RubeRad

Everybody's got their own area of OCD/expertise, be it cars, computers, audio/video equipment, wine, flashlights, ... For us it's bikes, and that's why we're here!

bigbenaugust 

But to answer your question, the lid probably does make it (I only have one) more aerodynamic than without. It does, however, stick out like a sidecar. That is the price you pay for plenty of room and the sturdiest piece of bike luggage ever.

steedjerald

Here is the question for those commenting about OCD. Ever thought that normal people improving and expanding their knowledge about things look at those that keep obsessing over what they are studying and posting about somebody's OCD when they are the ones with the symptoms? Think about that! Ha...

Good data and time well spent wphamilton...

wphamilton

No worries Jerald, cyclists have to be a bit OCD in the first place I think. It rained all day Saturday, along with my wife's birthday I couldn't get out to do the other tests but I plan to do some tomorrow, with hopefully more interesting results than the first one.

wphamilton

Short version: A little difference with panniers vs Backpack, significant improvement strapping it to the rack.


It was the same protocol as before, four tests each with panniers, backpack, and strapped to rack. I threw out the runs contaminated with passing cars and averaged the remaining runs, smoothed with a 3-point moving average. All downhill runs began from a dead stop and without pedaling. The riding position is on the hoods.

Here is the test bike, my foul weather beater: (don't laugh at my ride. It's served me well for a lot of miles)

with panniers: these are actually canvas bags strapped together and held over the rack with a bungee cord. The panniers are holding slacks, shirt, socks, belt, shorts, phone, pump and the backpack, the same as if I were commuting except for the extra backpack. Altogether it weighs about four pounds.

backpack: which also contains the pannier bags. This time the backpack is full and at commute weight. The straps are short so it rides high on my back.

For the last set, the backpack was simply strapped on top of the rack.

The chart shows that there is almost no difference between wearing the pack and having panniers. Strapped on the rack however there is about a .6 to 1 mile per hour improvement at 25 mph, and that is a significant difference. Anyone care to calculate the ratio of CdA, or I will later (and then the traditional "saves x watts at 25 mph" format).

Zooming in on the highest speed range of the tests:

You can see that the panniers actually beat the backpack by up to about .3 mph, and the rack was up to 1 mph better than the backpack.

wphamilton

The big difference in this and the first test is the backpack. The first time it was about half full with loose plastic bags and held low, which made no measurable extra drag. This time a pack stuffed completely full and held high resulted in the most drag of any method.

first test, half full backpack:


second test, full backpack:

Bjforrestal

Sweet. So trunk bag least drag, Then panniers, then backpack? I was assuming backpack to be second and panniers to be 3rd but with the legs in front I could see this. Now can you do it in a wind tunnel? Seriously though, thanks for going to all the trouble!

wphamilton

I've been wondering about it for a long time. I had assumed the same way you did, but now we know. If not certainly (because of experimental errors), at least with some confidence. I'm surprised at the size of the advantage of trunk bag over backpack.

Wider panniers of course would mean more drag, but for the size of a minimal commuter load I can't argue with the observed results: we're better off with just strapping it on the rack as far as power is concerned.

GuitarBob

I absolutely love that you did all of this. Thanks, from a dyed-in-the-wool empiricist!

martianone

Very nice data !!!
Have short legs and very long torso, wide shoulders - my upper body is a sail.
i wear an old fashion pack basket while commuting - I know it looks odd, but is very comfy & probably doesn't help the aerodynamics, also commute riding position is fairly upright. So I ponder the affect at my normal commuting velocity - around 20 kph. The drop data indicated velocities around 40 kph. So with resistance increasing with square of velocity ( and energy needed to overcome that resistance increasing by approx cube), wonder if there is any pragmatic impact on my commuting wattage?

seafood

Fantastic experiment! Thank you for putting this together. I believe the main reason for the rack is such an improvement is because it plays a role in smoothing out turbulence behind the rider. In most applications, proper aerodynamic drag improvements tend to happen most at the rear, which is a little counter-intuitive.

idc

Just wanted to post and say I love these experiments. Nice data.

spivonious

Great work! I'm not surprised that the rack bag was the least drag. It's pretty much directly behind your torso.

It is worth pointing out that the drag differences were very small until the bike hit 20mph.

wphamilton

And at 15 mph there is no measurable difference between any of the options!

bigbenaugust 

I had read somewhere that 19mph was the magic speed where the aerodynamic term of the thrust v. drag equation started to matter. Maybe it was Bicycling Science. I will have to dig that book up tonight and check.

alan s 

You don't really need to run tests to determine which is more aerodynamic. Just look at how much is exposed to the wind, and you can easily tell. If you sit upright, a backpack will add nothing to windage, but it you ride in a lower position, it will increase your windage. This difference is measurable at any speed, but the differences are magnified at higher speeds due to the exponential relationship between windspeed and drag.

wphamilton

Sorry but I disagree! The reason is, the shape and texture can matter as much as the reference size, and additionally what the air is doing at that point. Is it attached to the surface in a smooth flow, a momentary separation and reattachment, turbulent, low pressure? What about parasitic drag? And then we have real world considerations such as what happens when the wind shifts even a few degrees. It can become complicated very quickly and the situation we see on a bicycle is more dynamic than we'd think at first grasp.

alan s 

I completely forgot about parasitic drag. Never mind.

no motor?

Maybe that's why I feel faster with the backpack when compared to the pannier - most of my time is spent under 19mph.

droy45

I have found panniers to win hands down in every case. I am not so concerned with aerodynamics as long I don't have a sail sticking out there. I don't like backpacks because it adds a lot of weight to your butt/saddle which uncomfortable and its just too hot depending on the length of your commute. On my 31 mile round trip, a backpack would kill me. Make the bike do the work for you.

Aravilar

Semi-necro, I know.

Did you adjust the data for the fact that your acceleration is positively impacted by your mass? Given a specific CdA, you will actually go faster downhill if you weight more.

fietsbob

The FIM Superbike Race leathers include a hump at the top of the back, so the trailing edge of the rider's helmet has a bit more laminar flow

rather than the turbulence of a round helmet .. those pointy tail time trial bike helmets are working with the same principles .



Galileo theorized that a feather and a hammer in a vacuum would fall at the same rate, , it took a Moon Landing to prove him right.

Gnosis

Actually, Galileo’s theory was proven long before the Apollo 15 manned moon landing via an evacuated tube using a feather and a one guinea coin by John Miller back in 1761, which was 123 years after Galileo’s great work “Dialogues Concerning Two New Sciences” was first published in 1638. In the evacuated tube, the feather and the coin would repetitively free-fall at identical rates thus having proven Galileo’s theory. It’s known as the “Guinea and the Feather” experiment and has been witnessed by countless physics students over many centuries now.

Apollo 15 astronaut commander David Scott was merely the first human to perform the experiment on a heavenly body that was void of an atmosphere.

There are various places where the information I provided can be found, not least of which locations is the NASA website I’ve provided a link to below. If interested, do a “Find on this page” at the NASA website using “John Miller” as the search word…

Hammer and Feather

When it comes to physics and the history thereof, I’d never give anyone a bum steer.

wphamilton

I hadn't seen this until someone revived the thread, but if you're still reading the total mass was the same for all tests.

Aravilar

Earth's atmosphere is not a vacuum. Conceptually, the gravitational force scales linearly with mass (since gravitational acceleration is constant, for this anyway) but the air resistance (drag) does not.

Longer explanation is here: https://sites.google.com/a/mpstraini...nlighterriders