Crank compatibility
 

I've been looking to swap out my triple crank with a double, I saw a couple good deals lately. However, I have a few questions.
First, I read somewhere that you can set a triple shifter to work for a double crank. In my case it's an older shimano 105. How well does this work?
Second, I have a shimano 9 speed cassette. The crank I'm looking to get is an FSA with s-10 chainrings. From what I can find the "s-10" chainrings is only compatible with Shimano or Campy 10 speed cassettes. Has anyone tried to use one successfully with a Shimano 9 speed? Can I just change the chain to a narrow 10 speed chain?
My bike is a specialized allez if it matters.
Any help would be appreciated, thanks.

Hi,
triple shifters will work well, the middle shift will be what you can use to trim the chain. Will need adjusting but you would have had to do that anyway. 9 speed chains will work with ten speed chainrings, so no problems there.
If you've got 9 speed 105 you'll possibly have an octalink BB, so might want to check and make sure you don't have to replace that too.

BTW you can run a triple crank as a double - just adjust your limit screws and take the smallest ring off. LBS will know how to if you don't.

My current crank is a hallowtech with 68mm threads. The FSA comes with a 68mm bb so I think I'm good either way. I was considering just dropping the small ring but I think I'd rather upgrade to a carbon double crank, there seems to be plenty of deals online on some, although they appear to be one or two model years old.

I got the new crank yesterday and had it installed by my lbs. I didn't get a chance to ride it much yet, just a short test ride to make sure it all works. It's definitely an improvement over the old 105 crank. The only thing I;m not crazy about is the slight chain rub when I'm in both of the smallest chainrings, I didn't have any issues with the old crank as it was a triple and I was usually using the middle ring, guess I'll just have to get used to it.

You're cross-chaining here. This is generally something you don't want to do, but you should be able to adjust your FD cable so that you can use the shifter's trim to avoid this. Note also that triple and double FDs are not the same thing. You may have a better experience switching to a double FD.

Here's a quick recap, just in case someone didn't read the entire thread and/or I was unclear. I just replaced my old triple crank with an fsa sl-k pro DOUBLE crank. When I was using the triple I was mostly on the middle or 42t cog, I never had any issues with noise. Now, on the new double, when I'm on the 39t, or smallest cog, and the smallest cog on the cassette, 12t, the chain rubs against the inside of the tall cog, 53t, on the crank. It doesn't rub on the derailleur, at least not after I trim a little. The reason for the rubbing is that the small chainring sits closer to the frame of the bike then the middle chainring of the old triple did. Also, the new chainring is ramped on the inside of the 53t cog for smoother gear changes. I guess this is the trade off of riding a double crank vs a triple, lose some weight, get some noise. It only rubs when I'm in the smallest cog on the cassette, if I go up one ring to the 13t (at least I think it's 13, maybe 14), there is no rubbing. I'll just need to adjust my shifting strategies to compensate for the noise.

Unless I missed something, if you are running a 10 speed crank with a 9 speed chain, rub is more likely to happen between the rings when you go small/small. I did this with a 5600 crank and 9 speed everything else. From my experience the 10 speed chainrings are a teeny bit closer together and the 9 speed chain is a teeny bit wider in this environment. Just don't crosschain.

As mentioned above, you are cross-chaining. Small-small is to be avoided.

Yes, I am aware that I'm cross chaining. I was merely stating that I didn't have cross chaining issues with the triple crank cause I was always on the middle chainring...... I asked at my lbs if the 10 speed chain would be better, they said that it wouldn't reall help, is that true? Also, I don't think the 10 speed chain would work with a 9 speed cassette, or would it? I believe I read somewhere that fsa makes 9 speed chainrings for their cranks, I think they're called "S-9", would I still have cross chaining issues with them? Or, as mentioned above, do I simply avoid cross chaining?

Technically the 10 sp chain would be slightly narrower so it might help a little. But the real answer is still to avoid cross chaining. Even if it doesn't rub it still puts a strain on the chain and 10 sp chains already wear out faster.

Precisely. You weren't cross-chaining before, or at least not very severely, because the middle chainring was closer to the small cog. But now you are cross-chaining because your new small ring is farther away from the smallest cog. The solution to the rubbing you have now is to simply stop crossing your chain. You will stop the rub and your chain will last longer.

Will a 10-speed chain help? That depends on what you consider 'helping'. It may be a bit less likely to rub since it's not as wide as your current chain. But if you're still riding small-small, you'll still be cross-chaining, which can have a greater effect on the narrower 10-speed chain than it does on your current 9-speed one. I don't believe there will be any problem with using a 10-speed chain with your 9-speed cassette.... it's usually using a bigger chain that will not work (i.e. an 8-speed chain on a 10-speed cassette). But again, the ideal solution is simpler than any of that: stop crossing your chain.

This may be a total waste of time, but: some spoke protectors (pie plates) intrude on the hub somewhat, act as a spacer. Take off the cassette, remove the plastic, reinstall, you might pick up a few mm on the cassette's shift inward. Readjust the RD. You'll need to be lucky....

Good idea but it's already been removed. I'm going to do some research about those other chainrings that FSA recommends for 9 speeds, I'll post the results. Thanks everyone for the quick responses.