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 illustrate a bit of Bafang mid drive minutiae, 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.

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

Why do we need more than one?

You don’t. 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. At the very least I can confirm they work reliably as I have been using them instead of the Bafang magnets on several bikes due to their lower weight and thus kinder/gentler attitude towards my precious DT Swiss or Sapim spokes, and my wheel balance.

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. Not a surprise given I am quadrupling the signal sample rate.
  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 (even if you place the Bafang magnet opposite the valve stem to even out the two weights).
  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 cost/effort. I can’t help but think that two magnets means two points of potential failure rather than four. So I went back down to two magnets and gave that a try.

Long Term Conclusion

What I found over time with both 2- and 4-magnet installations was that apparently there is more signal-reading failure going on than we realize, and the Bafang controller has a way of gracefully dealing with this on your display. However when you use two or more magnets that failover procedure is no longer seamless to the eye. You can see oddball, cockeyed shifts in the speedometer reading as you ride along once you exceed about 22 mph.

This speed threshold is probably more about rotation count of the wheel more than it is actual physical speed. I suspect a 20″ wheel would evidence the issue at a lower speed, and a 29er at a higher one.

A 4-magnet setup is more susceptible to this than a 2-magnet setup. And remember I have verified that these Cateye magnets are perfectly reliable over literally a period of a year or two in a 1-magnet system, so they aren’t the problem.

So…

You can take this experience as useful in a couple of ways: A cheap, lightweight, stronger magnet replacement is a good thing, and while many sources say multiple magnets are not possible, they are. But they are not advisable on a bike that exceeds 20 mph (Class 1/2 speeds). For countries with a 25 km/h speed limit this may be a nice little spiff. You may as well be able to see a smooth display since you are going so slowly you have plenty of time to glance down and admire the view :-).

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.

7 thoughts on “Bafang Mid Motor: Multiple Speed Magnets”

  1. Hello, I’m just setting up my BBSHD, and I put two magnets, and set the “Speed Meter Signals” to 2 in Bafang Config Tool. However I get exactly half a speed on the display. I can see both magnets work, they make the LED on the speed sensor alternate when the wheel is spinning.

    I used the config tool updated by Stefan Penov, I’m not sure if that’s related to the “issue”.

    Do you have an idea why does it work like I set the value to “4”? If I can’t find a solution, I’ll need to bring a laptop near bike and set it to default, and remove the extra magnet.

    1. That sounds like you have one good magnet and one bad one. Even if you can see the light on the speed sensor. There’s nothing more to it than a) using two magnets and b) setting Speed Meter Signals to ‘2’. Since speed is decreased by half, that tells you the setting is working. That only leaves the magnets themselves as culprits.

  2. Another cool possibility with this is to have 2 magnets, and whenever your on private property, you could sway one magnet off to the side, effectively doubling your top speed 😇

    1. Well, if you can get into the settings to set it to use two magnets, you can just move to the screen with the speed limit and adjust that instead. If speed is set to the ‘display’ value, try setting it independently. If your locked display somehow still gets in the way… get one that isn’t locked. They are widely available. Having the magnet loose enough to twist on the spoke probably will cause more trouble than its worth

  3. I did so before I red your blog. I’m using for magnets(all the same) and programmed the speed meter signal variable thing to “4”. Works great til 17km/h. Then the speed is just dropping to the half of actual speed. And that very precisely. Anybody any clue?

    1. I saw a speed/reliability issue as well with 4 magnets. If I remember right it was about 20 mph where the issue evidenced itself, and I was able to reproduce it reliably. My best guess was that there is more magnet read errors than we realize and whatever error recovery process exists in the Bafang software to output to the display is optimized for a single magnet. I ended up with long term reliability with two magnets but at that point I didn’t see a compelling reason to do this mod any more, for that minor of an increase in display response.

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