Brushless Motor Acting Weird

Question

I recently got a brushless motor upgrade set for my RC car (I have previously asked questions about it), and there seems to be something wrong with it...

The steering servo works fine. The brushless motor runs fine when the tires aren't touching the ground, but once the tires are on the ground (or any stronger resistence on the wheels), the motor, at slower speeds, stutters and doesn't drive forward (until the motor goes faster... which most of the time it will just jump forward).

Does anyone know what might be happening? I didn't see anything in the instructions I got with the brushless set that said anything about this. The only thing kind of close, was about the anti runaway or something... Does anyone know what this is? Will it fix my problem?

If it will help, here is the translation of the instructions I got (they were originally in Chinese):

Answer 1

[Sensorless] brushless motors cannot be easily started from a stop. There is no way for the ESC to know where in the rotation the motor is, so it must make a guess, and "bump" the motor until it gets magnetic feedback from movement of the motor. At that point, it can continue to commutate the motor.

Starting the motor is difficult under load, so this is the stuttering you're seeing. Allow the motor to jump to get spinning, then you may be able to reduce the throttle to a slower speed.

Alternatively, get a sensored brushless motor and ESC. This setup uses hall sensors to exactly sense the motor's position, and start turning without the jump.

Answer 2

My guess is that the motor has too high a Kv for your car, and the ESC is trying to accelerate it too quickly. The stuttering are pulses of power from the ESC arriving out of synchronisation with the magnets - if the magnet hasn't moved close enough to the next coil when the power arrives, it gets pulled back to the previous one instead of forwards.

Changing to a smaller pinion gear (or otherwise reducing the gear ratio in the car, even smaller wheels) would allow the motor to accelerate faster and keep up with the ESC.

Most ESCs have some setting for how aggressively it attempts to accelerate. It's usually called 'Advance' or 'Timing' and can be expressed in degrees, or just as 'high/medium/low'. Setting this to a lower option would probably fix the issue. It's possible that this is the 50/70/100% setting mentioned in the instructions, although that sounds more like a power limiter that you might use to restrict a car so a child could drive it safely.

It's frustrating that this setting may not be adjustable, or that the instructions are so poor that we can't work out how to adjust it. It might be worth looking around for similar-looking ESCs with better instructions - these things are generally copies of something better, so instructions from another ESC might work.

Failing that you could try changing the ESC for one that does have a Timing setting (and better instructions) or even switching to a motor and ESC with position sensing.