most aerodynamic/fairingiest handlebar bag or basket?

General cycling advice ( NOT technical ! )
Brucey
Posts: 35979
Joined: 4 Jan 2012, 6:25pm

Re: most aerodynamic/fairingiest handlebar bag or basket?

Postby Brucey » 20 Sep 2019, 1:42pm

the illustration below compares Cd values for shapes of equivalent frontal area. The values are valid between Reynolds numbers of 10000 to one million, so won't apply to every bicycle part/cyclist at cycling speeds.

Image

airflow is in all cases left to right. The shapes on the left (1-11) are rotationally symmetric 3D objects. The ones on the right (12-22) are 2D shapes, i.e. they have the same cross section in the Z dimension (out of the page). Where a dotted line is indicated, this indicates a hollow section, so for example 2) is half of a hollow sphere, whereas 3) is half a solid sphere.

There are many surprises and anomalies even in this simplistic set of test data; for example (comparing 3) and 2) again) a solid half-sphere facing into the breeze is about 10% more draggy than a hollow one; presumably the wake vortex can swirl in a more useful way in the latter case. Note also that a flat disc has the same Cd value as a hemisphere with the flat side facing into the breeze (comparing 7) and 8 ) ).

Anyway this data suggest that if you are going to have a crude fairing, (eg a hollow shell of some kind) it is liable to work better as a front fairing than a tail fairing.

Things are more complicated at lower Reynolds numbers; the flow is more likely to remain attached even to fairly blunt objects so the Cd values are different.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

hercule
Posts: 969
Joined: 5 Feb 2011, 5:18pm

Re: most aerodynamic/fairingiest handlebar bag or basket?

Postby hercule » 21 Sep 2019, 1:53pm

Anecdata warning...

6 or 7 years ago I fitted a HP Velotechnik Streamer fairing to my Trice QNT recumbent trike. The QNT is a narrow track machine, and the Streamer is only slightly narrower than the trike’s width. I did think about fitting a tail fairing but so far have never got round to it, though I have most of the hardware for a Correx fairing.

It does add weight... at a guess 2-3kg. That said my QNT doesn’t tear up hills anyway (not with me riding). On the flat or downhill it’s barely noticeable.

It’s noisier... not so much from the fairing itself (I made sturdy attachments, much more robust than the original manufacturer’s parts), but from reflected transmission noises. You forget about it after a bit.

I don’t really see much advantage at wind speeds below 16mph head on. Above that the advantages grow such that a usual 35mph descent on the “naked” trike became 42-44mph. I’ve not been brave enough to explore beyond this!

The italics are important because it’s the relative wind speed that counts. Into the wind I notice sizeable gains. My main reason for getting the fairing in the first place was because the ride over to my parents was always hampered by grinding into the prevailing wind on the way home, the time is now much reduced and even comparing to other cyclists I’m passing fit cyclists in full crouch battling the wind. They don’t get much of a tow from me either, so the turbulence I leave behind can’t be too bad.

There is also an effect that a quarter wind can give the fairing a bit of lift so that even if you are partly heading into the wind you get a bit of welcome assistance. With a tailwind it’s like a sail. Even riding in sting winds conditions I’ve never felt that the fairing has adversely affected handling or safety.

This of course applies to trikes; I’m not sure I would be confident to fit the Streamer to my recumbent bike.

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Tigerbiten
Posts: 1923
Joined: 29 Jun 2009, 6:49am

Re: most aerodynamic/fairingiest handlebar bag or basket?

Postby Tigerbiten » 21 Sep 2019, 5:37pm

my understanding is .............

Rolling resistance has a linear increase with speed.
Air resistance has an increase with the square of the speed.
And it's around 15 mph where air resistance starts to become really bad.
It's a bit lower speed if your on a sit up and beg dutch bike and a bit higher speed if your on TT bars.

It's around 7 mph that a good recumbent starts to become more efficient than a bike.
Below this speed then the advantage of the extra power from being able to get out of the saddle is too great.
Once your in the zone where air resistance is dominate then a good recumbent is around 20% faster than a bike for the same energy input.
But also due to your position on a recumbents seat, your maximum sustainable power output may be a little lower than on a bike.
So at high power outputs you may not see the full aero speed advantage of a recumbent over a bike.
So unless the route is very hilly then a recumbent advantage only increases as audex routes get longer.
The fastest JOGLE time is still on a fully faired Windcheetah recumbent trike.

Luck ........... :D

KM2
Posts: 401
Joined: 23 Oct 2008, 5:38pm

Re: most aerodynamic/fairingiest handlebar bag or basket?

Postby KM2 » 21 Sep 2019, 7:50pm

Didn’t Pakit make a set of front panniers which were aero in design?