ABS System in Cars

that was a good answer and i cant really add any more. but i will look around

Hi Blink,

The sentence here describes not ABS, but traction control, which is just ABS in reverse. "when the ECU detects it is turning significantly faster than the others, brake hydraulic pressure to the wheel is increased".

One wheel without traction, even on a 4x4, will lose traction to all the wheels because of the differentials. If a load is applied to that wheel then the remaining wheels will apply torque to the road.

regards

Chas

The sentence here describes not ABS, but traction control, which is just ABS in reverse. "when the ECU detects it is turning significantly faster than the others, brake hydraulic pressure to the wheel is increased".

Yes, pretty close to correct. In actuality, traction control automatically applies the brake to a spinning wheel, whereas the statement, taken out of context, would suggest, in saying "increases," that the brake is already applied. And of course the article is intended to be about ABS rather than traction control.

But you're right, traction control is roughly the inverse of ABS. Add a few more sensors, and you can get stability control, and a few more, rollover control. So inexpensive has this become that all 2012 cars for the US market will have stability control, by law. In my first car with ABS, it was a $1000 option.

I don't think the law requires ABS even here in Spain but as far as i can see all cars have it, including the base model fiat panda bought for one of our electricians to run around in.

My first car with ABS was about 10 years ago and cost about $2,000 extra. I had had a shunt with the previous car, not too major,which would have been avoidable with ABS and even with fully comp insurance the uninsured losses still came to that sort of figure, so the choice of the ABS was a no-brainer.

And in all actuality ABS increases the traction. I can't remember the exact theorem or its name but the properties of rubber and a typical concrete or asphalt road yield the greatest frictional force at 20% slip. Good ABS systems aim to hit this mark. You notice when ABS kicks in if its sole purpose was to regain full traction it would only reduce the braking force to that wheel until the skid ceased and then maintain that braking pressure. However ABS systems pulse multiple times per second to ideally maintain 20% slip (usually fluctuating in a sinusoidal fashion due to the electronic controls). This actually has been proven to yield shorter stopping distances than maintaining full traction with the road.

I'm not sure what properties of rubber and road yield better braking with slip. But I think it has something to do with the transition between coefficients of staic and kinetic friction being maximized at that point.

-T

Read the below and you will know all you need to know

http://www.ukcar.com/features/tech/others/Anti-Lock-Brakes/

http://www.allpar.com/fix/ABS.html

Dear Rahul Vijayan: The reply from Blink is excellent, but I would like to make the following fine points about your questions:

"1. ABS is a system used to intermittently apply brakes under conditions where vehicle is likely to lose its traction."

No, only the driver applies the brakes with available power assist; the ABS intermittently restores rotation of a wheel that is beginning to lock up. This assures maximum braking efficiency and retains directional control. The peak axial and longitudinal forces that a tire can generate depend on the normal force (wheel loading) and the tire compound and the road surface. The tire forces will always fall inside of the 'friction circle' that describes the maximum available force for the (rolling) wheel and the contacting surfaces (rubber and road). Generally static (rolling) friction coefficients are a little higher than kinetic (sliding) friction coefficients. The significance of the 'friction circle' is that you can trade some slowing force for some steering force or vice versa, but the wheel will suddenly stop turning if you apply more than the force available inside this 'friction circle' with your driving choices. A well tuned ABS can often stop a vehicle in a shorter distance than a non-ABS system. A slightly increased stopping distance is also possible if the ABS is not tuned well for the vehicle, but directional control is retained to aid the driver in avoidance maneuvers and, this may save your life. A locked wheel has no steering effect and the 'friction circle' has become a narrow box in the direction of travel unless the tire turns at near road speed.

"2. There is a reduction in braking efficiency, which is the basis of this system."

Not necessarily, see above descriptions. With ABS equipped hydraulic brakes as found on most vehicles under about 12,500 pounds GVWR, typically a 3-way solenoid valve is momentarily pulsed by the ABS system to vent a very small volume of brake fluid from the wheel cylinder involved in an impending wheel lock up. The slight decrease in braking force allows the wheel to quickly spin back up to near road speed where the higher static friction coefficient again applies without a large increase in stopping distance. The vented fluid is eventually returned to the master cylinder reservoir. If a driver in a non-ABS vehicle attempts to manually reduce pedal force and then quickly reapplies pedal force to keep his braking and steering control, the brake power assist may diminish due to lost of vacuum, further increasing the stopping distance. A good NASCAR driver for example, will be able to brake at maximum effectiveness without ABS because his front to rear braking forces are being tuned from inside the vehicle to match the track conditions and ABS is rarely used on asphault oval tracks.

