250W geared motor performance

[Kermit]

Could you please advise if what I experience is right. I have been using 250 W geared rear motor (I believe it is mxus) with Kt controller for a year now. Works fine however what I see is that when I reach certain speed (depending on the charge level of the battery) speed meter stops at one value even if I am driving faster (I. e driving down the hill). In addition I 've got the feeling that the motor is stopping me (acts like a brake). When speed drops and motor starts working again everything is back to normal. In other words I have an impression that the motor makes some resistance. I am wondering what are your thoughts.

 

[Benjahmin]

Geared hubs have a clutch built in that disengages the motor and allows it to freewheel. It's the same sort of mechanism as the rear cogs on a standard bike. There can be a feeling of 'cycling through treacle' when you exceed the speed limit of the motor (legally 15.5mph) but this is merely you feeling the weight of the bike. Otherwise the bike should ride just like an ordinary one, albeit a heavy one, with no resistance from the motor.
There can be drag from rubbing brakes. To test for this, hold the wheel off the ground and spin it. It should spin freely. If it stops quickly check for binding brakes.
Don't know why your speed reading is out. Think you'll have to give more details of display and controller to get help with that.

 

[Kermit]

Controller is Kt type bought on Ali from C&D seller. Display is LCD 6 from the same seller. Motor is Mxus, refurbished, don't know the history.
Breaks do not rubs, wheel can rotate freely.
The biggest unknown is the motor I guess however I thought maybe I will find some clue here for C and P parameters setting.

 

[vfr400]

Does the speed shown on the LCD go to zero when you freewheel down a hiĺl?

Lift the wheel off the ground and turn it by hand in both directions. One way should be free and the other way, you should feel the motor turning.

 

[Kermit]

Yes. Speed goes to zero as you said. There is no external speed sensor. The speed is read from the mosfet. Maybe I don't know of the basis of how this system works.

 

[vfr400]

The motor has a maximum speed. If you go downhill faster than what the motor can spin, the speed shown will be the motor's maximum speed, not the bike's speed.

When the motor is driving the bike, it feels like the resistance is low because the motor is doing work. When you drive the bike faster than what the motr can drive, there is no help from the motor, plus the air resistance is higher, so it will be hard to maintain motion in that situation and you will feel the difference in resistance.

The motor has a clutch which disengages it from the wheel as soon as the wheel is turning faster than the motor, so the motor cannot make a significant contribution to resistance in that situation.

 

[RossG]

I think it all depends on what your setup is though doesn't it vfr. On my folder I've set the assist to it's max setting of 60 Kph (37 Mph), it won't drive me along that fast without me pedalling of course but it doesn't cut out either.
With the bike on the work stand I've turned the crank as fast as I can and the speedo just reads higher and higher, the motor doesn't disengage I can hear it whirring over the sound of the gears. Also I have a volt/amp meter attached, that was no mean feat on a hidden battery, and the current still flows.
Obviously Kermit has a different system to mine though.

 

[vfr400]

Quite correct, however, I'm not talking about the maximum speed set in the LCD. Instead, I'm talking about the motor reaching its maximum RPM, which depends on its copper windings and the battery voltage.

If your motor is still whirring at 37 MPH, something is wrong with your bike, like the clutch is stuck, or do you have one of those 40 mph DD motors? If your motor could actually reach 37mph on its own, it would be extremely inefficient at normal speeds, like 12 mph. It wouldn't be able to pull you up a hill and it would over-heat nearly all the time, and your range would be about 1/2 of what anybody would get on a similar bike with a similar battery.

The speed shown on your LCD is always correct up to the LCD's maximum of something like 72 km/h, but OP doesn't have a speed sensor, so his LCD is getting the speed from the motor, which is why his indicated speed never goes above the motor's maximum speed.

None of this applies to crank-drive motors, where it's the gearing that affects the speed, not the motor.

 

[RossG]

I'll have to carry out more testing on my bike, lousy weather again today sadly. I don't have a separate speed sensor so the speed is measured off of the motor or the rotating rear wheel anyway.
If I pedal under my own steam with assist switched off I still get a speed readout on the display, as long as the LCD is illuminated I can see how fast I'm moving. It's a Bafang geared motor BTW.
On assist levels 1-4 the motor cuts out at certain speeds but on setting 5 it never stops running no matter how fast the wheels go around.

 

[vfr400]

On assist levels 1-4 the motor cuts out at certain speeds but on setting 5 it never stops running no matter how fast the wheels go around.

