The Bicycle (and More) Explained

They say a picture is worth a thousand words, but an animation, then, must be worth a million. Make that animation interactive, and… well, we don’t know how many words it is worth, but it is plenty! That’s the idea behind [Bartosz Ciechanowski’s] blog where he uses clever interactive animations to explain the surprisingly complex physics of riding a bicycle.

The first animation lets you view a rider from any angle and control the rider’s pose. Later ones show you how forces act on the rider and bicycle, starting with example wooden boxes and working back up to the original bike rider with force vectors visible. As you move the rider or the bike, the arrows show you the direction and magnitude of force.

You eventually graduate to close-ups of the tires, the handlebars, and the sprocket. These kinds of animations are to physics what an oscilloscope is to electronics. You don’t have to have a scope to understand electronics, but it sure does help.

Once you are done with the bike — and that’s going to take some time — click on “Archives” and see similar posts on mechanical watches, GPS, internal combustion engines, and more. Really great stuff.

There was a time when beautiful educational animations were very expensive to create. They are easier now, but we still don’t see them as often as we’d like for topics like Fourier transforms.

25 thoughts on “The Bicycle (and More) Explained

  1. There’s one mistake in the demo simulations:

    Two boxes get pushed at equal forces, but at different distance from the center of mass. One box starts spinning faster, but both have equal linear acceleration; this is wrong: the box that spins faster should accelerate slower because of is angular moment of inertia. Giving it more angular acceleration should reduce from the linear acceleration. The force is effectively divided between the two: accelerating it around and accelerating it in a straight line – otherwise you get energy out of nothing.

    Imagine a weight that is dropped just off the side of a table, and another equal weight that is just touching the table by its edge when it’s dropped so that it picks up a spin. Which one reaches the ground first? Both experience the same G force, but the one that spins falls behind because it has to pick up angular momentum to clear the table.

      1. If there is ideal transfer of kinetic energy from the bullet to the block, then from an energy balance point the spinning block cannot go as high because some of the energy was stored in the rotating motion. That’s just basic physics and conservation of energy.

        However, the transfer of energy is not ideal and it’s an impedance matching problem, so the results can vary, or, the difference is too small to measure in that setup. After all, the block doesn’t spin all that fast so it only ends up with a tiny amount of energy.

        To lift a 100 gram block of wood up 1 meters you need 1 Joules of energy. To make it spin at 10 Hz, assuming the block of wood is equivalent to a 20 cm wide disc, is also 1 Joules. A small pistol shot is like 150+ Joules, so there’s 100 times more energy going “somewhere” in either case.

        1. And sure enough, if you pause the video at 0:43 with the red vertical line drawn over, the block that doesn’t spin does end up with its center of mass (the nail) ever so slightly higher at the top of the travel.

    1. The simulation is correct; your assumptions are not. The force is equal in both cases, not the energy. The extra energy in the rotating case comes from the fact that the force is acting upon a part of the body which is moving faster in the direction of the force therefore, since work = force • displacement, the larger displacement means more work gets done, thereby imparting more energy.

  2. What I want to know most about bicycles is why some frame geometries feel a hell of a lot easier for riding in urban space, one example is pre-1970 Raleigh Roadsters, feels like you can get them up to about 20mph real easy and just cruise at around that speed for miles. Whereas I guess a racing frame is more efficient to go faster, but you’re down in a tuck you can’t appreciate your environment from and also seems to involve a lot of side to side weaving to get up to speed rapidly, where the roadster doesn’t. Most later flat handlebar roadbikes seem to be racer compromises, as do most mountain hybrids. I think a lot of European standard type bikes from the low countries have similar frames that are very pleasant for upright riding, but then they go and spoil them with coaster brakes. Anyway, feels like something has been lost in frame design for utilitarian and lycra not wanted or required cycling.

    1. Very very few bikes in the Netherlands have coaster brakes and I don’t think they’re very common at all in bicycles for adults in western Europe.

      There’s a lot written about bicycle frame geometry and why some designs fit better for MTB, road-racing, etc. But the gist of it boils down to “nobody really knows precisely why bikes work in the first place” and that then leads into nobody being able to define exactly why some geometry is better than others. Mountainbikes have been getting slacker and slacker headtube angles and steeper seat tube angles in recent years. Until a few years ago “everybody” “knew” that a very slack (below 71 or so degrees) head tube would make a bike “lazy” and impossible to steer. Now everybody is riding 67 degrees and some downhill bikes are even going to 65. Road bikes have been getting a little slacker too but not much. However they went with a much more downward posture with the seat often slightly above the top of the bars. The earlier bikes are far less “head down”.

      The reason for the ‘cramped’ head down posture on road-racing bikes is aerodynamics. It gives a much smoother aerodynamics profile while not loosing too much in efficiency. But an upright posture is slightly more efficient when accelerating and allows for putting power down more easily, while the upright posture allows easier breathing and better cooling. So it just feels more comfortable and for short, relatively slow rides in the urban environment you don’t need the aero advantages of dropped bars. If bicycles are going to get more widely adopted in the rest of the world, the “standard” image of the lycra clad speed-demon needs to get dropped.

      1. Pretty much every bike with hub gears has a coaster brake, whereas derailleur gear bikes almost universally don’t. People prefer them on casual city bikes because rim brakes tend to squeal horribly when wet, and stop working, whereas coaster brakes pretty much just always work.

        1. And of course disc brakes would be the best, but they cost more, and the usual 7-speed Shimano hub comes with a coaster brake anyways. More expensive hub gears can be fitted with discs. Arguably, since the rear wheel lifts under heavy braking, the coaster brake acts a bit like ABS since it’s got less stopping power.

          1. The europeans for a long time went for cheap, and more than half the bikes had such brakes, but now all of the (western-)EU+UK is going electric and those all have other brakes, and are much more expensive, and are most often made of aluminium even, at a time when aluminium is at a peak price too.

        2. Having only used cheap American bikes, I tended to like one thing about the rim brakes – every time they didn’t work, if you increased the tension they would work again, unless you needed new ones. XD

    2. The most likely answer to that is how vertical you are on the bicycle it is why most mountain bikes and hybrids feel more comfortable for riders especially newer ones. The fact that the rider is more vertical means it is easier to breathe and easier to apply force into the cranks easier. The down side is aerodynamic drag increases exponentially while most other forces increase linearly compared to speed so as speed increases drag becomes the greatest factor, probably around 12-15mph for most riders. The position on an aero or most race derived bike frames put a rider in is developed to overcome drag more than to allow an efficient pedal position or keep the rider comfortable.

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