I have an Allied Alfa Disc with SRAM Dub and Red AXS Crankset (with Quarq pm).... and I want to switch all my components onto this 2020 Trek Madone SLR 7 Frameset. My LBS says I have to buy a new SRAM crankset with a GXP spindle.
Is that the best option? Can I do something else like use different bearings or this kit?
Modern cranksets either have 24mm-ish width spindles, or 30mm-ish spindles. DUB (28.99mm spindle) falls into the latter category. GXP spindles (24mm, tapered to 22mm on one side) fall into the former. Almost all press fit bottom bracket shells can take cranks with 30mm or similar spindles. Trek’s BB90 bottom bracket standard appears to be an exception. That is described in this Cyclingtips article and it appears confirmed in this BB compatibility matrix by Ceramicspeed. The article was written before SRAM introduced the DUB standard with its 28.99mm spindle, but the Ceramicspeed document seems to indicate that BB90 shells aren’t compatible with DUB spindles. Consider that the BB90 shell has the smallest internal diameter of all the press fit shells. It has a 37mm internal diameter, while the next smallest shell has a 41mm ID. There is not be enough space to fit the DUB spindle and adequately sized bearings in a BB90 shell.
The BB90 bearing kit you linked to will fit GXP spindles only. You can’t cram a DUB spindle in there. It does appear that your only option is to get a power meter crankset with a 24mm or similar width spindle. You could also switch to a different power meter type, e.g. pedals, which are definitely worth considering if you’re already using Look pedals. The BB86 bearing kit you referenced in comments will physically not fit in your BB shell; BB86 shells have an internal diameter of 41mm, but your Madone's BB90 shell has an ID of 37mm.
As a side note, I suspect this is part of the reason that Trek is moving toward the T47 threaded bottom bracket standard. They did so on several 2020 bikes, including the Domane and Emonda - however, they did not update the Madone (the aero road bike) in 2020. For the record, T47 uses the same internal diameter and width as PF30 (46mm and 68 or 73mm depending on road or mountain bike). However, the interface is threaded, and the bearings sit just outside the frame, whereas most PF30 bearings are just inside the frame. Placing the bearings and their cups externally enables them to be bigger, and the wider spacing should mean lower loads on the bearings. Both should mean longer bearing life. I am not convinced that 30mm spindles are noticeably better than 24mm spindles, but the market did demand 30mm, and it was always a problem that Trek's BB90 couldn't accommodate those spindles. In theory, someone might be able to design a DUB BB90 BB, but there is not much space to play with. If it is physically possible, there's not likely to be much demand for that application because Trek is moving away from BB90, and that standard was proprietary to Trek.
