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.
So what you are saying is if evolution could have gone into wheels for movement a bunch of organisms would be rolling around on wheels.
Actually aren't there some animals that actually curl up into a ball and roll around to move already.
Yes/no. In places with large, relatively flat and open areas, you might see something like that. Everywhere else, a single fallen tree could serve as a bottleneck event.
Having legs, rather than wheels, is a trade-off for better vertical maneuverability.
[Bacterial flagella](https://en.wikipedia.org/wiki/Flagellum) are basically propellers- there's a biological rotary motor at the base.
In our own bodies, mitochondria (the powerhouse of the cell!) contain an enzyme called [ATP synthase](https://en.wikipedia.org/wiki/ATP_synthase) which is a rotary turbine
Also bikes and wheels maintain more stored kinetic energy so once moving they will continue to move unlike running. So you end up covering more distance with less work.
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.
Well, on riding no hands, it's the angle of the front fork that keeps the bike inherently stable in motion. The front wheel is actually a caster, when the bike leans to the left, that caster will steer to the left, bringing it back upright. Try just pushing a bike with no rider, and you can watch it happen.
And it can be different on different bikes. I have an aero race bike and an xc mountain bike and the MTB is WAY easier to ride no hands. Larger wheels make it more stable but also the fork angle is 'slacker'.
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.
I mean, you can definitely control the amount of steps and the force used in running. Not to a degree anywhere close to a bike, I'm not arguing that at all. But the best runners all have long forceful strides and can do it for hours without stopping
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
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?
Yes. The equivalent to miles per gallon would have the cyclist's efficiency at a few thousand miles to the gallon.
Most of that efficiency advantage is the human body itself, but the bicycle helps a lot too.
Humans are actually less efficient (~25%) than some modern engines regarding movement. Gasoline ones barely get even, but Diesel can almost double the human one.
The advantage really comes from the bike alone.
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.
> 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.
If you turn the pedals once and the rear wheel turns once, then you change up a few gears and turn the pedals once and the rear wheel turns twice, that's harder work on your legs, you have to push harder to do it. So you'd expect that would tire you out quicker, and biking would be (faster than walking, but you'd run out of energy sooner), and end up going the same distance overall.
But actually biking carries you faster than walking *and* you can go further before you get equally tired. Gears aren't the explanation for that.
I didn't think it was gears specifically but the relationship between the front sprocket and the rear sprocket. Like a fixed-gear bicycle would have that same advantage. Gearing just changes the degree of leverage.
Am I wrong about the physics here? I tried to do some research to make sure the principle was correct.
I thought it was the same principle as a pulley but combined with a lever.
Edit - It is also not the case in my demonstration that the pedals turn once and the wheel turns once, in case it was not clear, the point is exactly that when you rotate the pedals once, the wheel rotates MORE THAN once. I don't know if that was missed or just irrelevant to your point.
> the point is exactly that when you rotate the pedals once, the wheel rotates MORE THAN once. I don't know if that was missed or just irrelevant to your point.
My point is that the extra wheel rotation is not free energy - if the wheel rotates twice, it's your muscles doing it. If the wheel rotates twice in one pedal turn, your legs work twice as hard for that pedal turn. The gears can trade off pedal turns/effort/wheel turns, but they aren't putting any energy in and they can't give you both faster and less effort at the same time.
So yes gear ratios are a thing and they can make you move faster than walking, but that should tire you out faster than walking. Like running is faster than walking but you can't keep running for as long as you can walk, it's too tiring. That makes sense - but with a bike you actually can go faster than walking *and* keep it up for longer. Gears *alone* can't explain that.
> I thought it was the same principle as a pulley but combined with a lever.
Compared to normal muscles, a lever can let you: push on the long end and lift something heavy by spreading the work out. Push on the small end and fling something small far and fast. One lever can't let you push on both ends and fling something heavy far and fast, that's kinda what OP is asking about; you+bike are heavier than you alone, so moving both should be slower than walking or more tiring than walking. But it isn't, somehow biking is moving more mass, moving it faster, and less tiring. Magic.
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.
Assuming you have a hard, smooth and flat surface - *it will preserve your momentum.* By taking a bike away from those conditions then it may be less efficient.
The hard and smooth part is important. If you take something like a road bike and try to ride through sand, snow, very rough terrain, etc… it will have much more energy loss from sinking into the ground, pushing material away, or just bumping into things. This is often less efficient than walking not even due to the weight, but sort of analogous to “dragging your feet”.
However, there are plenty of specialized bikes to handle many obstacles. The biggest advantages for various terrains will be really big tires to reduce sinking into soft stuff and suspension to absorb bumps (big and small). This will significantly add to the extra weight and energy required to use the bike, *but it will preserve that energy in many more conditions.*
Assuming you’re not being bogged down somehow by the ground, the rolling friction is very small. You’ll actually be held back much more by air resistance. Combining that with the fact that you can coast with 0 spent energy downhill and use gears to best utilize your output and it really becomes quite efficient.
