mywaracfirfoyff
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Re: Scary braking problem

BrendanDolan wrote: 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).
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.
BrendanDolan
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Re: Scary braking problem

mywaracfirfoyff wrote:
BrendanDolan wrote: 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).
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.
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.
Brendan Dolan
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Boardwalk Nissan - Redwood City, CA
mywaracfirfoyff
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Re: Scary braking problem

BrendanDolan wrote: 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).
Here is a good article which covers all of this:
http://www.theautochannel.com/mania/beh ... g_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!
TEG
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Re: Scary braking problem

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.
TEG
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Re: Scary braking problem

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.
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TomT
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Re: Scary braking problem

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 wrote: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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Re: Scary braking problem

TEG wrote: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.
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 wrote: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.
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.
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TomT
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Re: Scary braking problem

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 wrote: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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ttweed
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Re: Scary braking problem

mogur wrote: Everyone should be required to take a high performance school.
BrendanDolan wrote:I wish the licensing process involved more accident avoidance, and went through the electronic "nannies" in the car in better detail.
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
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Re: Scary braking problem

BrendanDolan wrote: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.
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.

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