Scary Braking Revisitied

[Spindoctor]

My recent experience. -

2012 Leaf - exiting freeway and coasting. At 25mph hit brakes to slow for right hand bend. Brakes completely locked up, no ABS "chatter," just complete lock up. The car slid straight and into the end of a guard barrier.

Car totaled, no injuries. Airbags work.

 

[donald]

The Euro NCAP ESC tests for the Leaf stability control systems appear to show it losing control and heading the wrong way with ESC off, and still going off in rather the wrong direction with it on (I believe it's meant to stay straight-ahead).

http://www.youtube.com/watch?v=apKpuPI2ma0" onclick="window.open(this.href);return false;

Basically, looks like a car with significant oversteer to me, which might go some way to explain why the recommended tyre pressures are on the low side, which tends to induce understeer, in this case to compensate for the oversteer (?) (for low rolling resistance tyres, they should normally be set to 42psi like reinforced tyres, plus or minus a little).

So, this is just a suggestion that this might be the key to this conundrum. If there is any instability in the car causing one side to become unloaded and consequently lock (which sounds like the OPs first scenario) then this would confuse the ESC and it'll do the wrong thing. With the weight balance all upset and some 'dumb' electronics in the loop, anything could happen.

In the OPs second scenario he actually confirms the car was going around a corner.

I recognise the efforts of the other poster to attempt to reproduce this, including some gentle turns whilst doing it, but I tend to suspect that there is some sequence of steering inputs that might be aggravating the Leaf's 'brake-assist' function. Maybe it is a sudden turn of the wheel from having been held in the other direction, then with sudden braking?

 

[EVOldtimer]

SpinDoctor, glad you are ok. Had you ever had the ABS activate on this car previously? It seems odd that it would completely not work suddenly. Can you tell us more about the circumstances?

Assuming you were not driving too fast, and the conditions were not excessively slippery. Here is my read on your crash. First, the ABS never activated. The SAS could be a party to this mess only if you were hearing ABS activating, but without the noise, it is unlikely the SAS was involved. If the ABS was working properly, you should have been able to continue down the roadway and not have slid off. Next, you either held the brakes locked (as we are taught to do with ABS). or the brakes stayed applied as mine sometimes do, and off into the wall you went. This sounds like two failures. ABS inactive for unknown reason and continued excessive braking either by you or the car itself.

Donald, thanks for the input. Understeer vs oversteer is not even an issue here. The car departed the roadway straight ahead. Besides the stability system can only work if the ABS is working. That is, if the brakes are applied and the SAS engages, you will hear the chattering of the ABS.

Additionally, my ABS problem was resolved by adjusting the pedal stroke sensor. My abs works properly now, and the wheels do not lock-up any more. Now my complaint is that the brakes stay applied, just like SpinDoctor's apparently did.

UPDATE: This winter was so bad that I had to remove the Ecopias and put on Nokina WR G threes. I am really having a hard time making myself do further tests with my nice tires on the car, but this post is going to make me get moving on this problem again. There has not been good enough traction to make the car stop abruptly since October, it just activates the ABS (as it should).

Donald, I have had collision avoidance experiences with SAS systems on other cars, and we really don't seem to be dealing with an SAS problem here (with my car). I have gone out and recreated the continued braking after pedal release many times, but for Nissan, I have to make the car continue braking long enough to stop by itself repeatably. Therein lies the rub. I might be able to get it to fully stop once in a session if all the conditions are right. I have already tried and failed with a dealer rep riding along. I have tried the brake slams both turning and straight and level, and the results are the same. Typically, I initiate a panic stop at 40 MPH and only sometimes the car completely stops by itself from about 35 MPH. (Yes, creepy!) I lose about 5 MPH with my foot on the pedal...

The brakes on my Leaf always stay applied too long, and lots of people think this is normal. I think that is nuts-total BS, but an not going to waste a bunch of time debating it. I am trying to find a way to repeatably make this symptom so severe that the car hauls itself to a full stop with my foot off the pedal completely. I have done it probably ten times or so ,but cannot make it do it upon command. AArgh!

I expect that this will get resolved somehow someday, I just wish Nissan would figure it out and let me enjoy my car..

 

[donald]

Donald, thanks for the input. Understeer vs oversteer is not even an issue here. The car departed the roadway straight ahead. Besides the stability system can only work if the ABS is working. That is, if the brakes are applied and the SAS engages, you will hear the chattering of the ABS.

Well, it's your car and your thread, but it is not at all correct that the stability control only runs with the ABS, if that is what you are saying. Yes, if the ABS has failed then the ESC will also be non-functional, but the ESC should operate whether you are braking or not. It might already have cut in if you made a sudden, albeit small and/or sub-conscious, steering input in these circumstances that you might not have realised you were making. The whole point of the system is that it responds to cues from your inputs and the attitude of the car that you do not even realise is happening (else you'd do something about it for yourself and there's no point making a car that does that!). Clearly, the NCAP test video shows the ESC working overtime to try to keep the car straight ahead, and no braking was going on...

The issue of oversteer is not simply about steering. Even in a straight line your tyres still need to 'steer', albeit in a straight line. The issue is that the rear tyres become unloaded when the car is performing dynamic manoeuvres. So a car that will tend to oversteer will also need to have a very light bias on the rear brakes, because the whole point of oversteer [in a FWD - i.e. not due to loss of traction due to driven wheels spinning] is that as the weight shifts around on the tyres that the rear tyres in an oversteering car become more easily unstuck than a comparative car with little oversteer tendency. A car with an oversteering tendency will lose stability in a heavy straight-line braking manoeuvre more readily that a car will less oversteer tendency.

