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Cycling Forces & Physics - Help!

  1. May 10, 2005 #1
    Hey guys,
    I'm a student at high school in Australia. I currently have a physics assignment that states i have to explain the forces and physics of an object. I have chosen cycling. I know the basic s=d/t and a little bit on airflow and using the "teardrop" shape in helmets and frames, but otherwise i'm a bit stuck on what concepts to explain and use in my paper. I can explain stuff about the equipment (clothes, helmet, frames, cleat-shoes) or the speed, drag, momentum, terminal velocity etc (like formulas, they could be really helpful!) Any help on anything to do with cycling and physics would be greatly appreciated.

    Thanks all the way from oz.
    Last edited: May 10, 2005
  2. jcsd
  3. May 10, 2005 #2


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    Gold Member

    I can't actually help with anything other than to mention that one of the most interesting things about a bike to me is the gyroscopic effect of the wheels. Don't ask me to explain it, though.
  4. May 10, 2005 #3


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    Staff: Mentor

    You mean the myth that the gyroscopic effect of the wheels keeps a bicycle from falling over? :wink:

    The stability of a moving bicycle actually has mostly to do with the geometry of the front wheel and fork, and a factor called "trail." There must be some Web pages about this, written by physicists, but I didn't look very hard. (That's Nelson's job!) I did find this page which looks like a fairly good non-technical description, and which might serve as a starting point for further research.
  5. May 11, 2005 #4
    thanks jt... this site was great. I have looked around a bit and decided to further study the airflow of an object (there's heaps of windtunnel studies on the net) , downhill speed and terminal velocity (can it be reached due to stability?). Thanks guys

  6. May 11, 2005 #5
    Terminal velocity could be interesting... this will depend on the slope of the hill, air resistance, and rolling resistance of the tyres on the road.

    Remember that terminal velocity is achieved when the forces balance out - when the force of gravity pulling the bike down the hill is equal to the forces of friction and drag (air resistance).

    You will need to figure out:

    The force pulling the bike down the hill. This will be a component of gravity, and will depend only on the angle (steepness) of the hill and the mass of the bike + rider.

    The air resistance, which will depend on the speed and aerodynamics of the bike and rider. Here is a page discussing how to approach this: http://www.kineticbooks.com/physics/16270/20217/sp.html [Broken]. The Drag Coefficient is tough to work out theoretically. I wouldn't go there if I were you - I'd just make up a drag coefficient and fudge it until it gave a nice result. You could have a couple of drag coefficients if you like - a lower one for the rider with teardrop helmet, lycra suit, nice bike with thin tyres and disc wheels, and a higher one for Bruce in his cap and boardies on his BMX.

    The mechanical resistance of the wheel bearings. I don't know how to figure this out.

    The rolling resistance of the tyres. This will also have a related drag coefficient, which depends on the quality of the bike tyres, the tyre pressure, road surface, and also on the normal force between the tyres and the road, which will be another component of the force of gravity on the bike and rider. The normal force depends on the mass of the bike and rider, and on the angle of the hill.

    I think that's probably enough... you should check with your teacher if this is the right level. You don't want to bite off more than you can chew!

    edit: Here's a really relevant site I just found for you: http://www.sportfit.com/christopher/triathlete/bePrepared.html [Broken]
    Last edited by a moderator: May 2, 2017
  7. May 11, 2005 #6
    How about the relation of linear to rotational motion. Have you learned v = wr where v is the tangental velocity (which is the same as your s as long as your going in the same direction) , r is the radius of the wheel and w is the angular velocity in rad/s. remember 2pi radians = 360 degrees. A bicyle at its roots is a simple machine which changes rotational motion into linear motion. You can extend this into gear ratio's and crank versus tire angular velocity ratio. This is just kinematics. I'd stay clear of air friction unless your just looking for qualitative study at your level. We barely touch quantifying air resistance in AP Physics. Have fun and feel the motion.
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