M3 AWD in reverse?

[SomeCurry]

Morning guys, I've tried to search for this but nothing is coming up, surely this has been asked or noticed before.

So - bit of snow from Storm Eunice - car is great going forward but in reverse the front wheels are slipping but the back do not appear to be powered? Is that correct? AWD only works in front drive mode not reverse?

I only noticed as I have a sloping drive to reverse on to with a bit of snow at the front so the back tyres absolutely had dry ground (from where the car stood last night) but they didn't attempt to turn and the front wheels just spun on the snow. The slip mode had no effect either (in reverse).

Thanks.

 

[Avendit]

Might be one to ask in a wider forum? Sounds like the kind of shenanigans that some of the Scandinavian owners would measure, graph and know about. Worth posting in the general M3 forum rather than UK specific?

 

[Godsense]

This appears to be normal behaviour. I have scan my Tesla and front always seems to engage only regardless of the situation. I haven't had snow to test that scenario but on other low grip surfaces / trying to force RWD I have not been able too

 

[Irata]

This is a bit unexpected and not desirable, only the front motor can truly freewheel as it's induction. The rear is permanent magnet.

There should be some power for the rear, otherwise it would instead be trying to generate electricity. That would indeed encourage the fronts to spin and this does appear to be what happened.

I would have expected this to have been widely reported by now, so I am thinking there is more to this.

 

[Roy B]

Firstly "trying to generate electricity" is a normal function of the motors. It's what regen is, and when it's being controlled by the car's electronics, it's not harmful but actually desirable.

Secondly, the inverters are perfectly capable of driving the rear motor in a zero torque state (i.e. freewheel). This is exactly what happens when you hold the accelerator at a point between applying (positive) torque and applying breaking torque (i.e. regen) - at that accelerator position both motors are in a zero torque state, so effectively freewheeling.

The difference is what happens when you power everything down. To achieve a zero torque state, a permanent magnet motor requires the inverter to rotate the magnetic field to match the rotation of the rotor. If the car is without power (motors and inverters powered down) then rotating an induction motor will have no effect, but rotating a permanent magnet motor will feed back voltages into the (powered down) inverters which they are not designed for.

But, bottom line, I wouldn't worry about it, I'm pretty sure Tesla's motor designers know what they are doing. Reversing is not going to fry your motors

 

[Irata]

But, bottom line, I wouldn't worry about it, I'm pretty sure Tesla's motor designers know what they are doing. Reversing is not going to fry your motors

The original issue raised is nothing to do with the motor designers and was never raised into question. There has been never any suggestion of frying the motor either.

It would all be the software controlling the situation.

There does seem an opportunity to improve matters here. There is always an opportunity to improve.

 

[plisken]

I would guess what you are seeing is the "computer" detecting rear wheel slip and sending torque or drive to the front and so it appears that the fronts are spinning and the rears are stationary. I know we're dealing with two motor's here but the principle must be the same, surely?

I've only had mine a couple of months and personally, it's been terrible in the snow (save all the learn to drive comments). Even on a slight incline, I've found the rear struggle for grip and instead of spinning the wheels as one may expect, power to the wheels is reduced, meaning I could probably open the throttle (or equivalent) fully and the car wouldn't move. I've also noticed this is a snow covered car park, when reversing as it happens. The car simply ground to a halt and no matter how much throttle I applied, there was no movement and no spinning of wheels, rear or front.
Never tried "slip start" but would assume this would disable or limit this and allow for some spinning of the wheels.

 

[Xdama]

Have you tried turning enable wheel slip on? Not sure if it would work or not.

 

[Adopado]

Have you tried turning enable wheel slip on? Not sure if it would work or not.

I think you maybe missed this in the original post ...

 

[Xdama]

I think you maybe missed this in the original post ...

Doh

 

[MrT3]

Never tried "slip start" but would assume this would disable or limit this and allow for some spinning of the wheels.

Yes, that is the purpose of the feature. In standard mode the vehicle will stop all wheel spin.

 

[rincewind]

The underlying issue seems to be the tyres. On winters or even all season this would be less likely to happen. Summer tyres + snow = slip/sod all traction. AWD will make no difference if all the wheels have no traction.

 

[MrT3]

The underlying issue seems to be the tyres.

I think we have gone off track here.

The issue is that the vehicle appears not to try and use the rear motor when in reverse, even when the front wheels experience slip. When going forward the rear motor is predominately used but the front will assist if rear traction is lost.

 

[rincewind]

The issue is that the vehicle appears not to try and use the rear motor when in reverse, even when the front wheels experience slip. When going forward the rear motor is predominately used but the front will assist if rear traction is lost.

Ah, yes, I scan read the thread. Seems to be more of a software control issue then?