Thread: Gymkhana
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Old 07-13-2013, 11:56 AM   #1259
lnewqban
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Joined: Jan 2012
Location: Florida
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Quote:
Originally Posted by Vulfy View Post
........I'm still very hazy on the whole concept of how sprocket size affects top speed, torque and acceleration.
Imagine that your transmission has only one gear combination with which you have to live.

Sprockets are nothing but levers.

That lever attached to the output shaft (front sprocket) can do with the twisting force coming from that shaft (output torque) only what levers do: to apply a tangential force at its free end (tension on the chain), more if the lever is short (few teeth), less if the lever is long (more teeth). For the same reason, that chain will move slower in the first case and faster in the second case. As transmitted power = force x speed, it remains the same for either case.

The lever attached to the rear wheel (rear sprocket) can do with the tangential force at its free end (tension on the chain) only what levers do: to apply a twisting force to that rear tire (wheel torque), which becomes a rearward pushing force at the contact patch. That force will be less if the lever is short (few teeth) and more if the lever is long (more teeth). For the same reason, the bike will move faster in the first case and slower in the second case. As transmitted power = force x speed, it remains the same for either case.

For brilliant acceleration, you want big force at that contact patch (a = F / m).
That twisting force coming from the transmission shaft (output torque) reaches its peak value only for the range of rpms' at which the manufacturer of the bike claims that maximum torque is achieved by the engine.

The right combination of sprockets should put your engine in that sweet spot for the power demanding part of the particular Gymkhana exercise.

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