Bicycle Mechanics - Using trim on a compact but not a triple; what gives?

Hi

Could someone kindly explain why trim is used often on a compact crank (shimano ultegra in my case), but I never need it on my triple (shimano 105).
They have both been adjusted optimally by the LBS and work flawlessly; just wondering why the difference...

Thanks!

The need for trimming has many variables including chainline, bottom bracket spindle length, chainstay length, chainring sizes, chain design, derailleur design and adjustment, etc.

The need for trimming has many variables including chainline, bottom bracket spindle length, chainstay length, chainring sizes, chain design, derailleur design and adjustment, etc.

Hmmm... more variables than I imagined.

Is there a "recipe book" out there for trim-free - or almost - drivetrain configurations?

A question, and as stupid as it might sound what is "trim" ?

Hmmm... more variables than I imagined.

Is there a "recipe book" out there for trim-free - or almost - drivetrain configurations?

Yeah. The better the chainline, the less gears up less gears in the rear + closer spacing of both = low to no trim.

A question, and as stupid as it might sound what is "trim" ?

Small shift that is not intended to move the chain onto a different cog/chainring but is used to eliminate rubbing that is common on larger gear range where the chain runs at an angle. Basically, it gives you more indexed positions on the front derialleur to eliminate rub.

Yeah. The better the chainline, the less gears up less gears in the rear + closer spacing of both = low to no trim.

Trim is a "half-shift" that is used for the front derailleur.

As you shift down on your chainring, say going left to right in layman's terms, at some point the angle of the chain will begin to rub on the "outside" of the front derailleur cage.

At that point trim can be used to move the derailleur outwards to eliminate the rubbing but not change gear.

Yeah. The better the chainline, the less gears up less gears in the rear + closer spacing of both = low to no trim.

Therefore a combination of 50/34 with a 12/27 will produce more rubbing (and the need for trim) than a 53/39 with a 11/23.

Is that correct?

Therefore a combination of 50/34 with a 12/27 will produce more rubbing (and the need for trim) than a 53/39 with a 11/23.

Is that correct?

Yes

There is also a problem with the chain rubbing the side of the big ring when the chain is on the inner ring and the right side of the cassette. This is more common with 50-34 compacts. Trimming the derailleur will not help this problem. Sometimes a bottom bracket spacer can be used to correct the chainline and eliminate this type of rub, but these cannot be used on all types of bottom brackets.

Yes

There is also a problem with the chain rubbing the side of the big ring when the chain is on the inner ring and the right side of the cassette. This is more common with 50-34 compacts. Trimming the derailleur will not help this problem. Sometimes a bottom bracket spacer can be used to correct the chainline and eliminate this type of rub, but these cannot be used on all types of bottom brackets.

Thanks Al, makes perfect sense. :thumb:

I also noticed the following when I'm in the 50 ring (using a 12/27 cassette):

Gearing:
50 ring with 27 = no rub
50 ring with 24 = no rub
50 ring with 21 = no rub
50 ring with 19 = rubbing = applying trim creates more rubbing
50 ring with 17 = rubbing = applying trim removes rubbing
50 ring with 15 = no rub
50 ring with 14 = no rub
50 ring with 13 = no rub
50 ring with 12 = no rub

I would expect rubbing on one of the middle cogs, and then apply the trim the quiet it out.
In this case I can hear it over 2 cogs; i've played with the cable tension and all it does is move the rubbing down to the 15 gear and still spread over 2 gears. Really strange.

FYI - ultegra derailleur front and back, fsa 50/34, ultegra 12/27, basic Shimano chain (not Ultegra); all of these are brand new.

Would you happen to know what the problem could be?

Yeah. The better the chainline, the less gears up less gears in the rear + closer spacing of both = low to no trim.

Thanks Op, explains the trim-free triple I have on my other bike.

Thanks Al, makes perfect sense. :thumb:

I also noticed the following when I'm in the 50 ring (using a 12/27 cassette):

Gearing:
50 ring with 27 = no rub
50 ring with 24 = no rub
50 ring with 21 = no rub
50 ring with 19 = rubbing = applying trim creates more rubbing
50 ring with 17 = rubbing = applying trim removes rubbing
50 ring with 15 = no rub
50 ring with 14 = no rub
50 ring with 13 = no rub
50 ring with 12 = no rub

I would expect rubbing on one of the middle cogs, and then apply the trim the quiet it out.
In this case I can hear it over 2 cogs; i've played with the cable tension and all it does is move the rubbing down to the 15 gear and still spread over 2 gears. Really strange.

