Scary braking problem

timk said:

mogur said:

With ABS, the last thing you want to do is pulse or pump the brakes. In a panic or hard stop you want hard and constant pressure on the brake pedal.

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Thanks for the tip. Why is pulsing or pumping the brakes bad? Does it confuse the ABS system? I guess I'm going to have to unlearn my skills that I learned with non-ABS brakes.
Still, I think this is a safety issue and I'm going to report it to Nissan tomorrow when their support number is available.
Tim

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Tim, it isn't a safety issue;it's a 'learn how to use ABS brakes' issue. I'm surprised you didn't know how to use ABS properly. It's been on cars for years and the first car that had them for me was a '90 Toyota Celica GT-S. I can't tell you how many times they saved me on ice, snow, and emergency braking. In emergency braking, such as you described, you NEVER pump the brakes, but hold your foot on the brake until you're completely stopped. While you are holding down the brake pedal, you can steer. My ABS NEVER locked up on me and always stopped straight, which these will do also if you use them correctly.

LEAFfan said:

I'm surprised you didn't know how to use ABS properly. It's been on cars for years

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That's a bit unfair! It hasn't been on all cars for years, and in fact I think about 10% of new cars sold today still don't have it. If Tim has been driving an older economy car it is quite possible he has never encountered ABS before.

Ray

planet4ever said:

LEAFfan said:

I'm surprised you didn't know how to use ABS properly. It's been on cars for years

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That's a bit unfair! It hasn't been on all cars for years, and in fact I think about 10% of new cars sold today still don't have it. If Tim has been driving an older economy car it is quite possible he has never encountered ABS before.

Ray

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Thanks for the comment, Ray. Up until about three years ago, I drove cars without ABS brakes, so it's what I'm used to. In an emergency situation, I typically don't have the time to think "hey, wait a second, I have ABS brakes, so I'd better not pulse them." I've also been rear-ended hard twice, so I'm very skittish about people coming up behind me, which is more reason I tap the brakes repeatedly to flash the tail lights.

I called Nissan EV customer support this morning to report the issue and they strongly recommended that other people having similar issues report the problem as well so they can gather more information. Please call 877-664-2738 if you experience something similar.

It turns out that another person in our local EV community had this problem as well. So at least three people have experienced this. I don't think he pulsed the brakes like I did, so the pulsing may have nothing to do with the problem. Perhaps it has something to do with the ABS brakes activating but not fully deactivating if the car is still rolling at above a certain speed (like 30mph). I work on embedded control systems similar to the ones in cars, so I'm not surprised that this system might have a corner case that isn't covered. I'll try to reproduce this problem with the dealer soon...

Cheers,
Tim

There is also "brake assist". This is a system that attempts to detect an emergency situation and compensate for not pressing hard enough. A few years ago Mercedes found that in an emergency, people pressed quickly and rapidly but didn't press hard enough so they developed brake-assist to press harder and longer when emergency braking was detected. Several car manufacturers now use brake-assist.

The problem described sounds more like brake-assist is kicking in rather than ABS.

Apparently a few similar Leaf brake problems have been reported to the Feds:

2011 Nissan Leaf Permalink
SERVICE BRAKES, HYDRAULIC
Date received by NHTSA: July 26, 2011Incident date: July 24, 2011
City: PortlandState: OR
Injuries: 0Fatalities: 0Crash: Fire:
Description:
WHEN DRIVING AT 60 MPH DOWN THE FREEWAY, I HIT THE BRAKES HARD TO AVOID A COLLISION WITH THE CAR IN FRONT OF ME. I DECELERATED TO ABOUT 30MPH WHILE VEERING TO THE RIGHT TO AVOID HITTING THE OTHER CAR. TO INFORM THE RAPIDLY APPROACHING CAR BEHIND ME, I TAPPED MY BRAKES ABOUT 4-5 TIMES A SECOND TO FLASH MY TAILLIGHTS. AT THAT POINT THE BRAKES LOCKED UP AND MY CAR RAPIDLY DECELERATED EVEN THOUGH I HAD REMOVED MY FOOT FROM THE BRAKE PEDAL. AFTER ABOUT 1/3-1/2 SECOND, THE BRAKES RELEASED AND I WAS ABLE TO REGAIN CONTROL OF THE CAR AND CONTINUE DRIVING. AFTER CHECKING ONLINE FORUMS AND MY LOCAL EV GROUP, IT TURNS OUT THAT TWO OTHER PEOPLE HAVE EXPERIENCED SIMILAR BRAKING BEHAVIOR WHERE THE CAR DECELERATED RAPIDLY EVEN THOUGH THE BRAKE PEDAL WAS NOT PRESSED.

