BSA, BB30, Press Fit, 24 or 30 mm shaft, Boost rear triangle, plus various bottom bracket widths: The variety of bottom brackets and matching cranksets is now as great as the milk and coffee combinations in modern hipster coffee houses.
Sram is going its own way and standardising the diameter of bike crankshafts and bottom brackets. These are currently offered with a diameter of 24 mm (GXP) or 30 mm (BB30, PF30). Now, however, one of the two sizes is not simply being dropped without replacement. Instead, the new standard for MTB drivetrains from Sram will be exactly 28.99 mm everywhere from the coming season. DUB (Durable Unifying Bottom Bracket) is the name of Sram's new bearing technology, which of course involves more than just the standardised shaft diameter.
Rather, the diameter is the result of intensive research work. The search was on for the optimum compromise between low weight, high rigidity and high durability. The new diameter turned out to be ideal, argues Sram and enthuses: "A DUB crankset is simpler, more universal and more durable" than any Sram crankset before.
Sram DUB: What's changing?
New 2018: Sram DUB - changes at a glance:
- Standardised oversize crankshaft diameter (28.99 mm)
- Aluminium crankshafts
- DUB bottom bracket (available in almost all bottom bracket designs) with new seal for increased durability
- New, lighter carbon tuned crankset (XX1, X01)
- New, lighter CNC Eagle SL chainring (XX1)
The shafts are now all made of aluminium (in various alloys) and pressed or, in the case of the high-quality X01 and XX1 cranks, glued to the left crank arm. In order to at least maintain the rigidity of the 24 mm steel shaft and at the same time become lighter, the diameter of the hollow aluminium shaft had to be correspondingly larger. Why not use the familiar 30 mm? According to Sram, the bearings for 30 mm shafts always lacked the space for a proper seal. With the now 1.01 mm thinner shaft, this space has now been gained in order to be able to use a new solution with an additional O-ring. This should increase the durability of the bearings.
In addition, a new carbon layout for the DUB XX1 and X01 cranks has enabled additional weight to be saved. The in-house CNC Eagle SL tuning chainring, which is also new, allows a few more grams to be shaved off. According to the manufacturer, the Sram DUB XX1 Eagle crankset weighs just under 80 g less than the previous GXP version and weighs a record-breaking 498 grams (manufacturer's specification) with 175 mm crank arm length, 32 mm chainring and bottom bracket.
Compatibility with frames is maintained
Nothing changes on the outside, the interfaces to the frame remain compatible. Sram DUB bearings are available for the following bottom bracket types: BSA 68, BSA 73, BSA 100, PF89.5, PF92, PF121, BB30 73 and PF30 73. Also the same: compatibility with Boost 148 rear triangles is still guaranteed via chainrings with either 3 mm (Boost) or 6 mm offset. The prices for the bottom brackets range between 38 and 52 euros.
There are no DUB bearings for frames with Super Boost Plus rear triangle (157 x 12 mm) and BB30 Ai, Cannondale's own bottom bracket type. There are also no changes for downhill cranks for the time being. The Sram DH cranks will only be converted to DUB in the future.
Conclusion
Sram DUB is designed to strike the perfect balance between lightweight construction and durability. To achieve this, Sram is switching all MTB cranks in 2018 to aluminium cranks with a standard 28.99 mm (instead of the previous 24 or 30 mm). Despite the same or improved stiffness, Sram was able to reduce the weight of some cranksets to record levels. The lighter DUB bottom bracket and the new Eagle SL chainring also help here. New seals and tighter tolerances should also increase the durability of the bearings compared to GXP bearings.
Sram is going its own way with the new shaft size. From now on, only Sram cranks will fit in Sram bottom brackets, and vice versa. It is not possible to swap just one of the two components. However, if you change your frame in future, you can keep the new Sram cranks, regardless of which bearing standard the frame requires. Exceptions: Cannondale BB30 Ai and bikes with Super Boost Plus rear triangle.
The first Sram DUB cranksets and bottom brackets will be available from February 2018, some models later.
Info: www.sram.com
Sebastian Brust
Editor
