The Great Honda Eulogy Part 2: The Seat Belt

Los Alamos

A couple weeks ago my wonderful Honda became crashed. She is no longer with us. So I did what any sensible person would do in this situation – write a small novella and send a chapter off to the local paper every week like it’s the 1800s again.

There is a lot of fascinating engineering that goes into cars that nobody ever thinks about. Materials science comes into play in the blend of plasticizers used to increase the lifespan of the polypropylene plastic used on all modern car interiors. Electrical engineering is used to reduce the amount of wires that carry signals throughout the car with multiplexing (although that didn’t show up until the 1998 model year Accord). Mechanical engineering determines the steel hardness, location, and torque of all bolts in the chassis in order to not fail even when a particularly careless owner loads the spacious trunk with 1,300lbs of gravel and sand for a 200 foot french drain install. It takes a lot of brainpower to create the masterpiece that is a 1996 Honda Accord – not to mention accidentally engineering it to get to 351 thousand miles until it dies through no fault of its own.

Every piece of a car has a quiet history and quite interesting design given the constant pressure from accountants to cut the price as low as possible. One of those pieces I’d like to talk about is the seat belt retractor. You’ve used it every time you get in your car and grumbled at it every time you try to adjust your seat belt while coming to a stop only to find that it locked in place. When the retractor locked for me, it prevented me from slamming my head into the window and getting a concussion.

Early seat belts in the 1940s were of the type you found on airplanes – dangly hip belts – and it was all the rage to smash your head into the steering wheel in the event of an accident. The first patent for a locking retractable seat belt was in 1955, when a system was proposed where the sudden yanking of a seat belt squished a pneumatic bulb that pressurized a locking cam that prevented further extension. It never got off the ground, presumably because the rubber pneumatic bulb couldn’t stand up to the extreme forces found in an accident.

The next attempt at a locking mechanism was in 1956, wherein you had to take the entire length of coiled belt, pass it over you, and buckle it into an anchor near the door. Then, in the event of an accident, a cam would press against the belt, locking it into place. Presumably it also failed because nobody wants to handle 2 pounds of belt every time they get into their car.

In 1963, electricity was discovered and a new style of locking mechanism was invented which used a solenoid to push a lever directly against the belt, locking it into place until current to the solenoid was cut. This was not an ideal design, as during an accident you can’t assume electricity will be there to provide a safety feature that we nowadays see as rudimentary.

1973 saw the first modern self-locking seat belt retractor, one that both used a purely mechanical acceleration sensor to detect sudden changes in travel and offloaded the stopping force onto a ratchet mechanism instead of pressing a cam directly onto the belt. This is important because the webbing of a seat belt is designed for tensile strength, not compressive – instead of having a cam press directly onto a belt and weakening it, the offloading of a severe tension shock onto a ratchet wheel allows the belt take the brunt of that tension. There have been few substantial improvements to this design, so it represents the basis of all seat belt retractors in use today.

The next time you get frustrated at your seat belt because you want to adjust it while coming to a stop and it locks up on you, think about all the engineering that went into that dumb, overlooked feature. A simple spool of belt that’s designed to work for more than twenty years with no maintenance, able to lock up at a moment’s notice with no electricity, and can withstand several times your body weight to prevent you from slamming into your steering wheel like everyone was doing in the 40s.

My Accord’s driver side seat belt retractor saved me from a rather annoying trip to the ER, and it did so thanks to over half a century of overlooked engineering – found on even the most milquetoast of vehicles.