Originally Posted by alfred mcdougal
Hello all... It's been a long time.
I'd like to swap out my current double chainring set-up, which is a FSA 46t/36t, with a FSA triple 42t/32t/24t. I have a Shimano XT M770 9-Speed Cassette and Shimano 9 speed bar end shifters. I use friction shifting in front and indexing in the rear. I'm aware of chainline and it's importance. I'm also aware I might have to swap out my front derailleur (it would remain friction).
I called FSA to get info about the swap; FSA claims their "42t/32t/24t SUPER TYPE CHAINRINGS FOR SL-K BCD 104/64MM triple" configuration will only work with a 10 speed rear cassette.
I'm skeptical... I don't see why it wouldn't work. Am I forgetting something?
Nobody who designs components has the time or money to see that they will work with every possible combination. The best that can be hoped is that the components will work for the limited number of combinations that the manufacturer states. Here are some possible considerations behind FSA's disclaimer.
1. Your current setup uses a crankset and chainwheels that are 110 mm BCD. The setup you want uses 104/64 mm BCD.
1a. This means you must change the crankset to one that will accommodate the new chainwheels.
1b. FSA's website states for all their triple cranksets "Compatible with 10 speed systems MTB."
They will not go further than that statement.
Here are some possible reasons for their reluctance.
2. There are different widths for 9 and 10 speed chains (6.4 vs. 5.88 mm)
2a. The spacing between chainwheels on FSA cranks reflects this difference. Therefore a 9 speed chain may not easily fit between two chainwheels on an FSA crankset designed for 10 speeds. N.B. this is different than using a narrower chain on a crankset and/or cassette designed for a wider chain.
3. If you use your 9 speed chain, you may have to place shims on the chainwheel mounting bolts to move the large chainwheel further out and the inner chainwheel further in. You may also have to move drive side bearing further out (with another shim), if the small chainwheel does not clear the frame.
3b. At this point you are definitely not using FSA's crankset and chainwheels as designed. Whether it works depends on your skill not their design.
4. If you change to a 10 speed chain, you will be able to use FSA's crankset and chainwheels as designed.
4a. It's most likely that you will have to change your cassette because of wear.
4b. It's most likely that your new 9 speed cassette will work with your new 9 speed chain. However, the new chain's narrower width will mean that your trim setting will have to be dead on.
There are other considerations regardless of which width chain you choose.
5. The new chainwheel set has an 18 tooth difference rather than a 10 tooth difference.
5a. You may need a new front derailleur with a longer cage because of this bigger difference.
5b. Your rear derailleur may not have an additional 8 tooth wrap capacity. It too might have to be changed.
Given all the components that might have to be changed, you should check the costs and decide whether it's worth it to you.