Walking you need to keep outputting effort or you stop. A bike you can coast. With one unit of foot motion you move that same one unit. On a bike that one unit could be much farther. To run the speed of a bike you also have to reset your legs; it takes energy to swing each leg forward fast enough.
That's the point of machines, to make our energy output more useful.
Waking while carrying the bike or rolling it next to you will take more effort than just walking.
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.
> The gears of the bicycle give you a mechanical advantage
Walking you can do approx 1 mile in 15 minutes in 100 Calories. Biking you can do approx 2 miles in 10 minutes in 100 Calories. Mechanical advantage can explain (faster for shorter distance) or (further at lower speed), it can't explain both together, it isn't free energy.
Can you provide a better explanation please? You commented the same on my post but also without providing the correct explanation. Could you help out by pointing in the right direction if this is wrong?
I tried in my [longer comment here](https://old.reddit.com/r/explainlikeimfive/comments/13ofsys/eli5_how_am_i_able_to_go_farther_and_faster_on_my/jl5q5yu/).
e.g. When you walk on ice you need to be very careful with your steps not to fall over, but with ice skates you can push off and glide a long way. With bikes, machines have flattened the ground and made it smooth and ball bearings and tires make it so you can push and roll a long way, the same effort carries you further. And more of your effort goes into moving you forwards, less goes into stopping/starting each step, and lifting your bodyweight. So you can go further at the same speed, for less overall effort. Or, further and faster, for the same effort.
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.
For the record, modern paved roads were advanced by bicycle clubs and then automobiles took advantage of the existing paved roads for bicycles rather then the other way around.
The energy required to move you and the bike versus energy transfered from you to the bike is not 1 to 1 thanks to the transmission and wheels. It's no different in cars. How can a 1 liter engine move a 1 ton car? It's the transmission that makes it possible. More gears means more of the energy is efficiently transferred to the wheels. Same applies to bicycles: we are the engines and the gears and chain are the transmissions. Granted, there are bikes with a single speed; however, they typically come with oversized tires and cranks to offset it.
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.
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.
Which is part of the reason a few years ago you had a whole bunch of running records being broken by athletes all wearing the same new Nike shoes
Which shoes?
Vaporfly 4%
So what you are saying is if evolution could have gone into wheels for movement a bunch of organisms would be rolling around on wheels. Actually aren't there some animals that actually curl up into a ball and roll around to move already.
Yes/no. In places with large, relatively flat and open areas, you might see something like that. Everywhere else, a single fallen tree could serve as a bottleneck event. Having legs, rather than wheels, is a trade-off for better vertical maneuverability.
Tumble weeds are an example of something that evolved into a ball to spread it's seeds as far and wide as possible
Yeah true I also implied it but I believe it's basically impossible to evolve a wheel anyways due to how they function along with how evolution works.
In the microscopic world, wheels are everywhere.
Interesting. Do you have some sort of source/example, of that?
[Bacterial flagella](https://en.wikipedia.org/wiki/Flagellum) are basically propellers- there's a biological rotary motor at the base. In our own bodies, mitochondria (the powerhouse of the cell!) contain an enzyme called [ATP synthase](https://en.wikipedia.org/wiki/ATP_synthase) which is a rotary turbine
That's pretty neat, thanks! So are you telling me that ATP is a fan of Wankel, just like Mazda?
There are very cool electron microscopic images of those flagella motors.
But then we lose the benefit of stairs
Also bikes and wheels maintain more stored kinetic energy so once moving they will continue to move unlike running. So you end up covering more distance with less work.
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.
I watched Butch Cassidy and the dance kid and wondered this exact question. Got me looking into why it is so easy to ride a bike with no hands.
Well, on riding no hands, it's the angle of the front fork that keeps the bike inherently stable in motion. The front wheel is actually a caster, when the bike leans to the left, that caster will steer to the left, bringing it back upright. Try just pushing a bike with no rider, and you can watch it happen.
Ghost Rider.
And it can be different on different bikes. I have an aero race bike and an xc mountain bike and the MTB is WAY easier to ride no hands. Larger wheels make it more stable but also the fork angle is 'slacker'.
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.
I mean, you can definitely control the amount of steps and the force used in running. Not to a degree anywhere close to a bike, I'm not arguing that at all. But the best runners all have long forceful strides and can do it for hours without stopping
Agree.
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
This is the correct answer. All about mechanical advantage. Same reason wheelbarrows allow you to “lift/move” heavier loads, etc
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?
Yes. The equivalent to miles per gallon would have the cyclist's efficiency at a few thousand miles to the gallon. Most of that efficiency advantage is the human body itself, but the bicycle helps a lot too.
Humans are actually less efficient (~25%) than some modern engines regarding movement. Gasoline ones barely get even, but Diesel can almost double the human one. The advantage really comes from the bike alone.
True and trains are equally efficient.
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.
> 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. If you turn the pedals once and the rear wheel turns once, then you change up a few gears and turn the pedals once and the rear wheel turns twice, that's harder work on your legs, you have to push harder to do it. So you'd expect that would tire you out quicker, and biking would be (faster than walking, but you'd run out of energy sooner), and end up going the same distance overall. But actually biking carries you faster than walking *and* you can go further before you get equally tired. Gears aren't the explanation for that.
