http://cyclingtips.com/...ero-disc-road-bikes/ According to Yu, earlier Venge ViAS Disc development mules with post mount brake hardware had already lessened the aerodynamic drawbacks to rim brake bikes, but it was the switch to flat mount that closed the gap entirely.
“[On the post mount prototype], there was a super small difference in terms of aero — 2-4 seconds slower at low to moderate yaws (below 5 degrees). Once you got out to 15 degrees, it grew to around 8 seconds. What we found in more recent testing on production frames and forks with more finalized shaping and flat mount hardware, that gap consistently shrunk to the point where I’m comfortably calling it essentially the same aerodynamics between the two fully built bikes.
“The difference at those same low-yaw angles, say 0 to 5 degrees, is effectively zero to maybe 2 seconds max over 40km. At yaws of 15 degrees, it was no more than four seconds on one side, and the other side was essentially zero seconds difference. When we test fully built bikes, our margin of error is in the neighborhood of a second and change so we’re really pushing into that margin of certainty, anyway. That’s how close we feel the two bikes are.”
At least according to Specialized’s in-house testing, the main culprit is the rotor.
“It’s mostly the rotor, and the fact that the hubs are usually a little bit bulkier. In that handful of seconds difference among most of the bikes we tested, the difference is greater in a crosswind where the rotor is more exposed. And it’s not just the rotor; it’s also the carrier and all that other material that’s directly exposed.”
Not surprisingly, then, it’s this area where Yu (reluctantly) admitted the most benefits can be potentially gained.
“I would concentrate more on the rotor, and the carrier, and the hub interface — that entire area — before I would concentrate on the caliper. Maybe even how the fork plays with all of those things, too. That entire area near the rotor, the design focus is better used addressing the rotor and hub area rather than the caliper.
“If I ticked those boxes, then I would look at the caliper to see if there’s something I could do there but I wouldn’t spend resources there. I think you can squeeze a lot more from the same effort out of the rotor and hub.”
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