The Anti-Lock Braking System (ABS)

     Most cars have ABS standard these days for safety, but what is ABS exactly?  How does it work, and how does it help us?

     ABS stands for the Anti-Lock Braking System, and what it basically does is prevent our car from skidding.  It obviously is connected to our braking system, but it only activates when our wheels are about to lock.  What does that mean?

     First of all, the car’s TIRES stop the car, not the brakes.  Remember that.  It’s the tires grip that both allows our to car to accelerate, corner, and stop, no matter how slow or fast we’re going.  With that out of the way, let’s move on.  What a brake system does is basically convert our kinetic energy (energy that’s in motion) into heat energy by applying friction.  When a car is moving, it has a certain amount of kinetic energy.  When a car is at a standstill, it has no kinetic energy.  So what we do to get from x amount of kinetic energy to 0 kinetic energy is to convert it to heat energy (since energy can neither be created nor destroyed).  The brakes are connected to the wheel hub, so that a brake disc is rotating along with the wheel.  And when we press on the brakes, a clamp pushes on the rotating disc, which results in friction (we’re using friction to convert the kinetic energy to heat energy), and as the brake disc slows down, so does our wheel, and in turn our car.

     Now, a tire has only a certain amount of grip.  And depending on the tire, we can have anywhere from craptastic grip in the snow, to glue-like grip on tarmac roads to confidence-inspiring grip in the rain.  Also, depending on the car, tire, weather, and road, we’ll have different levels of maximum grip available to a driver at different times.  This is where ABS comes in.  When we press really hard on the brakes, we assume that we’ll use the full braking power to use all our tire’s grip to stop our car and keep us safe.  But the problem is that the force the braking system is applying might be too much for our tires to handle.  So in this instance, our brake force is too much for our tire’s grip, and thus “lock” the wheels, meaning that the brakes have suddenly “clamped” the wheels stiff.  So imagine here a car going 60 miles per hour with wheels that don’t rotate, but are rather stiffly glued onto the body and can only skid across the pavement.  This is no good, as we’re not using any tire grip to slow down the car, and in certain circumstances, will spin the car out, flip the car, or something else.  The fastest way to stop a car, is to have fulling rotating wheels using all of the grip available to stop the car.  So what happens when our wheels “lock”?  They make the car skid and it becomes dangerous.  ABS was created to prevent that.

     Now imagine our car up on jackstands.  The car is up in the air, with nothing touching the ground.  Pretend we’re revving up our car, and our wheels are turning at a pretty high rate.  If we stomp on our brakes, they’ll almost immediately stop the wheels.  Because our car is on jackstands here, the brakes have to only stop the momentum of the spinning wheels since the car’s not moving.  But if the car was in contact with the ground, and was moving at a rapid speed, the brakes would have to first slow down the wheels which would use their grip to then slow down the car.  That takes alot longer than the “almost immediately” braking time of a car that’s suspended in air.

     So as our wheels are decelerating, a sensor is making sure the wheels don’t decelerate too much (too much meaning the wheels have given up their grip on the road and have decided to clamp stiff).  And when it does sense a quick change in wheel deceleration that’s not normal, it then lessens the brake forces (which are causing the wheels to lock in the first place) and lets the wheel stay gripped to the road.  It then reapplies the brakes (remember our foots still on the brake pedal since we want to stop) within milliseconds and rinses and repeats until our car is either at a full stop or we take our foot off the brake pedal.

     What is basically happening here, is the ABS is braking “for” us.  Instead of our foot constantly applying a braking force that will cause the wheels to lock, ABS interferes and makes sure they don’t lock by “tapping” the brakes for us.  This can all be felt by the driver, as the brake pedal feels like its pulsing.  That’s the ABS doing its job so that your tires don’t lock up and end up causing you to skid.

     Remember, ABS is only at work when we’re applying a brake force so high that it’s about to cause the wheels the lock.  That only happens when we’re braking really hard, or the road surface is really slippery.  Under light, or medium braking, our tires should have enough grip to slow down without the aid of ABS.


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April 2009
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© 2009 Rusi Li

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