The Best Advert for ABS

evilally · Post by **evilally** » 19 Dec 2009, 11:30

cachaciero wrote:
RichardW wrote:Real reason for ABS: Designed to allow you to steer whilst braking hard (enough to lock the wheels up)

Common misconceptions:
It improves stopping distances (it doesn't, in may well INCREASE the stopping distance)
r.
Could you give us a citation for that statement? given that a well designed ABS system is little different to that fitted to aircraft where they definitely reduce stopping distance.

Cachaciero

It's true, if you can eek out every last bit of braking effort manually without locking the wheels, you will stop faster. Also on snow you are often better to let the wheel lock and "dig in". ABS is primarily designed to allow the driver to keep control of the car and be able to steer it around any potential collision. It's the old ABS debate, personally I think ABS on a road car is a good thing though.

deian · Post by **deian** » 19 Dec 2009, 14:03

ABS has saved me three times, most recently about two week ago when some mercedes decided to pull into a junction right in front of me, he did not look whatsoever, i had swerve around in front of him with my foot slammed on the brake pedal, the abs certainly did help me steer around him as well as I come to a stop on a grass patch, they guy came out and apologised, and admitted wholeheartedly he was in the wrong, and shook my hand, i asked him to go apologise to my wife who was still in the car, he obviously made a mistake (as we all do), he has my respect for being a man about by stopping to apologise, shake hands, apologise to my wife and check my car over, not many people would even bother stopping!
wife said she was proud of me for avoiding the accident, and i said to her, i am proud of the car!

HDI wrote:Missed it by a few feet and the Xantia barely twitched !! No wheel lock up or sliding. I almost found religion and the Xantia has my utmost respect.

- Very true for all my ocassions too, wife feels te same now!

another time was when i was on the m56 going past those chemical works where the chevrons are, going nicely in foggy weather and all of a sudden red lights in front, slamming the brakes at 70mph in a car without ABS would have killed me (without doubt), and naturally i swerved towards the middle lane and then the left lane with abs still juddering on and off, it was one of those moments when you feel you should have been driving with more care and your face goes all red and you fell all hot and sweaty, i nearly cried in joy and fear and embarassment! didn't make any contact with any other vehicle, no1 was hurt but my ego lol it was one of those moment

I definitely think if i did not have ABS i would be a dead man a long time ago! I think they are as essential as seatbelts nowadays!

As RichardW said, it isn't guaranteed to help braking distance on all occasions, on these newer systems they are just as good as non-abs, but the older ones will not be so good and may increase your stopping distance but ABS is not so good on loose surfaces, such as gravel, snow and will then increase the distance.
That info is from wikipedia, search for all occurrences of the word distance and judge for yourself.

Snow is bad stuff, avoid it everytime unless it's impossible, i.e wife giving birth or something.

I think abs would have saved you here, the system on the xantia does not kick back to startle you enough for you to come off the brakes and carry on crashing, you may even have been able to swerve to a safer position with abs.

As for retrofitting ABS, it would seem an overly complex, you'd need ecu, wheel sensors, abs pump/disto block, new pipes etc.

cachaciero · Post by **cachaciero** » 19 Dec 2009, 15:07

evilally wrote:
cachaciero wrote:
RichardW wrote:Real reason for ABS: Designed to allow you to steer whilst braking hard (enough to lock the wheels up)

Common misconceptions:
It improves stopping distances (it doesn't, in may well INCREASE the stopping distance)
r.
Could you give us a citation for that statement? given that a well designed ABS system is little different to that fitted to aircraft where they definitely reduce stopping distance.

Cachaciero
It's true, if you can eek out every last bit of braking effort manually without locking the wheels, you will stop faster. Also on snow you are often better to let the wheel lock and "dig in". ABS is primarily designed to allow the driver to keep control of the car and be able to steer it around any potential collision. It's the old ABS debate, personally I think ABS on a road car is a good thing though.

"if you can eek out every last bit of braking effort" That is precisely what a well designed ABS system should do. The maximum amount of braking effort is obtained just before the tyre starts to skid AND NOTE that is not the same as the wheel being locked.
The ABS system should average out all four wheel speeds to give a speed reference, from this reference a value will be calculated which is the minimum rotational speed of the wheels, any wheel below this value will be considered to be skiding and the brake on that wheel should be released until the rotational speed reaches the minimum speed again.

The big problem as applied to cars is that the co-efficient of friction differs widely between wet and dry surfaces and between different tyres as indeed it does on aircraft but aircraft braking regimes are different and have ways of taking that into account. So in a car system the friction co-efficient used for the purposes of calculation must of necessity be a compromise, it will not be ideal for either the road surface or the specific tyres fitted. So I suppose in theory someone who was really good with cadence braking techniques may stop quicker than the ABS system particularly on a dry road where the ABS calculations will be less than optimal but I don't believe that will be the case in the majority of cases hence my interest in a citation for the assertion, i.e it would be nice to see the results of tests carried out under controlled conditions, until I see those I would not believe that the average driver will out do the ABS in terms of straight line braking distances or any other distances for that matter.

Cachaciero

deian · Post by **deian** » 19 Dec 2009, 16:11

Cadence braking by a human is be very hard, I don't think I would be able to think that clearly enough in an emergency situation to even give cadence braking a try.

Repetitive task such as ABS is well suited to computers, it is not foolproof though as with every machine, they are good at repetitive tasks but without the intelligence humans can apply. But ABS is a simple idea in theory, and even on a simple level I would assume it has saved millions of lives already.

HDI · Post by **HDI** » 19 Dec 2009, 18:11

As a matter of curiosity , does anyone know if the ABS system on Xantia's goes through a self diagnosis and valve exercise process at start up ?

Sid_the_Squid · Post by **Sid_the_Squid** » 19 Dec 2009, 18:57

I'm absolutly in favour of ABS on road cars, definatly saves lives. However I think RichardW has raised some very good points about ABS which often get overlooked.

ABS is detrimental in snow/ice, glad Xantias have the handbreak on the front wheels.

ABS *may* increase your stopping distance, espically if your car has a particulary 'sensitive' setup, which kicks in to early, I've seen some cars behave this way.

does anyone know if the ABS system on Xantia's goes through a self diagnosis and valve exercise process at start up

I certainly thought all abs systems did a self diagnosis at startup, if any faults are found the ABS light remains illuminated, dont know about a valve exercise though.

lurchy666 · Post by **lurchy666** » 19 Dec 2009, 19:07

personally the abs issue is as follows abs is great on wet/greasy roads, but as soon as it is icey or snowing may as well just look at the brake pedal casue thats as much help the abs systems (ford granada/ volvo 850/ sprinter lwb/ lt35/ ) i'm used to help, press the brake pedal and nothing happens what a good idea NOT

on the granada's and volvo as soon as it got icey i disconnected the abs (usually by disconnecting the fluid level sensor cable) normal brakes are 1000 times better than abs on snow in my opinion, the amount of near misses i've had cause of abs is stupid,

on my xantia the one and only time i have had it out in the snow the normal beakes are way to powerfull, i ended up only brakeing when it was in gear so the wheels couldnt lock cause of the engine ,

addo · Post by **addo** » 19 Dec 2009, 20:50

cachaciero wrote:...The maximum amount of braking effort is obtained just before the tyre starts to skid AND NOTE that is not the same as the wheel being locked.

That's in direct conflict with what a mate who is a road engineer told me; admitted he was speaking of dry roads. He stated that maximum "grip" is achieved at the point where the tyre has just begun to skid (not yet readily identifiable as a skid to the driver or observers) and rubber contacting the road has softened to a slightly sticky form.

Post by **myglaren** » 19 Dec 2009, 21:09

addo wrote:
cachaciero wrote:...The maximum amount of braking effort is obtained just before the tyre starts to skid AND NOTE that is not the same as the wheel being locked.
That's in direct conflict with what a mate who is a road engineer told me; admitted he was speaking of dry roads. He stated that maximum "grip" is achieved at the point where the tyre has just begun to skid (not yet readily identifiable as a skid to the driver or observers) and rubber contacting the road has softened to a slightly sticky form.

Sounds right BUT that's not going to happen on snow or ice.

addo · Post by **addo** » 19 Dec 2009, 21:44

Exactly - so to cover for situations where grip is going to be marginal at best, the ABS systems are "advanced" to lock/release before this state is ever achieved.

cachaciero · Post by **cachaciero** » 19 Dec 2009, 22:03

addo wrote:
cachaciero wrote:...The maximum amount of braking effort is obtained just before the tyre starts to skid AND NOTE that is not the same as the wheel being locked.
That's in direct conflict with what a mate who is a road engineer told me; admitted he was speaking of dry roads. He stated that maximum "grip" is achieved at the point where the tyre has just begun to skid (not yet readily identifiable as a skid to the driver or observers) and rubber contacting the road has softened to a slightly sticky form.

I don't think that I would necessarily disagree with what he is saying because there are many factors which will come into play at that point where the tyre surface velocity starts to diverge from that of the surface upon which it is running. For the rubber to have softened implies heat which implies that the tyre has been skidding for a period of time for that heat to build to a point at which it will soften the rubber. If you analyse it what you would get is an initial scenario where the tyre / road friction is at a value which will be maximum just before the tyre and road velocity start to diverge, at that point the tyre heats because of the friction caused by the skidding this then causes the rubber to soften and the road / tyre friction to increase to a degree but as this continues the friction will decrease again as the rubber becomes liquid.

