Shifting under load and skipping
 

I have a shifting problem.

The bike has a 9 speed drivetrain with low miles. The fd/rd/crankset are Ultegra 6500. It is a triple crankset. The rd cable is new. The shifter is a 7700 bar end.

When I have the bike in the middle ring, and I pedal hard (like standing up), the chain skips on to the small ring on the crankset. The problem has gotten worse over time, so that I can reproduce it fairly easily now. The chain does not skip when it is on the largest (outer) chainring.

I am trying to diagnose what the problem is. The list of potential causes is:

• Bent chain
• fd adjusted improperly
• bent chainring
• crud in the chainring
• loose chainring
• loose crankarm

In addition to these causes, what are the other ones that I need to look for?

The FD adjustment is most likely, especially since it started with the cable replacement.

When you stand and pedal hard the tops of the rings flex outward. So if the FD trim is a bit inboard, it might be OK on the stand or when pedlaing lightly, but will cause a shift when the rings move out (same as if the FD moved in). If you have an inline adjuster, trim the FD outboard a bit more and it should (hopefully) resolve.

You are going about things backward. Diagnosis is not a matter of multiple choice, unless you want to spend all day trying each possibility. You need to go through a logical process to narrow down possible causes before you go to checking them. First determine the exact condition under which it does or does not occur. Then look at the items that would be relevant under the conditions where the problem occurs and that don't apply when the problem does not occur.

BB shot, frame cracked.

Fork's bent.

While I agree with the comment about going about things backward (i.e., if you've thought about those things, why haven't they been eliminated yet?), I can see why you might want to gather all possibilities before trying to troubleshoot.

Here's another possibility-- there is not enough friction holding the shifter in place. I'm assuming the FD is friction shifting only. It's possible that in the large ring positions, the shifter has enough friction, but has a low friction spot where its position corresponds to being in the middle ring.

I actually had this problem with a Shimano 77xx series bar end shifter when mounted on a Paul thumbie mount. I had to torque the pivot bolt pretty hard to prevent the FD from slowly migrating toward the small ring.

I'd be interested in why you say that. If you were looking for a new road bike, would you assemble a list of all the possibilities first, or would you narrow your search by eliminating some items?

Some problems on a bike can have dozens of possible causes, and randomly trying each one is just wasteful. In fact I have seen people do just that and end up "fixing" a click by replacing pedals or a bottom bracket when it may have been caused by insufficient tightening.

Interesting that this same possible problem, low shift lever friction, surfaced in another thread recently. AS in that thread, TimmyT has a simple experiment to try to see if der adjustment or cable/lever holding are part of the problem. Remove the der and place the chain on the middle ring then test ride. Reinstall the der and leave the cable off but wind in the limit screw so the cage is clear of the chain while on the middle ring and again test ride.

I have seen worn chains and rings do this auto shifting thing when under power. Replacing both (and often the cassette too) has fixed it before. Do I have a pat explanation? No but it has worked. Andy.

what has helped me is to aim a GoPro camera at the front crank and play back in slo-mo to analyze what happens.

Many times, it's multiple factors that contribute:

- loose BB
- loose chainring bolts
- FD trim / cable-tension
- riding style

This last item was helped by having riders behind me show me videos of myself. Learning to push and pull at the same time out of the saddle allows me to keep the bike and my body perfectly upright. None of the power is wasted flexing the frame laterally and no rubbing of the chainrings or auto shifting.

Not exactly. At first, I thought the RD was skipping, so I swapped the RD, changed the cassette, and replaced the rear cable. As it turns out, it's the front that's the problem.

The process of elimination starts with finding the options, and then eliminating them.

BB is about a year old. The frame is in good condition.
AAAAAaaaaaaaaggghhh!!!! Why didn't I think of that?
I looked at this after you posted. I think it's OK. The front shifter is intact with no problems. The screw holding them on is tight.

I don't have a go pro, but that sounds like a good thing to do.

The bb seems OK. The chainring bolts are tight. FD trim is OK, but it's angle is less than perfect. Nevertheless, I did a lot of miles with the fd at that angle.

Right now, I'm leaning towards a worn middle ring. I was looking at it last night on the train, and it seems like the chain is not sitting right in the hollows of the ring. When I compared the middle ring to the small ring, it's clear that there is a lot of wear on the middle. I ordered a new ring. Hopefully, this does the trick.

Thanks for the help, guys! I appreciate it.

Of the options on your list, which ones have you eliminated?

You were given good advice early on, but you choose to reject or ignore it and go about this your own way. That's OK by me since I'm not nearly as invested in the problem as you are. So I wish you luck, and leave you with one last bit of advice you and take or reject.

Your problem is shifting off the chainring under load, so focus your attention to what happens at the top of the ring when under load. One simple diagnostic is to look down through the FD cage and see what's happening there.

