Largest 2 rear cogs auto shift ONLY under load
 

2007 Specialized Roubaix 53/42/30 Shimano Ultegra chainwheels 12-25 Shimano 9 speed cassette SRAM chain - entire driveline has about 1000 miles on it and was all replaced at the same time.

The bike has about 8000 miles on the frame.

Ultegra RD6500 rear derailleur - The problem started about 300 miles ago when I replaced the rear derailleur cable and housing.

I've been doing my own maintenance for 40 years and many bikes but can't seem to figure out this problem

Rear gears shift smoothly and effortlessly - everything works perfectly while spinning

When climbing (only climbing) under load in either the 23 or 25 tooth cog the chain wants to skip back and forth between the two. It does not matter which gear I start in the chain partially shifts to the other gear then skips back.

The chain is not skipping on the selected cog - I can see it moving from cog to cog.

If I spin while climbing the problem does not occur - only if I put more than several hundred watts into the pedals does the skipping occur.

The problem occurs with any chain wheel.

The problem does not seem to occur if I stand and stomp on level ground but I can't do that for very long in such a low gear.

I have fiddled endlessly with the rear derailleur but can't find the correct adjustment.



What am I doing wrong?

Sounds like the "low" limit screw is too tight (restricting). Just swapped components from one frame to another. Rear der was fine but bobbled in the high gear. Good tension, nice shifts but only in the high (small cog). Released limit screw a bit and works great now.

you might want to look at problems involving torque and the rear triangle.

loose wheel, loose bearings, even loose bb or chainring, broken frame or spindle, bent cog or frame...

in effect anything that can change the angle between the rear cogs and the crank under load.

and the fact that you can't get it to happen on the flat is not too surprising. as you mentioned it is difficult to create enough torque under those conditions. too much mechanical advantage and too little resistance.

A bit of a long shot, but my bike did exactly the same last week. The next ride the the rear derailleur cable broke right at the shifter lever. Turns out the cable was frayed internally, preventing the shifter from pulling the cable all the way to precisely shift the largest three rear cogs.

this sounds very possible, or something similar, especially since the OP stated that he had just changed the cable and housing...

"this sounds very possible, or something similar, especially since the OP stated that he had just changed the cable and housing... "
or the housing was not properly prepared when it was replaced.
Sailor: Did you replace the housing loop which enters the rear derailleur, and are all the housing runs properly sized and the ends prepped with the correct ferrules installed? It is shifter housing and NOT brake housing, right?

Answers to you helpful questions

- I've tried moving the low limit screw but can't seem to find the correct setting. When shifted to either the 23 or 25 tooth gear - the jockey pulley looks to be in perfect vertical alignment. i've followed the Shimano recommended procedure

- I did not replace the short cable housing that enters the RD - the new cable moved smoothly thru it and both ends of it were still clean. Is there something more I could examine there?

- the torque and changing angles seems to me to be the most likely culprit but I don't know what more to examine and can't figure out how to duplicate the problem except on very steep roads. I check the crank bolt torque on a regular basis but will check the BB tightness.

- I'm not sure what more to do to prep the housing ends other than have clean cuts. I used Clark shifter cables and used the ferrules they supplied - what more can I examine in that area?

Thanks for the ideas

Sorry, I can't be too specific here because I'm no mechanic, I took my bike into the shop when the cable broke. the mechnic there is very experienced, he works with our (New Zealand) Pure Black pro team. he says that design of the Shimano brake/shifter lever is a problem because the inner cable does a sharp bend where it attaches internally (sorry, can't remeber the correct wording he used) to the shifter mechanism. Cables tend to break there, and if it's slightly frayed or not well lubricated it can cause problems with the gear shifts. Hope this helps, sorry for my lack of correct terminology. Good luck with the problem.

When it auto shifts, does the rear wheel wobble in time with the shift? If so, your axle might be either loose or broken.

Can we ascertain that the hanger is actually straight, secure, wheel is seated properly in dropouts BEFORE tinkering with any limit screws?

Just fixed a bike with a similar problem for a friend. Straightened the der. hanger and readjusted the cable. Works now.

More answers
 

The low limit stop MAY have been a little inside (between the 25 & 23 tooth) and the B screw MAY have had the guide pulley a touch too close to the 25 tooth cog.

Everything works great riding around the parking lot of the marina where I live.

I'll have to ride a ways to find a steep hill to try the sytsem under sustained load.

QUESTION:
The shimano adjustment manual/guide says: "The end of the outer casing which has the
aluminum cap should be at the derailleur side." but the cable guide that came with the new bike has plastic caps on both ends. Is that important?

I was waiting for the "B screw" tip to come up. That's the only other thing I could think of. Seems like you've addressed that.

Dumb question knowing that your rear Der is pretty new that I think this may have been addressed above but I'll ask anyhow: Any side-to-side play in the rear derailleur arm?

I think I have your same cog (On a Roubaix no less!) and I think each cog only adds one tooth until you get up to the last couple where they jump by 2 teeth. Could the fact that 23 and 25 have a larger difference in their radii than the smaller cogs have anything to do with this happening there, and in no other gears?

Also to clarify, you mentioned it happens all chain rings, but could it occur more in one than another, or are we looking at the same frequency all around?

I only use the 23/25 cogs with the 30 or 42T chainwheel and the problem was the same in both cases.

I rode a bit on level ground yesterday and everything worked perfectly. I worked the 42/23 & 42/25 pretty hard under load (front brake dragging - standing and pedalling hard) and had no problems.

Today we are under "hazardous weather alerts" so I probably won't ride - we might get 1/4" inch of rain, the sun won't shine, the high temp will only be 61, and the wind might get up to 15 mph - in San Diego that is a bad winter day and everyone stays home in front of the fireplace.