Motor Cutting Out

[stever1957]

One little thing to ask you about:

Every now and then, on pedal assist, the motor of the Big Bear cuts out for a split second, then kicks straight back in. More often than not this happens twice within a very short time, then doesn't happen again for a long time. This never happens when using throttle.

It occured to me that this might be due to some local electromagnetic interference, but this never happens with my wife's Petite bike, and she is always right behind me, so I guess it rules that out.

Any ideas?

Best regards,

Steve.

 

[anotherkiwi]

PAS disk slipping?

 

[Woosh]

check the air gap between the magnet disc and the pedal sensor, it should be about 3mm.
There is also another situation where the motor may skip on the Big Bear.
When you are on a low gear, the power ramp up is quite strong, accelerating your bike too quickly, the controller will then cut the power for a second until your pedaling catches up with the assist speed.

 

[stever1957]

Nothing thus far suggested matches the facts, I'm afraid.

This will happen (to offer an example from today's outing) when using assist level two, halfway along a very gentle climb, in third gear, with no variation in pedalling rate.

 

[Woosh]

Hi Steve,

does it happen on gear 6 or gear 7?

 

[stever1957]

In the period since this first happened in third gear, I have not used those gears.

 

[stever1957]

Actually, as far as I can recall, this has only happened on assist level two and in second or third gear.

 

[stever1957]

By the way, this is no big deal - unless it gets worse, of course.

 

[Woosh]

3 may be too low for flatish roads, try 5, 6, and 7. The Big Bear can climb quite well even in those high gears.

 

[stever1957]

.......The Big Bear can climb quite well even in those high gears.

Maybe it can, but......

 

[stever1957]

In any case, third gear has not proven to be too low for flattish roads until the last few days - not at all - particularly when using no more than assist level two.

 

[Woosh]

Please check that the magnet disc behind the chainring is still parallel to the ring and the air gap between the magnet disc and the sensor head is at the recommended 3mm distance.
no programming algorithm is perfect on a cadence sensor because all the controller sees is the assist level and pulses from the cadence sensor.
The controller tries to keeps the speed constant. All other parameters equal, when the controller sees faster pulses, it reacts as if you want more speed, ramps up power then settles at your new speed. So pedal assist power comes in waves.
For 15mph on flat road, the bike gives out about 200W, the keep current is about 7A-8A. For 10mph on flat road, the bike gives about 100W, keep current 3A-4A.
So the changes in assisted power (ie current) are more apparent (wavy) at low speed than high speed, more apparent at low gear than at high gear, also at high assist level than low assist level, on flat road more than when climbing.

 

[stever1957]

Thank you for the reply.

Its welcomeness is matched only by its incomprehensibility

I'll check the disc and learn to expect a little 'waviness'.

 

[Woosh]

its incomprehensibility

I meant to say that the controller reacts by changing the amount of current it puts through the motor whose rotational speed is proportional to the current. At some point, a drop in current may cause the motor spin slower than the speed of your bike, that causes the clutch to disengage. When the current rises again, the motor's speed increases, engaging the clutch. That phenomenon makes the motor feels a little like stop/start. The thing is, this phenomenon is more noticeable in a bike fitted with a high power controller when the speed is low and you are on a low gear, or you go down a hill.

 

[stever1957]

Ah, thank you.

 

[stever1957]

On a thirty-five mile ride today, it happened just the once and yes, it does feel very much like something disengaging and then immediately re-engaging, rather than stopping per se.

As the thing only lasts for about half a second, and occurs so very infrequently it really requires no further mention, whatever it is.