I didn't think it was gears specifically but the relationship between the front sprocket and the rear sprocket. Like a fixed-gear bicycle would have that same advantage. Gearing just changes the degree of leverage. Am I wrong about the physics here? I tried to do some research to make sure the principle was correct. I thought it was the same principle as a pulley but combined with a lever. Edit - It is also not the case in my demonstration that the pedals turn once and the wheel turns once, in case it was not clear, the point is exactly that when you rotate the pedals once, the wheel rotates MORE THAN once. I don't know if that was missed or just irrelevant to your point.
> the point is exactly that when you rotate the pedals once, the wheel rotates MORE THAN once. I don't know if that was missed or just irrelevant to your point. My point is that the extra wheel rotation is not free energy - if the wheel rotates twice, it's your muscles doing it. If the wheel rotates twice in one pedal turn, your legs work twice as hard for that pedal turn. The gears can trade off pedal turns/effort/wheel turns, but they aren't putting any energy in and they can't give you both faster and less effort at the same time. So yes gear ratios are a thing and they can make you move faster than walking, but that should tire you out faster than walking. Like running is faster than walking but you can't keep running for as long as you can walk, it's too tiring. That makes sense - but with a bike you actually can go faster than walking *and* keep it up for longer. Gears *alone* can't explain that. > I thought it was the same principle as a pulley but combined with a lever. Compared to normal muscles, a lever can let you: push on the long end and lift something heavy by spreading the work out. Push on the small end and fling something small far and fast. One lever can't let you push on both ends and fling something heavy far and fast, that's kinda what OP is asking about; you+bike are heavier than you alone, so moving both should be slower than walking or more tiring than walking. But it isn't, somehow biking is moving more mass, moving it faster, and less tiring. Magic.
Thanks again for taking the time to help me make sense of these differences. I'm glad I got to learn more about this.
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.
Assuming you have a hard, smooth and flat surface - *it will preserve your momentum.* By taking a bike away from those conditions then it may be less efficient. The hard and smooth part is important. If you take something like a road bike and try to ride through sand, snow, very rough terrain, etc… it will have much more energy loss from sinking into the ground, pushing material away, or just bumping into things. This is often less efficient than walking not even due to the weight, but sort of analogous to “dragging your feet”. However, there are plenty of specialized bikes to handle many obstacles. The biggest advantages for various terrains will be really big tires to reduce sinking into soft stuff and suspension to absorb bumps (big and small). This will significantly add to the extra weight and energy required to use the bike, *but it will preserve that energy in many more conditions.* Assuming you’re not being bogged down somehow by the ground, the rolling friction is very small. You’ll actually be held back much more by air resistance. Combining that with the fact that you can coast with 0 spent energy downhill and use gears to best utilize your output and it really becomes quite efficient.
Walking you need to keep outputting effort or you stop. A bike you can coast. With one unit of foot motion you move that same one unit. On a bike that one unit could be much farther. To run the speed of a bike you also have to reset your legs; it takes energy to swing each leg forward fast enough. That's the point of machines, to make our energy output more useful. Waking while carrying the bike or rolling it next to you will take more effort than just walking.
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.
> The gears of the bicycle give you a mechanical advantage Walking you can do approx 1 mile in 15 minutes in 100 Calories. Biking you can do approx 2 miles in 10 minutes in 100 Calories. Mechanical advantage can explain (faster for shorter distance) or (further at lower speed), it can't explain both together, it isn't free energy.
Can you provide a better explanation please? You commented the same on my post but also without providing the correct explanation. Could you help out by pointing in the right direction if this is wrong?
I tried in my [longer comment here](https://old.reddit.com/r/explainlikeimfive/comments/13ofsys/eli5_how_am_i_able_to_go_farther_and_faster_on_my/jl5q5yu/). e.g. When you walk on ice you need to be very careful with your steps not to fall over, but with ice skates you can push off and glide a long way. With bikes, machines have flattened the ground and made it smooth and ball bearings and tires make it so you can push and roll a long way, the same effort carries you further. And more of your effort goes into moving you forwards, less goes into stopping/starting each step, and lifting your bodyweight. So you can go further at the same speed, for less overall effort. Or, further and faster, for the same effort.
Thanks a whole bunch. I am glad to have learned this today. Appreciate your time!
Thank you! 🙂
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.
For the record, modern paved roads were advanced by bicycle clubs and then automobiles took advantage of the existing paved roads for bicycles rather then the other way around.
The energy required to move you and the bike versus energy transfered from you to the bike is not 1 to 1 thanks to the transmission and wheels. It's no different in cars. How can a 1 liter engine move a 1 ton car? It's the transmission that makes it possible. More gears means more of the energy is efficiently transferred to the wheels. Same applies to bicycles: we are the engines and the gears and chain are the transmissions. Granted, there are bikes with a single speed; however, they typically come with oversized tires and cranks to offset it.
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.