Question on ABS braking system.


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Canew

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#1
ABS - Antilock Braking System, was designed to prevent the wheels of a vehicle from locking up in the event that hard braking was applied. ABS prevents locking up of the brakes in order to enable the driver to steer the vehicle even though that brakes are fully applied.

The way that the wheels are prevented from locking up is that the brake calipers open and close at a very high frequency.

Q1. What is the frequency of the open/close cycle?

Q2. Since the brakes open and close, does that mean that a vehicle with ABS will take a longer distance to come to a stop as compared to another vehicle without ABS? (assuming that the identical vehicles were travelling at the same speed and brakes are applied at the same time/location)

TIA. :)
 

V

vince123123

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#2
1) depends from car to car.

2) the ABS car will stop faster cos the nonABS car will be skidding all over the place and can't stop.

Canew said:
ABS - Antilock Braking System, was designed to prevent the wheels of a vehicle from locking up in the event that hard braking was applied. ABS prevents locking up of the brakes in order to enable the driver to steer the vehicle even though that brakes are fully applied.

The way that the wheels are prevented from locking up is that the brake calipers open and close at a very high frequency.

Q1. What is the frequency of the open/close cycle?

Q2. Since the brakes open and close, does that mean that a vehicle with ABS will take a longer distance to come to a stop as compared to another vehicle without ABS? (assuming that the identical vehicles were travelling at the same speed and brakes are applied at the same time/location)

TIA. :)
 

Del_CtrlnoAlt

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#3
Canew said:
ABS - Antilock Braking System, was designed to prevent the wheels of a vehicle from locking up in the event that hard braking was applied. ABS prevents locking up of the brakes in order to enable the driver to steer the vehicle even though that brakes are fully applied.

The way that the wheels are prevented from locking up is that the brake calipers open and close at a very high frequency.

Q1. What is the frequency of the open/close cycle?

Q2. Since the brakes open and close, does that mean that a vehicle with ABS will take a longer distance to come to a stop as compared to another vehicle without ABS? (assuming that the identical vehicles were travelling at the same speed and brakes are applied at the same time/location)

TIA. :)
A1- maybe search google might have some more detailed answer, but to a layman like me, i can hear the drum brakes on the rear goes... pah pah pah pah pah... about 1/2 sec or 1 sec interval.

A2- yes.... when i first got a ABS car, i find the braking distance abit too far, esp against those older non abs cars...
 

Canew

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#4
Del_CtrlnoAlt said:
the drum brakes on the rear goes... pah pah pah pah pah... about 1/2 sec or 1 sec interval.
Oh! Honestly, I didn't know that drum brakes have ABS too. All the while I thought that only disc brakes have ABS. Thanks for the info.
 

Del_CtrlnoAlt

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#5
Canew said:
Oh! Honestly, I didn't know that drum brakes have ABS too. All the while I thought that only disc brakes have ABS. Thanks for the info.
got lah, ABS is just a system integrated to the braking system... u wan ABS on lorries also can...
 

#6
vince123123 said:
1) depends from car to car.

2) the ABS car will stop faster cos the nonABS car will be skidding all over the place and can't stop.

I am no expert here ....

but based on my first hand experience, definitely ABS not only stops your car faster, but also gives you some amount of control.

I had to jam brakes once on a downslope on a rainy day to avoid a motorcyclist, boy with the ABS I was able to actually control the vehicle around the motorcyclist even as I felt it skidding! If there had been no ABS ...... I hate to think of the outcome!
 

hwchoy

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#7
I am not a mechanic and probably should do some research before saying this, but I think there are several misconception of what ABS is in the previous replies.

ABS does not just apply and disengage the brakes at "high frequency". The brake sensor senses when the wheels are "locked" i.e. when the wheels have stopped rotating while the vehicle still has a forward motion. When this happens, the ABS releases the brake to allow the wheels to rotate again.

Hence, the braking and releasing is determined by the sensor and not a predetermined frequency.

Now WHY should the ABS allow the wheels to rotate? It is a measurable facts of physics that the greatest stopping force occurs when the wheels are almost stopping but NOT YET sliding against the ground. At this point the kinetic energy is discharged at the brake disc/brake pad interface (so it gets thinner, yay!) while the rubber contact patch of the tyres still grips the ground without sliding. Once the tyres stops rotating completely while the car is still moving, the brake disc/pad interface no longer functions (disc not rotating against the pad any more) and the kinetic energy is discharged at tyre/ground interface. The tyre surface is being dragged over the ground and this is much less efficient in dissipating energy, plus the tyres has lost their grip on the road surface thus reducing the amount of control the driver have over the car.

