How does the apparently small amount of material which comprises the spokes provide the required sturdiness to handle the physics of a human on a bike?
In: 136
Have you seen the London eye? Spokes don’t have to be thick. In fact they don’t even have to be rigid. There’s a company that makes flexible bike spokes. See the thing some people don’t realize about them is that the support is not in compression, it’s in tension. The hub is hanging from upper side of the wheel rim, and the strength of the rim transfers the weight to the ground.
The axel is actually hanging from the rim by the spoke which allows them to be thin. Look at spokes more like wires, youre using their Tensile strength not compressive strength.
Tension.
None of those bars is ever pushed, even resting they’re stretched. When someone puts weight on the wheel, the spokes at the bottom see less tension, and the ones at the top see more, which effectively means that the center is hanging from the rim while all the spokes on the sides and bottom are keeping the rim circular.
Actually, let me address it another way-
Inline bikes are fundamentally different that 4-wheeled vehicles in that they handle VERY low side forces. When you lean into a turn, the force vector mostly follows the lean.
BMX bikes are different, though- people like to do tricks where they land hard at an angle and orient their body to provide torque on the handlebars and pedals to offset the impact. That produces large side loads, and the lighter spoked wheels start to fail.
4 wheeled vehicles’ wheels, however, do not “lean into” the curve. There is much more side load potential that lighter wheels cannot tolerate. Bonus, though- motorcycle and bike wheels must be round-bottomed so the contact patch can shift around. 4 wheel vehicle tires are flat-bottomed because they don’t have to lean. Flat bottom gives a much larger contact patch and much better traction in general.
The strength isn’t actually in the spokes. It’s the rim, it has enormous compression strength but only when pretensioned by the spokes.
If you take the rim without its spokes and lean on it, it will collapse pretty easily.
We can put MUCH more tension on a thin steel rod than compression. In tension, the spoke can take a hundred pounds with no perceivable stretch. But if you compress it, a couple of pounds and the middle is bend out and the endpoints are inches closer to another.
So notice how the sides of the bare rim bulge out as you lean on it, a bit before it buckles? What if we tie the two sides through the middle with 2 horizontal spokes? Actually that significantly increases the force required to buckle the rim, but it next starts bending a bit above and below those 2 spokes. So, spokes are light and cheap, let’s cover those, and cover all the rim.
With the rim’s shape held in place by rigid, high tension spokes, the rim’s compression strength is now stronger by orders of magnitude.
So, the rider’s weight is all pushing down on the axle. The force is in tension across the top half of the rim. The sides are held in place to avoid buckling out.
Now the bottom spokes are a bit more complicated. The rider’s weight is borne through tension on the spokes in the top half, then put the entire rim in compression. But the spokes on bottom are pretensioned so much that they don’t go into compression even when fully loaded. If it went into compression, that would be apparent as the spokes would bow outward
A lot of people think that bicycle tires are like wagon wheels because they have spokes and rims and something connecting them both. But bicycle wheels are actually pulled tight by the spokes to suspend in the center of the rim. So the transfer of forces is actually a bit counter intuitive:
If someone that is too heavy for the wheels gets on the bike, gravity will cause their weight to press down on the bike frame.
The bike frame will press down on the axle.
The axle will pull down on the UPPER spokes.
The Rim will pull back on the spokes to keep the spoke in place.
One final explanation is that if one were to use a machine to press the axle down until the wheel failed, the bottom spokes would bend easily and either the rim would need to bend or the upper spokes would need to break like guitar strings in order for the axle to move down.
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Many Eli5 posts end up being answers I would expect to read, or may already have some idea about the general subject. I read the top few comments to verify if the information I already knew is correct and if there are any added details I did not know or was misunderstanding or confused about.
This is the first Eli5 in quite a while which I had never thought to ask the question and had no idea about the answer. So thank you OP for asking the question, and thank you random strangers for giving incredible Eli5 answers.
this seminal book on bicycle wheels by the late great Jobst Brandt is – or was, at least, when i was getting started – required reading for bicycle mechanics: https://www.scribd.com/doc/261771508/The-Bicycle-Wheel-Jobst-Brandt-3rd-Ed
spokes arent in compression, they are in tention, the ones actually holding your weight are at the top of the wheel, all the rest at that moment in time are there to keep the wheel circular. infact tension is so important there is someone making string based bicycle [spokes](https://www.youtube.com/watch?v=XTSJ2gmrkpo)
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