Advertise on Bikeforums.net



User Tag List

Page 1 of 3 123 LastLast
Results 1 to 25 of 60
  1. #1
    rugged individualist wphamilton's Avatar
    Join Date
    Apr 2011
    Location
    Alpharetta, GA
    My Bikes
    Nashbar Road
    Posts
    5,939
    Mentioned
    17 Post(s)
    Tagged
    0 Thread(s)

    Back-Pack Drag - Revealed!

    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:

    Comparison.jpg

    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:

    TestRuns.jpg

    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 http://goo.gl/maps/Y3sDY

    The bike: bike.jpg and recorder recorder.jpg
    Last edited by wphamilton; 09-15-13 at 05:40 PM.

  2. #2
    Allez means go. bengreen79's Avatar
    Join Date
    Jul 2008
    Location
    Two Rivers, WI
    Posts
    815
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  3. #3
    rugged individualist wphamilton's Avatar
    Join Date
    Apr 2011
    Location
    Alpharetta, GA
    My Bikes
    Nashbar Road
    Posts
    5,939
    Mentioned
    17 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by bengreen79 View Post
    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.

  4. #4
    Senior Member
    Join Date
    Feb 2011
    Posts
    1,320
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    Cyclist OCD in full bloom.

    J.

  5. #5
    Senior Member
    Join Date
    Jun 2013
    Location
    Oxford, OH
    My Bikes
    Scattante R670
    Posts
    170
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  6. #6
    Senior Member
    Join Date
    Jul 2013
    Location
    South St. Paul, MN
    My Bikes
    Trek 520, Peugeot PX-10
    Posts
    260
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Back-Pack Drag - Revealed!

    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

  7. #7
    rugged individualist wphamilton's Avatar
    Join Date
    Apr 2011
    Location
    Alpharetta, GA
    My Bikes
    Nashbar Road
    Posts
    5,939
    Mentioned
    17 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Bjforrestal View Post
    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.

  8. #8
    Senior Member
    Join Date
    Aug 2012
    Location
    Lancaster, PA, USA
    My Bikes
    2012 Trek Allant
    Posts
    1,445
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    Yeah, I'd be interested in seeing backpack versus panniers. Nice to see someone putting an Arduino to good use!

  9. #9
    Senior Member himespau's Avatar
    Join Date
    Jun 2008
    Location
    Louisville, KY
    Posts
    7,499
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    Cool idea. Thanks.
    Punctuation is important. It's the difference between "I helped my uncle, Jack, off a horse" and "I helped my uncle Jack off a horse"


  10. #10
    Senior Member alan s's Avatar
    Join Date
    Oct 2007
    Location
    MD/DC/VA
    Posts
    2,817
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  11. #11
    New Orleans
    Join Date
    Jan 2006
    Posts
    2,473
    Mentioned
    4 Post(s)
    Tagged
    0 Thread(s)
    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

  12. #12
    Senior Member no motor?'s Avatar
    Join Date
    Dec 2005
    Location
    Chicagoland
    My Bikes
    Specialized Hardrock
    Posts
    3,896
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by spivonious View Post
    Yeah, I'd be interested in seeing backpack versus panniers. Nice to see someone putting an Arduino to good use!
    Me too. I know I certainly feel faster when using a backpack than with a pannier.

  13. #13
    coprolite fietsbob's Avatar
    Join Date
    Jun 2010
    Location
    NW,Oregon Coast
    My Bikes
    7
    Posts
    40,172
    Mentioned
    28 Post(s)
    Tagged
    0 Thread(s)
    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 ..

  14. #14
    Senior Member gregjones's Avatar
    Join Date
    Aug 2012
    Location
    West Georgia
    My Bikes
    K2 Mod 5.0 Roadie, Fuji Commuter
    Posts
    1,979
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Bjforrestal View Post
    Have you done it with rack and panniers, or basket, rack with trunk, etc?
    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.
    Last edited by gregjones; 09-16-13 at 12:55 PM. Reason: added stuff

  15. #15
    derailleurs are overrated bigbenaugust's Avatar
    Join Date
    Feb 2005
    Location
    KIGX
    My Bikes
    2009 Motobecane Fantom CX, 2012 Motobecane Fantom Cross Uno SSCX
    Posts
    1,729
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by gregjones View Post
    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.
    --Ben
    Carrboro Bike Coalition - putting the "bike" in "CARrboro" :)
    2011 Motobecane Fantom Cross Uno, 2009 Motobecane Fantom CX
    Previously: 2000 Trek 4500 (2000-2003), 2003 Novara Randonee (2003-2006), 2003 Giant Rainier (2003-2008), 2005 Xootr Swift (2005-2007), 2007 Nashbar 1x9 (2007-2011), 2011 Windsor Shetland (2011-2014)
    Current Linux Usage (by machine): Arch: I openSUSE: III

  16. #16
    Keepin it Wheel RubeRad's Avatar
    Join Date
    Aug 2011
    Location
    San Diego
    My Bikes
    Surly CrossCheck, etc
    Posts
    2,368
    Mentioned
    7 Post(s)
    Tagged
    0 Thread(s)
    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.

