Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Racing physics

  1. Aug 12, 2008 #1
    Motocross has seen a rise in the number of racers who "scrub" their speed, or whip the motorcycle to the side. They claim that whipping the bike sideways off the jump lowers the hangtime so they can get back down to accelerate, therfore having a faster lap time. It is also claimed that sliding the bike off the jump helps make a faster lap by not jumping as high.

    here is a video of a scrub http://www.youtube.com/watch?v=4IMZZXLXPTU&feature=related

    The scrub just does not look as aerodynamic as jumping in a straight line. I would think that the whip would add more wind resistance therfore loosing more foreward momentum than jumping straight. I also would think that sliding up the jump loosing traction would slow the rider down. I do beleive it does get them back on the ground faster because of loss of momentum, but I dont see how this would make them any faster compared with going straight. I actually think the opposite. I know the rider would have a longer hangtime by jumping straight, but I dont think the rider would loose as much momentum.

    I would like to do a controlled study on this one day, but for now this is my theory. What do you guys think?
  2. jcsd
  3. Aug 12, 2008 #2
    It looks like they get off the ground before the peak of the jump, so they likely won't go as high and hit the ground faster. Turning the bike sideways just looks cool.
  4. Aug 12, 2008 #3
    If you're expending energy on anything but forward motion, you're slowing down.
  5. Aug 12, 2008 #4
    To me it seems that the real effect what the rider achieves by letting the bike go sideways before the peak, is that his center of mass is lower: He is going over the peak without raising his center of mass unnecessarily. This will result in smaller hang time.

    Suppose you are running and you want to jump over some obstacle. Of course you are going to use a kind jumping technique, that you bend you knees somehow and pull you legs up while in the air. If you tried to use a kind of jumping technique, that you legs are hanging straight down while you are in the air, you would need to get your center of mass lot higher to get over the obstacle. Okey, this is very trivial, but to me it seems that the motocross rider is just doing the same with his bike.

    Why the smaller hang time result in better lap times is another question. Perhaps, if the bike falls from high to the flat ground, you lose speed there? So it is better to get down quickly on the slope, so you can accelerate there.
  6. Aug 13, 2008 #5
    It seems like the aim of the scrub is to encourage the bike to go forwards rather than upwards.

    If the bike hits the jump head-on, it wil be launched into the air at the incline of the jump. But if it hits it at an angle the inlcline will be reduced, getting it to the ground faster.

    The key is to hit the jump at speed but follow as straight a line as possible, the sooner you hit the ground the sooner you get going faster in a straight line.
  7. Aug 13, 2008 #6
    It seems that air resistance would be increased by going sideways. I wonder if the air resistance offsets the benefit of distance instead of height compared with the lower center of gravity. Thanks for the replies.
  8. Aug 13, 2008 #7
    I don't think the air resistance increases enough to do any major damage. How fast do these guys tend to go?
  9. Aug 13, 2008 #8
    If the goal is just to get back down on the ground quickly to accelerate again, then this sounds legit to me. Any way you can minimize how high you jump, or even possibly ruining the aerodynamics of the bike to cause it to fall like a stone could decrease your hang time. Now how aerodynamics can be used to decrease hang time? <shrug> I'd have to run some bikes in a wind tunnel to find out if they are creating lift in the jumps normally. Sorry don't have a wind tunnel in my pocket.

    But the concept of slowing down to get back to accelerating faster does make sense if you can get enough acceleration to make up for the distance you've lost compared to someone who did not slow down. Motocross seems to be all tiny bikes with almost limitless torque, so sounds plausible.
  10. Aug 13, 2008 #9
    yea, the motorcycles weigh aproximately 93 kg with engine sizes 250cc and 450cc. The top speed on a smooth surface is around 55mph, not sure of average speed on a race track. l own a 250cc bike and it pulls my 165lb's around with no problem. The bikes rpm's go to around 13000rpm
  11. Aug 13, 2008 #10
    I suppose if the bike is tilted a certain way it could act as a wing and bring the bike down faster.
  12. Aug 18, 2008 #11
    hey guys im a 85cc seinor class motocross racer and im happy to answer your questions!
  13. Aug 18, 2008 #12
    i usually go about 35 around corners and 40 to 50 on straightaways
  14. Aug 18, 2008 #13


    User Avatar

    Staff: Mentor

    That's pretty much it. Air resistance doesn't come into play for the most part. You can only be accelerating while your back tire is on the ground (see avatar), so the more time you spend in the air, the slower your lap times. The only time you want to jump higher and farther is if there is an obstacle that you are clearing. Many jumps are tabletops or doubles (two peaks), and you generally want to clear and land on the down-slope of those kinds of jumps. But as long as you are clearning the jump or obstacle, you want to stay as low as possible over the jump and in the air.

    Here's a fun thread from another forum where I'm berkeman as well:


    (scroll down to post #14 for some great pics)

    Edit -- Ack, that TT thread is old enough that only the first image in post #14 is still available. There are a couple good images also in post #39, though. Or do a Google Images search on "Bubba Scrub".
    Last edited: Aug 18, 2008
  15. Aug 18, 2008 #14


    User Avatar

    Staff: Mentor

    Like this one:

    http://www.275mxvideos.com/Images/pictures/Stewart1_Stone_768.jpg [Broken]

    Last edited by a moderator: May 3, 2017
  16. Aug 19, 2008 #15
    thank you for all the good replies

    One thing to mention about that picture, I beleive the racer in that photo was "prejumping", or hitting a bump on the face of the jump to rocket into the air before the peak therfore giving him time to maneuver his bike to that position.

    This is a picture from a motorcycle high jump event where this rider has his bike positioned much like the picture above.http://www.motorcycle.com/images/content/Event/tn_047_07_XG13_Friday_1.jpg It seems to me the benefit of doing this is what jostpuur explained above, like a kinder jumping technique.

    another picture of a high jump. http://www.motorcycle.com/images/content/Event/2(3).jpg (the goal of the high jump is to attain enough height and forward momentum to clear the pole.)

    there are not many times when you see a racer whipped this much in a race. http://a.bebo.com/app-image/6545983874/i.idlestudios.com/img/q/u/08/04/22/bubba_scrub.jpg [Broken]

    here is another video of a scrub by one of the best racers. http://video.google.com/videoplay?docid=4067086731514992064 [Broken]
    Last edited by a moderator: May 3, 2017
  17. Aug 19, 2008 #16
    On another note I will be getting my first chance at off-road biking soon, a whole day through rivers, mud, grass and anything else that might happen to present itself!

    Can't wait!
  18. Aug 19, 2008 #17
    yea I hope you like it. It can be a blast! have fun!
  19. Aug 19, 2008 #18
    thanks motomax99 for the mph
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook