question about hub motor

[tilopa]

This is a simple question that I could not find the answer to about how a hub motor works. If I have a front wheel hub motor at full throttle and the bike then begins to descend down a steep hill, will the motor limit the speed of the wheel that it would otherwise travel at because of gravity? Or will the wheel go faster than the hub motor and the motor and gravity are augmenting each other.

In other words does the hub motor control the wheel directly or is the wheel free to spin faster than the motors applied torque?

Thanks.

 

[Drunkskunk]

Interesting question.

The controller is not limiting the motor to that speed directly. Its just feeding it a bunch of power, and letting the motor go as fast as it can on that amount of power. Thats why you slow down on hills, and speed up on decents.

But there is a catch. If you get going fast enough, faster than the top speed of the motor, faster even than the top speed of the motor if the wheel was spinning free in the air at full power, the motor's back EMF will start acting like a brake, and the faster you go, the more that will happen. But you have to be going much faster than the bike's top speed before you can feel that.

 

[wesnewell]

Depends on the motor type.
If it's a geared motor, then it will simply increase in speed if air flow resistance doesn't stop it from doing so. Once you reach top speed of the motor you might as well release throttle and let it coast. Keeping the throttle open will just waste battery power.
If it's a direct drive motor and you keep the throttle or cruise control engaged, once you surpass the motor speed for that throttle setting, it will then turn into a generator and start charging your battery pack. If you release the throttle it will coast a little better, but you also lose its generator function and there still be magnet resistance. IMO, it's more efficient to keep the throttle engaged with dd motors.

 

[d8veh]

This is a simple question that I could not find the answer to about how a hub motor works. If I have a front wheel hub motor at full throttle and the bike then begins to descend down a steep hill, will the motor limit the speed of the wheel that it would otherwise travel at because of gravity? Or will the wheel go faster than the hub motor and the motor and gravity are augmenting each other.

In other words does the hub motor control the wheel directly or is the wheel free to spin faster than the motors applied torque?

Thanks.

As it's a front hub motor, I'm guessing that it's a geared motor (a photo would help). Geared motors have clutches that disengage once you go faster than the motor wants to turn, so you're free-wheeling down a steep hill with the motor disengaged. It makes no difference whether you have the throttle open or not because the back emf from the motor cancels out it's supplied voltage, so it doesn't consume any power. You can see this if you have a watt-meter. You can hold full throttle, and if you can pedal hard enough or go downhill, the watts go down as you approach the max speed of the motor and eventually reach zero as you go past it. This is true for most 250w motors, but I've noticed that some controllers for bigger motors still provide a nominal amount of power.

 

[dogman dan]

Like Wes says, throttle down the hills. But I disagree that no throttle is faster coasting. WOT will result in a faster coasting speed down the hill. Because as Wes says, the motor will coast freely till the motor reaches the speed you'd nomaly go at that throttle setting. Then it will resist. It will resist, but only lightly with throttle off. Throttle off, resistance will build as you build speed, but at slow speed it's light resistance some don't feel. Throttle on, you get no resistance till you reach a faster speed.

DD will limit speed downhills. Even on the very steepest grades, typical dd bike hubmotor windings will not allow coasting above about 35 mph. It's kind of nice, and another of the reasons why I like a slower winding for a touring bike. A slow winding may limit speed to as low as 25 mph down a 7% grade, which can be nice on a loaded down longtail.

Be advised though, that motor makes heat coasting down the hill. So you might not roast your brakes, but you can roast a motor if you topped that hill very hot.

Plantetary gearmotors coast down the hill freewheeling, and will descend much faster.

 

[tilopa]

Thanks for all the replies.

So, Dogman you are saying if I have a gearless direct drive motor then going downhill the bike will be breaked by the motor thus limiting speed? Is this true if I have the motor switched off while going downhill?

Thanks.

 

[The fingers]

It is fun sometimes to descend short sections at WOT just to see how fast you can go, but I prefer to coast on most longer downhill sections to extend battery range and keep braking to a minimum. The DD motor does limit top speed somewhat but this results in more control, sort of like engine braking in a lower gear does in a motor vehicle.

 

[dogman dan]

Exactly. A direct drive motor will resist some going down the hill. How much resistance depends on the winding of the motor. Slow motors will start resisting at a slower speed, and tend to decend slower than a fast winding motor will.

It's part of why a slow motor works good for my cargo bike. Climbs better, but slower, and decends better, but slower. Saves brakes and makes a safer ride.

It will make heat coming down the hill though. A gearmotor makes no heat going downhill since it freewheels.