Bicycle Mechanics - 8-speed crank compatibility with 9-speed derailleurs?

I'm looking to replace a set of 9-speed cranks, and have found a good deal on a set of cranks designed for an 8-speed drivetrain. Will 8-speed cranks work properly with 9-speed derailleurs, or has the design changed enough to make a difference?

Thanks!

-chris

I think the inner chainring is slightly closer on a 9-speed crankset than it is on an 8-speed cassette.

For practical purposes, with bar-end or downtube shifters (and probably with Ergo too), there is no difference whatsoever. With STI, if I remember correctly, adjustment is slightly more finnicky and you run the slight chance of having the chain skate between the granny and middle ring. Not a real problem unless you race or stand up when shifting.

Regards,

This will be a double crankset, with STI shifters. I guess I'm not sure whether that changes your answer...

Thanks!

-chris

No. A 9 speed drivetrain uses a narrower chain. 8 and 9 speed cassettes are the same overall width, but the cogs on a 9 speed cassette are closer together and are narrower.

I'm using a 9spd Tiagra shifter with an Ultegra f. der. on a old, probably first gen, 105 crankset. Maybe someone knows if thats an 8spd or not, I would suspect it is most likely. The same 105 crank also worked great with the original 7spd setup before I went to 9spd. Anyways whatever it is works perfectly, and I know that typically on mountain bikes it doesn't much matter with regards to the front chainrings, the rear is where the 8spd/9spd matters more.

The 9-speed chain is narrower than the 8-speed chain, so it fits in with cogs that are tighter to eachother. However, the internal width of the chain is the same, and chainrings are the same.

As you have a double with STI shifters, I'm 99% sure it will work flawlessly. If you "succeed" in having the chain skate between both rings (which is very rare, unless you have a wide-range double like 34-52), you could push your luck by disassembling the rings and filing the spacers [bi]evenly[/i] by 0,2 mm or so. I know that it's not a "Shimano-approved" procedure, but you would then get 9-speed spacing between your rings.

P.S. I have bar-end shifters on my tourer (which means friction in front, therefore more adjustability). The crankset is an 8-speed XT with 44-34-22 rings, with a 105 road triple (not supposed to work well) and a 9-speed cassette and chain. Only 14000 km so far in 9-speed mode and no problems...

I'm using "6-speed" 105 cranks (including a biopace ring) with 9-speed shifters, cassette and chain, and have never had the chain jam between the rings. Lot of hooey.