ km, it again made no unusual noises, but half-way through the second ride of the same distance, the buzzing returned and was quite loud, even our riding companions could easily hear it. We got off of the bike and tightened both FSA cranksets, but they were not loose. The alignment of the belt did appear to be slightly off - a couple of milimeters to the inside of the flange on both rings, whereas it is normally right up against the flange. However, it was still not close to reaching the inside edge of the teeth on the rings, and it was not close to touching the left chain-stay or anything else on the bike. I re-aligned the belt to be flush with the outer flange again, but when we got back on the bike nothing had changed, the noise was still there and just as loud.

    Some experimenting showed that the loudness of the noise was directly related to the power that the captain was putting in, becoming rhythmic with each power down-stroke, and being silent if the captain was putting in less than about 100 watts. After that little climb was done, the noise and vibration suddenly went away for the next several kms, including a couple of other short climbs, then it came back for a couple km, then it went away again. It did not appear to be related to the alignment of the belt.

    There is nothing visual that I could see was causing it. The fact that this occurred after a complete strip-down to pack the bike into two suitcases and then re-assembly suggests that taking the belt, rings, and cranksets off and reinstalling them will also not fix the problem. Does anyone have any other ideas, please, or similar experiences to share? I'll email Gates and Co-motion about this as well to see if they have any suggestions.

    Further info:
    Bike - Co-motion Speedster co-pilot, size = 21/18 (28.5" rear top tube)
    Crankset - FSA Gossamer
    Last edited by Chris_W; 07-19-09 at 05:54 AM.

  2. #2
    hors category TandemGeek's Avatar
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    I'm gonna go out on a limb here and suggest that Co-Motion / Gates will be about the only one who can trouble-shoot that one. It's the intermittent nature that's got me baffled.

  3. #3
    PMK
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    Did you try flipping the belt direction? Not sure if they allow or even advise that, but it would be a simple test.

    PK
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    Quote Originally Posted by PMK View Post
    Did you try flipping the belt direction? Not sure if they allow or even advise that, but it would be a simple test.

    PK
    I was thinking that too. Are the belts directional and did it get reversed in reassembly some time?

  5. #5
    hors category TandemGeek's Avatar
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    Quote Originally Posted by PMK View Post
    Did you try flipping the belt direction? Not sure if they allow or even advise that, but it would be a simple test.
    Quote Originally Posted by dvs cycles View Post
    I was thinking that too. Are the belts directional and did it get reversed in reassembly some time?
    They are apparently directional; however, in talking with a dealer who has installed the belts any issue would become apparent during installation as described in Co-Motion's Gates Belt FAQs:

    If your belt is tracking properly, there should be zero to 1/2 millimeter gap between the belt and the outer guard on the sprocket. If you see a gap, this would indicate which direction to reset the lateral alignment of your crankset. You may want to try reversing the direction of the belt before making any changes- if you do so, remember to never derail the belt as you might derail a chain, always loosen the eccentric and slide the belt laterally off of the sprockets. There may be a slight difference in the cut of the belt when its driven direction is reversed, so we have found reversing belt direction to be effective in some cases. If you still see a gap, or the belt derails after reversing the belt, readjust your crankset's lateral alignment accordingly.
    The Gates FAQs also addresses belt noise, but it's the intermittent nature that seems to defy logic on the alignment issue:

    My drive is squeaking or making noise, what’s the cause, and how do I fix it?
    The belt can make noise when forced to run hard against the flanges on the sprockets. This can be due to sprocket misalignment, wheel misalignment, or run out in the drive caused by damaged components. Spraying water on the belt may quiet the drive for a few minutes, but the permanent solution is to correct the alignment or replace the damaged components. Another possible source of noise is a loose front sprocket. Using thread lock on the sprocket bolts alleviates this problem
    .

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    Just an outside of the box thought. Was there a lot of cross wind that could have been causing the belt to create a harmonic? I have not see the setup, nor have any experience with it, just wondering. I am a carpenter, and i have seen downspouts create a harmonic hum with wind. I have heard it on my tandem with the cables on rare occasions. Might account for the random nature of the issue?

  7. #7
    PMK
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    Quote Originally Posted by pablopsd View Post
    Just an outside of the box thought. Was there a lot of cross wind that could have been causing the belt to create a harmonic? I have not see the setup, nor have any experience with it, just wondering. I am a carpenter, and i have seen downspouts create a harmonic hum with wind. I have heard it on my tandem with the cables on rare occasions. Might account for the random nature of the issue?
    Very interesting thing to consider.

