Crankset Replacement

[Bristolian]

I have just been offered a virtually new Shimano 105 R7100 (12-speed) crankset with left side power meter. My bike is currently fitted with Sora 9-speed.

I know the 105 crankset will physically fit on the bike but will I have any problems with the operation considering the front derailleur and chain are 9-speed?

Thanks in advance

 

[Sharky]

Having googled a bit, my thoughts are, that a 9 speed chain on a 12 speed chain ring will probably be ok, but front gear changes may not be precise.

Are the chainrings on both crank sets the same bcd? Can you swap the chain rings and keep the 9 speed chain rings on your new cranks with the power meter?

 

[Big John]

Depends on what the offer was, I guess. If it's free then try it. If it works ok then brilliant and If it doesn't you've lost nowt. If, on the other hand, there's a price involved then you could do with knowing if it'll work before parting with your hard earned cash. However, if it's a mate then he should let you try before you buy.

 

[Ajax Bay]

At the least, you'll get interference (because 9sp chain well wider than 12sp) between the rings with any cross chaining (top (small) 3 sprockets). The chain is about 1.4mm wider (6.6 - 5.2). You could use a 10sp chain, and this is what I'd do. Using an 11sp chain you'd find the shifting at the rear on your 9sp cassette would be adversely affected. New chain: cheap enough to try and would still give you 2000km with or without new chainset.

 

[Bristolian]

Having googled a bit, my thoughts are, that a 9 speed chain on a 12 speed chain ring will probably be ok, but front gear changes may not be precise.

Are the chainrings on both crank sets the same bcd? Can you swap the chain rings and keep the 9 speed chain rings on your new cranks with the power meter?

Thanks for the reply. Unfortunately, my 9-speed chainrings are 5 holes @ 130BCD and the 105 are 4 holes @ 110 BCD so can't switch the chainrings over

 

[Bristolian]

Depends on what the offer was, I guess. If it's free then try it. If it works ok then brilliant and If it doesn't you've lost nowt. If, on the other hand, there's a price involved then you could do with knowing if it'll work before parting with your hard earned cash. However, if it's a mate then he should let you try before you buy.

Thanks for the reply. Defo not free but not a bad price and he's more of an acquaintance than a friend. I could probably sell the crankset on for more than he's asking if it doesn't work.

 

[Bristolian]

At the least, you'll get interference (because 9sp chain well wider than 12sp) between the rings with any cross chaining (top (small) 3 sprockets). The chain is about 1.4mm wider (6.6 - 5.2). You could use a 10sp chain, and this is what I'd do. Using an 11sp chain you'd find the shifting at the rear on your 9sp cassette would be adversely affected. New chain: cheap enough to try and would still give you 2000km with or without new chainset.

Thanks for the reply. I hadn't considered using a different chain but what your saying does make sense. I feel a suck it and see moment coming on

 

[wafter]

Chain width (i.e. number of "speeds") appears more important relative to sprocket spacing on the cassette than chainring spacing on the crankset.. and on the back you can typically get away with fitting thinner chains than intended; the only problem being fatter chains.

The internal chain width between all the speeds involved (9-12sp) is the same, while there seems to be quite a wide range of tolerance in chainring spacing relative to chain width and I don't think a direct relationship between them is set in stone as IIRC Shimano recently increased pitch between chainrings on some of their road groupsets.

So... you could try just swapping the chainset first and if you have issues look at going for a thinner chain. FWIW I'm running an 11sp chain on an otherwise 9sp drivetrain (3x9) with no apparant issues after about 500 miles; however I am running friction shifting so can't comment on how indexed shifting might be affected.

I'm considering a 12sp crankset on my otherwise 11sp setup (on cost & availability grounds) so will be interested to hear how you get on

 

[midlandsgrimpeur]

At the least, you'll get interference (because 9sp chain well wider than 12sp) between the rings with any cross chaining (top (small) 3 sprockets). The chain is about 1.4mm wider (6.6 - 5.2). You could use a 10sp chain, and this is what I'd do. Using an 11sp chain you'd find the shifting at the rear on your 9sp cassette would be adversely affected. New chain: cheap enough to try and would still give you 2000km with or without new chainset.

I was always under the impression also, that when swapping components across different 'speeds' that chain was the key factor as I had always assumed that the biggest obstacle to shifting was the varying chainlines and chain thicknesses.

 

[Dan Lotus]

Clearly you just need to buy a new bike to fit the crankset to.

You're welcome :-)

 

[Sharky]

Thanks for the reply. Unfortunately, my 9-speed chainrings are 5 holes @ 130BCD and the 105 are 4 holes @ 110 BCD so can't switch the chainrings over

One plus point for taking the 105 chainset, is that the 110bcd should take a 34t inner, whereas the 130bcd will only take a 38t.

 

[Bristolian]

Clearly you just need to buy a new bike to fit the crankset to.

You're welcome :-)

Don't. Wifey has reminded me (more than once) that I'm still sat on a small bundle of money she gave me last Christmas to buy a new bike The problem for me is that we don't have room for a second bike and I'm quite attached about the Allez, even though I haven't owned it for very long.

 

[Bristolian]

One plus point for taking the 105 chainset, is that the 110bcd should take a 34t inner, whereas the 130bcd will only take a 38t.

Yes, the crankset comes with the ubiquitous 50-34t 11-speed chainrings.