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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 05-12-06, 06:37 PM   #1
legstwelve
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chain drop when pedal to the metal

I've recently encountered a problem when hammering it up hills. I have a compact crank and 12-25 out back. When hammer and tong in the 34T and somewhere near the top of the cassette it appears as if the chain is dropping to the next lowest chainring of the cassette. This is not a very pleasant feeling as it is a great unexpected jolt which usually results in my shoes becoming unclipped and me almost getting knackered. Everything shifts fine when on the flats so i don't think it has anything to do with the derailieur. Could it be the angle the chain is on or something else?
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Old 05-12-06, 07:20 PM   #2
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Sounds like a RD adjustment issue.
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Old 05-12-06, 09:24 PM   #3
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could be flexing of the chainstay. symptoms sound right
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Old 05-15-06, 02:30 AM   #4
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so how do i fix it? It's a CAAD5 frame so its pretty stiff.
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Old 05-15-06, 03:20 AM   #5
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cable-friction...
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Old 05-15-06, 06:56 AM   #6
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Quote:
Originally Posted by legstwelve
so how do i fix it? It's a CAAD5 frame so its pretty stiff.
If the frame is flexing enough to cause an "autoshift", then you may have the right barrel adjuster a little too tight. Give it a little turn in the direction that effectively shortens the inner wire's length. This will cause the inner wire to be less tense. Keep track of how many quarter-turn adjustments you made.

Initially, you could test the adjustment while the bike is mounted on a cycle trainer with the roller locked out or on a mechanic's stand. If it shifts well through the entire cog (i.e., the rear cassette's gears) range, then test it on the road.

If this is not the problem, then the inner wire may be getting hung up somewhere. Look for broken strands within the working range of the exposed inner wire, especially under the bottom bracket or just after or before where the inner wire is protected by cable sleeving. Don't forget to give the wire in the shifter a look as well. If you don't see anything then it may be something hidden by the cable sleeving. Either way, if the wire is fraying, best to get it replaced.

You may be able to see more of the wire if you shift to the highest gear on the cassette, then use your hand to pull the wire where it's exposed anywhere before the bottom bracket.

Make sure that if you only intend to replace the inner wire (you usually do not have to replace the entire cable set) that the cable sleeving is given a good douching with teflon spray lube before you run the inner wire.

If no fraying is evident, then try some teflon spray at the bottom bracket guides and just before the inner wire runs into cable sleeving.

If none of this works, then perhaps it may be a worn drivetrain. I'd start by replacing the cassette and chain at the same time. Lastly, you could replace the chain ring.

Hope this gives you an idea of what to look at.
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Old 05-15-06, 07:46 AM   #7
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I'm with everyone else here, but it's also possible that your cassette is worn - chain skip on a worn cassette is noticeable mainly when hammering. Have you recently replaced your chain? Were you running a worn chain for awhile? If so, it's possible that the cog's teeth are somewhat worn out. But unlikely, because that's not the gear you'd use most often (and thus wear out the most when riding with a worn chain) anyway.
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Old 05-15-06, 08:30 AM   #8
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Quote:
Originally Posted by legstwelve
I've recently encountered a problem when hammering it up hills. I have a compact crank and 12-25 out back. When hammer and tong in the 34T and somewhere near the top of the cassette it appears as if the chain is dropping to the next lowest chainring of the cassette. This is not a very pleasant feeling as it is a great unexpected jolt which usually results in my shoes becoming unclipped and me almost getting knackered. Everything shifts fine when on the flats so i don't think it has anything to do with the derailieur. Could it be the angle the chain is on or something else?
my cannondale 3.0 did the same thing because it had a bent derailleur hanger. if i adjusted too far one way the derailleur cage would twang the spokes on very steep climbs and the other way would make the bike autoshift.

i put a thin spacer on the hub before the cassette to give me some space which corrected the problem but adjustment was still very critical.

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Old 05-18-06, 04:23 AM   #9
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I recently had a FSA compact crankset installed which is less than a month old and the whole bike is less than 6months old so i think the chain and cassette should still be OK, i had a powerlink installed with the crankset. I haven't checked the chain for stretch but probably should.
I didn't notice this problem before the new crankset so it could just need recalibrating.

Thanks for the detailed istructions NoRacer. I just got Zinn so i'll do some general maintenance. No bike stand or roller, i'll just pop it upside down and spin the cranks and hope for the best. Hopefully a quick turn of the barrel adjuster will do wonders.

I don't have any teflon spray, should i apply lube or grease to the inner wire near the bottom bracket?
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Old 05-18-06, 06:01 AM   #10
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Quote:
Originally Posted by legstwelve
I don't have any teflon spray, should i apply lube or grease to the inner wire near the bottom bracket?
IMHO, a wax-based lube might be better due to it's "shedding" property. Petroleum-based products may cause an accumulation of debris in the cable guides.
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Old 05-18-06, 06:07 AM   #11
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When Happens to me, its allways been the rear derailer cable.
It needs to be tightened, I Just turn the adjuster knob by the shifter and it goes away
(SRAM X.0 shifters/derailer)
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Old 05-19-06, 06:23 AM   #12
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I have wax lube so i'll give that a go. Turned the adjuster knob on my ride today and the problem seems to have gone away. Wow who would have thought it would be so easy. I'll give the bike a proper going over this weekend.
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