BryanD wrote:
I'm just going to sit back and wait. If the tunnel data proves the bikes are faster, that's good for all of us.
However, as Damon has pointed out, you have to be careful in how you define "faster"...remember, he said that plopping rim brakes on a frame designed around discs can be just as bad as plopping disc brakes on a bike designed for rim brakes. Apples to apples comparisons are important. As I've often said, you need to compare "best in class" to "best in class"...not just aerodynamically, but in braking performance as well!
If you TRULY want to know which is faster aerodynamically, then you can't compare any future "disc optimized" bikes to current frames that are not to the same level of integration...AND you need to make sure that you take advantage of OTHER features which one or the other system allows in order to reduce the overall system drag. For example, it's silly to do a rim vs. disc comparison with wheelsets with the same amount of spokes in each since the rim brake setup allows the use of less spokes due to the spokes not needing to transfer braking torque. The number of spokes in a wheel affects both the translational AND the rotational drag (don't forget to measure "power to rotate" in these tests!) Now then, we're talking about a disc wheel here, so that example isn't exactly applicable...but it IS applicable to whatever front wheel you use in the total system comparison.
We saw the danger of these sorts of comparisons when the disc Venge ViAS was released. According to the narrative given, the bike was originally started as a disc braked design...and then in the middle the focus was apparently changed to rim brakes, which obviously means that compromises were made in the overall aero performance. Then, that development led to some "lessons learned" in the frame design and layup which were applied to the eventual disc version. So, for the claim to be made that the disc version of that bike is only
slightly less aerodynamic and only
slightly heavier than the rim brake version is a bit misleading since it's comparing a "compromised" (design-wise) version #1 of the rim brake model vs. a basically "clean sheet" version #2 (incorporating improvements learned from version #1) for the disc model. Not really "apples to apples"...
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