Randonneur aerodynamics

On 3/16/2021 1:17 PM, jbeattie wrote:
On Tuesday, March 16, 2021 at 9:03:17 AM UTC-7, Frank Krygowski wrote:
On 3/15/2021 10:21 PM, Frank Krygowski wrote:


About the wind tunnel: In one issue long ago, [Jan Heine] did pay for wind tunnel
time. I should dig it out (assuming the guy I loaned it to has returned
it), but the points I remember are that drag coefficient is barely
changeable, so frontal area is the most important thing; and that
(surprisingly) fenders don't impose a significant aero penalty.

But I remember in that issue he attached a bit of cardboard to the top
of a handlebar bag hoping it would send air over the rider in a touring
position - which, of course it didn't. I thought the idea was ludicrous,
and showed that his aero instincts are not very sharp.

However, I agree with Russell that the handlebar bag can be an aid in a
full tuck. My homemade bag is larger than most and less boxy. I've long
suspected it's part of the reason I outcoast so many of my friends.

I recognize that this is in conflict with "drag coefficient barely
changes." More on that in another post.
OK, I found the old 2007 issue of _Bicycle Quarterly_ with the extensive
wind tunnel article. There are lots of measurements to evaluate
different sources of drag. I'll concentrate on just one chart.

Page 21, chart #6 has drag measurements for 22 mph (= 9.83 m/s = 35.4
kph) with and without a handlebar bag. Drag is in Newtons in the table,
but I'll convert to aero drag Watts at 22 mph, since we're more used to
thinking in those terms.

Rider on hoods, no bag: 256 W with bag: 262 W
Rider on drops, no bag: 238 W with bag: 249 W
Rider in aero tuck, no bag: 160 W with bag: 163 W

(Those Watt figures ignore rolling resistance and climbing power. And of
course, a rider can't pedal in an aero tuck. The rider was pedaling in
other positions.)

The handlebar bag he used is one of the large, sharp-edged boxy ones. I
have vaguely similar bags on a couple bikes, but the two bags I made
myself are more smooth, curved and tapered at the front. I suspect it
produces less drag, perhaps even a net benefit.

BTW, I mis-remembered one point: Drag coefficient dropped by 7% in the
aero tuck position; I had thought it stayed closer to constant. This is
in addition to the major benefit of the tuck, which was the reduction in
frontal area.

So unless I'm missing something, the testing shows that no bag is more aerodynamic and that there is more drag with a bag, viz., exactly opposite of Jan's claim.

Right. What he measured then is different than what he's claiming now.

I was going to swap my 16lb Emonda for a steel Renee Herse with 45mm tires and bags to see if I could increase my speed.

Try it and report back!

My prediction? Not much difference either way, assuming by "bags" you
mean just a handlebar bag. Panniers do slow you down.

Related: A few days ago we rode to get groceries. For the first time in
years, I failed to break the 25 mph speed limit on the downhill at
village center. The wind was blasting out of the north and the very
square shopping panniers were full to overflowing.

Today's grocery run yielded 26 mph, so once again all is right with the
world.


--
- Frank Krygowski