foreign
so
formations like these require long hours of intensive drilling and careful judgment
when the troop goes around the corner the riders on the outside of the turn have
to adjust their speed to keep even with the riders on the inside the man on the
outside has to ride a lot farther and a lot faster in order to keep up with the parade
the outside wheels must spin faster than the wheels on the inside because they have
a greater distance to travel in the same length of time when a wagon turns a corner
the wheels can travel at different speeds because each one can turn freely on the axles and in the
early automobiles the rear wheels turn separately and only one wheel was connected to the engine
but when only one wheel was driven by the engine it had to do all the work and it couldn't get
a good enough grip on the road to do its job properly so the one wheel drive was soon out
of date but if two wheels are locked on an axle so that they are not free to turn separately one
or the other has to slide so engineers had to find a way to connect both rear wheels to
the engine without sliding and slipping on turns the device which makes this possible
is a part of the rear axle it is called the differential because it can drive the rear wheels
at different speeds the differential looks complicated but once we understand its principle
it is amazingly simple these two wheels are mounted on separate axles and supported by a frame
so that they can revolve freely at different speeds let's fasten a spoke on the inner
end of each axle so that by turning the spokes we can turn each wheel separately
with a bar or cross piece we can turn both wheels in the same direction
at the same rate of speed let's get something to hold this bar in place
so that it will press against the spokes notice that this support is not locked to the axle
it turns freely now we can spin the wheels by rotating the support
this is fine as long as both wheels are able to turn at the same speed but let's see what happens
when we go around the corner with this arrangement we cannot drive one wheel faster than the other
and if we stop one wheel the other wheel won't budge
let's put this bar on a pivot so that it can swing in either direction
now the bar can still turn both wheels at the same speed and because it pivots
it lets one wheel turn even when the other is stopped but
if turned too far the bar will swing around until it won't drive the spokes that turn either wheel
we need another crossbar and more spokes to carry on the job when we stop one wheel the crossbars
will continue to push the spokes of the freewheel around as long as both wheels are free to turn
the bars do not swing on their pivot and the wheels move at the same speed
now we have the working principles of a differential
to adapt the model for use in an automobile we will have to make a few changes in order to
reduce the jerky action caused by wide spaces between the spokes we will put in more spokes
further filling in the spaces between the spokes gives steadier more continuous action and changing
the shape gives firm constant contact now we can make the gears thicker and stronger and we have
differential gears the edges are cut so that they will fit together more smoothly and silently and
another gear is added to share the work of driving the axles the principle is the same
in order to turn the support and drive the wheels we can fasten a large gear here
connected by a smaller gear to a source of power
notice that the power is connected to the differential at the center line
we can make our model more compact by moving the gears closer together
when we put our differential in an automobile we have to leave room for the drive shaft
which carries the power from the engine we may build the floor of the car above the drive shaft
but if we do we won't have much room inside unless we make the top of the car high too
of course we could lower the floor and ceiling but the drive shaft would be higher than the floor
this would have disadvantages a shaft in the middle of the floor of an automobile
would be inconvenient for passengers and would be awkward for carrying luggage today engineers have
found a way to make the car roomier and closer to the road without a clumsy shaft above the floor
the drive shaft from the engine to the differential is lowered out of the way and
the drive shaft is connected to the rear axle at the bottom the new low center drive makes the rear
axle quieter stronger and more durable because it gives better smoother contact between the gears
the automobile of today with the low center drive is stronger and more rugged
every part of the rear axle has been built to withstand strains
far greater than it will ever meet on the straightaway or around the corner
you