Precise details of how SRAM managed to get the weight of the new Red road groupset below 2kg became clearer at the Freidrichshafen show, where pre-production samples were available for inspection alongside Scott Addict frames running complete working groups.
Perhaps the most innovative of the several major components is the cassette, which gets under 160g despite being made almost entirely of steel. SRAM wanted to use steel for the cassette in order to maximise durability and opted to machine the cassette’s eight largest sprockets in one piece. The basic idea is not new, having been realised in aluminium by more than one small-scale CNC-machining specialist. Weight, however, is invariably the issue when working with steel.
To minimise the amount of material in the piece, the cassette itself is machined from a steel forging to make a hollow cone (called PowerDome) with a series of concentric steps on the inside, each of which coincides with the base of one sprocket. Eight individual sprockets are machined on the outside. The inside face of the largest sprocket, which forms the base of the cone, has an inset shoulder with tooth forms, which match the shape of the aluminium plate that forms the inside face of the cassette. A sleeve fits between the face plate and PowerDome to support the plate when the lockring is tightened.
The matching tooth forms transmit drive torque from the PowerDome to the plate, while the aluminium plate itself houses the splines that transmit drive torque to the freehub body. Therefore, drive in any sprocket on the PowerDome goes through the face plate to reach the freehub body. By making the internal splines on the plate roughly twice as wide as those on any standard 10-speed sprocket, SRAM can transmit maximum torque without risk of damaging the freehub splines.
The face plate is press-fitted into the PowerDome during manufacture, and the assembly will be supplied for fitment as a complete non-separable unit. The lockring and two smallest sprockets, made as individual pieces, complete the Red cassette. Expect to see a range of sprocket combinations with options from 11 to 26 teeth.