In the old days, cars could be called ‘one wheel drive’ vehicles since a car’s engine powered just one of its four wheels. The problem was that in such cars, if the powered wheel ended up in a place with low or no traction, say because of snow or ice or mud or dangling over a ditch, you were literally stuck. The development of two-wheel drive vehicles that sent power to the two rear wheels improved this situation since if one wheel lost traction, the other could pull you out of it. But this created a new problem in that when you turn a corner, the outer wheel on the axle has to rotate faster than the inner wheel since it traverses a circle of larger radius.
It was to solve this problem that the differential was invented that enabled two wheels connected to the same axle to both receive power and yet rotate at different rates. Via Mark Frauenfelder, I came across this 1937 short video that gives a remarkably lucid explanation for how the differential works its magic. The first part is a long set up involving some synchronized motorcycle riding and the real explanation starts just after the 1:50 mark
With four-wheel drive vehicles (sometimes referred to as 4×4) power is sent to all four wheels and there is a differential for each of the front and rear axles. This causes some jerkiness when taking corners on dry surfaces since the front wheels travel slightly longer distances than the rear wheels and thus the four-wheel drive should be engaged only when necessary. The newer all-wheel drive cars have an additional differential between the two axles to take care of that problem.
So we have come full circle, since the four-wheel or all-wheel drive cars, like the old one-wheel drive cars, can move even when only one wheel has traction. But the major difference is of course that with the new cars, any wheel can serve that purpose.
Differentials are expensive. When it comes to trains, they have many axles, each one having two wheels rigidly connected to it, and no differentials. So how do they take corners? The ingenious solution to that lies in the design of the metal wheels that have a slight conical shape. This article explains how they work and below is Richard Feynman’s explanation of how it works, that I linked to four years ago.
This solution for trains only works for a limited range of curves which is why the curves on train tracks have to be much more gentle than for roads.
These are quite ingenious solutions.
Engineers. You gotta love ’em.