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
Generally yes. A product’s advertised maximum weight is not the weight at which is will completely collapse. How much margin is left depends on what the product is and how dangerous a failure could be. Things like lightweight hooks don’t need much of a margin since they aren’t holding anything heavy enough to be dangerous. Safety gear such as climbing ropes and rescue equipment often have a 3-to-1 margin meaning they can actually hold 3x what they are not to exceed in a safety operation. The air bottle fires fights use for example, usually get filled up to 4,500 PSI and are testing at more like 15,000 PSI.
Those calculations are made first in theory. Material strength is its own sub-field where you determine how strong something will be based on the materials used. Then once it’s built it gets tested in the real world if possible. And example of this would be a crash test for a car. In more permanent structure like a building, you can’t really test the thing until it’s built so again, a lot of margin is factored in and institutional knowledge is relied on.
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”
I am a licensed structural engineer so I like to think I can shed some light on this. I work primarily on residential projects so I’m going to give some examples of that.
Let’s say you have an uncooked spaghetti noodle that you’re holding. If you press at both of the ends, then it’s going to bend and buckle a little. If you Press enough it’ll snap. Now instead of pressing at the ends, just hold it flat and have a friend press straight down. Again, it’ll bend until you press it enough and it’ll break.
This is similar to how the posts/studs in your walls and the beams/joists in your floor work. Now sometimes we have spaghetti noodles that are much stronger, such as steel or engineered lumber. These can resist a lot more force before they bend too much or break. How do we know how much they can resist? There has been a LOT of testing done for products before they are accepted by the government, and there are guidelines for each of wood (NDS), concrete (ACI) and steel (AISC) all of which have loads of empirical data (meaning actual tests, not just math) for almost everything you can imagine.
So we use those, along with math, along with a little bit of creativity when required to make sure that your house won’t fall down.
There’s also wind and seismic but those are a lot more complicated so I won’t get into that for now. Hope that helps!
Build the structure, stack weights on structure until it collapses, record weight, rebuild structure. Easy.
Your assumption is not wrong, except that instead of testing the “bike”, engineers tested standard geometries using different materials and the engineering community keeps a huge database of those tests. By making use of failure theories, they can predict how desired shapes will behave under different loads and boundary conditions. Naturally, this is the start of the design process.. so it is not trying and error, in the case of a bicycle they will test the end product to validate their calculations, but in the case of a bridge for example, a direct test may not be feasible.
I recommend the following video
you figure out from the material properties, and dimensions, and pick a range for what loads it can handle. then you use a safety factor of 2-4x the breaking point.
materials like steel & aluminium have well know strength’s that allow us to estimate how much force it takes to bend, or break a specific piece of material.
People here are correct but it’s worth noting that the way these computer programs or tables people can use know how much weight a material with a given cross section can take is that an enormous number of tests have been done on that material to see how much force it can take before deforming.
So in a sense, yes. An engineer knows how much weight will deform each individual component of the bike from destructive testing and can then design based on that.
That is knows as the Calvin’s Dad method (drive heavy trucks over a bridge until it collapses then rebuild it). No, engineers do not use that method. Well geotechnical engineers sometimes do, especially for pile load tests, but we all make fun of them. Silly soil guys and their nonhomogeneous semi-infinite half-space.
There is an ASTM code for design of exercise equipment that determines allowable user weight.
As an engineer, basically the process is the following steps:
1. Draw out your system or product in a force diagram, here you make assumptions on where the force is applied and how it cascades through the system.
2. Figure out what the forces are through out the system, this can be done by hand but software is mainly used FEA is the tool( finite element analysis)
3. Take you force results, typically this is stress strain values to find your weak point.
4 adjust your design/ material until it shows that it can withstand the forces applied.
5. Apply the Safty factors and readjust your design. And then boom your done!
For life time of a product you can look at S-N curves that can give predictions on life time vs cycles.
That’s basically it though.
A lot of engineering is about testing things and seeing what breaks. We then turn that into math and calculations (in general, it’s not one engineering doing this every time they design something, it’s based on decades or centuries of data and work). We take that data, usually conveniently published in books or other resources, and figure out how much weight a structure could hold. We then use a safety factor (in civil we typically use anywhere from 1.25 to 3.0 depending on what we’re doing) to give the rating.
For example, if all our data and math says a piece of circular metal with x inner radius and y outer radius holds 500 pouinds, and if our factor of safety was 2 (I don’t know if it would be, that’s not my area, just a convenient number), then we’d say the maximum load would be 250 pounds.
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