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Drive Train Skipping Under Load

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Drive Train Skipping Under Load

Old 10-18-19, 01:41 PM
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SuperK
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Drive Train Skipping Under Load

I am at a loss here and any help would be much appreciated! I am trying to work on a ~2002 9 speed Ultegra 6500. I have replaced all cables, cable housing, and barrel adjusters. I have replaced the entire drive train (chain, chain rings, cassette, jockey wheels, and bottom bracket), re-aligned rear derailleur hanger, reset limit screws, and seem to have cable tension where it should be. On the surface every thing seems to work smooth if. I ease into things everything is really smooth the system can take any load. However if I start pushing the torque the chain starts to skip on the cassette. Is there anything else I should take a look at?

Thanks!

( !?!)
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Old 10-18-19, 02:42 PM
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I'm sure you thought of it, but I'll ask anyway: You're using a 9-speed chain?
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Old 10-18-19, 03:34 PM
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Yup, be sure your chain and cassette are compatible. Often skipping is blamed on "worn" cogs and/or chains, but often it's just a mismatch.

Usually it's safest to go with Shimano chains for Shimano cassettes, but it's often tempting to go with the lower priced and perfectly fine KMC and SRAM. But check the specs to be sure they're compatible. I use KMC on all my bikes now, but since they revamped their catalog and model designations it's tricky figuring out which chains are compatible with which freewheels/cassettes.

An example is trying to use anything other than a Shimano or compatible "narrow" chain on Shimano MegaRange freewheels. Non-narrow chains will climb over the smaller cogs under torque, even with brand new freewheel and chain -- been there, cussed that, followed a tip from an Amazon reviewer, problem solved.
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Old 10-18-19, 04:35 PM
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Above info is a strong culprit. But I'm just dropping this info "in case" someone else comes across this later with same symptom or if one of the above doesn't fix it. I assume the freewheel here is new, so one probably doesn't apply.

Two other common causes of skipping under load are bad freewheel (either internals or worn cogs allowing slippage) and incorrect chain length. Bad freewheel is easy enough to diagnose, just swap on compatible known-good wheels. Most often I've corrected this for friends, it's because of incorrect chain length, often too short.

Granted, there are some other specific cases. Like there's a certain Mavic rear derailleur which requires you change the routing of the cable for indexed vs. friction operation
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Old 10-18-19, 09:57 PM
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Check for proper chain wrap at the cassette. A B adjustment screw thats cranked in too far and is pulling the derailleur too far rearward can adversely affect the chain/cassette engagement.
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Old 10-18-19, 11:27 PM
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One more possibility is that one of the rear derailer's tensioning springs has broken, or similarly that the chain is too long, or that the tensioning capacity of the rear derailer has been exceeded.
There needs to be a certain amount of tension in the chain in other words.

Note also that the upper "B"-pivot bushing is anchored at just one end inside of the knuckle, and that the attachment is known to fail under impact loading, leaving the pivot with an enormous amount of wobble/play.

Make sure also that the pulley bolts are secured tightly and that both pulleys turn freely.

A tight link in the chain is yet another common cause of chain skipping.
And check that the special connecting pin is not protruding excessively where it might contact an adjacent larger cog.

Lastly, be sure that the cassette is being effectively compressed by the lockring such that no cogs are loose against each other. Turn the wheel backwards and note whether the cogs are all parallel to each other, sometimes a spacer issue prevents the cogs from stacking up parallel. Also check that the last cog has it's splines indexed correctly on the freehub splines.

Last edited by dddd; 10-18-19 at 11:38 PM.
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Old 10-19-19, 01:12 AM
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Could be dirty shift cable guides under the BB. If you apply high torque to the crank you increase BB flex. As it moves back and forth it can transfer motion to the cable, possibly pulling the derailleur out of position.
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Old 10-19-19, 06:44 AM
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I had a early 1980s Trek 700 (25.5" frame) with the very long stays and it was very susceptible to ghost shifting under load (standing on the pedals on a steep hill, with my Clydesdale sized body)

I attributed it to rear stay flex causing the shift cable to tighten and loosen.
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