2011 Nissan Leaf Permalink
SERVICE BRAKES, HYDRAULIC
Date received by NHTSA: July 26, 2011Incident date: July 26, 2011
City: Los AngelesState: CA
Injuries: 2 Fatalities: 0 Crash: Y Fire:
Description:
I WAS ON THE FREEWAY TRAVELING APPROPRIATELY 65 MPH. THE VEHICLE TO THE RIGHT AND IN FRONT OF ME SUDDENLY SWERVED IN TO MY LANE AND BRAKED HARD IN FRONT OF ME. WITH NO PLACE TO GO ON EITHER SIDE OF ME, I WAS FORCED TO BRAKE FAST AND HARD. THE ABS KICKED IN AS EXPECTED BUT ONCE I HAD DECELERATED TO A POINT WHERE I KNEW A COLLISION WAS NOT IMMINENT AND TOOK MY FOOT OFF THE BRAKE, THE CAR CONTINUED BREAKING VERY HARD. IT TOOK THE BETTER PART OF A SECOND WITH MY FOOT NOW COMPLETELY OFF THE BRAKE BEFORE THE BRAKES ACTUALLY RELEASED. IN THIS TIME, THE CAR BEHIND ME WAS NOT ABLE TO STOP AND HIT ME HARD, CAUSING INJURY TO TWO PEOPLE IN THE CAR WHO WERE NOT BELTED IN. THIS ACCIDENT WOULD HAVE BEEN AVOIDABLE IF THE LEAF HAD NOT FAILED TO DECREASE THE BRAKING EFFORT WHEN I RELEASED PRESSURE AND THEN ULTIMATELY TOOK MY FOOT COMPLETELY OFF THE BRAKE PEDAL. IT FELT AS IF SOMETHING LIKE THE BRAKE ASSIST FUNCTION WAS MALFUNCTIONING...

2011 Nissan Leaf Permalink
SERVICE BRAKES, ELECTRIC
SERVICE BRAKES, ELECTRIC
Date received by NHTSA: July 25, 2011Incident date: June 22, 2011
City: SherwoodState: OR
Injuries: None reportedFatalities: None reportedCrash: Fire:
Description:
I WAS ACCELERATING AFTER GETTING ON THE FREEWAY HEADING NORTH AND SLIGHTLY UPHILL I MIGHT HAVE BEEN PEAKING AT 55MPH WHEN THE TRUCK IN FRONT OF ME SUDDENLY SLAMMED HIS BRAKES ON. I DID THE SAME AND I HEAR MY TIRES SCREECHING , I PRESUMED THAT THEY WERE LOCKING. HOWEVER I DID MAINTAIN MY DIRECTION AND DIDN'T SEEM TO LOOSE CONTROL. WHEN I REALIZED I WOULD NOT REAR-END THE TRUCK I LEFT MY FOOT OFF THE BRAKE. I WAS ALARMED WHEN MY TIRES WERE STILL LOCKED AND REMAINED LOCKED FOR WHAT I THOUGHT WAS A VERY LONG HALF SECOND. BY THAT TIME I THINK I WAS UNDER 20 MPH ON A FREEWAY AND I WAS DARN LUCKY NOBODY WAS REAR-ENDING ME! I THINK THIS IS A DEFECT IN THE BRAKING RESPONSE.

Thanks, mogur, for the listings. The first report listed is mine.

I took my Leaf to the dealer yesterday to get the brakes checked out and the service manager, seeing that the ABS light wasn't on, refused to look at the car and sent me on my way. Apparently, four other people came to this dealer with similar braking situations and were told that this was "expected behavior" for the Leaf. He told me that the Leaf has a half-second lag time when releasing the brakes. If you press the brakes hard, they will continue braking hard after you release the pedal for about half a second unless you press the accelerator to release them immediately.

I'm not happy about this; however, it's interesting to hear that others have gone to the dealer with similar problems. As mentioned in a previous post, I reported this issue to the Nissan customer service center and they said they would follow up later this week.

If anyone else out there runs into this "brakes-keep-going" situation, please report it to Nissan customer service and file a report with the NHTSA. The more people raise awareness, the more Nissan might consider doing something about this.

Cheers,
Tim

And the rest of us should go find a remote road to go lock up our ABS a few times.
If this is "standard behavior", it's very broken.

GroundLoop said:

And the rest of us should go find a remote road to go lock up our ABS a few times.
If this is "standard behavior", it's very broken.

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I'll bet it's designed in on purpose. Check out this quote from brakeassist.com:

brakeassist.com said:

Usually, the driver decreased his/her pressure on the brake pedal after "slamming" it initially, causing a further reduction of braking force.

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http://www.brakeassist.com/diagram.html

I don't know why we don't just have the cars drive themselves and get it over with.