• Bent chain: I looked along the chain as I rotated it through. I did not see a bent link.
• fd adjusted improperly: This is probably still true, but there is no slipping in the shifter as Southpawboston suggested. The fd adjustment hasn't changed. I will readjust this later.
• bent chainring: I looked and didn't see it.
• crud in the chainring: I cleaned the chainring.
• loose chainring: The bolts are tight. I checked them.
• loose crankarm: I put the hex key in the arm. It's snug.

Actually, I wound up back at Sheldon Brown's site after a web search. Worn chainrings often lead to skipping and the chain falling off the rings, particularly on the larger cogs when there is more lateral tension. This is something I had not considered. Further searching yielded more information at both mtbr forums and bikeforums. My problem is not unique.

No he wasn't. He was given advice by you and you're butthurt that you're looking foolish

Don't confuse good advice with the advice you give. Just because you googled it and regurgitated the results as a forum post doesn't mean its worth anything.

I wasn't referring only to my advice. The OP was given good advice by a number or people (read the posts 2-9) As I posted, I'm not invested in his problem, so I don't care whether he follows my advice or not.

But instead of criticizing others (me, if you prefer) about whom you know absolutely nothing, you might consider offering some positive advice of your own.

BTW- I like that part about googling and regurgitating.....You definitely have me down.

Yep. I know em when i see em.

My advice is simply not to take advice from self important internet heroes. Nobody in particular in mind.

No regurgitation!

http://img4.wikia.nocookie.net/__cb2...li_-_drunk.JPG

Number 1, You dont Have to shift under a Load, get ahead of the gear , then take the load off the chain and it will shift , easier..

See, FB, many, many years, of training and experience, just can't stand up to googled regurgitation.......................:rolleyes:

I guess I will have someone new to ignore...............;)

I'm still going with FBinNY's advice as the most likely. Some posts certainly contributed nothing. Nobody in particular in mind.

Fair enough. ITOH, I find George Carlin's advice to be more useful.

David, now that was funny!!!!!

No wonder FB replies more quickly and better than I would have, he just does a quick Google search and posts the first thing that shows up. I knew it :crash:

Not exactly, but close.

• The diagnostic process starts with looking first at symptoms and the conditions under which they occur, including any precursors.
• The next step is to look at the most likely known causes that are related to the particular symptoms and circumstances (NOT to all causes of similar symptoms).
• THEN one procedes to isolate and eliminate each of the pared-down list of causes, starting with the most likely and the easiest to check/resolve.
• If none of the "usual suspects" seem to be the problem one goes back to step one to hypothesize through observation what other causes there may be.

I'll take your initial list as an example, keeping in mind that one could probably create a much larger one for skipping under load.

• Bent chain - Does not happen on all chainwheels, would only occur when bad section of chain is at middle ring (less than every 2 crank rev's
• fd adjusted improperly - only if it's rubbing hard enough on the outer cage to shift down to the small chainwheel
• bent chainring - unlikely to cause a problem only under load and very easy to see/hear
• crud in the chainring - would not be a factor only under load
• loose chainring - Large and middle chainrings are secured by the same bolts
• loose crankarm - would not affect only the middle ring

Further, as it happens only on the middle ring it cannot be any component at the rear of the bike, as those would be factors on any chainring, and more so on the inner or outer - the middle is the easiest one for all rear cog combos to deal with.

I would take the process from the beginning but we don't have the info necessary to do so, including:

• How long "over time" did the problem progress - days, weeks, months?
• Any changes in riding habits or conditions, maintenance procedures, or any accidents/incidents prior to the problem arising?
• What rear cogs are engaged when the problem occurs?
• Does the rider spend a very large amount of riding time in the middle ring?
• What is the amount of chain wear?
• Are the middle chainwheel teeth very sharp or do they have a "shark-fin" profile?

In many cases a mechanic solves a problem through observation and logic, rather than having seen the exact problem and cause before. Those of us who were mechanics before the Internet age fixed bikes without forum, websites and often without repair manuals because we used the aforementioned "tools." Applying that do what we do know results in the following:

The problem happens only on the middle ring, therefore the most likely cause/contributor is a characteristic of that ring.
It happens only under high pressure, so another factor would be the chain, which transmits the pressure from chainwheel to the rear cogs. A third possible contribution is frame flex, which can change the chain angle, but that would seem unlikely - as noted above there are more extreme chain angles possible with the other two chainwheels. That leaves us with the chainrings and chain. Without having the full info noted above my first hypothesis would be that the chain is unshipping due to a combination of chain and middle chainwheel wear. A smaller profile on the chainwheel teeth make it harder for the chain to remain engaged when coming in at an angle, and chain wear would mean that the chain is also not engaging fully in the valley of the chainwheel. Solution would be to replace the middle ring and chain and most likely the cassette.


Of course I Googled the above....