Upgrading a crank set

Question

I presently have a 3x7 speed crankset with a sealed cartridge bottom bracket on my touring bike and want to upgrade to a new Hollowtech II bottom bracket and crankset, specifically, the SHIMANO Claris 3x8-Speed Road Bicycle Crankset (FC-R2030). My shifters are 3x7 speed indexed, and I will stay with the 7 speed rear cassette. Is there any reason why the 3x8 front crankset would not work?

Answer 1

A 3x8 crank will work with a 7 speed system. 7 and 8 speed chains are the same, as is the spacing between the chainrings.

The Claris crank specification can be found on the Shimano Manuals and Technical Documents site here. The FC-R2030 seems to only come in 50-39-30. If that is larger than you current crank you'll need to check that you have sufficient clearance with the drive side chainstay.

The specification says the FC-R2030 works with either a 68 or 73mm bottom bracket shell, so that will work whatever your bike has. Note it also says what bottom bracket you need (BB-RS500) that will fit either shell width.

The FC-R2030 provides a chainline of 45mm - that's the standard for road triples. You'll definitely want to check that that is what you bike runs as there's a chance it runs the mountain bike triple standard 47.5mm. Measure from the center of the seat tube to the middle ring. More info here https://www.sheldonbrown.com/chainline.html.

If you have 47.5mm chainline you'll need to look at a mountain triple crank instead of a road triple crank.

Answer 2

Reasons a different crankset may not work:

1. Different bottom bracket width means it doesn't fit on your frame at all.
2. Different chainring sizes can't be handled by the front derailleur.
3. Inner chainring on new crankset hits the chainstay (or some other interference).
4. Chainline differs (too far in or too far out), and the front derailleur can't move the chain to the chainrings.
5. Different speed means chain is too wide or too narrow to work.

This is not an exhaustive list - there may be other issues you can run into. But these should cover most potential issues.

Reason #1 can be avoided by checking the specs on your bike and/or actually measuring the bottom bracket - including the threading used (most bicycles today with threaded bottom brackets use English threading).

Reason #2 is pretty obvious and easy to address.

Reasons #3 and #4 are less likely, but can be hard to predict and can be a real problem.

Reason #5 isn't going to be a problem going from 7 to 8 speeds, as the chain width doesn't change at all between those two. But chain width issues can can be a problem with larger jumps - like trying to use a 6-speed chain on a 10-speed crankset. A too-wide chain might not be able to engage smaller, inner chain rings, and a too-narrow chain might be able to get into places it's not supposed to and seriously break things. A wrong-speed crankset may also have shifting problems because the front derailleur throw is too large or too small for the distance between chainrings. An 11-speed crankset may have its chainrings too closely spaced for a 7-speed front derailleur to reliably shift - shifting to the larger chainring might be prone to tossing the chain too far.

For your specific case, though? The difference between 7 and 8 speed is probably the smallest difference possible. If you get a crankset compatible with your bottom bracket width (no road crankset on a frame with a MTB bottom bracket...), and get the same size crankset, you'll almost certainly have no significant problems at all - you may have to adjust your front derailleur some, and might even have to tweak your rear derailleur a tiny, tiny bit.

That's by far the most likely scenario. In your situation, I'd buy that crankset from the cheapest source I could find and trust that it would work.

But it's impossible to rule out problems, especially remotely via text. Pictures would help, as would exact bicycle model.