Bafang C961 display speedometer problem greater than 27mph

[nukezero]

I have a BBS02-750w detuned to 18A and runs the C961 display.

The bike is a 16" folder. So when the bike hits about 27.5mph or greater, the display gets funky and it starts reporting the speed backwards.. like jumps to 14, 13, 12mph, while i'm doing 28+ mph.

When I put the bike on a stand, and yank the throttle, the behavior is observed. But what i notice is that the speed sensor LED is like.. on the whole time. Does this mean the sampling rate is too slow for the speed of the rotation of the small wheel??

You might ask how I can achieve 28mph with a 16" folder. I have a 52T crank, paired to a 16T rear 8 speed shimano hub.

 

[alfantastic]

It might be something as simple as the magnet is a tiny bit too far from the sensor. I've had similar malfunctions happen on normal trip computers.

They get quirkier as the wheel spins faster.

 

[rscamp]

I have a BBS02-750w detuned to 18A and runs the C961 display.

The bike is a 16" folder. So when the bike hits about 27.5mph or greater, the display gets funky and it starts reporting the speed backwards.. like jumps to 14, 13, 12mph, while i'm doing 28+ mph.

When I put the bike on a stand, and yank the throttle, the behavior is observed. But what i notice is that the speed sensor LED is like.. on the whole time. Does this mean the sampling rate is too slow for the speed of the rotation of the small wheel??

You might ask how I can achieve 28mph with a 16" folder. I have a 52T crank, paired to a 16T rear 8 speed shimano hub.

I have the exact same hardware and the same problem on a trike with a 20 in rear wheel but in metric units. In my case, when going over about 55 km/h it displays half the actual speed. Adjusting the magnet or pickup positions has no effect on this behaviour...

 

[nukezero]

The magnet is EXTREMELY close to the sensor. In fact, it is so close that I can honestly say they are like 0.2mm apart.

I think I know what the problem may really be. The issue is the controller sensor is not able to sample the magnet fast enough. The LED blinking is an indication of each sample. When the wheel spins so fast to the point where the LED stays on and doesn't get a chance to even shut down, that means the sensor's sampling rate seems to be saturated.

Now, 28 mph might not seem that fast because some people have hit 35-40mph, but keep in mind those guys are on 26inch to 29cc wheels where the rotation per revolution is slower than say, my 16 inch wheel. My 16 inch wheel has to rotate faster just to get reach the same speed as as a larger wheel. I think that is the conclusion I've come to. It is a design flaw.

 

[rscamp]

The magnet is EXTREMELY close to the sensor. In fact, it is so close that I can honestly say they are like 0.2mm apart.

I think I know what the problem may really be. The issue is the controller sensor is not able to sample the magnet fast enough. The LED blinking is an indication of each sample. When the wheel spins so fast to the point where the LED stays on and doesn't get a chance to even shut down, that means the sensor's sampling rate seems to be saturated.

Now, 28 mph might not seem that fast because some people have hit 35-40mph, but keep in mind those guys are on 26inch to 29cc wheels where the rotation per revolution is slower than say, my 16 inch wheel. My 16 inch wheel has to rotate faster just to get reach the same speed as as a larger wheel. I think that is the conclusion I've come to. It is a design flaw.

Agreed. It just can't keep up. I even tried a strong rare earth magnet and it made no difference.

It is what it is...