Bicycle Mechanics - Do 9 speed cranksets work with 10 speed drivetrains?

I recently got a compact crank for my wife’s road bike, a Shimano FC-4550 (Tiagra level) 50/34. This is specifically labeled a nine-speed crankset. Now we are looking at getting her a 10 speed triathlon bike, but the crankset on it is a 53/39. I would love to put the original triple back on the old road bike and move the new compact to the Tri bike and just sell the 53/39, but will a 9 speed Tiagra compact work with the new 10 speed drivetrain or am going to need to pick up another crankset? I didn’t know if there was something to be concerned about with the chainring spacing or chain widths. I know 10 speed chains are narrower than their 9 speed counterparts. Since this will be a racing bike, it’s important that it run smoothly.

yup, 9 vs. 10 speed cranks is mostly marketing. people tend to associate 10 speed cranks with outboard bottom bracket bearings because that is when they debuted. swapping between the two works fine.

Should work. Some folks have had problems trying to run a 10-speed chain on a 9-speed triple. But even that often works fine.

the inner ring is the same thing for tiagra, 105, R600 which are 9/10S. the only thing that seems to set them apart is the outer ring.

Thanks. Just so I can learn, what is the difference in the outer ring, and is it a difference that is worth replacing just that ring? Are the rings the same distance apart on the 4550 versus say, an R700 or Ultegra SL? This Tiagra compact is still light compared to the triple, so we don't sweat the weight, just want smooth shifting in race conditions. Seems like a cool little crankset, so we are hoping to keep it around for awhile, and it was a deal at under $80 from PBK.

You might want to take a look at the spreadsheet on DMF's thread: http://www.bikeforums.net/showthread.php?t=332840

Interesting, didn't see the 4550 or any Tiagra level 110 patterns, but it certainly illustrates that it isn't as straight forward as one might think. I guess I could just try it out. As long as I can't do any damage, the feel is what really matters no matter what the technical differences are.

I've used the tiagra 4550 on a 10sp bike. didn't seem any different compared to the R600 or 10sp 105 cranksets.

the shifting is real smooth even at the tiagra level.

Unless someone can say that the rings are actually thicker in the 9sp there is no reason at all that it won't work. I used a 6sp crank with a 10sp chain and there no problems.

Unless someone can say that the rings are actually thicker in the 9sp there is no reason at all that it won't work. I used a 6sp crank with a 10sp chain and there no problems.

Ditto. I used a Sakae half-step triple crank with Ultegra 10 speed drivetrain with no problems at all.

Thanks all, I'm very encouraged!

I have a 9v crank and upgraded the shifters and chain/cassette to 10v w/ no problem at all. I had a different problem that I had to take to the LBS and they said "you know Shimano says that won't work" and I said "they don't know what they're talking about". I've never had a problem with the crank and chain. Now I DID have a problem with a 9v chain on an 8v crank but that's a different story.