dogman said:
Back to the subject of efficeincy on hills,,
...
So even if the stokemonkey is a lot more efficient than the best geared hubmotors, it will still take some considerable watthours to get up the hill.
What is very interesting, is how long a hill could a stokemonkey climb heavily loaded without overheating? In summer, I found even the gearmotors got hot fast on a steep hill, like 10% or more. A truly more efficient motor system would make a lot less heat. That could be what really makes the difference rather than having more range. let us know when july comes.
Dogman,
Exactly. Climbing up a hill, it is all about speed, load and grade. To move a given mass up a given grade at a given speed will take a certain amount of energy, even if the sytem is 100% efficient. Motor efficiency is directly linked to motor speed and power input. That is, if a lot of power is going into the motor and the motor is not turning at its most efficient speed (which is near the max speed, if memory serves), the efficiency will be low. That is, you will be sucking a lot of battery power and not much of it will be getting to the tires. If you have a big enough motor, enough batteries, and can shed the heat fast enough, well, there you go! The reason I like the system I have is that the motor speed is relatively uncoupled from the grade if you pick the right gear. You can see this in the data above; on the "Timberpass" ride, the grade varied between 1 and 20% but the speed only varied between 4.1 and 5.2 mph, about 20%. In that gear, the max bike speed is close to 5.2 mph so 4.1 is pretty close. The thing is more like a tractor than a bike at that gearing.
I think the question about how hot a motor can get can be thought of in terms of where a person is operating with regard to the continuous load of the motor. I have the power limited to essentially the max. continuous rating of the motor. I think I could put 100 lbs of dead weight on the bike and climb a 20% grade until the battery went dead but I would be going about 3 mph at which point is is difficult to keep the bike upright. The ebike sim. says this would take about 414W which I think would be do-able even in July here in TN. On my system, that is less than 12 amps. I have taken my daughter up a 25% grade (she weighs about 80 lbs) and have barely even pushed the pedals. So as you say, it is the combination of speed, grade, and load that makes or breaks the system. For hills of any significant grade, the bike is not likely to be going very fast so that the power required to overcome the hill is going to be nearly completely composed of the power required to move the weight up the grade (that is, wind resistance is almost nil by comparison) at the given speed.
So in the end, I have a 36V battery. If I limit myself to around 500W, that determines the speed I can travel up a grade with a given weight. In the limit as the speed up the grade goes to zero, I can carry an infinite mass up a given grade on fixed power if I pick the right gear. Who was it that said, "Let me pick the fulcrum and I am move the Earth" (Archimedes?)
For me, it is the hill climbing that makes my non-motorized commute a biaatch. For people who really want to haul butt, it is going to take a certain amount of power and 500W will not cut it no matter how you gear it; the ebike calculator says I can go about 27 mph on that much power on my bike on level ground. On the system I have, I could go to work and back traveling >20% grades without touching the pedals and keep my power usage very low. I have in fact done this just to prove it to myself. It took nearly an hour. Or I can do as I do now and pedal moderately and do it in about 30 minutes for about 1/2 Ahr/mile.
I think the main thing is for people interested in ebikes to be able to TRY them so that they can compare their expections and needs to the ability of various systems. For me, I like to have bike speed effectively decoupled from the motor speed. If you are able to keep the motor at near its max speed, it will be efficient, regardless (within reason) of the system used to get the power to the wheels. Some people prefer to go with bigger batteries and motors that handle more power. Either way, you can get the job done. The average hub motor system can be both efficient and fast, but it can't be both efficient and fast and efficient and slow.
One last antecdote of why I like my ebike. Two days ago I was recovering from a cold virus. I had ridden my bike to work and had felt pretty good in the morning and the weather was perfect for riding; 60 deg. F and sunny. But I was on my feet a lot that day and by the end of the day, I was pretty tired. I needed to go several miles out of my normal route to pick up something on the way home. One of my graduate students could have done it and volunteered to. But I thought "This is why I have the system I have. I have a 15 Ahr batt so I can go out of my way if I need to. I have a geared system so that if I am zonked I can get home with very little effort (although not that quickly). I have an Xtracycle so I can carry stuff without worrying about carrying capacity." So off I went. I felt about the same when I got home as when I left the office. If I had not had an ebike, I would have been dead on my feet.
If anyone wants to suggest a test of the hill-climbing ability for geared systems, I'm game. As long as I get to pick the fulcrum and lever (and if speed is no object). I don't know about the ability to shed heat but I assume it is similar to any system. It has the potential advantage of spinning pretty fast even at low speeds. On the Stokemonkey, the motor is a modified hub motor so the "casing" spins around and the "shaft" is stationary. I thought that was a weird arrangement at first but it probably helps supply convective cooling at low bike speeds.
Lastly, I want to thank, one last time, all those who patiently answered my questions last fall and suffered my suggestions on standardizing hill climbing tests of ebikes.