FYI - ultegra derailleur front and back, fsa 50/34, ultegra 12/27, basic Shimano chain (not Ultegra); all of these are brand new.

Would you happen to know what the problem could be?

First I would look for any cable housing problems that could cause excess friction.
How old are the cables and cable housings, if they are dirty or corroded that might explain your problem.
Dirt in the short housing just before the rear derailleur is often a problem.
Check the cable routing at the fixing bolt on the rear derailleur. The cable needs to run through a small groove. If the cable is on the wrong side of the fixing bolt it will cause strange shifting problems.
Be sure the lockring is tight enough. A loose lockring can cause the cogs to float around enough to cause problems. It's not normal to have any chain rub with the chain in the middle part of the cassette.

Al

Hi, see my answers below... hope this helps to zooming in on the problem.

How old are the cables and cable housings, if they are dirty or corroded that might explain your problem.
- recently replaced front derailleur cable and housing.

Dirt in the short housing just before the rear derailleur is often a problem.
- recently replaced front derailleur cable and housing, so shouldn't be a problem.

Check the cable routing at the fixing bolt on the rear derailleur. The cable needs to run through a small groove. If the cable is on the wrong side of the fixing bolt it will cause strange shifting problems.
- EDIT: Did you mean front derailleur? fyi - the rear derailleur works like a charm.
- Hmmm. For the front derailleur - the cable is running on the outside of the groove (inside towards the bike, so to speak), and then is bent at a 30 degree angle at the top of the screw and pointing outwards (away from the bike).
Should be cable be running inside the groove?

Be sure the lockring is tight enough. A loose lockring can cause the cogs to float around enough to cause problems.
- Yes, retightened it yesterday.

It's not normal to have any chain rub with the chain in the middle part of the cassette.
- So chain rub should only be happening on the extremities?

Al[/quote]

Is there a "recipe book" out there for trim-free - or almost - drivetrain configurations?

I've got a 10 year old 56cm Trek 5500 with a standard Dura-Ace double crankset and various cassettes. I've never needed to trim the front derailleur. There is nothing special about the way it was set up. I think the frame and components were built to close tolerances and so the chainline is just right. There is no functional reason that a D-A double should be lined up better than an Ultegra or 105 double. I can shift through all gear combinations with no chainrub on the derailleur. In the small to small combination there is a slight chainrub on the side of the big ring, but I don't ride cross-chained anyway.

Al

A curse on indexed FDs. The non-indexed type did not need improving in my opinion and the introduction of indexed introduced so many problems. When I changed my 105 STI triple FD back to a Bar-end, all of my problems disappeared....and yes, I do know how to set them up before someone asks.

Okay. Everybody who participated in this thread, I owe you a beer.

I replaced the Shimano chain - model UG-something - with an Ultegra HG93.

Now I have only one gear where the fd rubs - 17 - and the trim eliminates it and any other rubbing thereafter.

The only other rubbing I have is inside the big chainring when I'm in the small chainring and the cogs are at 13 and 12; at that point I should have shifted to the big ring anyways.

A million thanks to everyone and we'll get that beer in somehow. :beer:

EDIT: I wanted to mention that I believe to eliminate the rubbing on the inside of the big chainring - I currently have FSA - could be done by changing to a Shimano chainring - just a hunch.

"Hi, see my answers below... hope this helps to zooming in on the problem."

Check the cable routing at the fixing bolt on the rear derailleur. The cable needs to run through a small groove. If the cable is on the wrong side of the fixing bolt it will cause strange shifting problems.

"- EDIT: Did you mean front derailleur? fyi - the rear derailleur works like a charm."

I meant the rear derailleur. The problem occurs when the rear derailleur moves the chain to certain cogs on the cassette. I'm thinking maybe the chain is not centering over those particular cogs due to poor rear derailleur alignment. You seem to have tried adjusting the front derailleur several times.

It's not normal to have any chain rub with the chain in the middle part of the cassette.

"- So chain rub should only be happening on the extremities?"

It's more common to have chainrub when the chain is running on a wider angle through the front derailleur.

Al