The Ceepo Shadow-R ( http://aerogeeks.com/...shadow-r-first-look/ ) would put a nod in the maybe camp, but probably only if designed as part of the bike. Seems fairly unlikely that standard fenders would be net positive (specifically thinking of the part that wraps around the back and seems like a wind scoop), but this is all armchair speculation.

If the fenders are any wider than the tires they cover, then very likely they add drag.

However, if the fenders are equal to (or less than) the width of the tires, then I think that fenders could be designed with aerodynamics in mind. For example, since fenders don't spin like tires, I can imagine a scenario where the front part of the fender is designed as an optimal leading edge, with the aft part of the fender designed as an optimal trailing edge. In that case, I think it's a realistic guess that fenders could definitely improve the wheel and bike aerodynamics.

But, standard, off-the-rack fenders reducing drag? About that, I would say no.

---

Advanced Aero TopTube Storage for Road, Gravel, & Tri...ZeroSlip & Direct-mount, made in the USA.
DarkSpeedWorks.com.....Reviews.....Insta.....Facebook
--

Hello avatar78,

Yes fenders can improve aero .... However for Ironman branded races or USAT races:

Ironman

(b) Fairings are prohibited. Any device added or blended into the structure that may
decrease, or that has the effect of decreasing, resistance to air penetration, or that may
artificially accelerate propulsion, such as a protective screen, fuselage form fairing or the
like, is prohibited; (DSQ)


USAT

(e) There must be no protective shield, fairing, or other device on any part of the bicycle (including frame, wheels, handlebars, chain wheel, and accessories) which has the effect of reducing resistance. Aerodynamic carriers for food, water, and or cycling provisions may be attached to or be an integral part of the aero-handlebars if they meet the following guidelines:

---

Cheers, Neal

+1 mph Faster

The forward extension of a front fender almost certainly results in less drag if it's made fairly long (extending past the top of the tire) than if it's made very short, since a long extension can have the same frontal area but actually shield itself versus just being a wind scoop. Which is why it's especially annoying that most fenders are made very short out front: they'd do a better job blocking spray as well if they were longer. In some cases this front segment by itself might result in less drag than having no fenders at all (which is the main basis for fender designs that are explicitly sold as aero fairings).
But it's generally agreed that the lower run of the front fender will add drag. And, you don't want to skimp on this part of a fender if you want it to be effective at its main job. Because of this, not even Jan Heine claims that front fenders designed for blocking water spray are ever better than aero-neutral on the whole.

I have no idea as to the effect of a rear fender.

As always, the exact impact will depend on the particulars: frameset, wheels, fenders, and how they're set up.