nz6666

I am upgrading my 1x9 drive train to 2x9 for some long distance tour and hills. Current set up is Shimano Sora 9 speed RD short cage, 9 speed chain, single chainring, Shimano Alivio 9 speed rear shifter. This is my BB:



I am looking at a sora 9 speed double compact crank set:
Shimano FC-3550 Sora 9-Speed Compact Chainset | Evans Cycles

Can my frame take this?
Also most I see most crank sets now are 10 speed, many people say 10 speed will work with the 9 speed system, but I also saw some people complaining shifting problems (I am not quite sure what they are talking about). So should I go with the sora one or try a 10 speed one?

Also I am really confused with the BB standards, so what kind of crank sets my frame can take? I only know mine is square taped. Thanks.

dr_lha

If your frame has a square taper BB then it's likely that you have a standard English threaded bottom bracket shell. This means you can run the Sora 9 speed crankset you listed, but you will need a new bottom bracket, as I believe it is a Hollowtech II crankset. This will do the trick:

Shimano Tiagra 4600 Bottom Bracket | Evans Cycles

willtf

Seconding what dr_lha said. You can probably run any setup designed for a 68mm (English) shell, but the main compatibility concern is usually between the BB and crankset. The questions on the page you linked indicate you need a Hollowtech II (external-bearing) BB.

I would stick with a 9-speed-designed crankset, but you may have issues running a compact crankset with your short-cage RD. Do you know how many teeth you have on your rear cogs? See and study: Understanding Rear Bike Derailleurs - Performance Bike Learning Center

nz6666

The cassette is Shimano 11-28 T 9 speed.

willtf

Doing the calculations on that page, (50-34)+(28-11)=33 max capacity required, while the max capacity listed for the short-cage is 29. The numbers can probably be stretched slightly (considering the page also says the short-cage RD can handle a maximum of 27 teeth on the largest cog), but someone else may know about whether a 4-tooth difference is too much. A standard 39/53 chainset would reduce that to 2 over the max recommended capacity, which may be more reasonable.

D1andonlyDman

On one of my bikes, I'm running that Sora RD on a 12-26 rear, and 51-39-29 triple in the back, and I don't think I'm close to capacity in terms of chain slack take up. Based on that, I'd say the front spread is not an issue, but it's possible the rear cassette is one tooth too big. But there's generally some conservatism in the way these things are spec'd, so it's probably going to work fine.

Camilo

Rear derailleurs have two specs - maximum cog size and chain wrap. If you're not changing your cassette, and the current RD works with the largest sprocket on there now, it will continue to work with that same largest sprocket, regardless of whether it exceeds the stated spec. Changing the crank won't change that issue at all.

But, as someone pointed out, you might be exceeding the short cage RD's chain wrap by several teeth - and my opinion is that it won't work with that much difference. But double check that spec, and make sure you have a short cage, not a medium or long cage RD.

If you do have a short cage RD and it doesn't have enough chain wrap capacity, you can easily find a longer caged RD which won't cost you a fortune, especially if you're satisfied with Sora level stuff.

As for the crank and bottom bracket: Simply find the crank you want, and get the bottom bracket specified for that crank. Shimano (and most other) cranks actually specify the BB to use. They're not expensive so don't worry too much, just get the one spec'd for that crank.

Don't worry if the crank is a 10 speed or 9 speed. If you set up and adjust your front derailleur properly, the shifting will be perfect with either. I've tried it at least a couple of times, and it really is no problem. Currently running a 10 speed crank with 8 speed derailleurs, shifters and cassette. Previous was the same, except everything except the crank was 9 speed. I don't understand why people report unsatisfactory shifting, except maybe they don't bother to check and readjust the FD when they install the new crank.

nz6666

Thanks for the replies. One thing is I found the capacity of Sora short cage RD is 37 T on Shimano web site:
https://bike.shimano.com/content/sac-...oad/sora1.html

But here says it's 31 T.
Wiggle | Shimano Sora 9 Speed Rear Derailleur | Rear Derailleurs

Maybe I should just give it a try and change to a medium if not working?

The other thing is the frame has a tab for FD so it takes braze on FD. I measured the distance between the lowest point of the slot on the tab to center of BB shell is about 14 cm. Will I be able to install the compact? I am asking because I know the frame can take 53/39 no problem but not sure if the lower FD position will be ok on the tab.

Camilo

Like your idea of just trying the RD, just try the FD. I would bet it will work fine. Why? Because I've installed two different compact cranks on my mid-80s frame that was obviously made for a standard size crank and both worked perfectly. Not only do I think it will work fine on a bike "designed" for a standard crank, I'd also bet that your frame, if it's fairly modern, will have been designed with compact cranks in mind.

It will probably work fine as-is because the braze-on slot will be long enough to get the derailleur low enough (there is a span of tolerance for FD height after all). Even if it seems to be too high, give it a try and see if it works. Both of the compacts I set up on my old frame worked fine - one a Shimano compact and standard FD, the other a Campy compact and standard FD. Both are a little higher than I have FDs on other bikes set, but it still works fine. I don't measure all that accurately, but my guess is that they're on the high end of spec, while I usually try to set them on the low side, and am used to the FD looking like that.

If it does not go low enough to allow the FD to shift perfectly, I'd bet it will be within a mm or two, and you can file the slot a little to allow the derailleur to go low enough.

fietsbob

You are referring to replacing a square taper BB and crank with an external BB and a whole different crank.

I have no idea of your shop tool access or skills , if you are in doubt pay the bike shop to do the work.

nz6666

Just an update. I have successfully installed a Shimano 5700 crank set (10 speed). My BB shell is 68 mm English thread. I got a problem during installation, which is the 10 speed FD cage too narrow (I bought a 5700 FD originally) so the chain (9 speed) will rub the cage wall on the small ring and big cog combo. Got a 2400 FD (yes 8 speed FD), which has much bigger cage so a lot better but there is still rubs. Added 1 mm spacer on BB on drive side and the problem gone. BTW I use a 4600 front shifter, but no problem it works perfectly.