Derailleur adjustment with compact crank

[lunacycles]

Chain wrap length has to do with the size of the large ring, the size of the large cog, and the length of the chainstay. It needs to be long enough to reach from the large ring to the large cog (along the length of the chainstay) without pulling the rear derailleur too far forward.

I can tell you that with a 34T inner ring, a 25T large cog and a smaller, compact race geometry rear triangle, that being one link too long will cause the rear derailleur pulleys to rub the rear cog when in that combo even with the B-screw tightened all the way (how my newest bike was built). There's nothing wrong with the derailleur hangar. I don't have this problem on a bike with longer chainstays or a bike with a 36T inner ring. With a compact, you still need a lot of chain wrap to get into the larger cogs while in the big ring, but then there's even more chain slack when in a 34T ring compared to standard cranksets. Of course being one link too long also causes a little bit of rub on the bottom part of the pulleys sometimes, but this isn't a problem functionally. Not like having the derailleur body bumping against the cog. I used a chain length calculator and verified that the chain was one link too long. Removing it still requires the B screw to be all the way tight, but there's no more bumping. There is more noise in the big/big combo, and I'm sure there's rougher tension on the chain when in the big ring by looking at the RD pulley angles, but it was necessary.

Howdy aicabsolut, thanks for your reply and for the opportunity to respond again.
Over 20 years of wrenching I have experienced my share of b tension screw problems (i.e., derailleur guide pulley bumps against largest rear cog in small/large combos), but in my experience I haven't found that removing a chain link is the best answer, particularly if it creates a situation in which the chain is potentially too short and can potentially cause permanent damage to the components or frame. I hear you had this problem on your current bike and you solved it by removing a link. That's impressive troubleshooting. But to say this is a general tendency on bikes with short chainstays and a compact crank is a bit misleading/generalizing, imo, particularly in recommending that one should consider running a chain that may be too short in an effort to solve it (per your post previous to this one). Given the bikes I set up (frequently with the drivetrain set up you describe) I personally believe your situation is more the exception than the rule.

As a manufacturer, I believe what is the more likely culprit in your case is that your derailleur hanger is either not correctly aligned in the fore/aft sense or your hanger is too short for your app. That is: the threaded portion of the hanger needs to be the correct distance behind the dropout or shifting issues like this can arise. And/or, if the vertical distance between the dropout and derailleur mounting hole is not long enough, you can have the problem of the derailleur pulley being too close to the cog when running a larger rear cog, regardless of b-tension screw adjustment (and one reason I tend to put longer derailleur hangers on the majority of my frames). A lot of road frames come with short hangers, and maybe the one on your frame just isn't designed for a 34/28 (although that would seem odd given the current road offerings). Anyway, big old thread drift putting readers to sleep is going on here, but I did want to clarify my explanation a bit.

[aicabsolut]

Ok, I understand what you mean. I used several gear calculators (math works a bit better for me to understand things), like the one you can find on the ParkTools site, and they all told me that my chain was put on one link too long. Taking it to the shop, the guys weren't crazy about removing the link, but it still seems safe in terms of not pulling the RD pulleys too taught. It's still not ideal such that I don't think I can run larger than a 25T cog on the rear with this bike. They said I just didn't luck out with the components I chose on this frame.

Anyway, I just wanted to point out that there can be some problems caused by erring on the side of a chain that is too long. Yes, it is can be catastrophic to have a chain that is definitely too short. I guess I am just advocating making sure the chain is properly sized for the individual setup.

[HillSlugger]

After Wednesday I backed out the limit screw a quarter turn; seems to be just enough to do the trick. It was all good today!

Thanks for all your help!

[amartha]

Having just converted a triple to a compact crank on one of my 5 bikes, I have learned a lot.

I needed a 165 mm 9-speed crank for my short legs. The only model available was the Shimano Sora. I liked it -- much lighter than the Truvative triple.

FSA 9-speed compact cranks only come in 170mm lengths which would cause me too much toe overlap with the front wheel.

The compact crank works with a double front derailleur and triple rear derailleur. I replaced a Shimano Tiagra triple front derailleur with a Shimano 105 double front derailleur.

The compact crank must mate with its bottom bracket. I replaced a generic Truvativ triple bottom bracket with a Shimano hollowtech.

The Shimano 105 triple rear derailleur works perfectly!

Depending upon the other components -- 10 speed or 9-speed rear cassettes and shifters, the compact crank doesn't need Ultegra level, especially if the rest of the compenents in the drive train are less than Ultegra-level in quality.

