This article is within the scope of WikiProject Systems , which collaborates on articles related to Systems science.
Observation
Thought this ancectodal evidence, I must drive, on a daily basis, a vehicle equiped with hydraulically-actuated, four-wheel drum brakes, of the dual-servo, self-adjusting type. Since re-shoeing all four wheels several thousands of miles ago, I have, in a wide variety of braking conditions, experienced no brake fade. Could it be the modern linings? My car is a piece of crap, you dont even know.
I also wish to point out that the brakes lack power assist, which the servo action seem to make up for, and, that braking control driving on ice and snow was much better than what I have experienced on ABS and non-ABS power-assist front disc/rear drum equiped vehicles.-WK-139.78.96.115 01:27, 8 March 2007 (UTC)
Safety during ABS failure
This is probably something that should be mentioned. Back when ABS was first introduced, cars still had a manual proportioning valve for braking. While this was not as important when a car was equipped with disc brakes on the front, and drum brakes on the rear.. it was absolutely ESSENTIAL when a car was equipped all around with disc brakes.
Without a proportioning valve (anywhere from 65%-75% for the front disc brakes, and the rest to the rear), such a car will receive a 50/50 split in braking power, front and back. This is _very_ bad, as you will soon find the rear of your car attempting to slide in front of you, as it is much lighter than the front. This is especially evident when trying to brake whenever the road is slippery, be it rain, snow or ice.
This is why such a proportioning value exists, and all pre-ABS (and most 80s and 90s ABS systems) still had a value in place. These days, however, ABS is more advanced. ABS can actually detect individual wheel skid, and apply pressure independantly on each wheel, something early ABS could not to. This has lead to the removal of manual proportioning, which means that each wheel in a modern ABS system recieves equal braking pressure, if ABS is offline.
This is bad, very very bad. If ABS malfunctions, not only is the driver (and likely a driver that has never learned to brake without the ABS crutch) without ABS, they now have a car that can not effectively brake in poor braking conditions. While a skilled driver can easily compensate for the rear sliding out of control, it does make the situation more difficult. Even a skilled driver would have to reduce braking power significantly, with 50% braking power at the rear. An unskilled driver, especially one braking without ABS for the first time, would find themselves in serious trouble.
This removal of the proportioning value is a relatively new thing for standard consumer automobiles. Subaru, for example, just started doing it in 2004, I believe. VW and others have been doing it for quite some time, but I believe well under a decade. While on the surface it may appear to make sense (hey, this means we can fully use the rear brakes), this is seldom the case. Front and rear brakes are tuned, matched to the car, the proportioning value meant to prevent skidding of the rear brakes before the front. Many cars had a value that would even auto-adjust dependant upon the weight over the rear axle.
This is all gone now, in an attempt to simplify construction and save costs. I deem it a serious and potentially fatal mistake. Effectively, the "backup system" that ABS had (reversion to manual proporitioning) is now gone. ABS fails, and braking turns into a very dangerous matter.
I think there should be some mention of this, in this article ....
Comments?
Nicely written. You'd have to do a comparison between the safety advantages of a full authority ABS, versus the safety advantages of an ABS that had stopped working but which operates better in that mode. Actually, I sort of agree with you anyway. My specific concern is that if we are cornering at say 0.4 g and apply 0.4 g of braking, without a proportioning valve we have not changed the balanace of the car, that is, it will tend to drift outwards (ie braking is neutral steer). With a proportioning valve we tend to overload the front tire more, so giving more understeer. In an emergency it is probably better to give more understeer to the average driver.
Greglocock 10:51, 22 June 2006 (UTC)
On 13 google hits for "Antilock brake controller". "Antilock brake system" gets 4,930.
+++Does it work at low speeds? Hi, I have read the car bibles article on braking, which says:
"The hidden gremlin of ABS - what they don't advertise. If you look at the statistics for crashes, a large percentage of them are "fender benders" - low-speed impacts that only do a little damage and so slow that the vehicle occupants are in no danger. Less than 15mph normally. I'll give you one guess what the typical "minimum activation speed" is for ABS. That's right. Your average ABS system is useless much below 15mph. Seriously. Try it yourself. Find an empty road on a slight downhill grade - even better if its on a dewy morning. Run your ABS-equipped car up to about 15mph and jam on the brakes as hard as you can. The car will skid to a stop and the ABS system will remain totally silent."
http://www.carbibles.com/brake_bible.html
Is this true? If so, should it be included in this article. Thanks!! --Turtle 18:38, 1 June 2006 (UTC)
Yes it is true. ABS is deactivated at low speeds because the SAFETY advantage is minimal, but the difficulty of making it work at low speed is very great. A locked wheel is the best way to stop on many surfaces at low speed.
Greglocock 01:31, 2 June 2006 (UTC)
+++Effectiveness.I don't think you can make the absolute statement that ABS will NEVER increases braking distances on tarmac, wet or dry. The reason I say this is that with ABS the slip velocity oscillates around the optimum. With conventional brakes a truly skilled or lucky driver could hold the wheel at the optimum slip velocity.
Also, the calibration has to be a compromise between wet and dry road conditions. I don't know how much the optimum slip velocity changes with surface water, but would hazard a very informed guess that the optimum slip velocity for wet tarmac is significantly lower than for dry tarmac.
However if you have data that shows otherwise I'd be happy to agree.
Cheers
Greglocock
Background
Maybe some words on what ABS is supposed to do would be in order. What happens when brakes lock and how does ABS counter this?
Good suggestion. GL
Invention
The Dunlop "Maxaret" system was introduced on the Jensen FF way before this, in the mid-late 1960s. Just because it didn't have the name "ABS" shouldn't mean the article shouldn't talk of it -- this isn't a Bosch ad. I'm going to be researching the facts on earlier ABS systems to get this article a bit more neutral. —Morven 10:31, Jun 12, 2004 (UTC)
Good call, the Bosch Automotive Handbook has an excellent overview of the Lucas system, which I suspect is somewhat based on the Maxaret. greglocock
++++++++++++++++++++++++++++++++++
First edit by John Gwynne
> How much such systems actually reduce braking distances on bitumen is a subject of debate and depends, in any case, on driver skill. A moderately skilled driver capable of cadence braking would get little benefit from an ABS system compared to a novice driver.
>The recommended technique for non-expert drivers, in a typical full-braking emergency, in a straight line on a highway, is to press the brake as firmly as possible and to steer around the obstructions. However, in real-world emergencies of this sort, a novice driver will rarely (if ever) retain the presence of mind to do anything but stomp hard on the brake pedal and steer straight ahead.
>Some automotive engineers argue that is a better practice to design a car so that it is more difficult to lock up the brakes in the first place. This is done by making the car lightweight and balancing the braking system so that the force applied by the driver during a "panic stop" would be insufficient to lock the wheels.
>In gravel or snow, there is no question that ABS increases braking distances. On these surfaces, the action of the wheels locking causes them to dig into the material and create a buildup of ahead of the locked wheels which helps stop the vehicle more quickly. ABS prevents this from occurring. Some ABS controllers attempt to mitigate this problem by slowing the cycling time, thus letting the wheels repeatedly, briefly, to lock and then unlock again.>Another effect of ABS on slippery surfaces is that it helps the driver to maintain control of the car under braking rather than going into a skid. With 4-wheel ABS, the driver is able to brake and steer at the same time in order to avoid an obstacle, without having to worry about entering into a skid. The problem is that in certain situations, it is actually better for the car to go into a spin in order to reduce the stopping distance by coverting forward motion into rotatin
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