With respect to the comments of lurchy666 with respect to ice and applying brakes and nothing happening, the ABS system is doing what it is supposed to be doing it will apply as much braking force as it can without skidding which on ice is not likely to be a lot, if you do it manually using cadence techniques you won't stop very quickly either

and if you have no ABS and don't or can't use cadence techniques and just prod the brakes where you stop and which way you will be pointing is generally in the lap of the gods

.
The same is true of snow and gravel, you need to think of what is actually between the tyre surface and something which isn't going to move i.e the tarmac, think of loose gravel or snow as a" lubricant" but the properties of this "lubricant" will be the same for braking with and without ABS so the end result of any comparison should be the same as braking on a dry surface i.e ABS should still outperform manual braking all other things being equal.
Having said that things arn't always equal.
In the gravel situation the extra heat generated and skidding action causing the tyre to breakthrough to the hard surface under the gravel may well be significant in improving friction and the "manual" stopping distance at the expense of a degree of loss of control.
Equally the snow situation if the tyre is on top of the snow it is effectively aquaplaning ABS action may well prolong this effect whereas locking the wheels may possibly cause the tyre to break through and find contact with the asphalt underneath but then again it may not

and if it does it may be ice the tyre finds and not asphalt.
As an aside aquaplaning is a potentialy big problem for aircraft landing on wet runways but they don,t resolve the situation with the brakes they just bang the aeroplane on the runway hard enough for the tyres to breakthrough the water film at first contact, can't really apply that technique to cars

As a thought provoker if you find yourself on ice or snow and the ABS system is not stopping you in the space you have available you were either driving too fast or too close for the prevailing conditions.

cachaciero

DickieG · Post by **DickieG** » 19 Dec 2009, 22:36

The correct reason behind and ABS equipped vehicle taking longer to stop on snow or gravel is that without ABS, locked wheels cause a build up of debris to form ahead of the locked wheel which effectively becomes a wedge and causes additional friction hence the shorter braking distance than a car equipped with ABS where the wheels continue to roll over any debris hence no wedge.

I think that in some of the posts above there is a little confusion between ABS and ABD. Extensive research of real life collisions has shown that one of the strange things that regularly occurs just prior to impact is that the driver does not apply maximum braking just prior to impact despite it being obvious to the driver that they are about to suffer a serious or fatal injury. It is due to this fact that Brake Assist, ABD (Assisted brake distrubution) or whatever a manufacture wishes to call it was devised, this system senses the manner in which the brakes are applied and then applies maximum braking if the driver fails to do so. When first fitted to cars (IIRC Mercedes was the first) there was a tendency for the system to be "over active" with drivers complaining of the car doing emergency braking when not requested. With improvements in technology this issue has been resolved and just like car stability systems the newer the car the more effective they are in real life situations.

At work I drive high end high performance cars and the difference in the operation of the newer systems is quite significant, with the latest being very subtle and very effective.

One thing to bear in mind with ABS is that there is a large proportion of drivers who have never felt what happens with ABS when it is activated (the pedal juddering and bouncing up and down) which can frighten some drivers and cause them to release the brake pedal, another reason why ABD has been developed.

Post by **myglaren** » 19 Dec 2009, 22:54

I wondered what was happening the first time the ABS kicked in on my Accord (Xantia didn't have it).

On the other hand, when the Brake Assist kicked in on the C5 I was very glad of it and extremely impressed. As was the Land Rover who had pulled out in front of me, straight from a lay by into the outer lane where I was approaching him at 110 and he was doing 30. I'd seen him in the layby, saw him move and pulled into the outer lane, never expecting him to do the same. Never signaled or looked until all the C5's lights started flashing and he swerved off back onto the hard shoulder in a very impressive zigzag fashion.

03:00, pitch dark and clearly not expecting anyone else to be coming up the road.

The C5 pulled up very quickly, no drama, dead straight and no ABS rattling away as I had expected.

Post by **xantia_v6** » 20 Dec 2009, 08:56

I did a bit of practice yesterday, of stopping the Xantia on ice ( a sufficiently wide and deserted road to make it safe, with enough downhill slope to make it interesting).

Just slamming on the brakes did not produce much observable retardation, the ABS was obviously modulating the wheels between locked and no braking.

Hitting the brake pedal firmly and often produced slightly more retardation, but the ABS was still working overtime.

Tapping the pedal very gently, and as rapidly as possible (2-3 cycles per second) gave significantly better retardation. I did not notice the ABS kicking in.

BTW, you may agree or disagree with what Wikipedia says on Cadence Braking and Threshold Braking

rmunns · Post by **rmunns** » 20 Dec 2009, 15:34

I just took my 90Hdi saloon out to check ABS on ice on a deserted straight wide-ish bit of road about a mile away.
{{Contrary to earlier postings in this thread, the French DO NOT in general salt the roads. They do the motorways (but not the slip roads!) and major roads round and in large towns, but not as a general rule the smaller roads}}.
I know this stretch of road very well, so I was happy doing the experiment.
I found antilock engaged at once, kept foot pressed and turned steering both ways. The result was very limited steering but the important thing is that there was some, at least. The antilock gave little input back through pedal, not as fierce as with an Audi I had a few years back.
With no abs and in panic mode, foot hard down, physics says you carry on in the same direction as when you started skidding, ± deviations due to spinning.