Intermittent rattle when climbing

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mogwaiboi
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by mogwaiboi

My OCD is going into overdrive trying to pinpoint this intermittent rattle on my Cervelo :shock:. It sounds very similar to the rattle when the rear cable tension is out of whack, but it only happens occasionally under load whilst climbing in the small chain ring and lower gears. It's very noticeable when it does occur. My first thought was maybe the chain is knackered, so I changed that but no difference. I checked the cassette torque and that's fine (the cassette is only about 2 months old). My HED Ardennes + wheels are only around 2 months old as well - not sure if the rear wheel is making the noise somehow...help!
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rms13
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by rms13

Maybe cables rattling inside the frame?

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mogwaiboi
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by mogwaiboi

Good point, but it's an older Cervelo with external cabling.
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MJB
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by MJB

If it's only when climbing is it by any chance when you are maxed out on the last cog/s. If so then it may be frame/wheel flex rubbing occasionally on the derailleur cage. I managed to do this with a pair of low spoke count wheels i once had, every time I squeezed out some extra power in my cranking if my bodyweight was just so the frame/wheel flexed just enough to create a slight (but initially mysterious) rattle.

The spoke alignment had bugger all dish remaining so I fixed the problem with a change of wheels :D

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mogwaiboi
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by mogwaiboi

No, unfortunately it happens 3/4 of the way up the cassette.
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MJB
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by MJB

Does it rattle for a full pedal stroke, half a stroke or multiple crank rotations?

Had the bike on the stand to check for frame cracking?

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mogwaiboi
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by mogwaiboi

It's definitely multiple crank rotations. I haven't specifically had it in the stand to look for cracks but I will do, thanks.
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uraqt
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by uraqt

Did you check the easy stuff? saddle bag, cable hitting the frame, quick release, stuff in jersey?

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mogwaiboi
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by mogwaiboi

Saddle bag is super tight so I don't think that would rattle. If you mean front der. cable that is OK as it hasn't changed position in a long time. Yes, I've played around with the quick release positions and no luck. The reason I think it may be cassette/chain/wheel related is because they are the only things that have changed on the bike recently...but of course you're right, it could be something else. I might get my mechanic to give my new wheels the once over to make sure that they are all good.
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mogwaiboi
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by mogwaiboi

From what I can tell by pedalling slowly and sighting on the work stand, the chain isn't aligning with the cassette properly in a few of the middle gears, and making a noise. The cable tension doesn't seem to eliminate it either. Dodgy cassette maybe? Hanger alignment?
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numberSix
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by numberSix

IME this shifting error is due to the lower limit screw. In each chainring, retest smallestCog to smallest+1 and back again. when on the smallestCog the Upper pulley centerline s/b 1-2mm to the right of the cog c/l. When this plus the cable start tension is right, the middle of the cassette will shift correctly.

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by oreoboreo

here is a odd idea but it was my problem and drove me nutts. If you happen to have extra pedal spacers to help with fit your pedals may not be screwed in enough and there may be movement. I solved my issue by using the park green grease on the threads of the pedals like normal but added it to the front and back of each washer. When you are looking you have to look at everything...

the group here has seen everything so good luck

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AJS914
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by AJS914

Another thing to check is the tightness of the cassette.

wingguy
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by wingguy

numberSix wrote:IME this shifting error is due to the lower limit screw. In each chainring, retest smallestCog to smallest+1 and back again. when on the smallestCog the Upper pulley centerline s/b 1-2mm to the right of the cog c/l. When this plus the cable start tension is right, the middle of the cassette will shift correctly.


I gotta disagree with you there - Hi and Lo limit screws shouldn't have any effect on how the mech shifts on the rest of the cassette. Only cable tension, hanger alignment and B-screw.

Worth checking the cassette too though, I did once see a brand new Ultegra 6800 cassette with a single bent tooth causing a rattle, and a couple of SRAM pulley wheels with the same thing.

greentimgreen
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by greentimgreen

Three trips to different LBS's and I found out my random knocking was my valves rattling against my carbon rims on each pedal stroke. Embarrassing, but at least it wasn't just me...
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