Feasible Idea?

I got to thinking about how to achieve higher speeds with an electric bicycle, you have to use a bigger motor, and in turn bigger batteries, to have a reasonable range. After thinking about it for a while, I came up with a solution that may work if the bicycle is mostly used on flat ground. What If you ran two motors inline with a 7 speed planetary rear hub, one motor with an internal gear reduction, and one motor ungeared, both with freewheel clutches. Whenever you wound the bike out in 7th gear with the geared motor, you could let off the throttle, put it back in first gear, and flip a switch to change power to the ungeared motor. With the inertia that the geared motor provided, wouldn't the ungeared motor have enough power to pull the bicycle even faster? Is this a feasible idea?

This type of thing (slight variation of it) has been discussed a bit here and there. Here is my take on it;

Yes that is feaseable. However, if you are running the motor through gears, the range of typical bike cogs or internal hubs is more than sufficient to not require two motors. Second, the issue for range is not so much one of accelleration gearing, it is much more a matter of wind resistance as well as drive line losses and overall power system efficiency.

For example;

My recumbent is geared for 40mph top speed with a fresh charge. It has no gear ratio shifting on the motor side. I have had it geared for 30mph or 40mph with no appreciable efficiency change at my typical 20mph average speed. However, geared for 30mph gives me more punch. Whereas geared for 40mph gives me more top speed. In either case, the bike still requires a similar amount of wattage to cruise at a given speed (say, 25mph). Now, that same bike requires 1400 to 1500 watts to cruise at 40mph. No matter how I gear it, there is a minimum amount of energy required to push me and the bike through the air at 40mph. You can achieve better or worse results by moving the gearing in accordance with the motor's efficiency curve. But, again, you still have resistive losses to overcome and (above about 20mph) wind seems to be the greatest of those losses. Wind is not affected by gearing.

So, that being said, I would put it this way;

For maximum accelleration AND maximum efficiency in one package, multiple ratios (gearing) or possibly two different motors in one system would be advantageous.

Matt

I wanted efficiency. But I didn't want to sacrifice performance anywhere. I made it powerful enough to have more than enough power to flip the bike over from low speeds, let still geared for a loft top speed and high motor RPM for efficiency. Gearing could get me more low-end torque, but it all ready delivers more than I can use at low speeds, so it simplifies the needs of the bike through a little extra motor power capability.