Short version: I didn't find any extra drag wearing a backpack!

I was curious about just how much extra drag was caused by wearing a backpack, so to test that I coasted down a hill while recording the instantaneous speed at each point, and then compare at the end. I started from a dead stop, no power supplied other than gravity and (unfortunately) the breeze from an occasional car. I did the coast-down several times, both with and without the backpack which was half-filled with plastic bags to puff it out. Since this is commuting, for the tests I dressed in knee-length cargo shorts and a loose cotton polo shirt. I rode down in the drops, not completely horizontal but my back is 10 or 15 degrees from flat. There was no wind today.

Here is the comparison, with an average of the backpack runs vs average of the non-backpack runs. These are also smoothed with a 3-point moving average. Distance down the hill on the bottom plotted against speed on the y axis:



I expected some difference if even only a little, but if anything the descent was faster with a backpack than without. The difference is within the error margin though.

The graphs of the individual runs:



As you can see they are all pretty consistent, enough that I don't feel any pressing need to analyze the deviations further. I had to throw out a couple of runs due to excessive traffic, not pictured here.

Regarding the data collection, if anyone's interested, I used the magnetic reed switch from an old bicycle computer tied into a microprocessor board (Arduino Uno) which saved the time of each switch closure to a micro-sd card. I used software de-bouncing and I decided on the leading edge of the closed segment for the time stamp. These switches aren't as precise as you'd think by the way, with early or late closures seemingly at random. Hence the moving average - an early closure makes one open segment shorter but the successive one longer by the same amount, and a moving average evens that out.

It's this hill https://goo.gl/maps/Y3sDY

The bike: and recorder

Thanks for taking the effort to do this - very interesting. I commute in bike specific clothes since I have to change anyway. I feel faster without a backpack but I really think it's mostly a perception thing.

Me to, and wearing the backpack is my last choice of all reasonable alternatives so I wanted it to show a measurable drag. Maybe I should stuff it completely full, wear cycling clothes, try a bunch more runs and include some on the hoods, but even if I do eventually get a measurable difference it won't be much.

My theory is that with the looser shirt and enough angle of my torso and head, the backpack is in a turbulent area behind me and doesn't contribute to drag.

Cyclist OCD in full bloom.

J.

Very interesting, thanks for doing this! I've wondered about how big of a difference it makes plenty of times. I would've thought that down in the drops there's at least some difference. In fact, I've always felt silly going down, thinking that the backback just fills where my upper body would otherwise be. Now I know.

Have you done it with rack and panniers, or basket, rack with trunk, etc? Really interested in other commuting standard cargo variations. Nice work with the backpack

I may try it again next week with my beater bike which has a rack and some home-brew canvas bags. I seriously should do one with a full backpack though or at least with a normal commute load, while riding on the hoods to simulate the normal commute. Having it only half full with plastic bags may have thrown it off.

Yeah, I'd be interested in seeing backpack versus panniers. Nice to see someone putting an Arduino to good use!

Cool idea. Thanks.

I've seen commuters with huge backpacks that definitely cause drag, particularly when riding in the drops. Kind of funny looking, because they are leaning over trying to get as aero as possible, but the exposing the backpack to the wind, losing any benefit. Most smaller backpacks would have no effect on aerodynamics. I'd like to see a study on sweat flow increases from backpacks.

Great stuff
The backpack is in your aero shadow- so you have more or less paid that aero penalty already

No point-other than convenience or perhaps too heavy-to carry small loads any other way

Me too. I know I certainly feel faster when using a backpack than with a pannier.

your pack, if smooth and rounded will not make extra turbulence

in the air that is already forced out your way by the body ..



there are intentionally smooth outside packs , that way by design ..

shopping time, if it matters ..

As fietsbob brought up, clean air vs. already in turbulence, or directly behind (part of) the wind punching object's profile, makes a big difference how much drag something will create.

Panniers are right behind a pair of thrashing legs, I doubt there's any clean air for them to push against.

Then you need bigger panniers.

Thanks for taking the time to do and post, this is fascinating!

Regardless of the results, I will continue to use rack&pans instead of backpack for reasons of sweat and comfort, but this is helpful.

It reminds me of something I read in Bicycling magazine how after Lemond kicked ass in TdF with aerobars, people did coast-down tests with aero- vs drop-bars, and it was in that case a clear difference. In this case, the results seem to make sense with the idea that the backpack is in the aero 'shadow' so it doesn't have an effect.

If that is the case, perhaps the tiny boost in speed you're seeing is due to extra mass. As even Galileo knew, bowling balls do drop faster than golf balls, because their greater momentum is less affected by air resistance. And as we all know, nobody can descend like a fat man! If you replaced the cargo with actual stuff, I bet you'd see another small speed boost.

I think I need to design and patent a system that has robotic arms to reach out to the side of the road and grab rocks for ballast and stuff them into my panniers whenever a bubble-level shows I am cresting a hill; then when I get to the bottom, the robot arms can pull all the rocks out so I'll be ready for the next uphill.

Thanks for the encouragement. I'm definitely planning on testing again at least with some panniers vs backpack.

In this case I don't think that the extra mass was much of a factor - that's one reason I used light plastic bags to fill and a light denim cloth backpack. I think the small difference seen was due to the uncontrollables in the test. Vagrant breeze, passing car, a rough patch in the road, position slipped, that sort of thing.

Assuming my first test was accurate (and I don't see a reason why not) my question now is how bulky does the pack have to be to make a measurable difference, if it ever does? It's conceivable that a larger, more rigid pack in the turbulent area could actually improve the aerodynamics in the same way a tailbox does, even with a greater reference area. We'll see.

that one with all the pockets Versus one that is smooth and external pocket free . too ..

Very interesting!

I may have missed this, but what was your body position during the test? I've always thought that a backpack could make a difference to a rider in a full tuck (flat back).

Does the lid make yours more aeordynamic?

That was my thinking, so I was in the drops relaxed not straining for flat back but about 15 degrees up from flat. I wanted to test "commuting style" so I didn't go for a flat back or full downhill tuck. Riding on the hoods would be more normal for commuting though so I think I'll do that for the rest of the coast-down tests.

Stop following me around!

those kitty decals are extra slippery so the wind just slips right past them.

Why do you suppose that cycling brings out our OCD tendencies?

It's kind of hard to miss a guy with one bright yellow kitty litter pannier.....