Bafang RM G020 speed sensor is reading VERY low.

[Mark Pace]

On a just-built 20" wheel, installed today, on a RM G020 motor, the speed reading is WAY off, showing (at top speed) about 4MPH instead of about 20--which I am pretty sure is the actual speed of the motor. This motor was shipped from Hangzhou China, purchased on AliExpress. The previous motor was identical (AFAIK) except for being a 12T version, and was purchased in 'kit form' on ebay; this new one with the apparently wonky speed sensor is an 11T version, and came as a bare motor, not part of a kit.
The first motor had an accurate speed reading, but this new one (I think) acts, maybe, as if only 1 of its 6 internal magnets is getting read. The reading it does give is very consistent throughout the assist levels--just consistently very low. I tried a another new display (an EKD01 instead of the DM03) just in case that might have been it, but it also gave the very same low reading, with the same wheel size input.
Any suggestions, much appreciated!

 

[amberwolf]

They probably only installed one magnet (there are usualy six spots, but different batches of different motors, and different manufacturers, populate some, all, or only one, depending on their whim of the day).

to fix the speed reading, you'd need to change the number of poles in your display. If tha'ts not an option, you can either open the motor and install five more magnets, or build a bit of electronics that mulitplies the number of pulses by six.

 

[Mark Pace]

Thanks for that! I'll investigate to see if a couple of the (to me) mystery settings in the setups of either of those two displays will fix it. That seems the easiest remedy by far.

I just wrote the seller; maybe he'll reply helpfully. (It could happen!) The spec diagram included in the ad does state that number, 6, for its "Speed detection signal pulses/cycle". This is the 14th G020 motor I've installed, and the very first one with any defect. All the others however were purchased in 'kit form' (on eBay from Chinese sellers in the US) complete with controllers, displays, etc.

Or I could just exercise my noggin and multiply everything by 6 -- simpler than KMs to miles; but I'd feel better if it would err or lie in my favor, like, say, in an amount I could divide by 2 ....

 

[amberwolf]

! I'll investigate to see if a couple of the (to me) mystery settings in the setups of either of those two displays will fix it.

E-HP has a thread for various displays with links to manuals for some, that mayhelp you witht hat.

 

[Mark Pace]

I found some pertinent manuals online, with slight differences between them; but none exactly like what my particular DM03 (a 'Varstrom' version) shows when accessing its advanced settings (using password, 1657.) Unfortunate, because this manual here { https://cdn.shopify.com/s/files/1/0..._dm03_display_user_manual_en.pdf?v=1661845940 } beginning on page 21 / 28 (dual page numbers!) shows 12 parameters that can be altered, including the very one I need. But my display only has the first 3 of those 12: nothing besides wheel size, speed limitation (from 6-61 in mph) and power assist display (3, 5 , or 9).

The seller answered my initial inquiry with "Change the number of speed magnets from 6 to 1." I replied that my display did not have that ability, and asked why the motor listing ad stated '6' magnets when it had only 1, and if they had any similar displays that could be changed to 1.

In the meantime I bought another DM03 from another [US-based] Ali seller (cheap! only $29) and hope maybe I'll get lucky. The only other display I currently have, the EKD01, also lacks that ability. With the present DM03 set on a 24" wheel (instead of the 20" it really is) it reads very close to 1/5th the actual speed, but below a reading of 1.4 (actual 7mph) it goes to 0. I guess a single magnet is going to be somewhat less accurate at low speeds than more magnets, but even a single magnet would at least not quit before 2mph, with a compatible display.

Woulda been a lot easier just buying a Shengyi SX2 & related parts from Grin, I guess! I don't get why some makers like Bafang do not use any sellers or wholesalers in North America (aside from Grin--and there, only handling a couple of their products) who really seem to know a lot about what they are selling, nor want (according to reps I've spoken to at QBP and Olympic) to go thru normal bike-shop distributors who tend to offer vastly better service and info on the products they sell.

Thank you again!

 

[amberwolf]

Unfortunately, since custom firmware can be created and installed on any display (or controller) by the manufacturer for any customer (presumably ordering enough of them), you will find some that either don't work with anything other than the system they came with, or don't have all the menus you expect, or the parameters within those menus aren't what you expect or they don't do what you expect when you set them per the manuals you can find. Etc.


regarding the low number of magnets vs low speed vs readout failure: The smoothing math used by various systems will all fail to display any speed below some above-zero speed, probably around 1-2mph with only one magnet. Part of that depends on the wheel diameter and where the magnets are relative to the axle (and thus how long it takes to complete a rotation).

That's why multiple magnets are normally used...I use three magnets on my front 26" wheel for SB Cruiser's speedometer, and get a speed reading on the Cycle Analyst down to a much lower speed, but it will still read zero while still moving once it goes below that.


Regarding product knowledge...it's tough to find anyplace, with any kind of product, or even service, where those working there know much if anything about them. Part of that is that many places don't carry the same things all the time, just whatever was cheap that they can turn over quickly and make a profit on. Part of it is that the *people* have a high turnover rate, pretty much everywhere, either because they need more money than they can get at that place, or because of how they're treated, or boredom, etc., or some combination of these (or any of a bajillion other reasons, but mostly those), or because they only work a few hours at that place and a few hours at one or more other places, because that's all the businesses are wiling to give them.

When you find a place that does have PK, it's usually worth paying the extra money for their products because you are supporting the people that know that stuff and keeping them there for your future reference. If most people just buy the cheapest stuff from the cheapest places, then the places with PK go away or they have to dump the expensive people that have that PK because they can't afford to stay in business.

 

[Mark Pace]

Very well put. Your insights are kinda-sorta like, or related to, those of philosopher-architect John Ruskin: "It's unwise to pay too much, but often even worse to pay too little...." John Ruskin and the Common Law of Business Balance | Life of an Architect