Monday, May 19, 2014

Hub Motors

Two motors?

Two identically geared hub motors with two controllers, could be an asset in slippery or extra steep conditions; 2WD will have better traction. I don't know how well two separate kinds of power systems would work together, but having an extra motor on the front hub can't hurt if it is geared low enough.

                                                        two speed hub motor

 Apparently I cannot over emphasize the need for gear enough. Hub motors that do not have a gear reduction inside must use more amperage to overcome the lack of gears. And that means that you will need a larger motor and battery, just to handle the increased amperage. It also means that you will end up turning your bike into an illegal motorcycle if you want to power 400lbs up an 8% grade. Or at least have worse mileage, and you could run the risk of burning out your controller if it is not big enough to handle the increased wattage.

When you use gears it really changes things. With gears you can move the "efficiency peak" around to match your mph. On a fixed hub motor you are permanently tied to your power-band and this presents a real problem at low speeds. Low mph/rpm on a hub motor places you into the low efficiency areas of the power-band. You can compensate a little if you have an ammeter and are really careful about using even less current than the current limit allows (so you are using almost no voltage down low in order to be more efficient) but you are still fighting a less than desirable situation.

Run a 40mph hub at less than 10mph under load and you'll cook it. My best guess is that a hub would have to be capable of better than 40mph to climb rough steep grades efficiently, and in that case you would not be able to do so slowly or you'd overheat the motor.

At 10 mph the efficiency is running about 50%. The good efficiency areas don't even exist until you are past 20 mph. So unlike with gears where you can "gas it" off the line and be in the good efficiency areas all the time, gearless hub motors have a real "issue" with quick starting. Quick starts on a gearless hub motor are really bad news for battery usage... (much worse than if you are geared right)

The only reason to even think about hub motors is because the speed of operation is lower (usually less than 500 rpm) so it makes finding sprockets much easier. An excellent example is the Elf car.
That's why they used one for the Stoke Monkey mid -drive.

Geared Hub motors verses Un-geared hub motors

Hub motor verses gears debate

How long does a geared vs direct drive last?

This is  a self-contained, but wimpy  machine, made for people that live on flat land, and need to stay dry cycling to work! But do not think it will carry you up steep hills with cargo!

While a single stage chain drive loses around 5-10%, a hub drive not operating near it's peak RPM is losing far more power than a properly geared motor would be.

San Francisco wheel?

If someone wants to make a dent in america's car culture glut, they are going to have to engineer one that can drive 400lbs up a 10% grade at legal speeds. It has the potential to use thicker wires for lower voltages when climbing hill, unlike normal hub motors, without planetary gears.

Only in america can we use a motor strong enough.

Geared hub motors are intrinsically complex. Most direct-drive hub motors only have a few moving parts, and a hub motor with a gear set has tens of moving parts.

This leads to the big 2 disadvantages of a hub motors: #1 they are more expensive, #2 it are more fragile than a direct-drive hub motor. But they are much better hill climbers than a non-geared hub motor of the same power

So not all hub motors are bad. Some are very well made and they charge for that durability.

Hub motors in general have a problem running low voltage in through the axle, because low voltage requires larger wire than will fit. This may limit the slow hill climbing trust. But you can check the performance ability on this Canadian motor simulator.

If you have steep hills, you are most likely going to need a geared hub motor. I needed a 6:1 gear ratio for about 600lbs on our steep hills. But I have not seen one that is more than 5:1 ratio.

There are other problems with hub motors, like sensitivity to road shock and and water. Personally I do not think they are worth the price. With steep hills and a lot of weight it would be better to use a single stage gear reduction from a large motor set to the side.

The enabling feature of the GTF is its 3:1 reduction gearing. The setup is similar to this planetary gear system, also produced by Italian aviation company, Avio.

 Hub motors aren't quite right for the purpose of hill climbing with cargo because they are happiest running at RPMs too high for regular bicycle wheels.
We can get hub motors that weigh 10kg and more, but we can't get them in a configuration that will efficiently drag a loaded normal bike up a steep grade at less than 1000W input power.

If done right for given terrain (ie: larger wheel for flat land or a smaller wheel for steep hills, or at least an oversized motor and controller that will give a wider range of efficiency), a large direct drive hub motor is the most efficient motor system. But if you have steep hills, and there is a legal limit to the size of motor you're allowed, driving around in a single gear may not be the most efficient.

This is for a law that limits motor size.
If the law allows a bigger motor go for a bigger motor

This book does not tell you how motors work,

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