Three-Point Seat Belt

Every test here uses a jointed model figure on a rolling sled. Never restrain a person in a home-made belt and never test on a real vehicle. Real belts are certified, load-rated components.

Bohlin anchors to the vehicle frame, not the seat, and describes a "continuous sling" giving "physiologically favourable" restraint. The word physiological is doing real work.

Make a small rolling platform with a seat back, and a ramp ending in a fixed stop. Every run starts from the same height so every deceleration is the same.

Build a figure with a hinged hip and neck and a weighted head. The hinges matter — a solid block cannot show you what a torso does.

Release the sled and let it hit the stop. The dummy leaves the seat entirely. Mark the impact point on the ramp — this is the baseline.

Strap a single band across the hips to two floor anchors. Run the sled again and watch what the upper body does.

The pelvis stays, the torso folds forward over the belt, the head still travels a long way. Measure how far. The lap belt has restrained a third of the problem.

Remove the lap section and fit only a shoulder diagonal. Run it. The dummy submarines — it slides forward underneath the strap, hips first.

Run a single length from a shoulder anchor, diagonally across the chest, down to a buckle at the hip, and across to the far floor anchor. Three anchors, one strap.

Same release height. The dummy is held at both the pelvis and the chest, and the head moves a fraction of the distance. Put the three head-travel numbers side by side.

Trace the diagonal on the dummy. It should cross the collarbone and sternum, not the neck, and the lap section should sit low across the pelvis, not the abdomen. Adjust the shoulder anchor height until it does.

Move the shoulder anchor up so the strap crosses the throat, and run it. Note that geometry, not webbing strength, decides whether a belt protects or injures.

Attach both lower ends to the seat and repeat. The seat itself moves, and the restraint slackens. Bohlin anchors to the frame for exactly this reason.

Fasten and release it repeatedly with one hand only. Anything slower than a second or two, or needing two hands, will not be used every trip — and an unworn belt protects nobody.

The patent. US 3,043,625, "Safety belt", filed 17 August 1959 and granted 10 July 1962 to Nils Ivar Bohlin, assigned to Volvo AB. Bohlin had come from Saab, where he worked on ejection seats — an unusually good background for the problem, because an ejection seat harness must restrain a body against enormous forces and be operable instantly by someone under stress.

🔴 This is the seat belt that the Claghorn patent elsewhere in this collection is not. US 312,085 of 1885, titled "Safety Belt", is routinely cited as the first automotive seat belt; it is a work-at-height harness for painters, sailors and telegraph linemen, with a friction hook for rope descent, from a period with essentially no automobiles. Bohlin's is the real thing, and the two make a clean pair: same words, seventy-seven years apart, entirely different objects.

Volvo made the patent freely available, and that is the remarkable part. Holding a patent on a device that measurably prevents death creates an obvious conflict, and Volvo resolved it by opening the design to other manufacturers rather than licensing it for profit. The three-point belt spread across the industry as a result, and is commonly credited with saving over a million lives. It is worth sitting with as a counter-example to most of this programme, where the interesting question is usually who got paid.

Why three points and not two. Steps 6 to 10 are the entire argument, and they can be run in an afternoon. A lap belt alone leaves the torso free to jackknife; a diagonal alone lets the body submarine underneath it. Only the combination restrains the pelvis and the chest together — and Bohlin's insight was to do it with one continuous strap and one buckle, so that the whole thing goes on in a single movement. Earlier four-point harnesses restrained beautifully and were used by almost nobody, because they took two hands and thirty seconds. Compliance is a design parameter, and step 14 measures it.

Loading bone, not tissue. The patent's word is "physiologically favourable", and it means the strap must cross structures that tolerate concentrated load — pelvis, ribcage, clavicle — and avoid the abdomen and neck. Step 12 shows how quickly correct geometry becomes dangerous geometry. This is why modern cars have height-adjustable upper anchors, and why a belt worn under the arm or behind the back is worse than useless.