[aicabsolut]

Why are you using a long cage rear derailleur?

[Cataboo]

maybe 'cause she had it from the triple that she was converting to a double?

[HillSlugger]

My FD has been well behaved on my last two rides but yesterday the RD decided that it wanted to skip past one of the cogs towards the middle of the set. Everything else seemed OK. What would cause this all of a sudden and what do I do to fix it?

[aicabsolut]

Did it do it repeatedly?

Sometimes, we just get the "throw" wrong of the lever. Other than excessive wear to the cassette or chain, that's a fairly good guess when you have skipping problems on one cog and towards the middle. Did it matter which side you were shifting from? (Did it skip only when shifting easier or harder into that cog?)

[HillSlugger]

Did it do it repeatedly?

Sometimes, we just get the "throw" wrong of the lever. Other than excessive wear to the cassette or chain, that's a fairly good guess when you have skipping problems on one cog and towards the middle. Did it matter which side you were shifting from? (Did it skip only when shifting easier or harder into that cog?)

Yes, repeatedly. Yes, in both directions. The chain, cogs, and cranks are all new, with less than 250 miles on them.

[aicabsolut]

Only in the middle on one cog? You might want to check that the cassette is still on there tightly.

Hmm... the only time I had a similar experience it turned out (weirdly) that the rear cable was installed too tightly. I thought maybe tightening it would help because it was pretty new, so when that didn't work, I loosened it and the problem was solved. I'm not sure that's typical, though.

[HillSlugger]

Yeesh, I just realized that I rode this week on my race wheels, the first time in a while. I guess the cogs aren't lining up quite the same as the other wheel?

[aicabsolut]

Yeah, the spacing can be different between wheels meaning you might need some derailleur adjustments to make shifting optimal when switching. Are you running the same cassette spread on the two sets? That's another factor, though mostly has to do with the B-screw.

[HillSlugger]

Yeah, the spacing can be different between wheels meaning you might need some derailleur adjustments to make shifting optimal when switching. Are you running the same cassette spread on the two sets? That's another factor, though mostly has to do with the B-screw.

Yes, same cassette on both (12-27). Didn't have this problem on my road bike with the triple/long rear cage.

I have discovered that my chain length *will* allow me to get onto the big-big combination. At the other extreme on the small-small (or next to smallest cog) the jockey wheels start to rub on each other.

I suppose I'd be much better off if I could HTFU and learn to ride a 12-25!

btw I made some adjustments to the tension using the barrel adjuster and the shifting was better behaved today, but not perfect. Instead of skipping past the 3rd cog (or was it 4th?) it was reluctant to get up onto it, but only when I was in the small chain ring. I suppose I should be switching to the large chain ring instead of going to this combo.

[aicabsolut]

Whether you have to make adjustments when switching between wheels seems to be bike geometry and individual derailleur dependent...so I'm not surprised you were ok switching with the triple but are now having problems.

When you say "get up into it" are you shifting harder or easier? If harder (which is what I'm thinking when you say you shouldn't be using that combo in the small ring), then loosen the cable by turning the barrel clockwise and see how that goes. The cable tension isn't as finicky as the limit screws so you can play with it more. Or you can start from scratch and loosen it a lot then start to re-tighten until it shifts properly.

If you're only having problems when in the small ring, then it probably isn't your limit screw on the RD. More likely a cable tension thing.

What do you mean that the jockey wheels start to rub on each other? Do you mean the chain is rubbing on the underside of the derailleur cage? That's not a big problem. That's what that part of the cage over the bottom pulley is for. Plus, you've got a fair amount of slack on the underside of the chain a lot of the time when in harder gears. It'd be worse for your derailleur and shifting if the pulleys/cage were rubbing on the large cog when in the small ring / big cog combo.

[lunacycles]

I always found my race wheels and training wheels were a bit different in terms of where the outer edge of the small cog sat in relation to the hub locknut, and shifting problems resulted. If this dimension is even .5mm different, you will have shifting problems when you switch from one wheelset to the other. It seems to me you need to have confidence about rear derailleur adjustment to deal with this regularly. You will probably have to re-adjust the high gear limit screw and the low gear limit screw to achieve precise shifting when you swap out wheels. Assuming you are using the same brand cassette on both wheels, you should NOT have to mess with the barrel adjuster. The barrel adjuster (cable tension) controls the distance the derailleur moves with each shift, and that should stay constant if you are using the same brand cassette on both wheels.