If a car with ESC is tending to yaw under heavy braking (all cars will tend to yaw under heavy braking - but the less a car's oversteer tendency, the less it will tend to do it), the ESC system will compensate and increase the braking force on one of the rear tyres. In a straight line yaw under braking, this, unfortunately, happens to be the tyre on the 'inside' of the yaw motion, which is already experiencing a shift of vehicle weight off it, thus elevating the risk of locking that wheel up. You might well have thought you were braking in a straight line, but the car might have been struggling to maintain a straight line at all and the locked tyre you mentioned was a rear unloaded tyre that the ESC was trying to actuate to keep you on that straight line. If it had been struggling in that manner then it would seem plausible the system maintains an 'elevated' level of braking force above your own input and that is what you might have experienced. I am not defending that as a good algorithm, clearly if this is the case then it is too easy to 'confuse' the ESC system and to put the car in a compromised attitude, which one way or another is a safety issue. The question is whether this is the sequence of events leading, or if not what is?

The test to reproduce this possible effect is very simple to try, if you can find some safe space: Just drive across a nice open space and give the steering wheel a small but sudden jerk (not enough to actually change direction but just enough as if you were shifting over within you running lane suddenly, like if you were dodging a nail or pothole you spot in the road at the last second) and then stamp on the brakes almost simultaneously but just slightly after the jerk of the wheel. See what happens.

 

[ttweed]

My recent experience. -

2012 Leaf - exiting freeway and coasting. At 25mph hit brakes to slow for right hand bend. Brakes completely locked up, no ABS "chatter," just complete lock up. The car slid straight and into the end of a guard barrier.

Car totaled, no injuries. Airbags work.

This is shocking news! I would really like to hear more details about the incident. When you say "coasting," do you mean you had the Leaf out of gear (in neutral) when this happened, or were you just in gear with a normal "lift throttle" overrun regen-braking situation? Was traction control enabled? Were you really only going 25mph? How far did you slide before impact and what speed was scrubbed off before hitting the barrier? Was this on a dry surface? Was the brake application normal for the situation, or were you late on the brakes and "stabbed" at them quickly? Was there any investigation of the car's performance data by Nissan to indicate why ABS did not kick in and the brakes locked up? This seems to be a total failure of the safety systems in the car, and you are very lucky that it did not happen at a higher speed, as the consequences could have been much worse. I find it very worrisome that some system failure may have occurred that might effect my own or other Leafs, and that Nissan owes an explanation about this incident and may bear some liability for it. What is your insurance company doing about it?

Thx,
TT

 

[EVOldtimer]

Please read my post more carefully. If the VDC (as Nissan calls it) is operating to yaw the vehicle back into alignment with the direction of movement, THE ABS WILL BE ACTIVATING. The VDC can only affect the trajectory of the car by applying and unapplying the brakes through the conventional ABS system. It is possible for the VDC to control the motor, but this is not applicable in our braking discussion. The motor has no control over the individual wheels for acceleration or deceleration. In other words, if the VDC is active, you will be hearing and/or feeling the ABS working , if you are a normal person.

UPDATE:

I had a different 2012 Leaf yesterday (doing an alignment on it). My client/friend and I decided to test his car for the over application of brake issue. From 40 MPH I braked hard, and immediately let off the brake-I would guess at about 35 MPH or higher. With my right knee almost up at the MFD screen level (for showing him what I was doing), the ABS kept chattering and with the car slowing at maximum rate, it finally stopped chattering and left us rolling at about 5 MPH or less. So the car braked by itself from 35 down to 5 MPH without my foot being anywhere near the brake pedal.

Next we tried the same thing but with a sudden acceleration as quickly as I could get my foot over to the throttle. So somewhere between 30 and 35 I had my foot into the throttle, trying to get the ABS to stop chattering and the car slowing. Despite rapid and deep application of the throttle, nothing was different for at least a second. When the throttle did start responding, it was mushy and barely responsive. This left us rolling at about 15 MPH when all was said and done.
So, basically his car is doing the same thing as mine. The car keeps braking for a ridiculous amount of time after the driver stops braking. This is not a car that I would be comfortable doing emergency evasive maneuvers in.
Not good...

For what it is worth, the VDC is not a factor in this--that is, it is not operating. By law, the VDC warning light must illuminate if the system is operating. I have never had the light come on wile doing any of these tests. I see it plenty in the winter, but that is mostly for fun :twisted:

 

[donald]

I could not have read your post more carefully, but I might not have interpreted some of its potential ambiguities in the way you had intended.

Irrespective, if the VDC indicator was not operating then I'd agree that is not a factor. Clearly, your tests indicate the system is designed to keep the brakes on in those circumstances, and I agree that this is not ideal from a precision-control point of view.

However, I believe research into these things show drivers often go for the brakes in an emergency in a timely way but do not necessarily hold the brakes on hard or long enough to be effective. It is annoying that the system is designed for the 'least capable' drivers, perhaps, but that's the world we live in. I suspect you could equally swap your car for a current model from a different manufacturer altogether and still find you have a similar 'brake assist' function.