BrendanDolan said:

I probably should know this (but I don't...), but I know plenty of modern cars, Nissans included, have brake assists that apply the brakes into the ABS when they sense you've been startled and went for the brake (snapping the throttle closed, immediately hitting the brakes hard).

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Yes, indeed. A friend and I were just discussing this the other day. He works on Toyotas and was telling me how the new ones will not only detect how far you are pushing the pedal, but how quickly you are pushing it. He said the purpose was for women and smaller people who don't have as much strength in their legs. So if the car thinks you are slamming on the brakes, it will apply extra braking assist. I wouldn't be surprise if the Leaf does this too. In fact, I believe I recently experienced something like that the other day when I hit my brakes and got a lot more response than I was expecting. The only difference between me and the original poster is that when I let off the brakes, the car responded and stopped braking.

Part of the issue is drivers training in this country is sorely lacking, thus people tend to do the wrong thing in an emergency situation. ABS has never been shown to reduce accidents, as most people don't use it for the manner in which it is intended: the continual ability to steer while braking hard.

Most people brake hard initially, and do odd things with the pedal. Pump, release pressure, don't hold it to the floor for fear of breaking something, etc. It's normal, since most weren't trained to do otherwise. That's why many cars now apply full braking if the pedal inputs show the driver is surprised and going for the brakes.

This is the first I've heard that the LEAF holds the brakes down after you release, but again, that could simply be a safety measure to keep the usual scared driver from releasing pressure.

Just remember, ABS does in no way reduce your stopping distance, and a correctly driven, you can threshold brake a non ABS car to shorter stopping distances (and I threshold brake ABS cars when I need to get with the stopping program in a hurry).

I personally recommend that everyone find a high performance driving school in your area, and spend some time on the basics of car control. You only know what you know, and you'll be surprised at how little you know about car control when you go to a clinic. By attending a one or two day program with professional instruction, emergency situations don't cause you to panic and tense up, rather they cause you to make the correct driving inputs based on the situation.

BrendanDolan said:

Just remember, ABS does in no way reduce your stopping distance, and a correctly driven, you can threshold brake a non ABS car to shorter stopping distances (and I threshold brake ABS cars when I need to get with the stopping program in a hurry).

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This statement strains credulity. Do you have a citation for this?
A rolling rubber wheel on concrete has much more traction than a skidding wheel.

mywaracfirfoyff said:

BrendanDolan said:

Just remember, ABS does in no way reduce your stopping distance, and a correctly driven, you can threshold brake a non ABS car to shorter stopping distances (and I threshold brake ABS cars when I need to get with the stopping program in a hurry).

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This statement strains credulity. Do you have a citation for this?
A rolling rubber wheel on concrete has much more traction than a skidding wheel.

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http://www.drivingfast.net/car-control/braking.htm

You are correct. A rolling wheel has much more traction than a skidding wheel. Threshold braking is applying maximum braking force, running the traction up to, but not past, impending lock up. If you lock a tire, you're not braking as effectively, and at that point, you're not on the threshold of impending lockup.

Now, threshold braking take a ton of practice, which is why most people were taught to pump the brakes (lock, unlock, lock, unlock).

ABS systems are great, but they do not decrease stopping distance, except for the untrained who will either pump the brakes, or lock the brakes. ABS's best intention isn't to reduce the stopping distance, rather it's to allow a surprised driver to brake and steer (and turning while threshold braking is not something for the uninitiated, as it requires less pressure on the brake pedal timed correctly with a steering input to not exceed available traction).

Again, spend the money on a nice high performance driving event/car control clinic and the instructors will show you how this works.

Companies like Lotus spend a lot of time on their braking systems so a keen driver can threshold brake without ABS intervention, but a panicked driver can still slam the middle pedal and not lock the brakes.

EDIT: Been looking for a link on the websites of the schools I've attended, but they mention teaching it in the course syllabus, but they don't give the full explanation online.

BrendanDolan said:

ABS systems are great, but they do not decrease stopping distance, except for the untrained who will either pump the brakes, or lock the brakes. ABS's best intention isn't to reduce the stopping distance, rather it's to allow a surprised driver to brake and steer (and turning while threshold braking is not something for the uninitiated, as it requires less pressure on the brake pedal timed correctly with a steering input to not exceed available traction).

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Here is a good article which covers all of this:
http://www.theautochannel.com/mania/behind/russ/cruss_wig_abs.html

Long story short, use of ABS will greatly reduce stopping distances for a majority of drivers.
ABS has not lowered accidents due to improper brake pedal operation.

Brake assist is designed to overcome this problem.

I agree with you that better driver training would also solve this problem.

Thanks!

