Quote:
Originally Posted by masterx1234
so if the honda gx35 gets a top speed of 30 mph and with only 1.5 HP on a bikemotorparts kit then a dax titan xc50 with 2.5 HP should get up to 40 MPH correct?

The short answer is YES! That is about right.
There are two components to drag. The rolling resistance is proportional to speed. The aero drag rises with the square of the speed. Once over 30mph, the aero component is by far the dominant factor for most bikes. So to keep it simple, I will ignore the rolling resistance:
Sqrt((2.5hp/1.5hp) x 30mph x 30mph) = 38.73mph
Or, assuming that 1.5hp gets you 30mph, how much power is required to do 40mph?:
1.5hp x (40mph x 40mph)/(30mph x 30mph)=2.67hp
In order for this to be true, the gear ratio must be chosen so that your speed corresponds to the peak power rpm of the engine.
The reverse of this is true. If this bike has a 2.5 hp engine but you gear it so low that it only goes 30mph steady state, then the motor is past it’s power peak and is only putting out 1.5hp steady state.
I tried to make this point in another thread on California law when discussing the 2hp, 30mph laws but it got off track for that thread. This seems like the better thread to make the point since it answers your question.