Hello I have a Trek Crossrip ltd 2015 gravel bike. The frame is alpha aluminium 100 series and the bottom bracket is Hollowtech II. What bottom bracket should I replace it with?
Welcome to Stack Exchange. The Trek Crossrip has a BSA threaded bottom bracket shell that is 68mm wide, which is very common for a midrange aluminum road bike. The current crankset (given that your bike has the stock components outfitted) is a compact Shimano Tiagra. This is a Hollowtech 2 type that features external bearing cups and the 24mm spindle attached to the drive side crank. Pinch bolts on the left crank arm secure the crankset in place, and a plastic nut within the left crankarm threads into the end of the spindle and preloads the bearings prior to being secured by the pinch bolts.
Obviously a direct replacement for the bottom bracket, which here are 2 external, cartridge bearing cups threaded into each side of the bottom bracket shell, is any Shimano Hollowtech 2 compatible, BSA threaded for road bikes. Many companies other than Shimano make their own version of this bottom bracket standard that will work. A compatible Shimano branded BB model would be SM-BBR60 or SM-BBR501.
Specs that must be adhered to for a direct & proper BB replacement: BSA threaded, road style, for a 68mm BB shell, for a 24mm spindle. Variations in compatible BB's that do not affect the proper fit and use, but do very much affect the price, include the type of bearings used (steel or ceramic) and their grade (a measure of how round and smooth the bearing balls are to a certain tolerance), amount and type of sealing used to protect from the elements, and material used for BB itself.
Please note that the bottom bracket choice is dependent on two things. First is the BB's interface with the bicycle frame (the bottom bracket shell). The shell can be different widths, have different sized bores, have threads or not have threads. Each variation in the type of shell and thus, it's BB requirement, represents a different STANDARD. Your Trek has the 68mm BSA threaded standard. The second item that BB choice is dependent upon is the choice of crankset you want to use. There are many standards of that part, most of which refer to the shape, type and width of the spindle the crankarms will affix to and thereby drive the bike. Your current crankset has the 24mm Hollowtech 2 standard.
I've assumed you want to continue using your current crankset. If so, a Hollowtech 2 (24mm spindle), BSA threaded for 68mm, road bottom bracket is what you'll require. A different crankset may require a different bottom bracket if it represents a different standard generally having to do with placement, size and type of spindle.