M3 LR severe fishtailing under heavy braking

[GavinR]

Hi all,

Wondering if anyone has had the same experience as myself.

I had to emergency stop from high speed (70) on the dual carriageway earlier due to someone pulling out.

The experience was not pleasant - the car severely fishtailed as I was braking, with the back going side to side, almost like a powerful RWD car struggling to get traction on a launch!

I have a 2021 M3 LR. I am hoping this isn't the normal behaviour of the brakes for this car!

Can people let me know if theirs does the same (or not) when heavy braking from high speed?

Cheers

 

[pgkevet]

I can't see many folk slamming their brakes on hard at 70 to test it for you..
wet/dry road? On the white line? Tyre condition - as in have you managed to flat spot them from past launches etc? Do you clean the brakes periodically by some firm braking if mostly 1- peddling?
Otherwise perhaps worth a ring to get logs checked in case you have a traction control fault.
Only time I really slammed on the brakes was on my test drive in an S and that was only an 40 - held it's line nicely.
I have experienced squirly behaviours round tight bends when one rear tyre was worn excessively

 

[Irata]

Given the car is so young, I think you just got unlucky with a very slippy road surface. I guess it was a wet road? The rear tyres lost grip and the car was fighting it. By the sound of things, the brakes worked just fine otherwise the tyres wouldn't have lost grip.

I've found the stock tyres to be really good in dry and wet, though luckily I have not needed to do an emergency brake from high speed.

On a positive, you avoided any accident.

It sounds more like the tyres were on the limit, not the brakes.

 

[Mattallica]

Braking shouldn’t normally cause any lateral motion. If friction was either extreme high or low you’d get ABS to prevent unnecessary skidding. Makes you wonder if front was braking and rear motor was still in drive mode? Do you think you hit both pedals?

 

[Irata]

ABS can't help if there is zero grip. The drivers instinct may have been to apply some steering, exaggerating the sway.

Will be interesting to hear what the road conditions were, currently for all we know it could have been flooded, etc. There are lots of unknowns.

 

[dinonugget]

Please repeat the 70 to 0mph braking on several road surfaces and report back to us, to shed light on if your initial fishtailing is due to road conditions.

 

[TM88]

Do you think you hit both pedals?

One of the first things I tried when I first got my car was to deliberately pressed both the acceleratior and brake at the same time; the braking took precedence over accelerating which is what I expected. If I remembered correctly, there was a warning sound and a message displayed on the screen.

 

[GlynG]

So many variables that could cause the fishtail, but the fishtail itself is because either or both wheels lost adhesion with the road surface, So could be tyre condition /pressures, load in the boot, Road condition or contaminant on the road.
ABS will apply and release brakes very quickly, plus stability control will kick in - neither can defy the laws of physics but the car should not be fishtailing so bad you could loose control.
Also consider - the stopping power of the brakes - that is the friction between the pad and the disk is affected by contaminants on the disk - so If you have recently cleaned your wheels and perhaps got some detail spray on the disk that will reduce friction initially, The regenerative braking pretty much negates using the brake pedal so the disk doesn't stay as clean as an ICE vehicles disks, indeed many Tesla have disks that look like they have surface corrosion on them - and this too will affect the coefficient of braking - so can produce an initial imbalance between the two brakes on the same axle.

 

[init6]

Mat from CarWow described something similar to you on this video:

Admittedly they'd been racing and he started from a much higher speed than you.

 

[GavinR]

Thanks for your comments all.

That video from Carwow is exactly how I felt!

To answer some general questions, road conditions were great, was sunny and dry, around 14 degrees or so. Road surface is spot on on the dual carriageway.

Car had nothing in the boot or anything heavy. I definitely only pressed the brake peddle hard. Tyres all good condition, and regularly use the foot brake when regen isn't quite up to power.

Car has only done around 2,000 miles, tyres good all round.

Might give Tesla a call in the week just to discuss. It was very disconcerting to say the least!

 

[Mattallica]

Thanks for your comments all.

That video from Carwow is exactly how I felt!

To answer some general questions, road conditions were great, was sunny and dry, around 14 degrees or so. Road surface is spot on on the dual carriageway.

Car had nothing in the boot or anything heavy. I definitely only pressed the brake peddle hard. Tyres all good condition, and regularly use the foot brake when regen isn't quite up to power.

Car has only done around 2,000 miles, tyres good all round.

Might give Tesla a call in the week just to discuss. It was very disconcerting to say the least!

What were your tyre pressures?

 

[Yev000]

It was very disconcerting to say the least!

