ELI5, How am I able to go farther and faster on my bicycle than by walking, considering I have to move the mass of the bike, as well as the loss of energy to friction from the gears and wheels?
In: 68
The bike has two advantages over your legs. One, it’s on wheels, which gives you the ability to coast on flats or down hills. Second, it has gears, which gives you mechanical advantage. At your highest speeds, each single revolution of your pedals results in around 5 revolutions of your back wheel
Turning of the wheels is one continuous motion. When the wheels spin, they keep spinning and most of the kinetic energy is kept.
When humans run, when pushing off the ground then landing, human muscles not being elastic means much of the kinetic energy when landing is dissipated as heat or tear on the muscles.
Long-distance athletes wear shoes that specifically are elastic in the physics sense for their legs to be best able to recover energy.
The extra mass of the bike and the friction is still less of a burden than how inefficient walking/running is. Just a smoother motion means a lot less friction. The gears also play a big part, think how tiring it’s to pedal on the wrong gear ratio. When you are running you don’t have a choice. Runing would be more efficient if you could be taking half the amount of steps but they required twice the force.
Walking loses a lot of energy. When you step forward, you first apply force to lift your leg, then move it in front of you, then fall forward a tiny bit, then use energy to stop yourself from falling, then push yourself up slightly. You do this repeatedly, and this allows you to move in a particular direction. When you catch yourself, you also lose most of your speed.
This is considered normal.
Biking reduces this energy loss.
* Instead of lifting your leg, you rest your leg on the pedal and let the bike do the work.
* Instead of pushing yourself up, you push the pedal down. This seems like a “no change” situation, however there’s a trick here worth considering. Instead of using that energy to keep yourself standing up, you’re using the energy to send yourself forward.
* Instead of having to stop to catch yourself, you just… don’t. Most of the speed you already picked up is still there, allowing you to add speed with each step instead of having to regenerate it.
Because you’re saving energy at most steps, you can use more of that energy to produce speed, and move faster than you would otherwise.
When you turn the pedals, it makes a big circle, but it’s connected with a chain to the axle at the end of the bicycle – a smaller circle. It is easier to move an object when you have leverage. The chain forces the smaller circle to move faster than your bigger circle. When your pedals make one rotation, the smaller circle has moved more than one rotation.
A bicycle takes the motion of your legs and multiplies their effort. So it is easier to get farther with one movement of the leg.
Add in gears and these circles can be manipulated for more and more leverage – bigger circle under your feet and smaller circle on the rear wheel for faster rotation.
Do note that on steep uphills, the bike is less efficient than walking. At some point the bike is pretty much just extra weight and slows you down, especially since you need to maintain a minimum speed to avoid falling over on the bike.
The gears of the bicycle give you a mechanical advantage. A baseball bat gives you a longer lever (and a faster moving mass) to hit the ball with instead of just swinging your hand assuming your hand was tough enough to swat a 90 mph pitch. The crank that the pedal is on is a lever on the front chain ring. The front to rear gear ratio gives each push of your leg an extra advantage compared to using that push for just walking.
The bicycle rider does have increased resistance from gearing losses and higher aerodynamic drag, but the pedestrian also has losses in moving the body mass slightly up and down with each step. But with good well inflated tires, the rolling resistance of a bicycle is low.
The bicycle is considered one of, if not the most efficient forms of personal transportation in terms of energy consumed per unit of distance transported.
I heard that a person on a bike is the most efficient form of transport on earth, and that only long-flight birds can go further on less fuel. For example, you can travel several miles on a bowl of oatmeal. Does anyone know if this is true?
Bikes offload a lot of their convenience to the environment – without smooth asphalt pathways flattened by fossil fuel burning bulldozers they would be much less useful. Imagine taking [an early wooden bike](https://i.pinimg.com/originals/c0/08/0a/c0080a1d0fd9573dd75cc052c0b365d8.jpg) out to a national park to hike up a hill over the grass and off the trails. Then it would be you pushing the mass of the bike up a hill, harder work than just climbing on your feet. On the way down if the ground was too rough you couldn’t ride the bike you would have to walk it, fighting to control the bike so it didn’t drag you down or fall over. Lose lose both ways. Even on a modern mountain bike people generally ride trails of smoothed out terrain. Check [this video](https://youtu.be/rdNANaYbkfI?t=589) he’s struggling to push the bike and it’s flat, smooth, clear of rocks and tree roots and potholes and bushes and brambles and tufts and thickets, it’s not uphill, it’s only wet and muddy and that alone is enough to turn the bike into a burden.
Bikes benefit from precision engineering that biology can’t do – ball bearings, grease, axels, pneumatic tyres; animals don’t have wheels – there isn’t a good way to get nutrients and blood flow from a body to a completely disconnected wheel, and there isn’t much use for wheels in overgrown wilderness/swamp/jungle. These things and pavement combine to give bikes very low rolling resistance and the ability to coast forwards while not pedalling, compared to walking, and that works best in urban environments with flattened roads.
As well, bikes keep you level – turning the wheels only moves you forwards not up and down. With walking when you step forwards and [your legs move apart, your torso lowers](https://image.slidesharecdn.com/sehs4-161114192204/95/sehs-43-biomechanics-ii-433-force-com-20-638.jpg?cb=1479151341), then to bring your legs back together you have to lift your bodyweight a little bit. Some of your walking energy goes to continually lifting your body weight against gravity.
Other answers are saying that bikes have gears which give you mechanical advantage, which is true but it’s not relevant. Leverage lets you turn “I can’t move this lump it’s too heavy for my muscles” into “I can move half the lump, twice”. You put in half the force, for twice as long. Same total energy, levers don’t give you free extra energy.
The easiest way I found to visualize it when i had that same question is to draw the path of the center of gravity.
On a bike it more or less makes a straight line.
When you walk it not only goes up and down but side to side.
The going forward costs you almost no energy except for friction and wind resistance.
The going up and down is ridiculously high energy.
Seriously, it’s a little far in ELI5 but the energy is an order of magnitude different almost.
So saving yourself the energy of moving your center of gravity up even the 3 or so cm that it does is worth almost 10% of your body weight.
Add to that the difference in friction of the wheels vs your foot coming to a complete stop on every step and to be less efficient a bike has to weigh about 20% of your body weight for the same speed.
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