The prediction is only as good as the model and I am pretty sure BBS isn't going to share exactly how they came up with the number.

A disc doesn't test as fast on the bike as in wheel on.y testing. I would look at the Tour magazine article where they tested TT bikes with discs and deep rear wheels. I remember that the Canyon Speedmax was faster with the ENVE 90 than a disc. The problem with these tests is that they don't account for the power loss for turning the wheel. There are multiple data sources for that, but they don't agree very well.

Jim Manton at ERO or Tom A. would probably be the best source for a ballpark estimate of the savings at low yaw because the Chung approach will mix the watts to spin value in with the rest your your drag.

BBS has admitted (on here) that they under estimate the benefit of a disc in their calculations.

My opinion is that they underestimate it because of rotational drag (watts to spin) which is something more difficult to quantify than translational drag.

---

blog