Here’s a secret: sometimes engineers pick an FoS based on “that feels right.” True story. In the 1950s, a designer for a car suspension just doubled the thickness of a spring because “it looked flimsy.” That was his FoS calculation. His car didn’t fall apart, and he got a raise. So if you’re ever stuck, just add 20% more material and call it “engineering intuition.”
But please don’t do that for a bridge. Or a ladder. Or your favorite chair—unless you enjoy a dramatic meeting with the floor.
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When FoS Goes Wild
There’s a famous example from the 1970s: a pair of ski binding manufacturers had an FoS of 0.8 on purpose. They thought, “If the ski doesn’t release easily, you’ll just fall over—no biggie.” That design caused thousands of broken ankles. The fix? Just increase the FoS to 1.5. Suddenly, skiers stopped looking like flamingos on ice. Moral of the story: don’t squeeze your parts in the name of saving plastic.
Also, surprises: the Apollo 13 mission’s FoS for its oxygen tanks was technically 2.0. But a tiny manufacturing defect made it magically become 0.5. That’s how you get the most famous “Houston, we have a problem” in history. So yes, your FoS is only as good as the person who didn’t sneeze while welding.
