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very rough estimate:
ignoring interference and shielding (from the tyre):
Front fender drag coefficient : 2.3
(shape 20)
area: 0,045m x 0,4m = 0,018m2
cda of front fender= 2.3 x 0,018 = 0,0414
which is (via drag to watts calculator at the bottom of the page)= about 14 watts at 30km/h so less than that :) (because it is behind the tyre)
if only considering the 2 cm it sticks out wider than the tire: about 6 watts at 30km/h for the front fender
edit:
or apparently buy Bicycle Quarterly Vol. 6, No. 1 and Vol. 6, No. 3. and find out ->
https://janheine.wordpress.com/2012/05/02/aerodynamics-of-real-world-bicycles/http://djconnel.blogspot.dk/2011/04/aero-road-frame-versus-front-fender.html
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Bicycle Quarterly done a test in a wind tunnel actually, will find the article when I can
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The more aerodynamic your front wheel is, the greater the impact of the front guard. A 45mm guard is almost guaranteed to catch the entire tyre-wake. Making the rear of the guard less round might.
The airflow's pretty messy by the time you pass the seat tube, so the rear guard itself probably won't change much, but the fittings might if the rest of the setup is clean.
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svenhöek
edscoble
Emyr
@TurtleRecall
Out of interest, does anyone know (or could make an educated guess at) what difference to aerodynamics a set of 45mm SKS Longboards will make?