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Automotive Aerodynamics in motorcycles

  1. Dec 18, 2014 #1
    What is the approx speed in motorcycles at which chassis aerodynamics start to play a role in power output. I need a value at which say a harley body and a yzf body assuming same weight and engine will start to show considerably different characteristics.

    Thank you
     
  2. jcsd
  3. Dec 18, 2014 #2

    Bystander

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    Figure drag of ~ 1- 1.5 hp per sq. ft. at 60 mph, and that increases (or decreases) with square of speed, or ratio of speed to 60 mph squared.
     
  4. Dec 19, 2014 #3

    rcgldr

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    If you mean ram air intake as used on some sport bikes, it adds about 5% power at around 300 kph (186 mph).
     
    Last edited: Dec 19, 2014
  5. Dec 19, 2014 #4
    rcgldr, honestly didn't knew that can be designed for bikes too. Not what I was after but you learn something everyday.
     
  6. Dec 21, 2014 #5

    jack action

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    I would personally guess a 0.8-1.0 hp/ft² at 60 mph. The lower value being representative of a full fairing bike with a driver lying low and the higher value being representative of a bike with no fairing and a driver sitting straight. Although, I wouldn't be surprise that 1.5 hp/ft² can be achieved in an extreme case.

    But these values are proportional to the cube of the speed, not the square.

    Assuming similar, typical, frontal area (but the Harley would probably be larger), the aerodynamic impact would be:

    [tex]
    \begin{array}
    \textbf{Speed (mph):} & 60 & 90 & 120 \\
    \textbf{Harley drag (hp):} & 5 & 17 & 40 \\
    \textbf{YZF drag (hp):} & 6.2 & 21 & 50 \\
    \textbf{difference (hp):} & 1.2 & 3 & 10
    \end{array}
    [/tex]​

    So, to refer to the OP's question, there is no particular speed at which the aerodynamic effects «begin»: The Harley requires about 25% more power at any speed than the YZF.
     
  7. Dec 21, 2014 #6

    Bystander

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    If I didn't know better, it would be embarassing to have dragged that square all the way through --- knowing better and typing it --- too many bugs in my teeth.
     
  8. Jan 10, 2015 #7
    from experience, about 80 mph
     
  9. Jan 10, 2015 #8

    Doug Huffman

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    LOL "A role in power output." Try a similarly shaped HPV with much less than a horsepower and all of it at sensible physiological cost. Drag is proportional to speed squared PERIOD.
     
  10. Apr 24, 2015 #9
    http://news.motorbiker.org/Videos.nsf/Motorcycle-Superman.jpg [Broken]
     
    Last edited by a moderator: May 7, 2017
  11. May 14, 2015 #10

    cjl

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    Sure, drag force is proportional to speed squared, but drag power is proportional to speed cubed (since power is force times velocity).
     
  12. May 15, 2015 #11

    rcgldr

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    So the YZF with a fairing has more drag than the Harley? I was thinking that the rider leaned over on a YZF would end up a bit more streamlined than a vertically oriented rider on a Harley.
     
  13. May 15, 2015 #12

    jack action

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    Sorry, by reading the text, it is obviously a mistake on my part. It should of read as:

    \begin{array}
    \textbf{Speed (mph):} & 60 & 90 & 120 \\
    \textbf{YZF drag (hp):} & 5 & 17 & 40 \\
    \textbf{Harley drag (hp):} & 6.2 & 21 & 50 \\
    \textbf{difference (hp):} & 1.2 & 3 & 10
    \end{array}
     
  14. Nov 14, 2015 #13

    RaulTheUCSCSlug

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    Did the OP just want the difference in drag, but not take into account the actual difference in hp and torque the Harley and YZF have? Also like another person said, whether or not you have a fairing is going to make a world of difference. Some Harley Sportser flat trackers are made to be pretty aerodynamic.
     
  15. Mar 22, 2016 #14
    I seriously beg to differ.. I've built every type of HD Sportster racer there is, from unfaired dragsters, to half faired Sporty road racers @ Daytona, to 883 series road racers w/ nothing but a flat # plate for aerodynamic's on the front, and the flat tracker's share the same flat # plate, in front. If you talking about the little semi "boat tail" on HD flat trackers (XR 750), that was designed more fore aesthetics, than drag reduction.
     
  16. Mar 26, 2016 #15
    The easiest thing to do is conduct coast down tests. For example, get the Harley up to 80 MPH on a straight, level road on a low or no wind day and record the time-to-speed (video camera works just fine). Do the same with the faired bike. Now plot and extrapolate a bit.

    Aerodynamics play a role almost right away. Bicyclists will attest to this. There's a lot more drag at 15 MPH than at 10 MPH on a bicycle.
     
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