Braking while downshifting

[freinando]

Today I was driving in the highway in Sport+, the car was in second gear. Traffic is building up ahead, so I start braking to reduce my speed. My speed was around 50mph when I let the brake pedal go and let the car coast for a split second before engaging the brakes again.
It turned out to be a scary moment. When I stepped on the brakes again, the car wouldn’t brake. It took a greater force on the pedal to get the car to reduce speed as I was getting closer to the traffic on front. The sensation I got was like the car had decided to downshift to first gear that split second I let the brake pedal go, and the motor didn’t know what to do, either sync with the transmission for first gear or go into recuperation mode. I wonder if others have experienced this. It was a scary moment for sure.

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[prj]

During the shift torque can not be transmitted from the rear motor whether positive or negative torque. Otherwise it would strip the gears in the gearbox.

So in that moment there can be some loss of recuperation force.
Shouldn't be the case after the shift though.

 

[freinando]

Shouldn't be the case after the shift though.

Right. Normally I have no issues with recuperation braking, unless the downshift is in the middle. It also isn’t the first time this happens to me, this one was just too big to forget creating this post. Maybe my car takes longer to sync gears than others if no one else has had this problem.

 

[l33cher]

Is there a chance it was cold where you were driving? A lot of us have experienced in low temperatures during a an almost interrupted drive to have a very weird breaking experience. The break pedal feels very soft where you would expect the recuperation to start and then it’s very stiff and needs to be pressed really hard.

 

[KLHubb]

I find that on wet, cold days, the brakes are a bit greasy feeling when coming to a stop.
I attribute that to having cold damp brake pads that require warming up.
The situation mitigates as one drives.

 

[prj]

Is there a chance it was cold where you were driving? A lot of us have experienced in low temperatures during a an almost interrupted drive to have a very weird breaking experience. The break pedal feels very soft where you would expect the recuperation to start and then it’s very stiff and needs to be pressed really hard.

I would also wager that this is what is happening - especially if the car has PSCB or PCCB. My PCCB were horrible for this.

Most likely you are mostly regen braking and then when it's downshifting it needs the friction brakes in that instant, but since those are cold or wet they take a moment.

Try what happens if you drive the car more aggressive - e.g. if you do a few harder brake applications before, does it still happen? If it doesn't happen then it's a brake problem not a shift problem.

 

[KLHubb]

On wet days, I start out doing some hard braking to clear up the greasiness.

 

[Dee]

My PCCB were horrible for this.

I have PCCB too but never had any problems with braking whatsoever.
Only when driving out of a car wash of course.

 

[prj]

I have PCCB too but never had any problems with braking whatsoever.
Only when driving out of a car wash of course.

Car wash is especially bad, but also in wet if you go through some puddles.
In good weather they worked fine.

 

[Dee]

Car wash is especially bad, but also in wet if you go through some puddles.
In good weather they worked fine.

Never had any problems in wet weather either.

 

[prj]

Never had any problems in wet weather either.

It depends how you drive it, if you drive it very aggressively and brake hard so they get used there is no problem, but in very wet weather I had the regen -> physical brake transition be very far from ideal. There is a delay there. Not as bad as car wash, but same type of issue.
It's quite widely reported as well.

 

[j.w.s]

I find that on wet, cold days, the brakes are a bit greasy feeling when coming to a stop.
I attribute that to having cold damp brake pads that require warming up.
The situation mitigates as one drives.

Wet and cold PSCB brakes are just scary. Luckily I live in California, which is generally dry, and I know this will happen, so a few stabs at the brakes after washing the car makes it all good.

I have never experienced the problem on downshift mentioned.

 

[Dee]

It's quite widely reported as well.

And were fixed by a recall/dealer visit.
Something with brake booster not according to spec.
https://static.nhtsa.gov/odi/rcl/2021/RCLRPT-21V697-6539.PDF

https://carpokes.com/viewtopic.php?t=1047

I don't see these reports anymore, you do?

 

[prj]

And were fixed by a recall/dealer visit.
Something with brake booster not according to spec.
https://static.nhtsa.gov/odi/rcl/2021/RCLRPT-21V697-6539.PDF

https://carpokes.com/viewtopic.php?t=1047

I don't see these reports anymore, you do?

No, it's not the brake booster issue that I am talking about. Brake boosters don't randomly decide not to work well in the rain.

It's the inherent problem with the PCCB when they get wet. They soak up water and the first brake application isn't great.
E.g. highway driving in the rain the first application where you go beyond regen isn't as bad as the car wash but still not progressive with a slight delay.

I see a LOT of these reports to this day on the Audi e-tron GT facebook group for example, and others.

 

[freinando]

Is there a chance it was cold where you were driving?

Nope. Hot and sunny SoFlo.

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