Mid-Drive Technology
The EcoSpeed Electric Mountain Drive and Electric Mid-Drive units are the highest performance and most efficient electric assist kits sold in the world today. Our geared drive system with an efficient brushless DC motor gives top speed and hill climbing abilty that equals or exceeds that of hub motors having two or even three times the power.
Of course, the sophistication of the EMtnD and EMD comes at a price. So why would you want to pay one and a half to twice what a basic hub motor costs? Read on.
The graph to the right shows output power graphed against motor speed for a DC motor of about 600 Watts output. Power is on the vertical axis, motor rpm is on the horizontal axis.
In a fixed geared application such as a hub drive, motor rpm is directly proportionate to bike speed. The exact proportion depends of the fixed gear ratio chosen. An example of that would be something like 0 mph at 0 rpm (left end of graph) and 25 mph at 4000 rpm (right end of graph). It doesn't really matter for our purposes.
The relevant fact is that you only have peak motor power available over a narrow range of speeds. In our example that would be about 10 to 15 mph. That's exactly the speed range where peak power is least useful.
Where you want peak power available is at the ends of the graph. On the left end, corresponding to very low speeds, is where you'll be when you're slogging up steep hills. When you're cruising fast, you're at the right end and you need all the power you can get to overcome wind drag.
The next graph shows thrust at the rear wheel plotted against speed for a single speed hub drive and a multi-speed mid-drive. Both motors have identical performance characteristics.
Motor power is converted to thrust at the rear wheel and that is what actually pushes the bike forward. The more you have, the faster you accelerate, the steeper hill you can climb, and the faster you can go.
You can see from the graph that except for the one speed where their effective gear ratios match, the multispeed drive ALWAYS generates more thrust. And remember, the motors are identical, so the graph is just showing the effect of gearing. Also note that at lower speeds, where you'll be when climbing hills, and at higher speeds, the geared system produces several times the thrust of a hub drive.
That's why an EcoSpeed Electric Mid-Drive or Electric Mountain Drive can outperform hub motors with several times the power.
Finally, the last graph plots motor efficiency against speed. Again, you can see that the multi-speed drive is always more efficient in the critical low and high speed ranges.
Don't dismiss the importance of efficiency by thinking somehthing like "Oh, I'm not climbing hills very often so I can live with a little less range". When a motor is operating in an inefficient speed range the power that's not going to the wheels is going somewhere. In this case, it's going into heat in the motor windings. Many a rider has discovered to their chagrin that their expensive hub motor overheats and shuts down on a long grade.
At EcoSpeed we don't do regenerative braking for a very simple reason. What works on electric cars doesn't necessarily work on electric bikes. All electric cars have regen braking which recovers a small amount of energy from momentum during braking. Most estimates of the amount are in the 5% to 10% range. Electric bikes are light and travel at low speeds. As a result they have very little momentum so less energy is available to be potentially recovered. To recover that small amount of energy, regenerative braking systems either require that the motor be continuously engaged or that clutches be used to engage the motor when regen braking is needed.
No one, including EcoSpeed, uses clutches because of the cost and complexity involved. A continuously engaged motor means that there is always a drag caused by the motor's cogging torque. That doesn't matter to a car since the motor is going to be in use almost all the time. Bicyclists, on the other hand, have been known to
pedal. That means you are using your muscle power to overcome motor drag whenever you are pedaling without the motor turned on. The loss from that drag can easily exceed the power you get back from regen. In other words, on a bike, regen braking can actually cause a net
loss of energy! If you want the gory details look
here.
The last thing to note, is that for any electric vehicle, it's not the motor that supplies the power. It's the battery! Battery power is a precious resource. It's expensive and heavy.
Many hub motor systems supply brute force performance by using big motors and big batteries. That does work and you can match the performance of an EMD with a sufficiently large hub motor and a high enough voltage battery system. But you'll have to increase the weight and cost of the batteries you carry to feed that big motor.
At EcoSpeed, we like to create the highest performance at the least weight. Keep the weight down and your bike still feels and pedals like a good quality bike. That means you'll use the assist less than you otherwise would. And, of course, that means your overall efficiency goes even higher.
