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Thread: Wheelie / Stoppie physics

  1. #1
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    7th September 2004 - 16:18
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    Wheelie / Stoppie physics

    This mightve been discussed b4, but i couldn't figure what to search on...

    What actually causes wheelies / stoppies. Like i figure it's somehow about weight / balance / energy and momentum. I figure the "centre of rotation" that the bike moves around must become the axle in question?

    But when does hard braking become enough to rotate the rear end around the front and the same for wheelies - when does hard acceleration become front wheel skyward?

    Duh??

  2. #2
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    19th November 2003 - 18:45
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    for wheelies the torque of the engine induces a rotation in the opposite direction to the rotation provided by gravity about the back wheel, when this torque exceeds the torque created by the weight of the bike/rider then the front wheel will lift


    for stoppies the centre of gravity of the bike must be highish, the forwards momentum of the COG create a torque about the front wheel which is countered by the friction and stopping force of the brakes on the wheel so the bike pivots about the front wheel

    really need some force diagrams so off to paint back in a bit
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  3. #3
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    I was told, (and I'm not sure if it's correct) that during a wheelie, the front sprocket is essentially trying to "climb" the chain.

    Might be a load of shit, who knows.
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  4. #4
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    how can it climb? they're both stuck in the same motion.. kinda see the logic, but that's a 'cute' thing, just a bit of wank really

    a wheelie in brief - the rear wheel is providing more traction and torque than the front weight is in total, so it lifts the front wheel up. The bigger the bike, the more torque it needs to lift the heavier weight.

    a stoppie in brief - the front wheel is braking beyond the point of which the weight on the rear of the bike isn't enough, so it starts to lift, and thus the front wheel is doing all the braking (so that's 100% Mr LTSA-I-know-it-all bs).

    As for rotating the rear wheel round during a stoppie, that's nothing to do with the amount of braking really. If the wheel is up, it's up. You're then shifting your body motion to make the rear of the bike spin around. Same for a wheelie (ish). During a wheelie, you lean your body, and can use the handlebars to help steer.

    Heaps of topics on this, and heaps of 'how to's' on the net, but each rider has a slightly different way of doing stuff, and each bike will be different again. Ask Mr Skid to do a stoppie, then try a sprotsbike brakes..

  5. #5
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    Quote Originally Posted by bugjuice
    a wheelie in brief - the rear wheel is providing more traction and torque than the front weight is in total, so it lifts the front wheel up.
    huh, sorry this doesn't really explain it, in a wheelie the front wheel plays little part in the macro scale of things, sure the rotational inertia can be used to steer the bike and can impact the lifting/dropping of the front wheel, but not much.

    the amount of traction has no direct impact other than the amount of torque being induced from the tire would greatly drop once the tire looses traction

  6. #6
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    Quote Originally Posted by White trash
    Might be a load of shit, who knows.
    I do - you're full of it.
    (and I'm not sure about the physics)

  7. #7
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    without traction, there's no grip. no grip is no good, cos it'd just spin, and the front ain't gonna come up. Didn't mention anything about the front wheel playing a huge role, just mentioned it lifts the front wheel. Stick a brick on the front then, it'll lift that instead..

  8. #8
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    Ahh...

    3 points so far to Saslex, cos he has a lovely turn of phrase (physics wise).

    2 points to WT, cos unlike me he can actually do both... (and should be glad i asked the question - cos next time it goes wrong for him, he could do a quick mental calculation on the inverse square hyponuse of the torque to get it sorted.... or not)

    2 points to Bug, cos i didn't want him to feel left out... and cos he said "bit of wank" which is ... nice ...

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  10. #10
    It's all to do with stroke and grip,you have to get the speed right....and keep within the friction circle...invarably a mess to clean up at the end.
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  11. #11
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    lets just say that the front goes slower than the back.....it works for me.... and motu has another good explanation
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  12. #12
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    I did a stoppie the other day. Wasn't intentional. Had something to do with the cage infront of me slamming on their brakes. Didn't really notice the back end had lifted up until it came back down. Very exciting. I stopped within inches of the cages bumper

  13. #13
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    If you go back the the thread on chain/sprockets where it was stated the rear shock extends, the answer to the wheelie question is there. Simply put, once the shock is fully extended, the chain pulls the front wheel up as the bike tries to piviot round the rear axle.

    Stoppy, pretty much the front axle is dropping below its horizontal, so the force forward then turns into a lever around the front axle.
    The contents of this post are my opinion and may not be subjected to any form of reality
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  14. #14
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    Ahhh...

    Quote Originally Posted by TwoSeven
    If you go back the the thread on chain/sprockets where it was stated the rear shock extends, the answer to the wheelie question is there. Simply put, once the shock is fully extended, the chain pulls the front wheel up as the bike tries to piviot round the rear axle.

    Stoppy, pretty much the front axle is dropping below its horizontal, so the force forward then turns into a lever around the front axle.
    Nice (and 10 or something)

    Cheers

  15. #15
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    blah blah blah...........................drop the clutch open the accelerator figure it out for yourself

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