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B Question about bicycles and inertia

  1. Jul 28, 2017 #1
    In a bicycle, would I be able to pedal in a gear that I can’t pedal from a stationary position in if it was already started by another power source (engine, motor or by a stronger person) whilst keeping it rotating and preventing it from stopping?

    So If an electric motor started moving a bicycle in a high gear that I can’t start to move it in, will I be able to move it after it’s already started moving and keep it moving whilst preventing it from stopping?
  2. jcsd
  3. Jul 28, 2017 #2


    Staff: Mentor

    Sure, but you don't need an electric motor. Just start at the top of a hill and coast. When the speed gets high enough, you can pedal in high gear.
  4. Jul 28, 2017 #3


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    If it is a tricycle, so no balancing is involved, and the machine is frictionless, without air resistance, it would start and gather speed no matter how high the gear.
    Why should the torque applied by the rider be less when starting than when gong along? Our familiarity with IC engines makes us expect this feature to be universal, but muscle power and steam engines can apply full torque from zero speed.
  5. Jul 28, 2017 #4
    In a high gear the pedal torque required(proportional to the force applied by legs) is higher. The bike has some rolling resistance from terrain and tyres. There is a point where the torque required only to overcome rolling resistance is higher than the maximum you can do.
    A bicyclist should be able to deliver torque but also power is important. Power at pedal axle is torque times angular velocity. Where anyone can deliver reasonable torque even power for a limited time, competitive bicyclists can also maintain the power for a long time. That is they deliver enough torque at high axle angular velocity or enough torque at high speed. But I digress.
    Basically at a high gear ratio you should be able to deliver enough torque to accelerate the bike slowly. But I suppose you want to keep it for long distances, that means you will be able to deliver a lower torque. The idea is the electric motor will help you get to a speed where you can keep your balance. But the higher your velocity the harder(power required, not torque) it it becomes to pedal because you will have to deliver more power then after 20 - 30kph the air resistance rises considerably and it does that with the square of velocity. The only thing is that at low speed the bike you can't keep balance or at least is difficult. Without air resistance the a man can go up to like 200kph on a finely tuned bicycle but after that rolling resistance and gear, chain, bearings losses cannot be overcome anymore even by a competitive cyclist.
    A competitive cyclist can deliver more than 2000W during a short sprint where as an amateur can do like 550W. During long runs they do around 250W which is 8 times less than peak power and an average man around 100W which is 5.5 time less.
    If the terrain is not perfect and also until you break the friction in the gears (also at high speed the gear and ball bearings friction is slightly higher) the bike will require a higher torque but that is not a problem in case you don't intend to make it extremely difficult to pedal.
    Instead of using an electric motor only to set it in motion you can use helper wheels that can you retract after you reach like 5 kmph.
    Slopes will be very difficult to climb therefore a motor would be much more helpful.
  6. Jul 28, 2017 #5


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    The Torque is not the only important quantity involved. The cyclist can actually transfer very little Power to the wheels at zero speed. Power is the rate of doing work and you need speed in order to increase the Kinetic Energy of the bike.
    (Power transferred = torque times rotational speed of pedals)
    It's a sort of catch 22 because it will take you ages (assuming you don't fall off) to accelerate a stationary bike in a high gear. That's why you start in a low gear, which allows your legs to move faster and, thus, to transfer more Energy to the bike's KE per second. At high speed, your legs cannot supply enough torque whilst they are thrashing up and down so you then have to change up so that your legs are working at a decent rate and produce useful torque.
    Last edited: Jul 29, 2017
  7. Jul 28, 2017 #6
    If you plan to use the motor to take you at 25-30 kph its fine you can have some advantages like better acceleration more ground clearance (also less prone to break the pinion shifter) less likely for the chain to come off. But if you think the motor can take you to 40-50kph or more where you can coast, then it's not going to work.
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