In Reply To:
In Reply To:
Is that 50% a conservative estimate? Even with only a 30% reduction the front option looks pretty good at 8 mph. Bottom line is I already have a Jet 60/90 combo and don't want to buy two more wheels. It looks like I'd be better off with just buying a 90 front, in most cases.
I can't put a confidence number on the 50% figure. You'd have to poll the true aero experts like J Cobb, A Coggen, R Chung, Chicanery, or Tom A for a better estimate.
Don't overlook the handling consequences of a 90/90 combo. It's been said, and I've even bought into it myself that a rear disk moves the center of lateral pressure rearwards and improves handling. I don't quite buy that any longer. Yes, it does move the CLP rearwards, but it also increases the magnitude of the lateral force. Further, the rear disk location in the frame gives it no effect on steer tube torque. Nevertheless, it does have a greater gyroscopic stabilizing effect due to its higher moment of inertia. It's this higher MI that I believe is really behind the improved crosswind handling of a rear disk.
I'm not sure if you quite have the right take on the handling bit...here's my take:
A deep wheel on the front tends to put a torque on the steering axis that turns the wheel away from the side wind since the center of pressure is in front of the steering axis...BUT, what does that do? It actually causes the bike to turn INTO the wind due to the countersteering effect (i.e. to initiate a turn to the right, one actually starts the turn by steering the wheel to left. This causes the CG of the bike to shift down and to the right, initiating the right lean of the bike).
Now then, what happens at the rear with the disc? Well, the wind will tend to tilt the bike AWAY from the wind gust...thereby counteracting the front wheel wanting to tilt the bike INTO the gust. Make sense?
Of course, all of that also doesn't take into account any "splitter plate" effect of a solid disc on the rear of a bike, which can actually help decrease the drag at zero yaw.
http://bikeblather.blogspot.com/