When you’re driving, it’s rare to come across a van or lorry on the road without seeing the back of the vehicle covered in dirt and grime. In fact, the traditional white workman’s van is often synonymous as a target of light-hearted graffiti, carved into the filth on the back.
Thinking intuitively, it would seem that there’s no reason for the grime to collect on the rear of the vehicle – surely if it were to collect anywhere, it would collect on the front?
Stirring It Up
It’s no coincidence that these larger vehicles end up with dirtier backs – the main culprit is simple aerodynamics.
As the van is carving its way down the motorway, the air in its path is flowing smoothly over the relatively curved, aerodynamic front as well as the top and sides. But when it gets to the rear, rather than a smooth curve back down, there’s suddenly a sheer drop and a huge gap.
Negative pressure allows dirt to accumulate
Since the air pressure in the gap is lower than the area around it, the ‘negative pressure’ pulls air into the gap, which begins swirling around the back of the van. At high speeds this creates a vortex, which sucks in all the nasty dirt, grime and fumes of the road and dumps them all over the back doors of the van.
The lack of airflow over the rear of the van also means there’s no force to dislodge that grime – unlike a saloon car for example, where excess water is driven off of the boot and rear windscreen as the airflow passes smoothly across the surface.
It’s A Drag
This nasty air turbulence isn’t just a cleanliness issue – it also slows the van down, lowering it’s fuel efficiency. This is the reason you’ll occasionally see vans with spoilers, which are designed to ‘spoil’ this turbulence and smoothen the airflow.
Original and witty comments welcome!
And it’s not just a problem for vans and lorries. SUVs and even hatchback cars generate the same vortex when travelling at high speeds, reducing rear window visibility – so if you’re a saloon owner, thank your lucky stars that yours doesn’t need so many trips to the car wash. For hatchbacks and estate cars, it’s important to keep your wiper blades up to standard so you can maintain your rear visibility.
So how come saloon cars don’t suffer from the dirty rear window problem? Check out our next blog for the answer.
Author: ABD.co.uk
I searched up why do minivan windows get dirt not this
Hi Shawn,
Minivan windows will get dirty for the same reasons as detailed above. It’s all due to the airflows across the vehicle that pushes the dirt into the windows.