Say for example a 125cc motorcycle with 6 gears has a top speed of about 75mph, why can’t they make it with 7 gears and surely that’d increase the top speed?
In: 1
The higher the gear the more power required to turn it. Drag increases with the square of the speed, so to go twice as fast you need four times the power to overcome that drag.
Put another way, at 75 mph your motorcycle experiences about 0.1 psi of drag pressure, times the frontal area and the coefficient of drag. At 100 mph your motorcycle experiences about 0.18 psi. [Chegg suggests](https://www.chegg.com/homework-help/frontal-area-small-motorcycle-060-m2-drag-coefficient-062-ma-chapter-8-problem-41p-solution-9780521192453-exc) we can use 0.60m^2 for frontal area and 0.62 for drag coefficient. Passing this through my [favorite physics calculator](http://qalculate.github.io/) shows we need to overcome 60 pounds of force to go 75 mph and 103 pounds of force to go 100 mph.
0.5 × 0.00238slug/ft^3 (100mph)^2 × 0.6m^2 × 0.62
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