rebayona

I've been commuting for 23 years, over 100km/week, around 48.900mi in total. My bicycle was built by me, it has an standard frame with Shimano Alivio groupset M431 from 2006. The last drivetrain it’s from 2012: FC-M361 with 48–36–26 chainrings. 8spd.

It's time to replace it and I want to upgrade drivetrain with ST-M390 (9sp) keeping cranks and just replacing chainrings. My concern is about Shimano's manuals, on this document about compatibility M361 it's stated as 8spd.

Will I have troubles if I use this crankset? Should I upgrade the whole crankset or will I be fine just replacing chainrings? Is there a differences on chainrings labeled as "9spd" to those labeled for 8spd?

bigbenaugust 

I don't think that will be a problem.

dabac

Go for it.
I ran a 9-speed rear from a 7-speed crank w/o any issues I remember.

Tim_Iowa

Unless the 8 speed chainrings are worn out, you don't need to replace them. They will work just fine with a 9-speed chain.

My rule of thumb is one "speed" off on cranksets is no problem. Two is usually OK, but three may be asking for issues.

For example: your 8s crankset is perfect for 8s, fine for 9s, maybe OK for 10s, but problematic with 11s chain.

As an extreme example, my '88 KOM has the stock chainrings (6 speed Biopace!) and it's just fine with a 9 speed chain. Zero problems in 3 years.


Higher speed chains still have the same internal width as lower speed chains, so they'll fit just about any chainring. It's just the outer dimension that narrows with more speeds.


The risk is that the higher the "speed" of the chain, the narrower the outside dimension. So, a narrow 11 speed chain may risk falling in between the rings of a lower-speed crankset.

Tech FAQ: Chain width explained, compatibility queries answered | VeloNews.com

Darth Lefty 

Replacing the whole crankset is usually less expensive at this level, IME. Fewer bolts to tighten, too. I don't think I'd have any worries about compatibility between MTB triples. Aren't all the spacing differences for rub and skate on the doubles?

rebayona

Good to know, thanks
Great, that's good news. Thank you.
Got it, thanks a lot for the comprehensive answer.

Problem is finding the parts. Couldn't find Acera cranksets with 48T chainrings, neither separate chainrings in Bogotá, not even requesting parts to importer. One dealer said those were discontinued, as it happened with Alivio bigger chainrings a couple of years ago, and she said it's happening with Acera now as well. Is that true or simply Colombian suppliers stopped importing those references?

Darth Lefty 

I couldn't tell you if it's because you are in Columbia, but these parts certainly exist and are available in the States. The part numbers you list are one or two generations old. Some of the big-ring versions are numbered with a T for trekking instead of an M for mountain. Acera is M3000 / T3000. Look here for more details 2016-2017 SHIMANO Product Information Web

DiabloScott

The shop put a 9-speed chain on my 8-speed bike once because I was in a hurry and that's all they had.

Shifted fine in back but I could not get the FD to shift properly with it - no matter what adjustment I did, the chain would throw off to the inside on downshifts. Not between the rings, all the way past the little ring. I tried for a month to get it to work. Bought a new 8-speed chain and it was an immediate fix.

It wasn't a derailleur and chain problem; it was a chainring and chain problem. Maybe just my particular combination, but it can happen.

chas58

9 speed chain is narrower than an 8 speed. That means you can not use a 9 speed chain on 8 speed gears. To some degree you can use a larger chain on gears designed for a narrower chain.

When a bike shop accidentally gave me a 9 speed chain for an 8 speed bike, it was a mess shifting.

rebayona

Really useful link, thank you very much. I'll bookmark it

Good to know, thanks for sharing.

I followed your advice and finished buying the complete crankset. As you predicted, it was cheaper and easier that way. I bought FC-M361 crankset (48-36-26) but just to take those chairings and put them into my old alivio 175mm cranks. It was the silver version with chainguard, so I wanted to keep my black cranks and give the chainguard a chance (first time in 23 years).
I finished changing handlebar, bottom bracket, stem and also shift/brake levers (from alivio ST-M430), HG-53 chain, and 32-11/9speed cassette. Total was COP$415.000, that's roughly US$145.
I'm really happy with the outcome, thank you all for your help

physdl

I think 8 and 9 speed compatibility varies by brand. My bike is an 8 speed and I'm pretty sure the crankset could deal with a 9 speed chain even though it was the original 8 speed crankset. Right now it has 9 speed cranks and an 8 speed cassette, and the cassette handles it just fine. I never had any issues (in fact I remember reading somewhere that some people had better results with a 9 speed chain on 8 speed gears). I use KMC 9 speed chains, Shimano Deore cranks (the old 8 speed cranks were a no-name brand), and a SRAM 8 speed cassette.

My understanding is that 6 through 9 speed is pretty compatible, including derailleurs. It gets more complicated once you hit 10 or 11 speeds since the range of motion for the rear derailleur changes, along with the narrower chains.