    One thing I often wondered about the belts in this application is that if when fabricating the frame, the alignment tolerance was within spec but the BB axis were not true. That belt would follow a near impossible to align path.

    I was informed a while back that a pulley with a slight crown in the center would track true unless extremely out of alignment.

    PK
    2006 Co-Motion Roadster, flat bars, discs and carbon fibre fork, size 22 / 19
    2006 Ventana ECDM full suspension mountain tandem
    Some single bikes and a couple of KTM's
    And most important, someone special that enjoys them with me (except the KTM's)

  8. #8
    hors category TandemGeek's Avatar
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    Quote Originally Posted by Chris_W View Post
    There is nothing visual that I could see was causing it.
    Just a post script / thought....

    Have you been keeping the belt tensioned tightly? Again, from the Co-Motion FAQs:

    The Carbon Drive's belt tension should be set tighter than you might be accustomed to running your chain. The concentricity of the sprockets allows for more consistent tension that most chainrings. With about 10 pounds of force, press with your finger against the center of the belt between sprockets- with the belt tensioned properly the belt should deflect no more than 1/2" (12.7mm).
    How about cleaning? Given the number of miles that you've put on the belt, how often do you wash the belt and pulley teeth to keep road crud from collecting?

    The reason I ask these things is that while cleaning up our single bikes I looked up at our older tandem and remembered that I needed to rotate the timing rings to extend their life as the last time we were riding I put on a new chain and was getting a little bit of chain suck coming off the front timing ring: it's somewhat more pronounced on the daVinci cranks or other tandems that use 34t timing rings or smaller. Anyway, when it gets like that you get a very strange vibration that occurs under heavy pedal loads by the captain as each timing ring tooth catches a chain link.

    Anyway, like I said... just something else that popped into my head that may warrant some investigation as I suspect Co-Motion and / or Gates may ask similar questions as part of troubleshooting your experience.

  9. #9
    St Louis Tandem Team
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    noise in belt

    I think it may be debris in belt....I have had same odd issue

  10. #10
    Gear Combo Guru Chris_W's Avatar
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    Thanks for all of the suggestions, guys. I'll give them a try before and during our ride today, and report back. It's definitely not wind noise, it is definitely mechanical. It could be dirt, so I will try cleaning it. I've been keeping an eye on the belt tension from the start, and I've now been wondering whether I have too much tension on it, so I might try a bit less and see whether that improves things (I don't think I'd be able to put more tension on it). If none of those things work, then I'll try flipping the belt.

  11. #11
    pan y agua merlinextraligh's Avatar
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    It doesn't sound like the same noise, but ours has developed a clicking noise. I'm rather concerned because the same noise developed before the first belt broke.
    You could fall off a cliff and die.
    You could get lost and die.
    You could hit a tree and die.
    OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.

  12. #12
    Gear Combo Guru Chris_W's Avatar
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    Before yesterday's ride, I did everything suggested here. I tried cleaning the belt and rings, but they already looked clean, and the rag I used didn't really accumulate any dirt. I then flipped the belt, and reinstalled it with slightly less tension than before. During our 150 km (90 mile) ride with 3,000 m (10,000 ft) of climbing, it remained completely quiet, and the problem seems to be gone, at least for now.

    I think that flipping the belt was the key thing. Previously, the belt had occasionally been drifting away from the flange of the belt rings by a couple of mm, and then going back again soon after to be flush with the flange. As I said before, the timing of this movement didn't seem to be related to the intermittent nature of the the vibration / buzz. Even so, I believe that the movement and noise may have both been caused / solved by rotating the belt to be in the other direction. Now I just need to keep an eye on which way the writing on the belt faces when re-installing it each time.

  13. #13
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    Quote Originally Posted by Chris_W View Post
    Now I just need to keep an eye on which way the writing on the belt faces when re-installing it each time.
    Which way is the writing facing? Nothing on the belt I have to indicate a direction preference/requirement ...

  14. #14
    Gear Combo Guru Chris_W's Avatar
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    Quote Originally Posted by mburchard View Post
    Which way is the writing facing? Nothing on the belt I have to indicate a direction preference/requirement ...
    It seems to work best for us with the bottom of the letters closest to the tandem. Sorry for the late response, I've been away for a while.

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