So I recently asked about upgrading to a 9sp cranks with my 8sp cassette (I want to go to a compact crank). I noticed only 9sp parts I can find are quite low end. So the next question is does a 10sp crank work with and 8sp (or 9sp) chain?
Thanks,
Scott

the crank is not where the speed are the chain has to be thinner for all those cogs stacked up on the back .

So, if you put a wider chain on a thinner tooth it will work OK, about like putting an 1/8 wide chain on a 3/ 32"
thick sprocket ..

The Industry loves forcing the customer to Upgrade by pulling the last new !! improvement, from the market place..

So is there a physical difference between a 8, 9, or 10sp crankset?

Yes but small. 9-speed cranks have the chainrings slightly closer together than 8-speed cranks to keep the narrower 9-speed chain from "skating" in the gap between the rings. That said, with some care in shifting a 9-speed chain will work well enough on an 8-speed crank.

The spacing difference between 9 and 10-speed cranks is apparently insignificant. I've used 9-speed chains on 10-speed cranks and 10-speed chains on 9-speed cranks with no difficulty either way.

With my FSA carbon pro compact "9 and 10 speed compatible" crankset and 9 speed chain the chain rubbed on the big ring when using the small ring with the 3-4 smallest cogs thus producing no usable overlap between rings.

I measured the spacing, found it even tighter than Campagnolo 10 speed spacing, and shimmed the small ring inward to make all the gears usable.

Drew... What did you shim it with?

That's not surprising as FSA has had crank geometry problems in the past. My LBS replaced a bunch of FSA cranks with Shimano's a few of years ago because of shifting and interference problems like you describe.

Thin chain ring spacers made for the purpose by both Wheels Manufacturing and Le Tour.

I stacked a couple of .4mm shims (that's the difference between Campagnolo 9 and 10 speed spacing), although .6mm shims seem to be all that's available now

https://branfordbike.com/product/le-t...pacers-808.htm

If you measure the change in the distance between the tip of the teeth you will find very little difference between 9, 10 and 11 speed. Even though campy has machined the mounting surfaces of the big ring thinner by .4mm when 10 speed came out and another .6mm when 11 speed came out, the tooth to tooth spacing is NOT that much different. This has been misreported for many years. Someone just meaured the thickness of the mounting area and assmued that it translated into the same change in tooth spacing.

About tooth thickness. I have old steel rings from a 1990 7 speed hybrid bike that are the same thickness as modern 10 speed. 11 speed teeth are about .1mm thinner than 10 speed.

You're right, most of the time it doesn't matter. But there have been two different occasions in the last couple of years, once with a nine speed Ultegra crankset and once with a nine speed FSA crankset, where the chain would float over the smaller ring when shifting down and there was no fixing it, it simply wouldn't work consistently right. Replacing with ten speed cranks fixed both problems immediately and completely.

A 9-speed Ultegra crank (6503 triple) paired with a 10-speed Wippermann 5.9 mm chain is what I'm using on one bike right now and there has never been the slightest shifting problem so, as you say, the problem seems sporadic.