Rotational Inertia: Front end of an airborn bike drops when brake is applied

  1. So every mountain biker or motocross rider knows to never grab the front brake in the air. When you do the front end drops and can potentially ruin your day in a hurry. I figure that due to the loss of rotational inertia gravity accelerates the front wheel more rapidly, but I would like to see it described technically and mathematically... Any help?
    Last edited: Sep 11, 2011
  2. jcsd
  3. Borek

    Staff: Mentor

    Re: Front end of an airborn bike drops when brake is applied

    I don't think gravity has any effect here.

    The lighter the wheel, the lesser the effect, probably.
  4. For sure... The lighter the wheel the lesser the effect, also the heavier the bike the lesser the effect.
  5. A.T.

    A.T. 6,323
    Science Advisor
    Gold Member

    With zero external torques the total angular momentum of the bike must stay constant. If you stop the wheel rotation relative to the bike, the wheel's angular momentum is distributed among wheel and bike. The wheel still rotates, just slower with the entire bike rotating along.
  6. Ahh. Good point. That explains it thanks.
  7. Cm'on... keep some common sense...
    Angular momentum and energy of front wheel has no significance at all!

    If you grab the front brake why in the air, you just stop the wheel immedaitely, and keep it blocked - as soon as you touch the ground you either flip, or fall into front wheel slip - what is not something bikers like :(
  8. Borek

    Staff: Mentor

    Funny, how you correctly named dependencies, but failed to see it is about angular momentum. I was sure you got it after reading a hint.
  9. No the front end definitely drops, or the bike rotates rather... If you don't believe me go try it. I'll go with A.T. on this one
  10. Borek

    Staff: Mentor

    Have you read original post? It is about changing position while in air, not about what happens when you touch the ground. My understanding is that the effect will be observed even if you grab the front brake and then release it long before landing.
  11. Filip Larsen

    Filip Larsen 1,016
    Gold Member

    I have no experience riding a bike like that, but if it is due to conservation of angular momentum, as I would also think it is, then braking the rear wheel "in-flight" should give the same effect everything else being equal. In addition, revving up the rear wheel should have the opposite effect and "pitch up" the bike. Are these effects something you would observe in practice?
  12. rcgldr

    rcgldr 7,644
    Homework Helper

    Motorcross and supercross racers rev up or brake the rear wheel to control pitch during large jumps. The engine also has angular acceleration in the same direction as the rear wheel, but I don't know how much overall effect it has.
  13. berkeman

    Staff: Mentor

    On a MTB the wheels are too light to make much difference in pitch. On a dirtbike or MX bike, you routinely use the engine and rear wheel in the air to adjust your pitch. You tap your rear brake in the air to drop the front wheel a bit (conservation of angular momentum), and you rev the engine and rear wheel hard to raise the front wheel as needed.

    You don't usually use the front wheel for adjusting your pitch because 1) it is lighter than your rear wheel and drive train, and 2) it is a little dorky to land with your front wheel not rotating.
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