So the answers are:

A1. not a fixed frequency but triggered by the "wheel-locking" sensors
A2. ABS cars stops in a much shorter distance because it ensures the wheels are rolling at the point of greatest frictional force.

remember this science experiment, where you have a block on an incline plane, and the block is tied to a weight over a pulley at the top of the incline. and you have to keep adding mass to see at which point the block starts moving? this is to measure the amount of frictional force the block has over the surface of the incline plane. at that critical point when mass of the weight = the frictional force, the block does not move, but if you give it a small push thus overcoming the frictional force, the block would start moving and will not stop (till it reaches the top of the incline).

Here is why you must not let the wheels slide against the road surface because there is much less friction compared to when the wheels are "just" rolling (which means the contact patch is firmly pressed to the ground at maximum frictional force).

Q.E.D. :bsmilie:
 

hwchoy

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#8
catseye said:
I was able to actually control the vehicle around the motorcyclist even as I felt it skidding!

that's the whole point, you actually are NOT skidding. the contact patch of your tyres are in full grip of the wet surface so that when you steer it is able to change the direction of your car. if it is skidding when you steer you will go into a spin.
 

jbma

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#12
hwchoy is right. The is no frequency involved. Your disc brakes have sensor that detects the wheel locking and would release it and grip it back again. There are many a times when ABS has saved me from a collision. And because it doesn't skid it would take a shorter distance to stop.
 

hwchoy

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#13
in fact just happened to me this afternoon on the AYE outside NUS Science Faculty. The guy in front braked hard so I jammed on the brakes and the ABS came in. I wasn't so worried about hitting the guy in front, more worried about the taxi behind me not stopping. With ABS I was able to steer half a lane to the right (very close to the guard rail) so that the taxi has more room to siam to the left :)
 

Canew

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#14
hwchoy said:
in fact just happened to me this afternoon on the AYE outside NUS Science Faculty. The guy in front braked hard so I jammed on the brakes and the ABS came in. I wasn't so worried about hitting the guy in front, more worried about the taxi behind me not stopping. With ABS I was able to steer half a lane to the right (very close to the guard rail) so that the taxi has more room to siam to the left :)
A classic example how ABS saved the day. :sweat: My colleague has adviced me to look for a car with ABS if I were to buy one. No ABS, no deal. :sticktong
 

ycchen

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#15
It is really hard to say, must compare different cars..
for example formula one can do 0km~160km~0km in 6 secs :sweat:
 

user12343

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May 15, 2005
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#16
i tink the primary design objective of ABS is never meant to achieve a shorter stopping distance. it's designed such a way that in the event wheel lockup occurs during braking, the ABS kicks in at a predetermined frequency to let the wheel move abit and lock a bit. more importantly it's designed such that the driver still has control over the vehicle to "siam" the obstacle while preventing excessive fishtailing or skidding out of control due to the lockup of the wheels.

whatever the case, drive safely, observe a 2-second distance from the car in front of u, the last thing you wanna see is a big white bag inflate in front you and your face covered with white powder.... ;p
 

user12343

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#17
my car brakes have a grinding sound when the ABS kicks in, at least about 10 cycles per second.
Del_CtrlnoAlt said:
A1- maybe search google might have some more detailed answer, but to a layman like me, i can hear the drum brakes on the rear goes... pah pah pah pah pah... about 1/2 sec or 1 sec interval.

A2- yes.... when i first got a ABS car, i find the braking distance abit too far, esp against those older non abs cars...
 

DEADMETAL

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May 11, 2005
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#18
1. Interval is very short, witness is before but might varies btw cars

2. Distant on dry ground longer, distant on wet ground shorter.

With ABS u still can steer, without ABS, u will turn round if road is wet.
 

Ian

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Feb 20, 2002
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#19
ABS by itself won't save you from an accident, there are other factors to take in to account when determining the shortest braking distance, some of them include:

Condition of your tyres
Tyre composition
Brake rotor condition
Brake pad composition
Brake pad temperature
Brake pad wear
Suspension condition
Steering linkage condition

and last but not least.

Driver ability.
 

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