"Also.. i know that this intermittent operation is brought about by an electronic system of sensors, an ECU and actuators... what type of sensors and actuators are used here??? are there any mechanical ABS systems(in the sense which has not got any ECU or related electronics)... even if not available commercially.. any such concepts are available???"

See Blink's answer for wheel speed sensor description.

"ECU" is an Acronym for Engine Control Unit and I believe you may have meant CPU or MPU, Micro Processor Unit. No, I do not believe that any effective mechanical aftermarket ABS exists because the sensors are inherently electronic. A 16/32 bit MPU costs less than a dollar today and it is by far the cheapest way to monitor all four wheel speeds and prevent lock up on a per wheel basis.

Luther M

Hi Luther,

Rahul didn't ask if any "mech only" systems are available, only if they could exist.

While I would not bet my life on it, I have a strong recollection that the Jensen Interceptor FF of 1968, my dream car as a 17 year old, had a mechanical ABS. Wiki explains how the system works, which cures a 42 year curiosity!

regards

Chas

Dear ppl,

I thank all of u for educative discussions like this...

Being a student i m really glad to upgrade my knowledge base with such facts..

Coming to the topic.. Can ABS be fitted onto drum brakes???... though the system alters only the hydraulic pressure to the wheels... will the huge play(compared to disc brake-pls correct me if i m wrong) required for brake actuation in drum brakes be carried out so fast and accurately???

also.. if i wish to attach an ABS system to an existing disc brake set up.. which parts should i change or modify???

"Coming to the topic.. Can ABS be fitted onto drum brakes???... though the system alters only the hydraulic pressure to the wheels... will the huge play(compared to disc brake-pls correct me if i m wrong) required for brake actuation in drum brakes be carried out so fast and accurately???"

Rahul,

Yes, ABS will work on drum brakes, but you are right there is a much bigger mass which must move further to decrease braking force in the brake shoe mechanism than in equivalent direct piston operated disk brake pads . The amount of brake fluid removed during the less than 0.1 second pulse of the 3-way valve is still small on hydraulic drum style brakes. The other important limiting factor is the angular momentum of the wheel assembly. A disc braked wheel can have less weight and less angular momentum than an equivalent drum braked wheel. Lower angular momentum (aka polar moment) helps the wheel assembly speed up quicker when the brake force is decreased during the brief pulse applied to the 3-way valve in the brake fluid line to the wheel cylinder. Any modification of a vehicle's brake system could produce unintended consequences and is not advisable. This is the primary safety system! A quick search on Google confirms that no aftermarket ABS providers could be found. Your best bet is to keep the brake system fitted in top condition. Lighter wheels and tires can improve handling and braking. My advise is to go here for improvements.

Luther M.

Thank u Luther!!

In reply if you was to fit an ABS to your motor you would invalidate any insurance.

unless you got their permission

it would be possible to fit the abs but its complicated as you would need to also fit al the sensors and toothed gear units to each wheel.

are you also aware there is a limiter on the rear wheels that limits braking force to 25% so that 75% braking force goes to the front brakes?

if i was you i would learn cadence breaking so you can do the breaking in the event of a skid and not rely on an ABS,

Fitting ABS would be very complex and if it ever failed, Think of the consequences NO BRAKES!!!!

Additional links relating to how ABS works

http://autorepair.about.com/od/glossary/a/def_ABS_2.htm

http://www.mucda.mb.ca/aboutabs.htm

http://en.wikipedia.org/wiki/Anti-lock_braking_system

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/anti-lock-brake1.htm

http://www.aa1car.com/library/abs1.htm

http://www.aa1car.com/library/abs_bendix9.htm

All links were of good...

ThanQ Peter!!!

no problems glad to be of use

anti lock braking system:

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/anti-lock-brake1.htm

read this nd get urself cleared....!