Science Fun
The DIFFERENTIAL on Autos, Trucks & Trains

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When a car turns a corner or rounds a curve, the wheels on the outside of the curve must travel farther than the wheels on the inside of the curve.  In early autos, this was a problem – how to provide force on wheels that went differing distances and different speeds?  The earliest vehicles provided power to only one wheel.  But later vehicles, and today’s cars, put rotary force on both the inside and outside wheels.  How is this done?  It’s one of the great advances in the history of vehicles – how to provide rotary force to wheels on both the right and left sides of a vehicle that turns corners or curves.

First, you should know where the power of the engine is converted to turning force on the drive wheels.  In most cars more than 8 years old, the force of the engine was transmitted to a series of gears called the differential in a bulbous section of the rear axle.  In more modern vehicles, except trucks and tractors, the differential is somewhere near the center of the front axle.  Regardless of where it’s located, most differentials work in similar ways.  Their job is to transmit equal force to the two driving wheels, whether they are at the front or the rear of the car.  

Imagine that a small gear (we’ll call it gear A) is turned by the car’s engine.  It turns a larger gear, (we’ll call it B) to which are attached  two separate axles on which rotate smaller gears, C, which drag around in a circle gears DL and DR, which turn left and right axles, respectively.  Because left and right axles are turned by separate gears, they can turn at different speeds.  One of the great inventions in cars was the development of this way to turn both left and right wheels independently.  This let cars go around corners, keeping force on both inside and outside wheels.  It is the car’s differential that enables cars to turn corners and still have turning force on each wheel.  The differential was indeed a wonderful invention.  You can usually tell where the differential is in truck axles.  It is a bulge in the center of the rear axle.  In passenger cars it is more difficult to see because it is under the engine, mid-way between left and right drive wheels.  In most modern cars, it is near the middle of the front axle.   Sometime, when you are at a repair garage, ask the mechanic in charge if he can show you where the differential is, and what it looks like.  Be sure to thank the mechanic, because he must interrupt his own work to show you.

But what about the differential in trains?  Trains have right and left wheels, and on curves those wheels must travel different distances.  If they didn’t have a differential, the wheels would slip and screech and wear as the train rounded each curve.  But the wheels do have a differential – a different kind from that of autos and trucks.  If you can find a railroad car on a siding, you may be able to see the differential for yourself.  It is a “hidden” one, but if you know what to look for, you can see it.

On a train, a right and a left wheel and the axle that connects them make up what is called a “truck.”  A train truck is one solid chunk of iron.  When one wheel of a train truck turns, its partner on the other side turns, too.  One cannot turn without the other also turning.  How, then, does a train truck go around a curve without one wheel slipping or skidding?  Look closely at a train “truck” and you can see that each wheel, outside the rim or flange that keeps it on the track, is tapered or sloped.  It is smaller in diameter at the outside edge than it is next to its inside rim.  So when the train rounds a curve, it tends to “hug” the outside of the curve.  The outside wheel of the truck rides where it is largest in diameter – next to its rim.  The rim keeps the wheel from rolling off the track.

On the other side, which is the inside of the curve, the truck rides at its smallest diameter.  So the inside wheel of the truck travels a shorter distance than the outside wheel.  When the track straightens, the wheels will move right or left until both wheels travel the same distance with each rotation of the truck.  But the truck may have to move left and right a little for the wheels to find that position that makes each of them move the same distance with each rotation of the truck.  The squealing and screeching that you hear when a train goes by slowly is mostly the wheels trying to find that position on the track where the wheel on the left side travels the same distance as the wheel on the right side.

Do the wheels wear from seeking a position where left and right wheels travel differing distances?  Yes!  So from time to time a train truck must be returned to a special shop where the tapered part of each wheel is heated and pressed off, and a new “sleeve” with a restored taper is pressed onto the old wheel.  Quite different from changing the rubber tire on a car!

Look for the taper on the outside layer of a train truck.  You may be able to see it on a railroad car or engine at a railroad museum.  Or on a railroad car on a siding where the railroad cars are parked.  But because railroad tracks can be dangerous places, it may be safer and easier to look at model railroads to see if they show the sloping surface (taper) of the wheels on the model cars.  The cheaper ones may not, but the higher quality ones will!  If they don’t, they should, because train wheels wouldn’t last long without that taper!  If you ride a subway, watch for riders who stand, but sway back and forth.  Now you know why they do – it’s because the train’s wheels are sloped to take care of curves and the differing distances that sloping wheels travel on curves.