How do I know what gear combinations should work without chain rasp on front derailleur?

Question

I tried adjusting the front derailleur on a 3x10 trekking bike. After fiddling around a long time, I concluded that is probably not possible to make all cogs work with all chainrings. How do I know what gear combination should work? Should you for example be able to use the fifth gear with all three chain rings without chain rasp? Should you also be able to switch between all chainrings in the fifth gear?

Currently, the three largest cogs are only working with the smallest chainring. If I adjust the front derailleur to allow shifting on the second chainring, I lose the ability to downshift from the third to the second chainring in higher gears (the chain rattles but does not come down).

I also experienced something similar when I crosschain on a 3x8 city bike. I have never had any issues with a 3x7.

Answer 1

You should normally be able to access all gears at the back, whichever gear you have selected on the front derailleur. Then it comes down to how many gear combinations you can achieve noise free.

Shimano have a bunch of tech docs for setting up derailleurs, a general one is here: https://si.shimano.com/pdfs/dm/DM-FD0003-06-ENG.pdf (the link has rotten away; consider http://web.archive.org/web/20220402115619/https://si.shimano.com/pdfs/dm/DM-FD0003-06-ENG.pdf) and it has a good run through of front triple adjustment. You could follow this through and see if you manage any improvement.

See the note on page 4, they expect some rub when cross chained whichever ring you’re on and suggest you deal with it by changing the gear so you straighten the chain out.

Better mechanics can set up many derailleurs without chain rub in any gear combination. Learning from them expands what you think is possible. Personally, I expect (with a triple) to access all gears without chain rub when using the middle ring, and to have a little chain rub when fully cross-chained from the small and big rings. I.e. in small-small or big-big only. As you’ll avoid cross-chaining the extremes, this would be adequate for majority of riders.

Then again, there are so many variables, in the components, speeds, bike geometry, tyre widths etc, that knowing what is the best you can get from a set up depends on your preferences, your skills, and the bike itself.

Answer 2

There are two approaches here:

1. The standard rule is to avoid cross-chaining. It means that you don't use large with large and small with small. The large sprocket at the rear should not be used with the large ring at the front. In the same manner you should not use the smallest rear sprocket when your chain is on the smallest front ring. In both cases the chain runs quite diagonally or sideways when you look from above. It puts a lot of strain on the drive components and the chain is most likely to rub against the front derailleur plates. With a triple the use of the middle ring is less critical. It may work with any rear sprocket. But still, depending on the position of the chain, rubbing against either FD-plates is possible, so you should avoid the use of the two extreme sprockets.

2. The previous being quite old school, newer derailleurs and narrow chains are less affected by cross-chaining, if you leave out the higher wear of the drive train. Especially modern indexed front derailleurs don't have just three positions but two for each chainring or even three for the middle one. You can notice these if you carefully actuate the shifting. You will notice that when the derailleur is as far right as possible, a slight touch of the lever will have the FD move inboard ever so slightly without throwing the chain off. This position is meant to be used with the larger rear sprockets. It will only work with carefully adjusted indexing of the derailleur which may be fiddly. Some makes, like SRAM have FDs with a special geometry of the plates that doesn't require trimming where electronic shifters trim the position of the FD according to the position of the rear derailleur

Note: in the days of non-indexed shifting and down-tube levers it was customary to the adjust the position of the left lever slightly when the 'ear-meter' registered noise from the chainrings.

Answer 3

While it is not unusual to get some chain rub on setups using 8 or more speeds as @Carel has stated, in general cross-chaining should be avoided for the reasons stated. I have found that in some instances you can make adjustments that allow either the use of most of the smaller cogs or most of the larger cogs but not both. I have seen cases where you can reduce the chainrub on the derailleur but at the expense of only being able to drop to the smallest crank ring if the chain is on the 4 largest rear cogs. You need to decide which setup is appropriate for your riding style.

Answer 4

The chain is most likely to rub the front derailleur cage when:

It's at either its most inboard or outboard positions, i.e. lowest and highest gear ratios: small chainring/largest sprocket or large chainring/smallest sprocket. The chain will rub if the derailleur limit settings are too tight.

When the angle between the chain and the cage is greatest, i.e. during extreme cross chaining: large chainring/largest sprocket or small chainring/smallest sprocket. With the chain running across the derailleur there's less clearance on either side.

You really should be able to set up a modern 3x10 drivetrain to shift properly without rub in the lowest and highest gear ratios. Rub when cross-chaining is less of a concern as you should be avoiding that anyway. One thing to look at if you are having difficulty is the derailleur cage angle. If the cage is yawed in out out a little too much that may be the cause of you rub problems.