Condensed Answer: A 10-speed front derailleur can operate sufficiently well with a 9-speed chain. The main issue to look for is rubbing of the chain against the derailleur’s cage.
Of course, the front derailleur should match the number of chainrings at the front. If you combine a 2x derailleur with a triple crankset, you won’t be able to shift onto the big chainring.
The Effect of Chain Width
A chain has two widths – outer and inner. The inner width of the chain is the distance between the inner plates; the outer width is the distance between the external plates (image below).
The inner and outer width of a chain depend on the number of cassette cogs.
For example, an 8-speed chain has a larger inner and outer width than an 11-speed chain.
Why?
Those changes are necessary for the following reasons:
The overall width of a cassette does not change dramatically with each gear increase. This is done intentionally to ensure that a single hub can accept a multitude of cassettes.
The sprocket thickness gets ever so slightly thinner with each gear increase, but the change isn’t dramatic. Therefore, the only way to fit more cassettes within roughly the same dimensions is to make the distance between each cassette cog smaller.
This necessitates a thinner chain or else rubbing will occur. Since the inner width of the chain cannot be changed much (it depends on the sprocket thickness), the only option left is to make the chain thinner by thinning out the outer plates.
Front derailleurs and chainrings have to reflect the changes to the chain too.
The table below contains the inner and outer width of 7-12-speed chains:
| Number of Speeds | Inner/Roller Width | Outer width |
| 7 | 2.38mm | 7.3mm (Shimano), 7.1mm (SRAM) |
| 8 | 2.38mm | 7.3mm (Shimano), 7.1mm (SRAM) |
| 9 | 2.18mm | 6.5-7mm |
| 10 | 2.18mm | 5.88-6mm |
| 11 | 2.18mm | 5.5-5.6mm |
| 12 | 2.18mm | 5.3mm |
Since we are examining the compatibility between a 10-speed front derailleur and 9-speed chains, we need the values for 9 and 10-speed chains.
The inner width of 9 and 10-speed chains is the same. The outer width, however, is slightly different. 10-speed chains are narrower by about 1mm.
Consequently, front derailleurs designed for 10-speeds have slightly narrower cages too. The narrower cage makes the shifting more responsive.
When a 10-speed front derailleur is combined with a 9-speed chain, the chance of rubbing between the derailleur’s cage and the chain’s outer plates is higher due to the extra width.
This problem is more likely to manifest when the bike has a triple crankset because of the more extreme chain movement and positions.
That said, a 1mm difference is not enough to render the combination unusable. If the derailleur has a trim function (additional movement meant to remove chain rubbing) or the bike has a friction front shifter, then it’s fairly straightforward to stop chain rubbing via the shifter.
A 10-speed Cassette Is Not Compatible With a 9-speed Chain
A 10-speed front derailleur could be combined with a 9-speed chain, but a 9-speed chain cannot operate properly with a 10-speed cassette.
A 9-speed chain is wider than a 10-speed one and will rub against the cogs when used on a 10-speed cassette.
Consequently, a 9-speed chain and a 10-speed front derailleur are a viable combination only in a 9-speed drivetrain.
FAQ: What is a friction shifter?
Modern shifters are indexed. This means that the movement of the shifter is segmented into a number of clicks equal to the number of rear cogs on the particular cassette. Each click moves or releases a pre-determined amount of gear cable which in return moves the derailleur up or down the cassette.
Before index shifters, bikes had friction shifters. Unlike indexed shifters, friction shifters do not have a segmented movement. The lever moves freely. Consequently, it’s up to the rider to determine how much to move the shifter handle or knob to perform a shift.
Undoubtedly, friction shifters make the shifting experience more difficult. For that reason, the bike industry quickly switched to index shifting as soon as it was available.
However, some road bike models kept the front shifter as a friction unit or a hybrid that’s capable of operating in index and friction mode.
Why?
Because friction shifters allow the user to trim the front derailleur and avoid chain rubbing in extreme gear combinations.
Interesting fact: Some racers, including Lance Armstrong, raced with a downtube front friction shifter and a modern STI-brifter for the back. The combination provided the best of both worlds when climbing while also saving weight (the omission of the front STI shifter saved about 100 grams.)
FAQ: What about combining a 10-speed chain with a 9-speed front derailleur?
This combination is also possible. A 10-speed chain is 1mm narrower in total width than a 9-speed unit.
Thus, in this case, the front derailleur’s cage will be slightly wider than necessary. The extra width reduces the chances of rubbing but could also make the shifting less snappy. To what degree this phenomenon will manifest depends on the entire drivetrain.
Another pro of this combination is that it can operate in a 9-speed and a 10-speed drivetrain because 10-speed chains are compatible with 9-speed cassettes.
