- [Narrator] You might have noticed
that after making a turn,
when you release the steering wheel
it will automatically return to the central position.
You will be amazed to find out
that this steering wheel return ability
is not achieved by any complicated mechanism
using springs or valves.
Instead, engineers have achieved
the return motion of the steering wheel
solely by giving the front wheels
an angle called a caster angle.
Let's discover the interesting physics
behind how providing an angle to the front wheels
makes the wheels and the steering wheel
come back to their original positions.
First, let's have a closer look
at the steering wheel mechanism.
It is clear that the rotation of the steering wheel
is transferred to the rack and pinion mechanism
which then turns both the wheels.
If you observe closely, you can see
that the wheels are turning along a particular axis.
This axis is called the steering axis.
In the current demonstration,
you can see that the steering axis is perfectly vertical.
However, this is not normally the case in practice.
In practice, the steering axis will be slightly inclined
to the vertical as shown.
This angle is called the caster angle.
You will get a clearer picture of this angle
when it is viewed from the side.
Let's watch how this steering mechanism works.
At first you might not notice much difference
from the previous mechanism.
However, when you understand
the concepts of patch area and pivot point,
you will notice a big difference.
Patch area is the area where force
is transferred to the wheels.
It is clear that the wheel is pivoted
to turn around at the meeting point
of the steering axle and road.
Let's call this meeting point the pivot point.
In the first mechanism,
the patch area is in line with the pivot point.
However, in the second case,
the patch area is way behind the pivot point.
To understand the effect of this trailing patch area,
let's study more about the forces
acting on the wheels during cornering.
Assume that your car is making a perfect circular turn
in a level plane.
To make this turn, what the car needs
is a centripetal force.
In a level plane turn, this centripetal force should come
from the frictional forces at the wheel patch area
and this fact is clear from this snapshot.
Now let's analyze in detail
what this frictional force does to the front wheels.
In the actual steering wheel geometry,
we saw that the patch area
is behind the steering axle meeting point or pivot point.
If you no longer hold the steering wheel
in this turned condition,
the effect of the centripetal force
on the wheels is obvious.
It will produce a restoring torque
and the wheels will automatically realign to the center.
Let's watch it from the top view also to get a better idea.
However, for the initial geometry
with zero caster angle,
there won't be any restoring torque
since the centripetal force passes through the pivot point.
In short, just by giving a positive caster angle
to the steering mechanism,
engineers were able to attain the restoring torque.
We salute the brilliant minds that visualized
such an ingenious idea and avoided the need
for a complex mechanism.
It is clear from these discussions
just how critical the caster angle is
for a vehicle's straight-line stability.
The caster angle is not adjustable on modern cars
but if during the wheel alignment operation
any variation is found,
due to wear and tear of the connected parts,
the issue should be fixed.
We hope you will support our educational services
on patreon.com
and don't forget to subscribe.
Thank you!