The advantage of a transmission is more power at low speeds while at the same
time reducing the power needed. But once you are up to a cruising speed the
transmission does not any help except for allowing the motors to operate at
high efficiency at more than one speed. The disadvantages of a transmission
are increased drag, or lower overall efficiency and increased weight.
The advantage of a two motor system is that a second motor is
often, if not always, lighter, smaller, simpler and more efficient than a
transmission. The disadvantage of a two motor system is increased power
requirement.
Here is another way to look at it: if your main concern is high
efficiency at a particular cursing speed [e.g. highway travel] then longer you
travel the less efficient a transmission becomes and conversely the more
efficient a two motor system becomes. A transmission only increases efficiency
during acceleration once you are at a steady speed the transmission reduces
efficiency in terms of drag and weight. While a two motor system is only less
efficient during acceleration, so if you have little acceleration and a lot of
steady speed then two motor system is overall more efficient.
Here is a very simple model; if it gives good results then a
closer examination would be warranted: take a 25-mile highway trip, well give
a ¼ mile acceleration; more simply a 100-mile trip with 1-mile acceleration.
At any give time the motorcycle needs X power. A transmission is 5% less
efficient than a non-transmission so over 100 miles it will take 100 miles *
.05 = 5 units more power. The two motor system will take twice the power at
acceleration 1 mile * 2 = 2 units more power. But if you are commuting in stop
and go traffic then a 40-mile commute might have over a 1 of acceleration. So
a transmission is 40 * .05 = 2 units and a two motor system is still 2 units.
So if your steady speed is more 40 times longer than your acceleration then a
two motor system is more efficient. But if your steady speed is less 40 times
more than acceleration then a transmission is more efficient. A closer study
is needed and results will vary based on
rider/bike/weight/route/etc
but I think it will be clear that in short city
travel a transmission is more efficient, but in longer highway travel a two
motor system is more efficient. But since an electric motorcycle has a finite
range and cant be filled up at the pump then it should be built to be the most
efficient for the longest-range option. Hence once you get an electric bike
going over 50 miles on a charge [i.e. lithium] and you have some plans to
travel on the highway then a two motor system is the best choice. Further with
a 100+ mile range, city travel never be a concern, even with the lower
efficiency of the two motor system, but when a long trip on the highway is
needed the two motor system will shine as it will give you more range than a
transmission.
harry
Albuquerque, NM
http://www.austinev.org/evalbum/1179
http://geocities.com/solarcookingman
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