Is cogging normal?

Hi,

So I have a leafbike direct drive motor and controller with a 52V battery. When the bike has a load I hear "cogging" initially on take off, but it goes away when bike is moving and also when there is no load. Is this normal? Can it cause damage? Is there any best practice or way to get rid of cogging?

It is ´normal’ because it is common, but it shouldn’t.

Not much you can do since it is related to motor design and/or machining precision. But, you would greatly improve your motor running if you would feed it with a sinewave controller.

Could it be the sound of the trapezoidal waveform controller?
If not, then it can only be the rear freewheel.
If you mean it feels like its flat when no power is given thats normal, there is resistance, but you mention initially on take off, so that makes me think its the trap wave.

butters149 said:

Hi,

So I have a leafbike direct drive motor and controller with a 52V battery. When the bike has a load I hear "cogging" initially on take off, but it goes away when bike is moving and also when there is no load. Is this normal? Can it cause damage? Is there any best practice or way to get rid of cogging?

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I would not call this noise cogging.

But hub motors do grunt under the heaviest loads, and some grunt quite loudly. If you have a particularly loud one, its because of the controller, plus one looser wrap on a winding vibrates. Any noise is amplified by the cover, much like the box on a guitar.

But I betcha, all you have is normal grunting. Big motors do grunt. Even small ones grunt some. The lady wrapping the motor winds can only wrap them so tight, so there is still some wire vibrating in there. A sinewave controller will lessen the grunt, and feel smoother. That vibration is made worse by a crude controller for sure. But nothing wrong with that, if you want to, run that cheap controller all you want.


If your motor is cogging, I have had motors that ran fine, but did cog more heavily than usual. All dd motors cog some, and geared motors cog some when turned backwards. But this cogging is not much, until motor rpm increases, causing the normal cogging while coasting to increase. ( on dd's, geared has the freewheel inside)

When you blow a controller shorting the phases, you will recognize full cogging when you see it. Its like heavy brakes are on. If you experience that, its a short on the phases, like a cut wire at the axle, or shorts in the controller.

Speaking to the matter of real cogging - which as I understand it is an inaudible effect that's kind of like slight braking - it would be interesting to have some way to measure it. It's normal, but how normal? Lots of talk about the fact that direct drive hubs do it, but I don't see much about how bad they do it, or whether it's the same with every direct drive hub or radically different from one to the next. I suspect it's better now than it was with early models, but who knows. In practice, I think I can feel maybe a 15W loss, but the real pain comes from ascending grades, it's weirdly hard to pull that extra 40lbs or so up hill.

Anyway, I guess some kind standard procedure could cover it, like put it on a stand and run the rear wheel up to 10 mph, and then time how long it takes to coast down to a stop.

Cogging in acceleration/load, is a design flaw or an error in manufacturing precision. Two hubs of the same model can be different, one badly cogging in acceleration and the other almost silent. But, big motors are getting better. The 2 last ones that I received are very silent in hard acceleration.

Controllers too are getting better, and they can suppress this cogging effect that is common with our cheap Chinese motors.

I have found it impossible to tell what is cogging resistance with a DD, and what is the effect of just having a 15 pound hub, and 10 pound battery on the bike. The weight alone drags me down a lot if I just pedal a DD bike.

Since I find it impossible to hold a throttle any less than 50-100w, that's what I give a DD bike when I need to just limp it home, and don't want to have the effect of the weight or the cogging. I suspect though, that 75 watts is needed to overcome that 50 pounds. And only 25 watts gets rid of the cogging.

Big motor with huge magnets will cog more than a smaller 28 mm wide dd. Makes sense, more copper, more magnet, more generation if you turn it. Cogging dramatically increases with speed, like on a downhill. Cogging increases more, at the same speed, with a high turn count, slow rpm motor. I found a 5305 vs a 5304 almost eliminated the need to use brakes on a 5% grade downhill. Speed would max out at about 35 mph, while you'd hit 40 or more on the faster motor.

I never called motor grunting cogging, and I understood it was just wires jumping around in the windings vibrating, if the winding is not turning fast enough yet. Its wanting to move, and if the load don't let it, the wires try to move. A defect, loose wrap, will make a motor grunt like hill. I have no idea if this same defect would increase cogging when coasting. I never had a motor that grunted all that much. All about the same noise on take offs.

What WILL increase cogging is a short on the phases. A good short, and its like brakes on. Unplug the motor, and short any two phases and you'll see what I mean.

The one motor I owned that cogged like hell did not grunt more. But it cogged like hell. Its likely no coincidence that that motor was pretty toasted. Not totally blackened, but it was by no means golden wire anymore. I think it had a very tiny short in the windings, where the varnish melted. It seemed to run fine, but it cogged like mad.