  17. #17
    rugged individualist wphamilton's Avatar
    Join Date
    Apr 2011
    Location
    Alpharetta, GA
    My Bikes
    Nashbar Road
    Posts
    5,939
    Mentioned
    17 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by RubeRad View Post
    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.

  18. #18
    coprolite fietsbob's Avatar
    Join Date
    Jun 2010
    Location
    NW,Oregon Coast
    My Bikes
    7
    Posts
    40,172
    Mentioned
    28 Post(s)
    Tagged
    0 Thread(s)
    that one with all the pockets Versus one that is smooth and external pocket free . too ..

  19. #19
    johnliu@earthlink.net jyl's Avatar
    Join Date
    Aug 2006
    Location
    Portland OR
    My Bikes
    61 Bianchi Specialissima 71 Peugeot G50 7? P'geot PX10 74 Raleigh GranSport 75 P'geot UO8 78? Raleigh Team Pro 82 P'geot PSV 86 P'geot PX 91 Bridgestone MB0 92 B'stone XO1 97 Rans VRex 92 Cannondale R1000 94 B'stone MB5 97 Vitus 997
    Posts
    3,768
    Mentioned
    38 Post(s)
    Tagged
    0 Thread(s)
    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).
    Your signature contains too many lines and must be shortened. You may only have up to 2 line(s). Long text may have been implicitly wrapped, causing it to be

  20. #20
    Senior Member no motor?'s Avatar
    Join Date
    Dec 2005
    Location
    Chicagoland
    My Bikes
    Specialized Hardrock
    Posts
    3,896
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by bigbenaugust View Post
    Then you need bigger panniers.
    Does the lid make yours more aeordynamic?

  21. #21
    rugged individualist wphamilton's Avatar
    Join Date
    Apr 2011
    Location
    Alpharetta, GA
    My Bikes
    Nashbar Road
    Posts
    5,939
    Mentioned
    17 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by jyl View Post
    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).
    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.

  22. #22
    derailleurs are overrated bigbenaugust's Avatar
    Join Date
    Feb 2005
    Location
    KIGX
    My Bikes
    2009 Motobecane Fantom CX, 2012 Motobecane Fantom Cross Uno SSCX
    Posts
    1,729
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by no motor? View Post
    Does the lid make yours more aeordynamic?
    Stop following me around!
    --Ben
    Carrboro Bike Coalition - putting the "bike" in "CARrboro" :)
    2011 Motobecane Fantom Cross Uno, 2009 Motobecane Fantom CX
    Previously: 2000 Trek 4500 (2000-2003), 2003 Novara Randonee (2003-2006), 2003 Giant Rainier (2003-2008), 2005 Xootr Swift (2005-2007), 2007 Nashbar 1x9 (2007-2011), 2011 Windsor Shetland (2011-2014)
    Current Linux Usage (by machine): Arch: I openSUSE: III

  23. #23
    Senior Member himespau's Avatar
    Join Date
    Jun 2008
    Location
    Louisville, KY
    Posts
    7,499
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by no motor? View Post
    Does the lid make yours more aeordynamic?
    those kitty decals are extra slippery so the wind just slips right past them.
    Punctuation is important. It's the difference between "I helped my uncle, Jack, off a horse" and "I helped my uncle Jack off a horse"


  24. #24
    Cycle Dallas MMACH 5's Avatar
    Join Date
    Jun 2005
    Location
    Land of Gar, TX
    My Bikes
    Lucinda--2010 Jamis Aurora Elite & a few others
    Posts
    3,360
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by JohnJ80 View Post
    Cyclist OCD in full bloom.

    J.

    Why do you suppose that cycling brings out our OCD tendencies?
    That's gonna leave a mark.

  25. #25
    Senior Member no motor?'s Avatar
    Join Date
    Dec 2005
    Location
    Chicagoland
    My Bikes
    Specialized Hardrock
    Posts
    3,896
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by bigbenaugust View Post
    Stop following me around!
    It's kind of hard to miss a guy with one bright yellow kitty litter pannier.....

Page 1 of 3 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •