How a car differential works

Image for article titled This vintage film is still the best explanation of how a differential works

How does a car’s differential work? You probably have a vague understanding. Some things turn, some don’t, and somehow the result allows your car to roll through a curve without shredding its tires or chewing its guts. There are cogs and other cogs, and it’s basically magic. It was the extent of my own understanding, until I found this vintage educational film from 1937. This is still the best, simplest and most immediately understandable explanation of how a differential works that I have ever found.

I love this old black and white movie so much, I wrote about this some time ago to These guys. It’s worth reintroducing it to a new audience, because even if you’ve held the innards of a disassembled diff in your hands, you might still struggle to explain how this collection of ring gears, sprockets and spiders feeds the ground, and allows two wheels on the same axle to turn at different speeds.

Our instructor in this lesson is Henry Jamison”Jam” Handy, a fascinating character who has spent much of his career making educational films like this. Most of the time these were cleverly disguised advertisements – most of Handy’s automotive films were made at the request of General Motors, while others were sponsored by Standard Oil. Whatever the topic, a Jam Handy movie starts with a simple question of “how X works” and answers it with clear, intelligent, and immediately understandable visual aids. Typically, the last minute of the film is when it becomes an advertisement – for example, by taking advantage of the latest technological advances that you will find at your friendly neighborhood Chevrolet dealership. But everything leading up to this brief sales pitch is general-interest, brand-independent knowledge that is absolutely fascinating for car enthusiasts or anyone with an inquisitive mind.

So! Back to the differentials. Naturally, this 1937 film doesn’t delve into the voodoo of limited-slip differentials – we’re only talking single-tyre open differentials here. But the visuals and explanation are so simple a kid could grasp it.

So take 10 minutes and learn the inner workings of something you’ve always wondered about. Even if you already know the ins and outs of differentials, I promise this movie will give you a new perspective on how it all works.

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