Brake squeal when reversing?

It just stopped doing it on mine...no idea why! Been fine for the last few months.

I purchased my car 50k 1 year ago and for the last 2 months it has been screaming so loud when I reverse out of my driveway it’s so EMBARRASSING!!! A brand new car and the whole street staring at me… I am definitely worried about it , I never had this problem with any other car, I might as well be driving a car from the 90s! If it’s going to wake up the whole neighbourhood… MG replaced the brake pads and it literally did NOTHING .. I’m really upset about this, the wireless charging has also stopped working and the car play is temperamental. MG please do something about this ! I was really excited to be a ln MG girl and now I’m feeling disappointed.

Mostly caused by the friction brakes/discs slightly seizing . The downside of regenerative braking .
Not saying this is the cause in your case ...but possibly . What mileage are you at ?

I love it on mine - it's like a whale sound !

Eddie99 said:

I love it on mine - it's like a whale sound !

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This occurs from car using regen for braking over applying the mechanical brakes and disks subsequently rusting up through lack of use. Fix is to regularly charge to 100%, set regen to 1 and find a good hill to run down that will allow for heavy braking with minimal regen effect - cleans up the disks/pads and reduces any squeeling.

You can also use neutral "gear" when reversing down a slope. In my MG ZS EV 2021, this reduces the ferocious squealing 90-100%. In other words, it usually stops the squealing completely, but sometimes there is a much quieter and shorter squeal.

I have no idea why this (mostly) works.

Coulomb said:

You can also use neutral "gear" when reversing down a slope. In my MG ZS EV 2021, this reduces the ferocious squealing 90-100%. In other words, it usually stops the squealing completely, but sometimes there is a much quieter and shorter squeal.

I have no idea why this (mostly) works.

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That is strange, supposedly there's no regen in either reverse or neutral, so friction brakes should work the same? The only difference is the motor being engaged.
Big question is, "how does the motor contribute to the noise?"

The other surprise, is, if MGirl regularly backs out of her drive, why isn't that cleaning the disks?
Could it be a case of the reverse warning signal getting a massive volume boost somehow?
Be nice to have the car here so I could do a few tests.

decrep said:

The only difference is the motor being engaged.

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The motor is always mechanically engaged in full EVs, as far as I know. So this may be some weird oscillation of the motor controller's electrical signal, resulting in high frequency mechanical oscillations, which would result in that sort of squeal. Both neutral and reverse would be applying electrical current to the motor, but in neutral, it would only be magnetising current, no drive current (forwards or reverse). The squeal seems to have an essentially fixed pitch (frequency), which would be compatible with this theory.

I wonder if it's something to do with the torque on the motor crossing zero (i.e. when the torque changes from backwards to forwards or vice versa). On initial reversing, the torque would probably be backwards, to speed the car up. Though that might depend on how strong the slope is. Then as you ease off the accelerator pedal or apply the brakes, the torque goes positive to slow the vehicle down with regen. [ Edit: See next post; there is no regen in reverse or neutral. ] If this crossing zero theory is correct, then selecting regen level 1 might help. I suppose that neutral "gear" is like selecting zero regen (as well as zero drive).

But I find that if the front brakes are wet, the squeal doesn't happen. That suggests a mechanical cause, unless the slight change in friction is enough to prevent the electrical oscillation somehow.

I find that hard braking helps, but only a little. Neutral "gear" seems much more effective. Besides, why don't the brakes ever seem to squeal when moving forwards?

Edit: Added the word "essentially"; the pitch seems to vary a semitone or so.

decrep said:

supposedly there's no regen in either reverse or neutral,

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I missed that point. Do we know for sure that reverse has no regen?* I assumed that it would, but haven't checked when driving.

If there is no regen in reverse "gear", that would probably kill the "torque crossing zero" theory. Though I doubt that you can fully turn off regen in an AC motor, just aim for zero torque, and the motor controller does its best to achieve the closest set of near sine waves that matches the measured motor speed.

* Edit: From the MG4 user manual:

Coulomb said:

I missed that point. Do we know for sure that reverse has no regen? I assumed that it would, but haven't checked when driving.

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I know nothing for sure, it's all up for grabs!
But some things seem more likely than others, death and taxes for instance.

I just read somewhere that there was no regen in reverse, that's why some people advocate braking down a hill backwards to clean the disks.

Do we know how much of a slope MGirl's drive way is? If it's flat brakes probably aren't involved.

Coulomb said:

If there is no regen in reverse "gear", that would probably kill the "torque crossing zero" theory. Though I doubt that you can fully turn off regen in an AC motor, just aim for zero torque, and the motir controller does its best to achieve the closest set of near sine waves that matches the measured motor speed.

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Another question is then, would regen in reverse need to be enabled or turned off?
I can't see why MG would want to turn it off, but if it requires work to enable it, why would they bother, the percentage of reverse driving any body does would be very low.

I took ours for a backwards test drive.
Engaged OPD
Slight up hill slope, foot off the accelerator and vehicle reverses slowly, certainly no OPD. sped up then took foot off, no evidence of regen slowing the car, it just keeps going indefinitely, power meter is still positive.
Apply brakes, still slight positive power until car stops.
Convinces me ours at least has no regen in reverse.

decrep said:

I took ours for a backwards test drive.
Engaged OPD
Slight up hill slope, foot off the accelerator and vehicle reverses slowly, certainly no OPD. sped up then took foot off, no evidence of regen slowing the car, it just keeps going indefinitely, power meter is still positive.
Apply brakes, still slight positive power until car stops.
Convinces me ours at least has no regen in reverse.

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What's OPD ?

One Pedal Driving, surprised you haven't heard about it.
As far as I'm concerned, that's the significant difference between driving cars that don't have it.