Bafang Mid Motor: Multiple Speed Magnets

Some Bafang BBSHD and BBS02 programming tutorials say Speed Meter Signals must be set to 1 or bad things happen. They are right. They are also totally wrong.

This is going to be a quickee post to show off a little spiff you can do on a Bafang mid drive, and point out something wrong I see in some tutorials or forum posts about Bafang BBSHD / BBS02 settings. Specifically:

The Speed Meter Signal

Put simply: It does not have to be set to ‘1’. But I am getting ahead of myself. Lets start from the beginning:

At the root of the matter is the BBSxx Speed Sensor. If you buy one as part of a kit, or on its own, this is what you get for roughly US$20:

Figure 1: L to R: Sensor magnet, sensor base, sensor and underneath, the two screws used in the install

The speed sensor magnet attaches to one of your spokes. You attach the sensor to your chainstay (usually) and position it so the magnet passes close to it as the wheel rotates. The sensor detects the magnet’s passing and calculates your speed, via a rotation count and knowing your wheel diameter via a separate setting.

Figure 2: The speed meter installed on the chainstay.

So if you buy the typical sensor, you get one magnet, one sensor, and you need to set the Speed Meter Signal to ‘1’. Is that setting because you have one speed sensor?

No. Speed Meter Signals counts the number of magnets. Not the number of sensors. Each magnet is a signal. Got one magnet? Set it to 1. Got two? Three (for a 36-hole wheel)? Four? Change the setting accordingly and it works great.

Figure 3: The Speed Sensor settings from my Luna Black Box, with the setting in question circled.

Why do we need more than one?

The one magnet works pretty good as it is, so nobody really gets too deep into this. Plus nobody sells Bafang speed sensor magnets by themselves. So to do this you are talking about roughly $20 per magnet because you have to buy a whole speed sensor assembly.

But… what if you don’t? Cateye sells a sensor magnet all by itself. It is cheap, widely available and can be gotten cheaper in a 2-pak. Here’s one, close up:

I have found these can just be tightened onto a spoke by hand, and they do not need any thread locker to stay tight (adding some Vibra Tite would not be such a bad idea). Reportedly these magnets work over a much greater distance than their Bafang cousins, which is another benefit.

Get To The Point!

Fine here it is. Look for the speed sensor magnet in the picture.

Figure 4: How many lights do you see, Picard?

There are four lights magnets on this wheel. One every 8 spokes. I have the Speed Meter Signals reading set to 4. The improvement is not earthshaking but I do get the following:

  1. My speed reading on my display updates faster and more smoothly. Quadrupling the signal input is a good thing which is not a surprise.
  2. The Cateye magnets are smaller and lighter by a fair bit than the Bafang magnet assembly. This results in the wheel getting thrown less off-balance than when using that big heavy Bafang doodad (even if you place it opposite the valve stem to even out the weight distribution).
  3. Four magnets placed equidistantly around a wheel make for a more balanced wheel spin. Its minor. But when spinning the wheel with the motor when the bike is up on the stand the lesser amount of shaking is noticeable.

Using just two sensors (the 2-pak of Cateye sensors is only $9.95) gives a noticeable improvement as well. Enough that 4 sensors is not noticeably better or worth even the minimal the cost/effort. I can’t help but think that two magnets means two points of potential failure rather than four. This weekend I’m going to go back to two magnets.

So…

You can take this tidbit as useful in a couple of ways: A cheap, lightweight, stronger magnet replacement or a way to get a better speed signal to your display.

Its not a big improvement, but it is a nice little one.

Author: m@Robertson

I'm responsible for the day-to-day operations at my place of business: Leland-West Insurance Brokers, Inc. We do classic and exotic car insurance all across these United States. I'm also an avid auto enthusiast, a born again cyclist (i.e. an ebiker) and participate in medium and long range CMP and NRA sanctioned rifle competitions.

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