Latif

I've been looking at options to change the crankset gearing options on my Trek Domane SL5 gravel which came with 105, 50/34. I'd like to go with 46/30 but it seems all of the aftermarket options don't jive with BB90. I've had my mind set on Absolute Blacks oval chainring setup , although a pricey option. I see now Shimano has it's new GRX gravel line but can't find any information on whether their 600series 46/30 crankset will work with BB90. I'm a greenhorn as I've never had to navigate the world of cranksets and BB compatibility much less replace them. May use my LBS for that piece.

Thanks,
Latif

fronesis

Trek's BB90 bottom bracket standard is compatible with Shimano 24mm cranks. Since the new GRX cranks will be normal Shimano 24mm cranks, you should have no problem putting a GRX crank into a BB90 BB.

The question of installation is a separate one: depending on mileage, while upgrading the cranks you may want to replace the BB. Since BB90 is a press-fit-type BB (bearings are pressed directly into the frame), you will either need the proper tools for the job, or you can take it to your LBS. Key note here: installing a Shimano crank is EASY and requires no special tools; it's installing the press-fit-type BB that's not straightforward.

Latif

Thanks so much, sounds like it'll work. Treks BB90 is wide and I've been trying to get my head around how that works as my other road bike has older Shimano 9sp Ultegra crank but in a narrower threaded BB. If I understand the literature on the new GRX series I may need a new front derailleur too because of wider chainline issues. Sounds like the GRX line will be at dealers in a few days, will take it from there.
Bike is less than a year old and not a whole lot of miles so far so hopefully no BB bearing issues. It's been creak free so far.

fronesis

You can picture it pretty easily just by looking at your old bike: on it, the bottom bracket section of the frame is relatively narrow, but the BSA BB cups EXTEND the width of the overall bottom bracket when they screw in.

On the Trek, the frame part of the bottom bracket is much wider, but the bearings press INTO the frame, rather than extending out. So your crank doesn't know the difference: the total effective width of the bottom bracket is the same, because it encounters the bearings at the same location.

BB "compatibility" is mostly about the diameter of the crank axles, not the widths of the BB. Since the BB92 is designted to work with Shimano 24mm crank axles, you are fine. If you had a bike with BB30, the 30 refers to the axle diameter, and this would cause issues if trying to fit a Shimano crank.

That's my understanding from reading the literature: the GRX 2x cranks should be fitted with GRX FDs.

Latif

Once again, thanks so much! With your description I see it well now. It's time I do the work myself and I've got a great LBS to back me up if I get into trouble. I'll get the parts from them regardless.
The other piece of this conversion is a solution for the rear end from GearOops which allows me to take off the 11t small cog and use the 12t cog as 1st position with a lockring adapter. They supply a 36t that will go on the other end giving me a 12-36 11sp cassette. I don't ever use the 11t anyway and I'll end up with 46/12 and 30/36 which hopefully will give my aged tired legs what they need and cut down front shifting on our many rollers.