You may not realize this, but there’s an elaborate linkage just below the base of your windshield that allows your car’s wipers to move back and forth and, in some cases, up and down—and it is fascinating.

To learn more for our latest episode of David Dissects, I went to a junkyard to find out how three different styles of windshield wipers—pivot, opposed and mono-arm—actually work.

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If you think windshield wipers consist of just boring metal arm shoved at the end of electric motors, you couldn’t be more wrong! Wiper arms are controlled by “wiper transmissions,” intricate linkages that translate the unidirectional rotation of a motor into back and forth motion of wiper arms, which have rubber blades attached to them to clean water and grime from your windscreen.

I’ll let the video do most of the talking, but the idea behind how wiper arms move back and forth deals with using pin joints, linkages and bellcranks to restrict certain degrees of freedom, and allow motion only where you want it.

The primary mechanism for any wiper transmission is an electric motor whose output shaft is a worm gear. That worm gear spins a ring gear seen in the picture above (this gear-mesh multiplies the motor’s torque), which sits on a shaft that goes through the bottom of the casing.

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That shaft has a small metal link bolted to it, so as the motor’s worm gear spins the ring gear, the latter’s shaft actually rotates the link like a propeller blade on an airplane engine:

At the end of that “propeller” is a rod attached to a pin or revolute joint; that One Degree Of Freedom-joint restricts translation of that rod in any direction relative to the attachment at the end of the link, but allows for rotation about a single axis (into the screen in the picture above). What this means is that as the ring gear shaft spins its link, that pushes and pulls the rod, but because that link is free to rotate about its joint, the other end of the rod does not necessarily have to move up and down as the ring gear link rotates—it can just move left and right.

It’s a bit hard to visualize, which is why we have that video, but the main point is that, by restricting certain degrees of freedom, and allowing for others, you can translate unidirectional rotation into translation of wiper arms.

photo: justcarcrazy/YouTube (screengrabs)

As for the different styles: well, you’ve got your typical pivot-style mechanism shown above, which is cheap and simple, though a bit large and heavy, plus you have to use a different system based on whether the car is sold in right or left hand drive markets (you have to flip the direction).

gif: S|D/YouTube (screengrabs)

Then there’s the opposed style wiper system, whose wiper arms pivot about shafts located in the lower corners of the windshield. These systems are great, because they clean a lot of the window (including the corners) and the only part left un-wiped is behind the rearview mirror—a part of the windscreen that you don’t really need to look through anyhow.

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Plus, they can be used in right hand or left hand drive markets. But, they’re rather clunky, complex (seriously, check out this video of it moving in-vehicle) and expensive compared to pivot wipers.

gif: Josh Brinson/YouTube (screengrabs)

My favorite style comes from Mercedes, whose mono-arm wiper transmission is great because it’s a small package, cleans quite a bit of the glass (since the wiper blade actually extends towards the corners as it moves), and it can be used in a number of different markets. But it’s pricey and complex compared to other systems.

So really, like all engineering decisions, it’s a trade-off between packaging space, complexity and cost. But no matter what automakers choose, these wiper transmissions that move wiper arms back and forth to clean our windshields are a lot more interesting than I ever would have guessed.