I understand that Shimano groups (105, Ultegra, etc.) come in both 9 and 10spd varieties and that would require appropriate shifters and chain to match.

What I can't get my head around is how this would affect the crank and chain rings up front. I see 105 triple cranks with 9 and 10 speed designations for example.

How does the # of gears on the cassette change the front rings and crank?

Thanks
-Sean

The difference between 9 and 10-speed cranks is minimal to non-existant when it comes to chainring spacing and thickness. In fact FSA makes one crank configuration and says it's 9 and 10-speed compatible.

When Shimano went from 9 to 10-speed components they changed the crank/bottom bracket configuration. The 9-speed cranks were Octalink (splined) with a conventionally installed cartridge bottom bracket. The 10-speed cranks (named Hollowtech II) have the spindle permanantly fixed to the drive-side crank arm and the bottom bracket is just a pair of bearings that sit externally to the bb shell.

9s and 10s work interchangeably. Sure, the outboard bearing BB come about with the 10 speed upgrade, but I've used outboard with 9 speeds and Octalink with 10 speed. Anything else is just marketing munbo jumbo.

Thanks for the good info. I was curious and searched on eBay. Here's a "hollowtech", but he says it's an "octalink" crank.
https://cgi.ebay.com/Shimano-105-trip...QQcmdZViewItem

Is the seller confused?

Also aren't both types "splined"?

That's an Octalink crankset, the so-called "9 speed" crank. Uses a Shimano BB 5500 or 6500.

No, it can be Hollowtech and Octalink. Hollowtech means the inside of the crankarms have sort of a honeycomb design inside, to give excellent strength-to-weight. The newer outboard bearing stuff is "Hollowtech II." I don't know if the "II" means they came up with an improved version of the honeycomb-like design, or if it's the same honeycomb-like design on cranks designed for outboard bearings-

edit: I just checked Sheldon's definition of "Hollowtech," it says the crankarms are made up of two forged pieces welded together, nothing about a honeycomb design. So I may be wrong, maybe the crankarms are just hollow and not a honeycomb design (I haven't cut any open to check ). I thought I remembered some marketing stuff about the honeycomb thing when Hollowtech was first introduced, and they could still be as Sheldon describes and have honeycombing inside-

I've learned a good bit. To summarize what I now know:

shimano 9 speed cranks use a solid cartridge style BB like this:

shimano 10 speed cranks use the ""hollow, or "external BB like this:

Double cranks don't matter. In fact, my late 70s era Stronglight cranks work fine with Shimano 105 components.

You have it.