- [Instructor] When you learn to drive,
the most difficult control to master is that of steering.
Do you know there are some amazing physics
and mechanisms beneath the steering wheel,
which keeps the vehicle on the right track
without any skidding of the wheels.
This video will demonstrate why we need such mechanisms
for steering and how they work.
We will also understand the workings
of power steering mechanisms.
We thank brilliant.org for supporting this video.
The amazing thing about an automobile
is that if you just tilt the front wheels as shown,
the whole vehicle will make a turn.
However, why doesn't the vehicle carry on moving
in a straight line even after the wheels have tilted?
To get the answer to this question,
we have to understand the physics behind
the most significant invention of mankind.
The physics of wheels.
In a wheel which is rolling perfectly, the velocity
of the wheel at the contact point should always be zero.
This will make sure that there won't be any slippage
between the stationary road and the wheel
at the contact point.
But how can a moving wheel have zero velocity
at a point on its surface?
This condition is possible because the wheel
has two kinds of motion.
The wheel moves along in the direction of the car
and it also rotates along its axis.
When we sum these two components at the bottom
of the wheel, you can see they are in opposite directions
and they cancel each other out.
Just keep this simple principle of a rolling wheel
in mind and you will be able to perfectly understand
the steering mechanism.
Assume that even after the wheel turns,
that the car carries on moving straight.
Let's again check the rotational and translational
velocities at the contact point of the wheel.
You can see that the rotational velocity is inclined,
but the translation of velocity is straight.
Due to this, the velocities will not cancel each other out
and this will lead to skidding.
The only way to achieve the zero velocity condition
is to make sure the translational velocity is also inclined.
This is possible only when the whole car turns
with respect to a particular CenterPoint.
In this snapshot, this fact is shown perfectly.
Here, all four wheels meet the perfect
conditions of rolling.
One major thing to be noted here is that,
for such a perfect turn, the perpendicular lines
from the front wheels should meet the real wheel axis
at a common point.
This condition is the principle of steering.
If you observe carefully, you will note
that the angles turned by the left and right wheels
are not the same.
This means that for perfect steering,
the left and right wheels should turn at different angles.
The steering mechanism is used to carry out this purpose.
The most commonly used steering mechanism in modern vehicles
is the rack and pinion type.
Let's see how this mechanism manages to steer the vehicle.
A rack is at the center of this mechanism.
This rack is constrained so that it can only move
in a straight line.
The pinion which comes from the steering,
can make this rack move.
A part called a steering arm is attached
to both of the wheels.
This part is constrained so it only has a rotational motion
along the axis shown.
The steering arm is connected to the car frame
via a roller bearing.
This makes sure that it can only turn.
A tie rod connects the steering arm to the rack.
The tie rod can have both translational
and rotational motion.
Now just observe what happens to the wheels
when the rack moves.
You can see that the left and right wheels
are turning at different angles.
If you track the meeting point of the left
and right wheels, you can see the meeting point
always lies on the rear wheel line.
Thus the rack and pinion mechanism perfectly satisfies
the conditions needed for steering.
Due to this, the vehicle makes a turn without slipping.
The steering we have discussed so far was the manual type.
Nowadays, electric motor assisted power steering
is widely used on most cars.
Electric power steering makes the steering action
effortless and more accurate.
A column assisted power steering unit is shown here.
Here a Brushless DC motor drives the steering column
and the pinion.
The motor is capable of rotating in both clockwise
and anticlockwise directions.
An electronic control unit decides the amount of power
the motor should transfer to the column.
The ECU takes the following inputs such as the torque
the driver has applied to the steering wheel,
the steering angle, the steering wheel speed
and the vehicle speed and then decides the appropriate
torque to be supplied.
A haul of fact based sensor and torsion bar arrangement
is used to measure the drivers torque input.
Due to the motors power assistance, the steering wheel
rotation becomes effortless for the driver.
Do you think this is a perfect mechanism?
Wait for a moment and think about what happens
to the steering in the case of a motor failing.
You might be aware of the fact that a worm screw can turn
a worm gear, but a worm gear can not turn a worm screw.
This means that if the motor fails, any manual rotation
by the driver will be blocked by this constraint
of the worm drive.
In short, the steering becomes jammed.
To overcome this issue, the rotation
from the steering wheel is never directly connected
to the worm gear.
Instead, it passes through a planetary gear set.
Here the driver's rotation is given to the sun gear.
The output is drawn from the carrier and the worm gear
also becomes ring gear of the planetary gear set.
During normal operation, power from the ring gear
will be easily transferred to the carrier.
In case of motor failure, the ring gear cannot move.
However, you can see the driver's manual rotation
of the sun gear will be transferred to the carrier,
and finally to the pinion gear.
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