Question for the mechanics

stien · #1 03-26-2017, 03:34 PM

I'm typically a decent wrench but I'm stumped on this one and was thinking you guys might be able to help.

The wife got a new Cervelo R3 frameset (warranty) which I built with her previously used 6870 group. The only change being a wheels mfg BBright Shimano BB.

She rode it a few times this past week and raced a crit on it yesterday without issue but today while climbing I saw her RD twitching so we pulled off and checked it out.

It looks like the chain is at such an extreme angle in the small ring and two largest cogs that it's actually jumping to the outside of the lower jockey wheel and staying there until she backpedals. Rubbing the cage. This is how I killed an older 10s di2 RD so I'm concerned.

I've verified that the crank is completely flush with the BB on the drive side. It's a brand new chain. The group has 3k miles on it so the jockey wheels still look new. I might be able to space the cassette to the drive side slightly but shouldn't have to, you know? No crashes before or since.

Any assistance appreciated.

Cicli · #2 03-26-2017, 03:41 PM

Did you check the hanger?
They are almost always off, even on a new frame.

Check the hanger.

Tandem Rider · #3 03-26-2017, 04:17 PM

Have you done a quick check on frame alignment? Stick some tip tools or something like a rod in the rear, eyeball left side of head tube, left side of seat tube, to a spot on the tools, repeat for the other side. You can get within a mm or so this way. I had a frame warrantied because alignment was so far off (18 mm) it shifted badly and threw the chain frequently.

dustyrider · #4 03-26-2017, 04:58 PM

Sure it's not a limit screw adjustment? Checked hanger? Can you duplicate in stand?

Bruce K · #5 03-26-2017, 05:15 PM

My in house mechanic says BBrights are asymmetric

Is it possible you installed it backwards? (Offset to the drive side)

This could account for the weird chain line

BK

stien · #6 03-27-2017, 05:23 AM

I have not checked the hanger, thanks guys. I can duplicate on the stand and it's not the limit screw because it happens in the second largest cog as well.

The shop who handled the warranty put in the BB. I'll have to look up how it goes in. It's flush to the frame on the NDS and has a bunch sticking out on the DS.

oldpotatoe · #7 03-27-2017, 05:31 AM

Quote:

Originally Posted by stien

I'm typically a decent wrench but I'm stumped on this one and was thinking you guys might be able to help.

The wife got a new Cervelo R3 frameset (warranty) which I built with her previously used 6870 group. The only change being a wheels mfg BBright Shimano BB.

She rode it a few times this past week and raced a crit on it yesterday without issue but today while climbing I saw her RD twitching so we pulled off and checked it out.

It looks like the chain is at such an extreme angle in the small ring and two largest cogs that it's actually jumping to the outside of the lower jockey wheel and staying there until she backpedals. Rubbing the cage. This is how I killed an older 10s di2 RD so I'm concerned.

I've verified that the crank is completely flush with the BB on the drive side. It's a brand new chain. The group has 3k miles on it so the jockey wheels still look new. I might be able to space the cassette to the drive side slightly but shouldn't have to, you know? No crashes before or since.

Any assistance appreciated.

I'd say the hanger or the der cage is bent.

tumbler · #8 03-27-2017, 09:36 AM

I would check the RD hanger. I had the same issue on one of my bikes. It's surprisingly easy for those soft aluminum hangers to get knocked out of alignment.

Mikej · #9 03-27-2017, 09:41 AM

Does it use a directional chain?

nash · #10 03-28-2017, 10:04 AM

Take a closer look at the chainline. Easy to do, in a broad way, by putting the chain in the middle of the cassette, seeing how the chain lines up in either chainring. Or put in in big/small and see how that looks. Is the chain straight in that position?

stien · #11 03-28-2017, 11:04 AM

During the ride I had mentioned that the cage (the whole RD) looked twisted in. It hasn't been crashed since I swapped the parts so I'm assuming hanger is twisted a bit. Hoping to bring to the shop tomorrow as it's one of the only tools I don't own yet.

Directional chain is a good idea but I've got it going the right way.

Pretty sure the chain does straighten out once in the middle of the range. I'll need to put it in the stand to verify tonight.

ontarget · #12 03-28-2017, 12:24 PM

Funny you mentioned that a derailleur hanger alignment tool is one of the few you don't have. After stopping by my LBS a few times for hanger checking/alignment, I threw down the money for my own tool. Am I glad I did! The convenience of having my own has been great. I do more checking than aligning. You won't regret it!

Sent from my XT1585 using Tapatalk

OperaLover · #13 03-28-2017, 12:27 PM

[QUOTE=ontarget;2149218]Funny you mentioned that a derailleur hanger alignment tool is one of the few you don't have. After stopping by my LBS a few times for hanger checking/alignment, I threw down the money for my own tool. Am I glad I did! The convenience of having my own has been great. I do more checking than aligning. You won't regret it!

Agreed! Money well spent. A former pro mechanic recommended it as an indispensible tool. Park Tool works great.

Clancy · #14 03-28-2017, 04:23 PM

First, double check the B limit screw. Saving it happens with both top cogs does not elimate the B limit screw from being the cause. Seeing the derailleur "twitch" is a pretty good sign that the guide pulley is hitting a cog.

If B is OK, double check chain line.

The B limit screw and chain line are easy to check, takes a minute.

If those two are both ok, then derailleur hanger. And with a good eye, it's possible to sight and see if its bent, usually noticeable. Very slightly bent, maybe not.

Usually with a bent hanger the chains jumps from one cog to the next, ghost shifting, or hunting for engagement. But I've seen bent hangers cause weird things.

chiasticon · #15 03-29-2017, 07:23 AM

Quote:

Originally Posted by ontarget

Funny you mentioned that a derailleur hanger alignment tool is one of the few you don't have. After stopping by my LBS a few times for hanger checking/alignment, I threw down the money for my own tool. Am I glad I did! The convenience of having my own has been great. I do more checking than aligning. You won't regret it!

agreed there. it's a must-have with modern drive trains. eleven speed especially seems very prone to being just a little noisy in one or two cogs if the alignment isn't perfect.

Quote:

Originally Posted by Clancy

Usually with a bent hanger the chains jumps from one cog to the next, ghost shifting, or hunting for engagement. But I've seen bent hangers cause weird things.

agreed there, but isn't "hunting for engagement" kinda what he's talking about, that it's doing? he said "twitching" and that's what I pictured. anyway, not sure what di2 does when the hanger's out, but I could imagine "twitching" being the thing.