Just over fifteen years ago, the outboard-bearing bottom bracket and 24-millimeter crank spindle answered countless prayers of countless mountain bikers. But it begged the question: Could we do better? So of course, soon after, the 30-millimeter spindle was introduced, and it touched off a controversy that we may never see the end of. BB30 mandated a new frame standard and opened the floodgates to several more.
But riders with traditional bottom brackets still longed for soup-can-sized spindles, so some purpose-built cranks and very thin bearings emerged as a solution. Then, that solution created a problem. Though these cranks are lighter, stiffer and stronger, the retro-fit bottom brackets have a much shorter lifespan than those built for 24-millimeter spindles. In a perfect world, we'd have something that offered the structural benefits 30-millimeter spindle cranks with the durability benefits of 24-millimeter bottom brackets.
Enter SRAM's new crank and bottom bracket system, the Durable Unified Bottom Bracket, or “DUB.” Before we cover what DUB is, we should make it clear what it is not. It is not a new frame standard. DUB bottom brackets are available in BSA (threaded), PF89/92, BB30, PF30, as well as fat-bike-specific threaded and press-fit standards. All the leading boxes checked.
The size of the spindle is new, but whether that makes it a new "standard" is really a matter of semantics. And SRAM is betting big on DUB. Each and every Eagle-compatible 1x crank (that's; XX1, X01, GX, and both carbon and alloy Descendant and Stylo cranks) will now be available in the DUB configuration.
DUB crank spindles have the intriguing diameter of 28.99 millimeters. No need to reach for your calculator watch, that is indeed just one millimeter smaller than the 30-millimeter spindles that have long threatened to change our frames or shorten the lifespan of our bottom brackets. But SRAM claims that one millimeter helped make it possible to design the DUB bottom brackets to be even more durable as their 24/22-millimeter GXP. They use higher quality bearings and a fundamentally new system for sealing out moisture and debris. BB30 and PF30 cups are aluminum, and the thin-walled PF89/92 cups are steel. DUB aims to be stiffer and more durable, and the whole system is significantly lighter. You save nearly 100 grams at the XX1 Eagle level.
SRAM will continue to support GXP and 30-millimeter-spindle cranks, so don't expect to be forced into this new concept before you're ready. And if you are ready but you've got a non-boost frame, you won't be forced out of it either. SRAM boasts that DUB is "backward and forward compatible." But forward compatibility is hard to predict. Look far enough forward, and maybe our bikes won't even have cranks.