Originally Posted by
catgita
Someone has to mention Jan Heine! He did a wind tunnel test on a rando bike with and without fenders and found no difference. By memory they were 38mm fenders on 32mm tires. The article speculated that the increased frontal area was canceled by the smoother air flow. These were well fitted, permanently mounted, full coverage aluminum fenders. Poor fitting, clip on, open on the sides fenders may be totally different.
BQ posted the following summary of the tests on their blog:
"Well-mounted metal fenders do not affect the bike’s aerodynamics. The front section of each fender shields the tire and reduces the wind resistance, while the rear fender increases the aerodynamic drag. The two effects cancel each other."
While I understand the words, there is so much information left out of the summary that it really sounds like, "it depends" is what they're saying. But, I've never seen the test methodology, results, or findings, so the degree to which "it depends" is unknown to me, but I don't see how effects can cancel each other out all the time, in all setups, in all conditions, so it's a suspicious claim.