How do Seatbelts Work
Seatbelts save many thousands of lives across the world every year. Understanding how they work is not something most of us ever worry about. Which is fine, as long as one important, and highly technical fact is know: They only work when they are worn!
Now, assuming that you have heeded this piece of wisdom, lets take a look at how that clever piece of grey material is going to save your life in case of an accident.
To understand why we need seatbelts, we first need to take a little trip down memory lane to 10th grade physics. How many of you remember what "Inertia" is? Hands up? OK, well just for fun let's recap. Inertia is the tendency of everything to keep moving in the direction it is going, and at the same speed, until something exerts a force that changes the direction or speed. A car experiences inertia as it drives along the highway at 40mph. So too does everything, and everyone, in the car. When you are traveling at 40mph inside your car, it feels like you are moving as a single unit, but in fact your body has its own inertia to deal with. What this means is that if the car hits a tree, the car comes to a sudden halt. Your body however carries on moving forwards at 40mph, until something stops it. This is why if you are not wearing your seatbelt you may be catapulted into the glass.
If the force that stops your movement is the windshield meeting your head, your skull is going to take a great deal of force as the glass shatters. Seatbelts are designed to stop your movement by spreading that stopping force over a great area, thus reducing the intensity in any one place, and most importantly, applying the stopping force over areas of your body that are more able to deal with the pressure, such as your Pelvis and Rib Cage, which are far sturdier than your head.
In addition, the fabric of seatbelt, that webbing that is used in almost all cases, is designed to give slightly. Just enough to reduce the abruptness of the stop ever so slightly. This is why seatbelts are not made of chains (in case you were wondering!), or elastic bands. It is necessary for the material to give just a little, but not so much that you hit the steering wheel or the seat in front of you.
A car also has crumple zones that are designed to fold on impact, again reducing the suddenness of the stop experienced by the passenger section of the car. By spreading the stopping force over a larger area of your body, and a longer stopping time (from the webbing fabric and the crumple zones) the effects of the inertia are reduced and you are often saved from the unfortunate effects of continuing to move forwards after your car has stopped suddenly.
Now, to get technical about how seatbelts exactly work will need someone with more engineering expertise than I possess. However, the brief explanation is presented here!
Seatbelts are designed to move, and be flexible, allowing the passenger to lean forward and be comfortable while on the move. In a sudden impact or halt, the seatbelt ceases to be flexible and fixes in place, holding the person safely in their seat. There are two ways this is achieved. 1. Triggered by the movement of the car, or 2) Triggered by movement of the belt itself. In both cases, the spool on which the seatbelt moves freely under normal circumstances is locked in place when a sudden jerk engages a ratchet that holds the spool firmly.
In modern cars, there may also be an additional mechanism called a pre-tensioner. This device actively pulls the seatbelt back in, and thus moves the passenger further back into their seat at the time of impact, in an attempt to hold them in the safest position to survive a crash. A pre-tensioner is often wired to the same system as the cars airbag, and is triggered at the same time. Finally, some seatbelts are fitted with Load Limiters, which aim to reduce the incidence of seat-belted related injuries. In severe crashes the pressure which the seatbelt exerts on the body can cause injury, so load limiters allow the belt to give a little more than normal, to reduce the amount of pressure experienced by the body from the belt. The easiest example of this might be a small fold sewn into the fabric of the belt. The stitches would be designed to withhold a certain pressure, and after that pressure to break. In a severe crash, the stitches would break allowing slightly more "give" in the belt, and reducing the abruptness of the stopping force again.
After all of this somewhat technical look at how seatbelts work, the very first statement still holds true. If you do not wear them, then they will not work. Next time you jump in the car for a three-minute journey and figure it is not worth the bother of putting your belt on, just remember what you have learned about inertia and stopping pressures. You really do not want that kind of pressure applied to the top of your head - Put your seat belt on!
