Ever wonder why some cars have rear wipers and others do not? This is one of the endless stream of questions my daughter throws at me while I am chauffeuring her around the city.
With this question in mind, my own survey of vehicles seemed to indicate that rear window wipers are strongly correlated with a vehicle having some kind of rear door, such as SUVs and Hatchbacks. Cars with trunks do not seem to have them. I don’t have the statistics on this, but go ahead and test it out for yourself.
Going with the Flow
If you agree that this phenomenon is a thing, what might the explanation be? Aerodynamics seems to be the key. SUVs are not much different from a large brick in a wind tunnel. They leave a lower pressure zone behind them, so that the air flow forms eddies and junk gets stuck to the back end of the vehicle. This results in greater drag, a major reason why SUVs have poorer gas mileage. And it’s also why you need a rear wiper. The rear end of our formerly white SUV is by far the muddiest. I try not to drive it that often.
Bring Up the Rear
Why don’t they have rear wipers on sedans then? The rear of the car is more tapered and the flow over the windows remains laminar, so not as much should stick on there. It might still be useful if you had stuff fall or slide down the window, like snow or bird poop. But a rear wiper might disrupt the air flow and be less durable over time, especially if driving at higher speeds.