Don't most Specialized frames have a BB30 bottom bracket?

That’s something I don’t know. I read that with the Roubaix, they made some change around 2012/2013. Why I’m looking for answers before I just buy something. I do plan on buying used as well.

Let's talk about the transition first. The stock specialized S-Works crank pairs with Specializes OSBB or Over Sized Bottom Bracket. Your s-works crank has a 30mm spindle (BB30) and probably pairs up perfectly with what you have for a bottom bracket - which is BB30 as well. Here is a site with a chart you can use. Use the left column to pick your desired crankset and spindle (GXP or BB30, etc) and right column to find the adapter you need so they pair up with the OSBB if needed. Easiest thing to do would be to find the same crankset in 165mm and install it with the current bottom bracket. Use that strategy for both bikes. If you are switching spindle sizes/lengths then you will also need to change the bottom bracket to match the crank.
https://wheelsmfg.com/carbon-osbb-tech


Second, let's look at the 2012 S Works Roubaix. You say Dura-Ace crank so that means it's a 24mm spindle. That means your current bottom bracket has a 24mm hole aka SRAM GXP or Shimano Hallowtech 2. They mean the same thing- GXP/Hollowtech are both 24mm spindle bottom brackets. Which means you should hunt down another shimano crankset in 165mm or hunt down a SRAM 165mm crankset that has GXP in the title so it can be used with your CURRENT bottom bracket. If you find the crankset you want, and it has BB30 in the title, then that means you will need to get a new bottom bracket that is BB30--BIGGER than BB24 (GXP/Hallowtech 2) in order for it to work.

Both of your bikes have OSBB "over sized bottom brackets" which means they are just beefier and both have a native BB30 opening for bearings. Your Transition has that stock bottom bracket bearing still pressed in and it pairs up perfectly with your S-Works crank because that crank has a BB30 spindle.

Your Roubaix has an adapter for the bottom bracket to reduce the spindle size of the native BB30 bottom bracket to 24mm for the shimano dura ace cranks you are running.

You can't use a GXP crank in the Roubaix without changing the BB, GXP is 22mm on one side, 24 on the other.

Thank you for all your information!

I think the best thing for me to do is find the same crankset in 165mm, and leave the BB alone. Although, with the information you've given me, I'm not worried about changing it if I have to.

Thanks again!

You are having it backwards... given both bikes use some type of BB30, you should find the crankset you liked and then figure out what BB you would need to install the crankset. After all, you are doing this to get the crankset you want, not the BB you want.

Here is a few quick points and hope they help:

- if you bike already has BB30 bottom brackets, any BB30 crankset will work with your bike
- if you want to install either Shimano Hollowtech (all new Shimano) or SRAM GXP type of crankset, then you would need a what they call "conversation bottom bracket" - meaning the bottom bracket will convert the BB30 to accept Shimano Hollowtech or SRAM GXP cranks - both Praxis and Wheels Manufacturing make good conversion BBs
- if you are installing new BB30 bearings, make sure they do them right to avoid creaking down the road
- don't use adapter cups - they suck in general

Thanks for the tips! I'm going to find cranksets I want to run, and build accordingly.