Rear derailleur wants to shift under high load!?!?

[Pierre]

Hi gang,
This has been bugging me all summer. I thought I had fixed it a few times but the symptoms are back!

Here's what's happening: under normal climbing conditions, if I shift to my largest cog (28) all is well, no creaking, smooth as silk drive-train. Even if I stand up, all is good (as long as I'm not really givin' 'er).

If I stand up and exert some real power on steeper climbs, though, my chain will sometimes almost shifts into the next cog down. In fact on two occasions it did - but that was much earlier in the summer before I changed the rear shifter cable as a possible solution. More often, (again under really high power) the chain will make noise like it almost going to shift up a gear but doesn't.

I've tried changing the derailleur cable, tightening the drop-out/derailleur clamp, tightening the derailleur itself, tightening my chain-ring bolts, verifying that the RD is nice and straight...the problem seemed to get better after I changed the shifter cable (it has not actually upshifted since then).

I sometimes wonder if I'm getting so strong that I'm twisting the frame but heck, I'm not a power guy (FTP = 237, for instance). The frame is a year old Look 675 so I'm inclined to think that I've got a ways to go before I'm really twisting the rear triangle with enough strength to cause ring/cassette misalignment...
Any thoughts?

[Lewis Moon]

What shifters are you using? For Campy 10sp, that was the notice your drive train gave you that it was time to rebuild a shifter.

[Tandem Rider]

I had a similar problem with a bike, turned out that the frame was badly misaligned. The rear wheel centered fine, but somehow the cassette was almost a centimeter to the left of where it should have been. Carbon fiber meant no realignment, so they gave me a new frame, all ok now.

[eddief]

drive systems act weird sometimes with stretched out chains.

[cmbicycles]

I assume the limit screws are set right, if not the chain doesn't have quite enough room to settle on the top cog it could cause this issue. How worn, or how many miles on the chain and cassette?

[batman1425]

Check your BB cable guide. Road gunk and electrolyte residue like to build up in there and your cable won't slide freely. When you start laying down the power, BB flexes a bit and drags the cable with it - result is ghost shifting.

Would make sense that you saw an improvement with replacing the cable. If that doesn't fix it, I would look for cracks in around the BB shell or in the rear triangle somewhere that would allow enough frame flex to move cable guides far enough to cause ghost shifts.

[Pierre]

Thanks for the replies, guys. Chain is brand new, cassette is on 2nd chain (cassette was replaced in the spring).

The set-up is 7800 shifters with a 7900 drivetrain.

[jruhlen1980]

I had a similar issue on a couple steel frames. Then I lost 20 pounds and it hasn't happened again.

Nevermind, just re-read the op and saw what kind of bike you have.

[Pierre]

No, carbon. I'm at 160lbs now at 5'11" and previously rode steel without issue.

[mgm777]

Possible causes(based on my own experiences):
- Rear derailleur needs adjustment
- Bent derailleur hanger
- Cassette needs replacement(i.e. bad cog)

Hope this helps.

[benb]

You're probably not too far off power wise in terms of being able to create some of these issues on a frame but most of these modern carbon frames are super stiff and are very difficult to bend enough to cause these kinds of issues. I have about 40w and 5-10lbs on you at different times and have been able to flex quite a few frames of all of the different materials enough to hear things rubbing and get occasional odd behavior. Just cause your FTP is 237 doesn't mean you don't go above that. For all we know you can do 600w for one minute.. that will get things flexing.

Seems like you've already got plenty of things to try. I would go for checking chain wear, cassette wear, limit screws, and stuff like that next.

You didn't mention if you changed the housing, I'd have definitely done that with the cable change.

[Pierre]

thanks, Ben. Yep - housing changed as well.

[weisan]

Has this ever been a problem the whole time that you own this bike?

Or did it start doing that at some point?

[alexstar]

Sounds like a worn cassette that doesn't want to play nice with a new chain. I would swap it out and see what happens.

[lhuerta]

Possible causes(based on my own experiences):
- Rear derailleur needs adjustment
- Bent derailleur hanger (must be done with actual tool on 4-axis....no eyeballing)
- Cassette needs replacement(i.e. bad cog)

Hope this helps.

...adding to (and modifying) list above.

•*chain length (likely too short)
•*rear der cable loop (likely to short, where small amount frame flex is being transmitted in to cable pull)
•*stiff link or bent tooth on cog

[oldpotatoe]

I'd check der hanger alignment and upper limit screw(too tight)...

[cnighbor1]

If indexed you need the matching indexed rear derailleur
Upper pulley floats side to side to help keep chain aligned
check if it moves side to side Might have wrong pulley installed
I had same problem on a bicycle where I was trying to use a set of non indexed shifter Under load frame would flex just a very little bit and would shift down went to indexed shifter and no more problem

[thunderworks]

It could also be as simple as a derailleur cable that needs a touch more tension. Try backing out the barrel adjuster a tiny bit. Make sure it still shifts smoothly down to smaller cogs though.

[benb]

I'd check der hanger alignment and upper limit screw(too tight)...

That!

[Pierre]

keep the ideas comin'!
I did try putting in a longer loop of housing from the chainstay to the derailleur. I think this may have helped (that was a while back).

Yes, it has been doing this since I got the bike last year.

I took a good hard look at the limit screw set-up yesterday and tweaked the "L" screw to give the derailleur a bit more runway on the largest cog. I also re-checked the B screw set-up. Seems fine.

I'm curious about the short chain length idea. I will read up on that to make sure that I validate the length and adjust accordingly when the next chain goes on in the spring.