Shengyi SX hub motor suddenly "cogging"

[Mclewis1]

My 6 year old front drive SX based city bike has been virtually flawless for over 5000kms. My problem is that at the start of a short ride yesterday the motor has now started to "cog". At zero power levels (power is on and either I roll the bike forward or on a stand just push the wheel with my fingers) there's an obvious stop/start at what appears to be at passing over every magnet. The bike continues to operate just fine under power but no longer coasts the way it used to. I haven't had the bike up to full speed (45kph) since the problem appeared but it seems to accelerate just like it always has. The bike hasn't been wet for weeks and even then it's only been in minor/light rain events.

The setup is a full Grin kit bought in 2020 - Shengyi SX1 geared hub motor, Baserunner controller in a Hailong battery cradle, CA 3.14 with MFswitch, crank mounted PAS, twist throttle. 52v battery. No ebrakes. Battery is currently sitting at just over 52v.

Everything has been visually inspected, with no unusual wear points and all the cabling is in it's correct place. All the connections have been pulled apart and re attached and are visually in good condition (clean, no bent pins, etc.). No error messages on the CA and both the CA and the Baserunner have also been checked via PC connection - again no errors and the hall sensors cycle/alternate as appropriate.

I re loaded my CA config and it's 3.14 firmware (this hasn't been done since the bike was new). No config changes have been made to the CA for many years (the changes when it was new were the usual tire size, battery voltage, and I softened the power ramp up a bit).

Nothing mechanical has been touched on the bike for months (other than chain lube and the occasional wipe down). Spring maintenance was new brake pads and the hub motor was cleaned and re lubed (Mobilegrease 28), and I've put over 500km on it since then.

One thing that I think is different on the CA since the problem showed up is that while my wattage being used ramps up as normal it now seems to show negative wattage when it drops off (stop pedalling/ let go of the throttle). I'm not certain but I don't believe I've ever seen negative numbers (except for the -1 to -3w the CA draws), and now I'm seeing -50 to -60w immediately before returning to 0. It almost seems like the bike has regen - but of course the clutch prevents the motor from applying any braking effect.

I'm out of ideas ... any thoughts?

 

[neptronix]

Most likely a clutch failure, failing engaged.
An engaged clutch will make the wheel resistance identical when moving it backward versus forwards, with the power off. Is that the case?

 

[Mclewis1]

Thanks, That was my first thought while riding and I have a feeling that we're both right, that the clutch is somehow damaged. I initially didn't understand the symptoms and so went down the rabbit hole of an electrical problem.

With the power off the feeling is the same in both directions, and you can hear the gears.
With the power on but no forward power applied it now cogs when moved in the forward direction, but does not in reverse.
With forward power applied the motor appears to function normally.

I guess I wasn't thinking correctly about how the whole thing works and didn't realise that there's a difference with the power on (but no forward power to the motor) vs, the system totally powered off. I assume this is something that direct drive hub motor owners know?

I'm going to open the motor tomorrow and verify my assumptions. If the gears are intact (as they feel to be) I may just ride my "new" hub motor setup like it is for a while. Without freewheeling the overall efficiency is down a bit and the motor temps appear to be up a bit but I don't usually push things or regularly go very far on this bike ... it's my road ebike setup that gets the real mileage.

Predictably it's just my luck that Grin are currently sold out of both the replacement clutch/gear assemblies and the whole motor. The front version is in stock but the rear isn't and if I replace the whole motor I'm going to change things around and go with a rear drive setup.

Q - Does anyone know if the clutch/gear assembly described for the Shengyi DGW motors is the same as for what Grin calls the SX series?

 

[neptronix]

A working geared motor, with the power on or off, will have a high amount of resistance when turning it backwards ( motor engaged ) and virtually none when moving forward ( motor not engaged ).

This shouldn't change whether the motor power is on or not.

if there is cogging when moved forward with the power on, i'd make sure you don't have any kind of virtual freewheeling or regen turned on in the controller.