Originally Posted by
gregf83
I'd like to see the aero testing. I've never done a test myself but the fenders make the wheel significantly wider particularly with the 2-3" wide fender extension on the bottom of the rear fender. Maybe I'll do some tests this winter.
Bicycle Quarterly, Volume 6 Number 1,
The Aerodynamics of Real-World Bicycles. Chart 3 compares a bare bike vs. the same bike with full aluminum fenders: 8.21 Newtons vs. 8.20 Newtons at 22 MPH. Chart 4 compares a pedaling rider on a bicycle without fenders and with fenders: 26.06 Newtons vs. 25.59 Newtons. Chart 5 compares narrow tires vs. wide tires, both with and without fenders (including rider on hoods in all cases). 30.5 mm tire without fenders: 26.06 Newtons. With fenders: 25.80 Newtons. 24.5 mm tire without fenders: 25.78 Newtons. With fenders: 25.56 Newtons.
Of course, to keep it all in context, simply going from the hoods to an aero tuck drops more than 10 Newtons, so all of the fender vs. non-fender numbers are essentially meaningless - unless one is arguing that fenders cause significant drag. The bottom line appears to be that fenders act as fairings - even though they add width, they help smooth airflow over a very turbulent area.
<edit> Fenders with an additional mud flap at the bottom, whether molded in or added on, do make things worse. The BQ test showed a full leather front mud flap adding .18 Newtons of drag, bringing the total drag to .17 Newtons more than the same bike without any fenders. I'm frankly surprised it's so small; that big old mudflap on my rando bike makes me think I'm riding a semi. But in reality, I seriously doubt that a couple of tenths of a Newton are going to make much difference unless you're time-trialling.