Sorry, but that's not true because it's impossible. The motor has a maximum speed that depends on its copper windings, typically in the range 200 to 300 rpm. Mxus is normally around 260 rpm or 23 mph in a 26" wheel.

 

[anotherkiwi]

Controller is Kt type bought on Ali from C&D seller. Display is LCD 6 from the same seller. Motor is Mxus, refurbished, don't know the history.
Breaks do not rubs, wheel can rotate freely.
The biggest unknown is the motor I guess however I thought maybe I will find some clue here for C and P parameters setting.

P2 should be set to 0 - no external speed sensor.

 

[RossG]

Sorry, but that's not true because it's impossible. The motor has a maximum speed that depends on its copper windings, typically in the range 200 to 300 rpm. Mxus is normally around 260 rpm or 23 mph in a 26" wheel.

I think you got the posts mixed up vfr, I don't have a Mxus . In my case the motor doesn't cut out at 25 Kph as it's set to 60 Kph. The next time I get my power meter out and mess around attaching it which is a bind I can tell you, I'll take a video and prove it.

 

[vfr400]

Ok, your motor doesn't cut out at 15 mph, but it's still limited by its windings, so it's max speed will most likely be in the range 200 to 300 rpm. Motors for 20" wheels are normally about 330 rpm, which is still around 22 mph. There's no way in the world that your motor will spin up to 60 km/h, which is 482 rpm in a 26" wheel or 627 rpm in a 20" one.

 

[RossG]

vrfr, I've put my bike on a work stand switched on and gone through all PAS levels. As we all know a motor assists to a certain preordained speed then cuts out as indeed mine does all the way up to level five where it doesn't.
On it's stand I can only turn the crank so fast then it starts to get a little unstable, but I managed to get up to 29 Mph on the display and stand on me that motor had not cut out. It's set at 60 Kph and that's the point at which it will cut out. I can hear the motor still running over the noise of the derailleur and see current flowing via devices I have attached to the controller.
Just to clarify the motor isn't driving the wheel at that speed I am, It just does not cut out when it reaches 15.5 Mph because it's set at a higher speed.

 

[vfr400]

That's right. When unrestricted, the motor will spin as long as you're pedalling, but it's disengaged from the wheel, so not doing anything to contribute towards the driving force. If you have the wheel or hub speed sensor and the correct settings (p2=1), the speed shown on the LCD will be the wheel speed, which will max out at 72km/h regardless of speed settings. If you don't have a speed sensor and you set p2=0, the speed shown will be the motor speed, which will max out when the motor maxes out tegardless of speed settings.

If your different levels have speed limits, you have p3=0. You should change it to p3=1, which most people find much better, and there are no speed limits.

 

[RossG]

That's what I've been saying all along really, basically the motor does not cut out at 15.5 Mph as it's set higher. The suggestion was it stopped dead at that speed which it doesn't.
Just to add I don't have a KT controller or display so your suggested settings would not apply. My bike has a Bafang hub motor but there are no markings or codes on it so it may well be a built to spec job.
The bike certainly shifts though that's for sure.

 

[vfr400]

That's what I've been saying all along really, basically the motor does not cut out at 15.5 Mph as it's set higher. The suggestion was it stopped dead at that speed which it doesn't.
Just to add I don't have a KT controller or display so your suggested settings would not apply. My bike has a Bafang hub motor but there are no markings or codes on it so it may well be a built to spec job.
The bike certainly shifts though that's for sure.

Just a bit of misunderstanding. Maybe my explanation wasn't very good. I can't remember what I wrote, but if I said that the motor cuts out at its maximum speed, I meant that it makes no power contribution. It will still spin, but the current going through it is close to zero, so power is cut.

Your controller is working with speed control. I think that current/power control gives a much better ride and better control.

You only have to measure your motor's speed when unrestricted. You use first gear so the wheel can't spin faster than the motor, then lift the wheel off the ground, turn the pedals and measure the RPM and battery voltage. That will give you the Kv of the motor (RPM per volt) so you can calculate its max speed at any voltage.

 

[RossG]

Thank's for that useful info vfr all noted for future reference. One final question, 36v-250w-15.5Mph cut-off, so a 48v battery gives what is it now 30% more speed, so what about that cut off point is that raised accordingly as well, if so then it's electronically controlled not mechanically I assume ?

 

[vfr400]

If your motor spins to 22 mph at 36v, it'll spin to 28mph at 48v. On the road, those speeds would be more like 20 mph and 24 mph.

 

[RossG]

Ah, all understood now thanks again vfr. Learning more ever day.