8 speed vs. 9 speed chain rings.
What is the difference between 8 speed and 9 speed chain rings and crank sets? I need to replace my triple chain ring crank set on my commuter bike and am having a hell of a time finding one for an 8 speed drive train. The spacing between the chain rings and the thickness of the teeth and ramps on both my commuter bikes OEM 8 speed crank set (FC-M71) and my mountain bike's 9-speed cranks FC-M770) are virtually the same when I measure them with a caliper. Half of the bike mechanics that I talk to seem to think there is a difference between (6, 7 & 8) and 9 speed crank sets and the other half say that they are interchangeable. According to Shimano’s web site, their FC-M442 cranks are compatible with 7, 8 and 9 speed drive trains however; all of their mountain bike cranks like the Deores, LX, XT etc. are only for 9 speed drive trains.
The options I have found so far are a Shimano FC-M410 (8-speed) or FC-M442 (9 speed) with a UN54 BB for around $75.00 or a two-piece FC-M542 (9 speed) with integrated BB for $110.00.
Chainring and Cassette Cogs are the same thickness in 8 and 9 speed drivetrains. Its the spacing in-between that is different.
I think the main difference in the rings are the shifting ramps and pins. They are made to work with different width chains (9 speed is narrower). I believe the cranks are totally interchangable (they have been for me).
Originally Posted by bwalton
If you are just looking to replace the cassette, chain, and rings, and can only find 9 speed rings, you can run a 9 speed rings, 9 speed chain, and 8 speed cassette together.
What if I have an 7 speed cassette and an 8 speed chain, can I use 9 speed cranks successfully?
Yes. ... and no.
Originally Posted by GottaGo
Tooth profile and inner chain width is the same for 7 or more speed, so your 8s chain can certainly run on a 9s crank, and as you know it works fine on your 7s cassette. So that's a yes.
The only drawbacks of a narrower chain than necessary are generally higher price, faster wear, and in some cases the ability to ride the tops of cogs while resting against the face of the adjacent larger cog. If none of these are factors for you, feel free to build a 7-8-9 speed bike.
The key to solving any problem is to understand and address the underlying cause.
I'm confused. Why would a narrower chain ride "the tops of cogs while resting against the face of the adjacent larger cog"? I would think the narrower chain would avoid this issue because of it being narrower.
Originally Posted by FBinNY
On the other hand, the concern I had been working with regarding using an 8 speed chain with 9 speed cranks is along the lines of what you suggest but flipped around the other way - the 8 speed chain being wider would ride up on the chain rings because the rings are closer together (taking advantage of the narrower chain).
So is the 9 speed chain ring/8 speed chain issue the way I described it above real or the result of too much thinking on my part?