English subtitles for clip: File:Around the Corner (1937) 24fps selection.webm

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Formations, like these,

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require long hours of intensive drilling
and careful judgment.

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When the troop goes around the corner,

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the riders on the outside of the turn
have to adjust their speed

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to keep even with the riders on the inside.

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The man on the outside
has to ride a lot further and a lot faster

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in order to keep up with the parade.

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The outside wheels must spin faster
than the wheels on the inside

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because they have a greater distance to travel
in the same length of time.

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When a wagon turns a corner,
the wheels can travel at different speeds

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because each one can turn freely on the axles.

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And in the early automobiles,
the rear wheels turned separately

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and only one wheel
was connected to the engine.

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But when only one wheel
was driven by the engine,

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it had to do all the work,

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and it couldn't get a good enough grip
on the road to do its job properly.

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So the one-wheel drive
was soon out of date.

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But if two wheels are locked on an axle,
so that they are not free to turn separately,

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one or the other has to slide.

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So engineers had to find a way
to connect both rear wheels

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to the engine,
without sliding and slipping on turns.

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The device which makes this possible
is a part of the rear axle.

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It is called the differential,

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because it can drive the rear wheels
at different speeds.

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The differential looks complicated.

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But once we understand its principle,
it is amazingly simple.

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These two wheels are mounted
on separate axles

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and supported by a frame,

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so that they can revolve freely
at different speeds.

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Let's fasten a spoke
on the inner end of each axle.

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So that by turning the spokes,
we can turn each wheel separately.

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With a bar or cross piece,
we can turn both wheels

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in the same direction
at the same rate of speed.

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Let's get something
to hold this bar in place,

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so that it will press against the spokes.

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Notice that this support
is not locked to the axle,

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it turns freely.

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Now, we can spin the wheels
by rotating the support.

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This is fine, as long as both wheels
are able to turn at the same speed.

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But let's see what happens
when we go around the corner.

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With this arrangement, we cannot drive
one wheel faster than the other.

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And if we stop one wheel,
the other wheel won't budge.

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Let put this bar on a pivot,
so that it can swing in either direction.

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Now, the bar can still turn both wheels
at the same speed.

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And, because it pivots,
it lets one wheel turn

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even when the other is stopped.

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But, if turned too far,
the bar will swing around

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until it won't drive the spokes
that turn either wheel.

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We need another crossbar
and more spokes to carry on the job.

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When we stop one wheel,

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the crossbars will continue to push
the spokes of the free wheel around.

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As long as both wheels
are free to turn,

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the bars do not swing on their pivots
and the wheels move at the same speed.

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Now we have the working principles
of a differential.

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To adapt the model for use in an automobile,
we will have to make a few changes.

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In order to reduce the jerky action,
caused by wide spaces between the spokes,

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we will put in more spokes.

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Further filling in the spaces between the spokes
gives steadier, more continuous action.

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And changing the shape
gives firm, constant contact.

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Now we can make the gears
thicker and stronger,

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and we have differential gears.

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The edges are cut so that they will fit together
more smoothly and silently.

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And another gear is added
to share the work of driving the axles.

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The principle is the same.

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In order to turn the support
and drive the wheels,

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we can fasten a large gear here,
connected by a smaller gear

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to a source of power.

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Notice that the power 
is connected to the differential

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at the center line.

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We can make our model more compact
by moving the gears closer together.

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When we put our differential
in an automobile,

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we have to leave room
for the drive shaft,

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which carries the power from the engine.

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We may build the floor of the car
above the drive shaft.

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But, if we do,
we won't have much room inside,

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unless we make the top of the car high too.

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Of course,
 we could lower the floor and ceiling,

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but the drive shaft
would be higher than the floor.

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This would have disadvantages.

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A shaft in the middle of the floor
of an automobile

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would be inconvenient for passengers,
and would be awkward for carrying luggage.

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Today, engineers have found a way
to make the car roomier and closer to the road

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without a clumsy shaft above the floor.

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The drive shaft from the engine to the differential
is lowered out of the way.

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And the drive shaft is connected
to the rear axle at the bottom.

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The new low center drive makes the rear axle
quieter, stronger, and more durable

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because it gives better, smoother contact
between the gears.

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The automobile of today
with the low center drive

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is stronger and more rugged.

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Every part of the rear axle
has been build to withstand strains,

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far greater than it will ever meet
on the straight way

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or around the corner.

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Inside Wheel

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Outside Wheel

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Inside Wheel

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Outside Wheel

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Inside Wheel 

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Outside Wheel