eMTB stuttering, need help tweaking

Hello All,

  • MBS 200mm tires with 72T drive pulley
  • Turnigy SK3 190kv 6364 motor with 20T pulley
  • 9mm HTD5 belt
  • 10S3P LG MJ1 (10A) battery back + basic 40A BMS
  • Ollin VESC with Ackomaniac firmware (4.28) - also happened with stock Ollin/VESC FW 4.12
  • Nunchuck (wired)


As you can see in the video, the motor stutters a lot during startup and when moving. The video and sound should hopefully be more descriptive than words. You can also see the watts-up meter, which shows < 10A draw - doesn’t seem to be hitting the max amp threshold.

Doesn’t happen when there’s no weight on the board, only when I’m on it (150lbs).

I can’t tell if it’s mechanical (belts and pulleys slipping, etc), or electronic/wiring (power delivery, etc), or software (VESC settings, etc).

Does anything stick out for anyone?

Thanks in advance!

@chaka ideas?

All motors without sensors will stutter on startup. There may be some settings you can tweak to help but I am no expert… @Jinra do you have any advice?

Yea that’s a normal problem, not much you can do to change that than maybe up the startup boost to .30.

Thanks for the quick feedback. I can try adjusting the startup boost and see.

What about during riding (not coasting, but keeping speed)? Almost sounds like slippage. Would an idler to keep tension help?

Have you tried FOC?

@racidon I did, but couldn’t get it setup correctly. After applying all the parameters and rebooting, the nunchuck doesn’t spin the motor at all, or produce any sort of life. And I can’t detect hall sensors (detection moves motor a few degrees forward and back, but then errors).

BTW, with a 20T and 72T pulley, I’m supposed to get between 7 and 8 teeth of contact, which according to the several online calculators, should be plenty. Unfortunately my 2 pulleys are about 3-5mm from each other :sweat:

You’ll need sensors for smooth start up, I’ll check out the video when i get home

@GrecoMan’s suggestion of increasing the startup boost (I just made it 0.060, not 0.3) absolutely helped. But I’m getting awful jerking while moving.

@Jinra darn … already invested in more unsensored SK3’s for making this one a dual-motor, plus another one for another board.

I think for satellite it’s recommended that you stay below .50 startup boost. But having dual motors should help your problem a lot. It can also be solved with a simple push before you start riding

@GrecoMan I hope you mean 0.050, not 0.5?

Haha yes, .050

also @bmcm a wider belt will help with more torque transfer. 12mm or 15mm are the best

@GrecoMan yep, agreed. At the time I bought the drivetrain, SDP-SI did not have options for anything wider than 9mm for my ratio, specifically the larger drive pulley at 72 teeth. If I go wider I might have to 3d print my own out of nylon (not impossible since I have a Prusa MK2 3d printer).

with dual motors it should be fine, ive seen a lot of other people (I think ive seen @jinra use 9mm belts on a dual drive) use 9mm on dual with no problems

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Yea just got home and saw the video. You will either need sensors to help with the startup or kick start to get going. VESC has no idea the position of the rotor can so it can’t give you a smooth startup. You can try .1 startup boost to see if it’s any better. It may work better with dual motors, albeit unreliably.

Thanks, @Jinra.

Any advice, though, on fixing the jerking that happens when moving? It’s apparent after 40s into the video.

Is it that loud thudding sound? What is that? Does the jerking happen with you off the board?

@bmcm given that you’re running a MTB you should leave absolute max at the default 130 value. Also your battery min is way too high. Don’t regen above rated current of your cells.

Yeah, more and more I’m tempted to just bite the bullet and commit to dual motors. I just didn’t feel like spending another $250-300 (vesc, pulleys, belt, motor mount) but I might have to.

@Jinra Yep, that’s it. It happens when moving. Not sure - possibly the belt. I think it only happens when I’m on it.