Derailleur adjustment with compact crank

[HillSlugger]

Don't touch the limt screw (except maybe a tiny bit looser to improve shifting to the big ring)! One of the limitations of a compact is that, when you're cross-chained small to small, the chain will drag across the teeth of the large ring. Just don't cross-chain and you'll be fine

I can get to about the third cog from the bottom (14T) with the help of the trim before my chain drags on the large ring. I try to shift to the large ring sooner (15T or 17T), depending on cadence and terrain, but the best laid plans....

No, I was big/small, not crosschained.

BTW, when I was at the LBS the wrench told me that my chain was a bit too short because he couldn't get it to go big/big. I pointed out that I shouldn't ever be big/big!

[Becky]

No, I was big/small, not crosschained.

BTW, when I was at the LBS the wrench told me that my chain was a bit too short because he couldn't get it to go big/big. I pointed out that I shouldn't ever be big/big!

Gosh, I'm sorry! I'm not reading very closely again- my fault!

In that case, I'd loosen the high limit screw the tiniest amount (quarter turn or so), and try again. You're almost there, and soon it will be perfect!

[lunacycles]

BTW, when I was at the LBS the wrench told me that my chain was a bit too short because he couldn't get it to go big/big. I pointed out that I shouldn't ever be big/big!

Yes, but you want your bike to be able to do this, should you be in one of your less clear/ more tired moments and accidentally make that shift. If the chain is too short and you accidentally shift "big-big" while riding, it can literally rip the derailleur off the frame, potentially destroying your frame, at a minimum destroying the derailleur. A longer chain also just shifts better.

[aicabsolut]

You shouldn't be using trim at all to avoid rubbing in large ring/small cog combos. The cable should be pulled all the way tight (shifting completely). Trim when in the large ring will help you avoid rubbing the inner part of the cage in the larger cogs.

I would NOT mess with the limit at this point. You are getting rub in the smallest 2 cogs. This indicates that you've probably still got an alignment issue. Can you see where relative to the tail of the cage the rubbing is occurring? When you are standing near the rear of the saddle on the right side, facing forward, and when the bike is in the big ring/small cog, you should be able to look down and see that the chain is centered over the tail of the cage. If it's too far in or too far out, then you will have excessive chain rub despite properly set limits.

Also before you mess with the limit screws, check your cable tension. You are having symptoms of slow shifting and too much chain rub. That could be a limit issue, but I'd be hesitant to go there first given the bike's propensity to throw the chain off the outside. Try testing cable tension first. Hand shift by pulling the cable. Does the rubbing stop when you are pulling on it? If so, then maybe you just don't have the cable tight enough such that when you've got the lever pulled all the way, you aren't getting full movement of the cage (but you can get the additional necessary tension by pulling with your hand).

[HillSlugger]

You shouldn't be using trim at all to avoid rubbing in large ring/small cog combos. The cable should be pulled all the way tight (shifting completely). Trim when in the large ring will help you avoid rubbing the inner part of the cage in the larger cogs.

I would NOT mess with the limit at this point. You are getting rub in the smallest 2 cogs. This indicates that you've probably still got an alignment issue. Can you see where relative to the tail of the cage the rubbing is occurring? When you are standing near the rear of the saddle on the right side, facing forward, and when the bike is in the big ring/small cog, you should be able to look down and see that the chain is centered over the tail of the cage. If it's too far in or too far out, then you will have excessive chain rub despite properly set limits.

I think the alignment is OK, but I'm not completely sure.

Also before you mess with the limit screws, check your cable tension. You are having symptoms of slow shifting and too much chain rub. That could be a limit issue, but I'd be hesitant to go there first given the bike's propensity to throw the chain off the outside. Try testing cable tension first. Hand shift by pulling the cable. Does the rubbing stop when you are pulling on it? If so, then maybe you just don't have the cable tight enough such that when you've got the lever pulled all the way, you aren't getting full movement of the cage (but you can get the additional necessary tension by pulling with your hand).

Just checked, I can't make it go any further by hand.

[aicabsolut]

Yes, but you want your bike to be able to do this, should you be in one of your less clear/ more tired moments and accidentally make that shift. If the chain is too short and you accidentally shift "big-big" while riding, it can literally rip the derailleur off the frame, potentially destroying your frame, at a minimum destroying the derailleur. A longer chain also just shifts better.

This is true, but it is easy to have too long of a chain on a compact if the chainstays are relatively short. That's because when in the small/large combo, you don't have enough chain tension to pull the RD away from the sprockets. With some setups, tightening the Bscrew to its max won't fix that. So, to avoid that, the chain will be a tad short and noisy. But, it shouldn't be shortened so much that the RD is being pulled sharply forward when in the big/big. You still need enough chain wrap to reach this gear and sustain a bit of an S shape in the RD, even if it's noisy.

[lunacycles]

This is true, but it is easy to have too long of a chain on a compact if the chainstays are relatively short.

If your rear derailleur hanger is designed and aligned correctly for the gears and components you are using (e.g., "road") you shouldn't have the rear derailleur guide pulley bumping up against any cog with the b-tension screw adjusted correctly...just hasn't been my experience. Even if this were to be the case, I'd rather live with a little chain noise in the small/large combo than risk ripping my rear derailleur/derailleur hanger off due to a too short chain...

I determine chain length by "dummying it up" in the small/small combo, and ensuring the chain is just short enough that there is no chain rub against the bottom part of the rear derailleur cage in this combo. I then shift through all gears to ensure it can handle them, keeping in mind stated derailleur capacities as stated by the manufacturer. This has never failed me.

And I am trying to understand: what does chainstay length have to do with it? Thanks.

[aicabsolut]

If your rear derailleur hanger is designed and aligned correctly for the gears and components you are using (e.g., "road") you shouldn't have the rear derailleur guide pulley bumping up against any cog with the b-tension screw adjusted correctly...just hasn't been my experience. Even if this were to be the case, I'd rather live with a little chain noise in the small/large combo than risk ripping my rear derailleur/derailleur hanger off due to a too short chain...

I determine chain length by "dummying it up" in the small/small combo, and ensuring the chain is just short enough that there is no chain rub against the bottom part of the rear derailleur cage in this combo. I then shift through all gears to ensure it can handle them, keeping in mind stated derailleur capacities as stated by the manufacturer. This has never failed me.

And I am trying to understand: what does chainstay length have to do with it? Thanks.

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.


MDHillSlug, I guess you can try loosening the H limit now some, but do it slowly. Like 1/8 of a turn at a time, then re-check the shifting. You might want to put up with a little bit of rubbing if you don't use the small cog a lot, if that helps you keep the chain on.

[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.