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Why does the Motorcycle does A Wheelie when you Clutch-Wheelie

  1. Apr 24, 2013 #1
    I personally don't ride a motorcycle, but I want to know if that relates to Physics.
    When someone holds the Clutch and leave it fast or Clutch the motorcycle to make a Wheelie, What is that thing in the Clutch which makes the Motorcycle pop up? I think I even saw cars do wheelies in videos, so what's the reason in Physics that makes it pop up while clutching?
     
    Last edited by a moderator: Apr 24, 2013
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  3. Apr 24, 2013 #2

    berkeman

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    When you pull in the clutch, that allows you to rev the engine, so that when you let out the clutch, the engine is at a high RPM which provides more torque. It is the extra torque that helps to lift the front wheel.
     
    Last edited: Apr 24, 2013
  4. Apr 24, 2013 #3

    berkeman

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    BTW, if your bike has enough power (or more specifically, a high power-to-weight ratio), you don't need to use the clutch to lift a wheelie. Just roll on the power and the front end comes up... :smile:

    PS -- I don't do wheelies on purpose on public roads (illegal). I keep them on the racetrack and in the dirt.
     
  5. Apr 24, 2013 #4

    Danger

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    I would further recommend using caution when experimenting.
    I love bikes, but I was always a bad rider (vertigo problems) and quite timid. The only time that I popped a wheelie was by accident. I ended up flat on my back with a couple of hundred pounds of steel on my chest. There was no injury, but it could very well have resulted in a broken back, broken ribs, or a concussion.
     
  6. Apr 24, 2013 #5

    rcgldr

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    During heavy acceleration, at the point of contact, the rear tire pushes backwards against the pavement and the pavement pushes forwards against the tire. The forwards force from the pavement is below the center of mass of the bike, which experiences a backwards reaction force due to the acceleration. The forward force from the pavement and the backwards reaction force at the center of mass create a torque that results in a wheelie if the acceleration is strong enough.

    There's also the issue of the torque generated on the rear tire. If the rear tire was locked in place, unable to turn or move with respect to the pavement, then with sufficient torque being appied to the rear tire from the engine (and clutch), a bike could wheelie. Under normal circumstances (the rear tire rotating and accelerating), you have torques related to the engine and rear tire accelerating, but these would be relatively small.

    In the case of a chain drive, you have a tension in the upper part of the chain, and a compression along the swing arm. This would generate internal torques, but I'm not sure if this contributes to a wheelie. For a shaft drive, there are internal torques applied to the shaft drive and whatever is holding the shaft drive in place, but again, I'm not sure if this contributes to a wheelie.

    Also as mentioned by the previous posters, most high end sport bikes will power wheelie without using the clutch or sudden application of the throttle (to produce a jerk) if in first gear at full throttle. Leaning forward will reduce the tendency to wheelie, but it may not eliminate it.
     
  7. Apr 24, 2013 #6
    if you understand the physics behind it well, you can be more confident about doing the wheelie. you can also do power wheelies if your bike engine is powerful enough. since your question is about clutch wheelies, i am restricting my answer to those.
    as per newton's third law, just as the chain tries to rotate the back wheel, the wheel also exerts a reaction torque which will try to rotate the chain, and hence the whole bike, in the opposite direction. under normal riding conditions, this reaction torque does not exceed the torque produced by gravity, in other words the weight of the bike prevents it from wheelieing.
    the force of kinetic friction between the clutch plates should be greater than the engine torque for normal operational torques. otherwise, you would never be able to get the clutch plates to lock at normal operational torques. when you abrupty drop the clutch, the normal reaction is maximum, so the force of kinetic friction which you face is maximum. this force can be enough to counter gravity and lift the front wheel. if it doesn't work, try giving full throttle or increasing the rpm. this will increase the time for which the frictional force acts, thus increasing the time for which the wheelie lasts. or else try sitting a little bit back on your seat. remember you can stop the wheelie at any time by pressing the back brake, this will provide enough counter torque to stop the wheelie. but be careful as sometimes wheelies can happen in a fraction of a second not giving you enough time to control it.
     
  8. Apr 24, 2013 #7

    Danger

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    That's what happened to me. Even worse, the first thing that happened was that my torso was forced rearward. In my instinctive reflex to hold on for dear life, that opened the throttle more.

    By the bye... is your shift key broken?
     
  9. Apr 24, 2013 #8

    sophiecentaur

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    I have often thought that the throttle control is the wrong way round on motorbikes. Positive feedback is never a good thing in control loops.
    Also - the Hand control adaption on motorcars is daft. You push to stop (unlike bicycle brakes) and pull to go faster - same problem as with a bike throttle, especially for a driver with not much body strength.
     
  10. Apr 24, 2013 #9

    rbj

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    it seems to me that the throttle control is the correct way for negative feedback.

    if i am riding the motorcycle and popping a wheelie, and you are on my right observing my right flank, the sense of rotation of the bike is the same as the sense of rotation of my right-hand grip. but given Newton's third law, i would expect that the rotational jerk of the bike popping up would tend to cause my hand to twist in the opposite sense.
     
  11. Apr 24, 2013 #10

    rcgldr

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    What the clutch does is add the torque related to angular decleration of an engine to the torque produced by the engine through combustion, increasing the overall torque for a brief period.
     
