H/T Mental Floss.
I have noticed these chains but never knew what they were for.
Getting stuck behind a service vehicle like a fire truck, school bus, or ambulance can give you time to think. (If it’s a school bus, it can also give you time to make faces at the children, or ponder the loss of innocence as a 9-year-old gives you the finger.) If you have a car that sits low enough to the ground, you might be able to catch a glimpse of chains hanging from underneath the truck. Why are they there?
Unlike many of life’s disappointing explanations for mysteries, the reason behind them is actually pretty neat. They’re stealth snow chains.
Known as automatic tire chains and sometimes referred to by brand names like Onspot or Insta-Chain, these chains essentially hang listlessly from the vehicle’s suspension until inclement weather arrives. When that happens, a driver can flip a switch that will lower the chains so they hang in front of the back tires. The system makes the chains spin in front of the wheels. When the tires rotate, they catch the chains, greatly improving traction on icy or slick roads. You can see how they deploy and work in the video above.
The automatic chain devices are typically seen on vehicles that need to be able to confront and cope with varying road conditions in a split second. (It’s not practical for a school bus or ambulance to stop so chains can be fitted on the tires.) And while these types of vehicles are often heavy and more easily driven in snow, the chains can assist when they need to drive uphill.
Now that you know, you can get back to taunting the kids staring at you from the back of the bus. Or you can start to wonder why there are tiny black dots on your car’s windows. There’s an explanation for that, too.