As much as I prefer Shimano's mountain bike components, they sure make it hard to supply a customer with exactly the desired component. Take the crankset, for example. Years ago, crank and chainrings were easily modifiable. Don't like the stock 26/36/46 ring set up? Easy, replace the ring you want to change with any by Specialized or Sugino (my all time favorite cranks and rings, by the way). And without ramps and pins, they shifted great. However, as six became seven which became eight ... and that became ten, chains got skinnier and shifting suffered so chainrings had to be profiled and ramps and pins were added to move the chain between rings.
And even that was no problem because you use an appropriate width chain to match how many gears you have and it all works. This brings me to my frustration with Shimano. Recently, I built a really nice bike for a customer - custom frame, hand built wheels, Shimano XT...a real dream bike for someone who had been through other bikes and now wanted to treat himself to something really special. We decided to go with an XT crankset with 38/24 rings instead of the 38/26 because of the 29" wheels. He's tall, so 180mm cranks were also in order. He also wanted to go with as many silver components as possible. No problem. Shimano's XT crank is available in both black or silver, up to 180mm length, and with 38/24 ring combo. Okay, that's a problem. Yes, all those options are available in an XT crank, just not all together.
Solution: Bring in the XT 180mm silver 38/26 cranks and swap out the 26t for a 24t. How difficult could that be. Maybe not so difficult if Shimano hadn't created different 38t rings based on if they were going to be used in conjunction with a 26t or a 24t. Okay, small set back. Time for more research. Go to the Shimano tech doc page and look up part numbers and check compatibility. Nope not listed. At face value, any old Shimano 10 speed 24t ring isn't necessarily compatible as a double with a 38t outer. They have these pesky two letter codes that need both rings need to have in order to be compatible. No distributor, and not even Shimano, have a 24t ring that is compatible with the 38t ring from the 38/26 crankset.
Now, if this was my bike, I would have put on a "non-compliant" 24t Shimano 10s ring and it would have probably shifted fine. However, this was someone's dream bike and they were spending big bucks on it and I wanted it to be right. In the end, the only thing I could do to make it exactly right was to bring in another XT crank (175, black) with a 38/24 ring combo and swap rings. You want your dream bike, I'll make it happen.
Check out the pocketing and ramp profile of the 38t rings: 38/26 on the left, 38/24 on the right. I'm sure Shimano with all their engineering expertise could have figured out a way to make one 38t ring that worked just as good with either a 26t or a 24t ring. They are smart that way. This is the kind of stuff that drives folks to their competitor, SRAM.
(What's playing: The Beatles Money)