If you have ever owned a matchbook car, you know that each pair of wheels are connected by an axle. In small toys, the axle is often a steel wire about the diameter of a paper clip.
But in a road car, connecting the wheels with a straight, rigid axle yields a terrible driving experience. Here is a fascinating video on the design and evolution of a steering differential. That’s the gear system that connects a drive shaft from the transmission to a split axle—allowing powered wheels to rotate at different speeds. This is necessary to accommodate turns and uneven terrain.
Without a differential, your tires won’t last long on dry pavement and they will wobble and feather during sharp turns. After all, the outside wheel is cornering a larger radius and so, it wants to turn at a higher speed.
With the exception of 3-wheelers, like the Polaris Slingshot) or the Campagna T-Rex (photo), differentials have been used on internal combustion autos, even in the 19th century. The mechanism was known to the ancient Greeks, and patented for use in steam-powered cars in 1827—long before gasoline engines. Even the Ford Model T had a differential gearbox. But not every 4-wheeled vehicle has a differential…
One Wheel and Rear-Wheel Drive
So, how do go karts, motorized toys and some very cheap cars deal with the need for opposing wheels to spin at different speeds? Answer: The engine is shunted to just one rear wheel, often using a chain drive. This leaves the other wheel free to spin at whatever speed is necessary. It also leaves the front wheels free to navigate turns without the complicated task of sending power through a steering linkage. Because the front wheels were not powered, they simply rotate freely on independent roller bearings at the end of a straight axle.
But one-wheel power lacks traction and skid control. And it becomes dangerous whenever one wheel has less grip on the terrain. That’s where a differential comes into play. Furthermore, rear-wheel drive tends to shove a vehicle into each bump and hill rather than pulling it up and over obstacles. In the past 40 years, most cars have been redesigned to power the front wheels rather than the rear wheels. This not only improves handling, it helps in snow and on uneven terrain.
Differential Design: Simple, elegant & efficient
This video was filmed in the 1937, as evidenced by a vintage, pre-war Chevy sedan. Although this educational film it was made more than 100 years after the differential was invented, I wonder how many additional innovations have been added since?
This video explains a differential. Set time to far left to add
3 minute preface explaining the problem with straight axles.
I’m not in the business of teaching auto mechanics. But if you have caught the bug, check out these additional drive train principles —