This is all very interesting and mildly disturbing.
I had noticed some odd braking behavior at slow speeds without invoking ABS at all.
This situation is something like this:
#1: Slowing for a red light.
#2: Light changes, let off of the brake pedal, but then the car in front slows to turn right and so I tap the brake pedal to slow down again.
But this 2nd time the brakes pull harder than I expect and the car stops instead of slowing, so I have to hit the accelerator again to get back with the flow.

It only happens sometimes, so I think it has to do with timing of the pedal presses, and how hard I pressed them, but I don't think I was ever anywhere near invoking ABS. It feels like a low speed issue, as I don't recall anything similar happening above 20MPH. Basically getting on and off of the brake pedal in rapid succession can sometimes result in an unexpected amount of braking. Perhaps an unintentional way that "brake assist" get invoked.

And by the way, in case it isn't obvious, I think that ABS is most useful for panic stops in the snow and heavy rain.
Also, more useful if you are running old/bald tires that have poor traction.

Also, threshold braking can not compensate for tires with differing coefficients of friction due to different road surfaces, etc.; ABS can. For 99.4% of drivers and situations, ABS will be better and stop quicker.

BrendanDolan said:

Now, threshold braking take a ton of practice, which is why most people were taught to pump the brakes (lock, unlock, lock, unlock).

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TEG said:

This is all very interesting and mildly disturbing.
I had noticed some odd braking behavior at slow speeds without invoking ABS at all.
This situation is something like this:
#1: Slowing for a red light.
#2: Light changes, let off of the brake pedal, but then the car in front slows to turn right and so I tap the brake pedal to slow down again.
But this 2nd time the brakes pull harder than I expect and the car stops instead of slowing, so I have to hit the accelerator again to get back with the flow.

It only happens sometimes, so I think it has to do with timing of the pedal presses, and how hard I pressed them, but I don't think I was ever anywhere near invoking ABS. It feels like a low speed issue, as I don't recall anything similar happening above 20MPH. Basically getting on and off of the brake pedal in rapid succession can sometimes result in an unexpected amount of braking. Perhaps an unintentional way that "brake assist" get invoked.

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If it happens again, think "how quickly did I release the gas and get on the brake?" The B6 Passat had a touchy brake assist, and back when I worked in sales I'd nail the assist all the time when moving cars around the lot in a hurry. If you snapped the throttle closed and touched the brake immediately after, there's your answer.

mogur said:

Also, threshold braking can not compensate for tires with differing coefficients of friction due to different road surfaces, etc.; ABS can. For 99.4% of drivers and situations, ABS will be better and stop quicker.

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Correct. A well sorted 4 channel is going to allow you to brake in a straight line on a varying surface with your foot to the floor. I realize for the average driver ABS is the way to go, but I always enjoy playing the devils advocate and help make people more aware about what cars can and can't do, and promote drivers training. I personally find it's comically easy to get and maintain a license in the US, and I wish the licensing process involved more accident avoidance, and went through the electronic "nannies" in the car in better detail.

Agreed, I autocrossed and ran in SCCA C Production for a number of years and a drivers school is the best way to learn what you and your vehicle can (and can not) do. Everyone should be required to take a high performance school.

BrendanDolan said:

I personally find it's comically easy to get and maintain a license in the US, and I wish the licensing process involved more accident avoidance, and went through the electronic "nannies" in the car in better detail.

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mogur said:

Everyone should be required to take a high performance school.

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BrendanDolan said:

I wish the licensing process involved more accident avoidance, and went through the electronic "nannies" in the car in better detail.

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I can't agree more with these views. Having instructed in the Tire Rack Street Survival School, I am always amazed to find that most teenage drivers have never felt the ABS system kick in when braking, or attempted an accident avoidance maneuver under controlled conditions before. This should be part of basic driver training before licensing. It is not necessary for every driver to know high performance driving or racing techniques , but basic car control and survival skills should be mandatory.

TT

BrendanDolan said:

I probably should know this (but I don't...), but I know plenty of modern cars, Nissans included, have brake assists that apply the brakes into the ABS when they sense you've been startled and went for the brake (snapping the throttle closed, immediately hitting the brakes hard).

I'll take my demo out and give it a shot and see if I can replicate it.

That being said, take it back to the dealer, and have one of their LEAF techs sit shotgun while you drive, and replicate the issue.

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I experienced the Brake assist the other day, and wow, it's really something! It made me a bit uncomfortable. a car pulled out right in front of me (an f'ing kid talking on a cell phone!) and I suddenly hit the brake with medium pressure to slow down and the car came to an abrupt stop, pulling the brake pedal down all the way for me. What I didn't like about it is that it felt like an over reaction, on the other hand, had the car that pulled out in front of me kept going, it would have prevented a collision. I guess someone has looked at a bunch of stats and decided that an over reaction like this results in fewer accidents... I'm skeptical, but do believe the braking system worked as designed.

g