  12. Apr 24, 2013 #11

    berkeman

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    If you are supporting your upper body position with your abdominal muscles, then your statement is true. But if you get surprised by a quick acceleration, your upper body rotates back with respect to the morotcycle, which pulls back on the top of the throttle grip. That can result in a bad positive feedback loop.

    So good body position and good throttle mechanics are important in performance riding. At least for riding a dirtbike or a sportbike, you tend to stay in a leaned-forward position and bend your elbows, so that a quick acceleration will not result in pulling the throttle open more (instead, your elbows just extend a bit until you can pull your upper body back into the forward position). You also operate the throttle grip by having your forearm more in-line with the axis of the grip, and twist your wrist like you are opening a doorknob. You don't have your forearm at a right angle to the grip and open the throttle by pulling/rotating back on the grip -- that's bad form. Think "twist the doorknob open" and lean forward and keep your elbows bent, and you will have a lot fewer problems with unexpected accelerations...

    (Glad you weren't hurt, Danger!)
     
  13. Apr 24, 2013 #12

    Danger

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    Thanks. Boy, but I wish that someone had told me that "posture" advice 40 years ago!
    Luckily, I was barely moving when the accident happened. It was on my friend's farm, and I was just idling up a bumpy trail (with a passenger, which upset the load distribution). The front wheel hit a rock and that's all she wrote.
     
  14. Apr 24, 2013 #13

    berkeman

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    Good way to lose a girlfriend. In more ways than one!

    :smile:
     
  15. Apr 24, 2013 #14

    Danger

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    Reminds me of one of my favourite poems (I've rewritten two words to modernize it from the original 1923 version):

    Ruth rode on my Harley
    On the seat in back of me.
    I took a bump at 95
    And rode on Ruthlessly.
     
  16. Apr 25, 2013 #15
    lol no. just avoid unnecessary punctuation.
     
  17. Apr 25, 2013 #16
    remember torso is also rotated during a wheelie. so body weight can stretch your hands. ive mostly heard this problem from beginner bike riders and never faced it myself. but ive never really tried to wheelie too high though.
    sudden braking is really more of a concern than sudden acceleration. normal bike riders are not expected to suddenly accelerate anyway.
    another thing which has bothered me about bike design is the fuel tank hump right in front of your privates. in case of an emergency if you have to brake suddenly you could end up banging your privates on the fuel tank. scooters appear better off in this respect.
     
  18. Apr 25, 2013 #17

    sophiecentaur

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    That was an interesting response. On first reading it I though "The man's mad" but then I thought a bit. . . . . It's actually quite complicated,
    There are two senses in which the feedback acts. Supporting your idea; if, when you rev the engine, you wrist dips down and the bike surging forward will tend to leave your body behind, raising the wrist and reducing the acceleration. This is certainly negative feedback and it would suggest that it's the 'proper' way to hold the throttle.

    Supporting my idea, if you hold the throttle with your hand 'over the top' , with your wrist elevated, then acceleration of the bike will drag your wrist down and increase the acceleration. If the acceleration is hard enough to cause loss of grip then as your body moves back, your hand will move further back over the throttle and produce further acceleration. This is positive feedback and would be the 'wrong' way to hold the throttle.

    Then there is the issue of what happens when the front wheel starts to lift. You are right about the initial negative feedback due to body rotation vs bike rotation but, once again, if you start to lose grip, the top of the throttle will still be pulled backwards, giving more positive feedback. There must be a temptation to loosen grip when a wheelie starts and this can only make things worse - the last bit of contact with the bike will be as your hand rolls back over the top of the throttle, putting it to Max!

    "Proper posture" and throttle grip are clearly very important
     
  19. Apr 25, 2013 #18

    rcgldr

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    I've witnessed a few incidents where new riders allow their upper bodies to get pulled back and in turn pull back on the throttle, causing it to go full throttle. On some bikes, you have to twist the throttle a lot in order to go full throttle which help, and one tip for a new rider would be to position their right wrist below the throttle before grabbing it to limit how far they can twist the throttle, and if they pull back and straighten their wrist, they shut off the throttle, unless they open their grip and their fingers pull the throttle open due via friction.

    Where the direction of rotation of the throttle makes sense is when you're blipping the throttle while downshifting and braking at the same time. I can blip the throttle just using my palm and thumb while squeezing the brake lever with all 4 fingers, while others only use 2 or 3 fingers on the brake lever (as long as the fingers remaining on the throttle don't restrict how far back you can pull the brake lever). If I'm lane splitting (California), I'll keep all 4 fingers on the brake lever and use my palm and thumb for throttle control so that braking reaction time is reduced.
     
  20. Apr 25, 2013 #19

    Danger

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    What on Earth is "lane splitting"? :confused:
    One other thing that should be mentioned, primarily for people who have not yet had any experience at all with bikes, is that there's a huge difference between 4-stroke and 2-stroke engines when it comes to throttle response. Most street bikes are 4, and most dirt bikes are 2. A 4 acts more or less the same way that a car does. A 2 is like a Tasmanian devil in a paper bag; you open the throttle a bit and it moves—open it a bit more and there's no change—open it a bit more and there's no change—opBLAMMO!
     
  21. Apr 25, 2013 #20

    OCR

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    Lol... "lane splitting"? ... AKA, whitelining.




    OCR
     
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