Example: An exercise bike that can support 250 lb. Does the engineer find a large enough weight that deforms the bike, then take a fraction of that as the amount it can support?
In: Engineering
One minor note – that 250 lb rating accounts for how the bike gets loaded *in use*. That means how much force is on it e.g. when you hit a pothole while pedaling as hard as you can at 25+mph (…if it’s a regular bike, of course).
With something like a bike (any product that you will mass produce, vs something like a bridge), you also do what’s called “verification testing” before you start production.
Context: when you start to design something, you first draft a list of requirements. Things like “must support rider weight up to 250 lbs” and “seat height must be adjustable between X mm and Y mm from bottom bracket” and whatever other critical functions you want the bike to have, that if it doesn’t have you’d consider it to not meet your expectations. Then you design it to meet that list of requirements. (This is where the engineers do all the math other people described. But an engineer experienced with designing a bike will already know the rule of thumb to use as a starting point for how to design it, then refine the design with math.)
After you have a design and build some prototypes, you figure out the production process. Maybe some features are too expensive to manufacture so you change the design.
After you nail down your production process and the design, you test it to make sure it meets all the requirements. This is the “design verification” test. That means durability testing (how many hours/miles can you travel before certain components break?) and discrete requirements testing (does the seat height adjusted within the desired range?). What this means is that you *can* skip all the math, if you just build it stronger than it needs – as long as it meets the strength requirement then it’s good. But if you add a “bike must weigh less than N lbs” requirement then maybe you have to bust out the math, to figure out where you can make it weaker/etc.
You could still do trial-and-error to tweak the design until it meets all the requirements, but good engineering tools let us bypass a lot of that with computer simulations, and be pretty confident that your first prototypes will be built to spec.
So, sometimes weight ratings are just “we spec’ed it to support X lbs, and it passed the Design Verification test to prove that it supports X lbs”
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