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The Physics of Motorcycles (or why motorcycles do that!)

Discussion in 'General Motorcycling Discussion' started by ginji, May 9, 2007.

  1. As some of you would have realised by now (I hope anyway) I'm a uni student! and the worst kind too, the know it all* physics type :p

    So I've decided to tell you all why your motorcycle behaves the way it does when you do certain things. If you don't understand what I've said, and want to, please tell me and I'll try and explain it again for you...

    First off...

    Why the hell does the bike lean to the left if I push on the left hand handlebar?

    This is called countersteering. What happens is that you push on the handlebar, and turn the front wheel in the opposite direction you want to turn. This changes the pivot point (the wheel's contact point) out from under the bike's centre of mass and in turn generates the lean.

    Next time, why your grip to the road really changes when you lean the bike over (hint, it has nothing to do with the contact area of the tyre ;))

    (BTW, this is what happens when I get bored at uni... And my GF called me a geek for actually posting this, so you're allowed to call me that too :LOL: )

    *Spelling and grammar excluded unconditionally.
  2. Yea, I know the diagrams are there, but I was going to make some more specific ones to motorcycle wheels ;)
  3. Geek ! :wink: :LOL:

    Well you said I was allowed to call you that ....
  4. Shouldn't it be "Why does the bike lean to the left if I push on the lefthand handlebar?" Which is the practice of counter-steering.
  5. what did he say? :?
  6. yep, fixed now...
  7. hey fizzix know it all.... shouldn't it be precession, not precesion? :p

    I could be completely wrong, but with it being precess, I'm making an assumption becaws i is erly leefer of skool and u made universtitty.
  8. Is there a God?
  9. Differing views on this question means that I am unable to postulate on the existence of one or many God(s)

  10. Thanks for the info Ginji, I've always wondered how that worked!
  11. Finally! Someone finds it useful :)

    I posted it because I didn't really think many people realised why it worked, just that it did. (Also to poke a bit of fun at myself :p)

    Time to draw up those diagrams...
  12. No, you are the 'worst' kind (heh, I'm a uni prof, so my geekiness trumps yours :p )

    Cool bananas though, and an interesting topic. It's also why bikes will stand up when they're moving but not when they're stopped. It's one of the cool things about bikes - they're inherently dynamical systems.

    Now drop and give me some vector diagrams!
  13. Logic tells you this is the case, but it's good to see the math behind it.
  14. Vector diagrams on their way... more interesting to make em then it is to pay attention to my chem lecture...
  15. Okay, let's imagine a snowbicycle - a pushbike with the wheels replaced by skis.

    You're riding this contraption downhill in a straight line at 60 km/h and you want to turn left. To initiate the turn, do you push on the left bar or the right?

    I ask because I suspect the gyroscopic precession business is a furphy. I reckon you'd steer the snowbike exactly the same way you steer a motorbike.

    Push on the left bar, the front ski grips and starts to move the front of the bike to the right. The rest of the bike (the bit from the headstem back) wants to keep moving straight ahead due to inertia but can't, so it falls to the left and initiates the turn.

    IIRC, someone once rigged up a pushbike with a second front wheel mounted directly above the normal front wheel, but turning in the opposite direction. The idea was that this would cancel out any gyroscopic effect of the normal front wheel. The bike still had to be counter-steered though.

    [Edit: here's a link to a webpage detailing various precession-cancelling bicycles built over the years to investigate this whole business. If nothing else, the experiments seem to establish that precession has nothing to do with the stability of 2-wheelers at speed.

    By the way, if precession is relevant to the steering of 2-wheelers, shouldn't it be easier to turn in one direction and harder to turn in the other?]
  16. You mean "applied physics" lecture. :LOL:
    (Chemists hate that... :LOL: )

    The hierarchy goes:
    Mathematics is "pure" science. :wink:
    Physics is only applied Mathematics;
    Chemistry is only applied Physics;
    and Biology is applied Chemistry... :p
  17. It steers differently, and as such it doesn't really apply to your argument of precession not applying to motorcycles. A skier steers by placing weight on their outside foot (so to steer to the left, they put the weight on their right foot). Doesn't even have anything rotating, which you need for precession.

    Well of course it has nothing to do with the stability, we're talking about leaning the bike, which any rider who is familiar with counter-steer will tell you that you push on the left to lean to the left and on the right to lean to the right.
    No, you apply a force on one side, and lean to that side. When you go to apply a force on the other side you're not applying the force on that side still. I don't know where you picked that idea from?
  18. My idea was to imagine something as close as possible to a 2-wheeler but without anything rotating. How would such a device react to inputs if you needed to steer it? The specific characteristics of snow may mean this isn't as useful a thought experiment as I'd hoped, but I note you haven't directly addressed the question I raised.

    Over about 15 km/h, a motorcycle turns when you lean it in the direction you want it to turn. But to initiate that lean, you must counter-steer, ie momentarily turn the handlebars in the opposite direction. Once the bike begins to lean (and turn), you steer the other way to manage the degree of lean.

    The hypothesis is that the reason the counter-steer works has less to do with gyroscopic precession and more to do with moving the part of the bike that's in front of the headstem to the side while the part of the bike that's behind the headstem is travelling straight ahead.

    You don't need precession to explain what happens here. It's exactly the same as turning the bars on a stationary bike to the right and then letting go of the bike. It falls to the left, even though its wheels aren't turning.

    If you read the last few posts, you'll see that at least one of them claims that gyroscopic precession is the reason bikes don't fall over when they're moving. Which is why I posted the link - I thought people might be interested.

    In any event, if precession is at all relevant to counter-steering and making a bike turn, how is it possible to ride a pushbike in which the effects of precession have been eliminated?

    I was asking a question, not making an assertion. Last time I was at a science museum and tried the device they'd set up to demonstrate precession (a rotating wheel on an axle you could hold on either side and turn in various directions), it was harder to turn the thing one way than the other. Seemed relevant, is all.