Derailleur adjustment with compact crank

[HillSlugger]

Instead of doing my Wednesday bike commute I took the tri bike out for a test ride this morning. The good news: no dropped chains! The bad news: I sometimes had trouble getting the chain to go onto the large ring and if I was on the large ring and using the two smallest cogs I had FD chain rub and couldn't trim out to stop it. Obviously I need to move the limit out a tiny bit. Do you think I should do this right away or wait until after my next ride on Saturday? And the stupid question so I can get it right the first time: to move the cage out do I loosen or tighten the limit screw?

[Becky]

Instead of doing my Wednesday bike commute I took the tri bike out for a test ride this morning. The good news: no dropped chains! The bad news: I sometimes had trouble getting the chain to go onto the large ring and if I was on the large ring and using the two smallest cogs I had FD chain rub and couldn't trim out to stop it. Obviously I need to move the limit out a tiny bit. Do you think I should do this right away or wait until after my next ride on Saturday? And the stupid question so I can get it right the first time: to move the cage out do I loosen or tighten the limit screw?

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....

[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.