It's a 1.8 ton car. Physics.... Just to give you an idea VW Sharan - a 7 seater family van weighs less than Tesla model 3 LR. And for all that the wheels are only 1" wider....

Sharan 6.5-7″ width

Model 3 8.5″ width

My BMW 7 series was this heavy and it has 9.6" on the rear (245mm).

You need a lot more expensive brakes and tyres if you want to slam it @ 70 and be stationary in anything reasonable.

And by the way "tyres in good condition" is pretty meaningless for stopping distance. "good condition" means you have groves for the rain to go through to stop aquaplaning. Why F1 dry tyres are bald... The more rubber you have in contact with the road the better.

 

[init6]

What's the physics of fishtailing? I can understand what you're saying about contact area, but what causes the car's rear to veer from one side to the other?

 

[Mattallica]

What's the physics of fishtailing? I can understand what you're saying about contact area, but what causes the car's rear to veer from one side to the other?

It could be several things, most expected reason would be that the back is trying to achieve greater speed than the front, either through acceleration, lack of grip, poor suspension setup (incl. toe/camber) or bad tyres (incl. pressures).

 

[Dilly]

I haven’t had to slam on at 70mph but I’ve done it twice at 60mph.
the first time was in my Ioniq Electric. Road was dry and an idiot pulled out of a junction turning left to join my lane. He couldn’t see me, he was hidden by a van waiting for me to pass so he could turn right. That‘s the first time I’ve felt ABS do it’s job. Car stopped just in time in a straight line.
had a similar thing happen in the M3 last year. Again, road was dry and car stopped in a straight line.
Given the higher degree of computer control in the M3 I would expect a straight line stop if the road conditions are reasonable

 

[tsh2]

What's the physics of fishtailing? I can understand what you're saying about contact area, but what causes the car's rear to veer from one side to the other?

Peak breaking on front wheels, minimal weight on back. Any variation in balance at the front will allow the back to swing.

 

[pgkevet]

I had a situation (not in a tesla) where my previous car's back end stepped out after sliding on wet leaves on one side at a bend. Traction control then tried to hold the new wrong line. I corrected that sharply and traction control then tried to hold the new, new wrong line -> a few fishtails while i'm holding it in my lane while being throttle-off to slow down since braking could have made things worse.
I can imagine a situation where one rear brake bites milliseconds first to set something similar off but lane keeping holds the fishtail for the driver?

 

[mpandrew]

I had the back end do something similar in the opposite scenario, too much go pedal joining a motorway from a slip road in the wet. I wasn't breaking any speed limits but was accelerating hard and the rear started fishtailing, this came on very quickly and was quite alarming. It had the feeling of a powerful RWD car rather than AWD but I accept the M3 dual motor implementation is quite different to a mechanically linked AWD system. It was resolved by lifting the pedal and counter steering but that was instinctive only and may in fact have made it worse by over correcting. In hindsight I suspect just lifting and letting the car settle would have been a better option. Without doubt this moment has made me more cautious putting the boot down, especially when damp.

 

[Mattallica]

I had the back end do something similar in the opposite scenario, too much go pedal joining a motorway from a slip road in the wet. I wasn't breaking any speed limits but was accelerating hard and the rear started fishtailing, this came on very quickly and was quite alarming. It had the feeling of a powerful RWD car rather than AWD but I accept the M3 dual motor implementation is quite different to a mechanically linked AWD system. It was resolved by lifting the pedal and counter steering but that was instinctive only and may in fact have made it worse by over correcting. In hindsight I suspect just lifting and letting the car settle would have been a better option. Without doubt this moment has made me more cautious putting the boot down, especially when damp.

Even the finest AWD ICE car will struggle to defy physics!

 

[Yev000]

What's the physics of fishtailing? I can understand what you're saying about contact area, but what causes the car's rear to veer from one side to the other?

Peak breaking on front wheels, minimal weight on back. Any variation in balance at the front will allow the back to swing.

This...

The centre of mass for the car changes, the back becomes 'lighter' and loses grip. Any kind of slight angle at the front will cause the back to try and turn. This changes the centre of mass again (to the side) which causes more grip on one side and you turn to the side with less grip....

Weight transfer - Wikipedia

en.wikipedia.org

To prevent it at higher energies (speeds) you need more rubber in contact with the road, stiffer suspension and of course the brakes not to be too hot (so probably better brakes).

This is why people like rear engine cars sports cars. Heavy engine at the back and nothing at the front means you can go fast and brake late for corners without the back end losing grip. Same reason F1 cars have massive rear tyres and tiny front tyres.