Why did Cervélo develop a floating rear derailleur hanger for its disc-brake frames?
Our RDH was developed to reconcile incredibly tight engineering tolerances with our manufacturing processes. How tight? The two rear dropouts must be parallel (or “flat”) to within 0.1mm of the bottom-bracket’s outside face. The top and bottom brake surfaces need to be similarly perpendicular to the dropout face, and the thru-axle holes must remain perpendicular to the dropout face as well.
To ensure a “running fit” that allows a thru-axle shaft to slide through easily and turn freely with no binding, these tolerances become even more extreme. Our initial goal of 0.04mm of drive-side to non-drive-side concentricity — that is, of forming perfect circles around a common axis — is exceptionally difficult to maintain between any two surfaces, let alone molded, bonded, and painted carbon-fibre ones! Establishing these tolerances was one thing; manufacturing frames to meet or exceed them was quite another.
These near-impossible values led us to explore alternatives. Thus was born the concept of the floating RDH (or as we engineers prefer to call it, the laterally decoupling concentrically co-locating RDH).
Here’s how it works: The hanger portion of the assembly is fitted into a recessed hex slot on the inside of the drive-side dropout, followed by the wheel. The thru-axle is then inserted and the threads loosely engaged, perfectly aligning the threads on the drive side with the non-drive side hole. The hanger nut is then installed and torqued down onto the male portion of the hanger, which fixes the alignment permanently and achieves a perfect concentric location of the left and right sides of the frame.
The result: A free-running, non-binding fit that satisfies our manufacturers’ needs for repeatable, achievable dimensional tolerances, and that gives Cervélo riders and dealers the ease-of-use, ease-of-assembly and ease-of-adjustment they desire.
This approach also allows us to avoid “tolerance stack,” whereby errors in tolerances on hubs, frames, RDH holes, brake holes and so on conspire to make bikes untunable. And because the RDH is physically located off of the axle — as opposed to a traditional QR setup, in which the axle/hub floats relatively freely in the dropout — we can ensure the best possible alignment of the RD mounting point with the hub.
In short, a relatively simple solution yields myriad benefits for the rider, mechanic and dealer.