How does ABS Work in Cars and Bikes?
Nowadays an anti-lock braking system is used in almost all modern vehicles. This system prevents accidents like this, where you lose control of the steering as you apply the brakes. With ABS, you will be able to steer the vehicle properly and it also reduces the braking distance.
What is ABS?
How is it able to prevent accidents due to braking?
We will explore these questions in this Post. To properly understand ABS, we first need to understand the principles of steering and braking. When you press the brake pedal, the brake pads on the wheel disks will be activated and press against the disks, and this will stop the rotation of the wheels. As the wheels stop, this causes slippage between the road and wheels, and the vehicle stops due to this friction.
Now let’s understand the steering mechanism. The whole steering mechanism is dependent on the simple principle of the wheels rolling. The wheels will always try
to avoid slipping on the road. So this means that the velocity of the wheel at the contact point will always be zero. The wheels have two kinds of motion.
They rotate on their axis and also they move along the direction of the car. Due to these two kinds of motion, they have two kinds of velocity: translational and rotational.
For non-slip rolling conditions, the sum of the translational and rotational velocities
at the contact, the point should be zero. Just keep this simple principle in mind, and you will very easily be able to understand the steering mechanism.In an automobile,all four wheels should be rolling at all times to avoid the vehicle slipping. In fact, this simple rolling principle is the reason why your car turns when you turn the front wheels.
Let’s examine this. Assume a hypothetical condition in which, even after turning the wheels, the car carries straight on. In this case, the rolling velocity will be inclined as shown, and the translation velocity remains straight. Therefore, if you add up
these two velocities, the resultant velocity will not be zero.
This will cause the car to slip. The only way to avoid this is by making the translation velocity also inclined. This is possible only when the whole car makes a turn as shown.
When the car makes a turn at the instantaneous center is shown, all four wheels can satisfy the condition of rolling. From this snapshot, you can clearly see how the velocity at the contact point of all four wheels becomes zero.
Now, let’s analyze the steering mechanism at the time of braking. Due to the brake pads being activated, the wheels stop spinning. Even though the wheels are turned, since they are not rotating, there will be no rotational velocity at all. In this case, the vehicle does not need to turn as this has become like a normal case of braking. The vehicle will no longer react to the driver’s changes to the steering and will slide in a straight line which can cause an accident. Apart from losing control of the steering, cars without ABS face another big issue, braking on surfaces with different levels of traction.
When the left and right wheels are on different traction surfaces, applying the brakes will generate different frictional forces on the wheels. This will produce torque and the car will go into an uncontrollable spin. Let’s see how ABS solves these issues.
An anti-lock braking system is very simple. What ABS does is to prevent the wheels from completely locking up. The ABS system includes speed sensors attached to all of the wheels. When the sensors detect that a wheel is about to lock up, a modulator unit partially releases the brake pad on that wheel. This way, the wheels will be allowed to spin intermittently during braking. You can immediately see the effect of this intermittent wheel rotation on steering. Here, the wheels will have a rotational velocity component. This will definitely allow the steering to function, and the driver will be able to steer the vehicle even during the braking. But the big question here is: How does the action of intermittently releasing the brakes reduce the braking distance? To get an answer to this question, we first need to understand how the coefficient of friction between the tire and the road varies with respect to slip.
In perfect rolling conditions, the value of the frictional coefficient is nearly zero.
However, when the wheel is 100% slipping, sliding friction comes into play.
You can expect the frictional values for other levels of slippage to vary like this.
But since rubber is a complex material that has strange behaviors during braking, the frictional coefficient between the tire and the road varies as shown with respect to slippage.
It has a peak value of around 12% of the slip ratio. And below and above this,
the frictional coefficient reduces. During braking without ABS, the frictional coefficient
which comes into effect is predominantly sliding friction.
However, in ABS braking, the clever algorithm adjusts the brake pressure
to keep the slip ratio near 12%, where the frictional value is at the maximum.
This significantly reduces the braking distance. Now back to the huge instability issue of cars. By intelligent electronic braking distribution, which is a subsystem of modern ABS, one can overcome this issue very easily. An EBD yaw rate of the car and the relative slip of each wheel is measured.
Just by reducing the brake pressure on the wheels with a higher grip, we will be able to reduce the frictional force produced by these wheels.
This will keep the yaw torque and the whole car under control.
We hope this post gave you a clear conceptual overview of the workings of ABS.
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