Why is it so much harder to ride a bike up a hill than to push it?

Question

Title says it all. From a physics point of view, gravity is a conservative force and so the amount of work done should be the same, and yet one option is a lot more tiring than the other. It's especially odd if we consider friction, since rolling friction is less than static friction, which should imply that cycling up the hill does (slightly) less work. I understand that it's because of friction that if the terrain is flat, cycling is easier than pushing a bicycle.

Here're some hypotheses I've thought about:

1. If a person is standing on an inclined plane, he's probably stationary (because of friction). But a bicycle will roll downwards. This constitutes an extra force that a cyclist has to overcome.
2. Something to do with the human body, i.e. if we replaced the cyclist with a machine then it'll indeed do the same amount of work both ways.
3. Something to do with the human body, in the sense that if we had an Olympic-level cyclist then he'll have no problems cycling up the hill. In other words I only feel more tired because I'm unfit.
4. Tiredness correlates to power (physics "power" - work done / time taken). One cannot ride a bicycle slowly up the hill, since the bike won't stay upright; conversely if one is riding the bicycle faster than pushing speed, there's a difference in power.

I find #1 unconvincing since it seems to be double-counting the "roll backwards" force together with gravity. #4 sounds reasonable but on flat ground things reverse, which is contradictory. The other two are plausible. Does anyone know if either (or both / neither) are correct?

Answer 1

The bike's gears are set so that a large force over a shorter distance at the pedal gives a smaller force over a longer distance at the wheel. This is the opposite to most machines, eg. a screw-jack or winch where you put in a smaller force over a longer distance to move a much heavier load.

Normally when you are cruising on the level you want this - the force needed at the wheel to overcome drag and friction is low compared to the force you can easily push with on the pedal. But going uphill you reach the point where to move fast enough to stay upright you have to put a lot of force on the pedal.

Some extreme mountain bikes have low gear ratios <1, so that less force on the pedal than at the wheel - you have to spin to move at a crawling pace.

Answer 2

Being a 65-year-old cyclist who lives at the top of a steep hill, I am well-familiar with this problem. When you are pushing a bike up a hill on foot, you are moving uphill very slowly and the power required to do this is less than that required to ride the bike (i.e., go fast enough to keep the bike upright) up the same hill.

On flat ground, things reverse, but for a different reason. Imagine you are trying to push your bike at a run and go as fast as you could while riding it. Even with maximum exertion, you simply can't do that because you can't move your legs back and forth fast enough, carry your own weight, and push the bike all at the same time. Because the bike lets you change gears as you go progressively faster, you do not have to whip your legs back and forth at some physically impossible speed to go fast on the bike- you just select a taller gear. In addition, you do not have to support your own weight while on the cycle, and you can take a break while riding by coasting momentarily.

Answer 3

Your legs were made for running. That’s what they’ve gotta do, or one of these days the lions gonna get a bite of you.

Somewhat different sets of muscles are involved in biking uphill or climbing stairs versus walking or running. When biking, your hip extensor (gluteus maximus) does most of the work. In relaxed walking, your hip flexor (psoas) swings your thigh forward, but the gluteus is lazy. The knee extensor (quadriceps) and flexor (hamstrings) are important in all of these activities.

Answer 4

Friction is defined as

$F=μ*Ν$

Where $μ$ is the coefficient of friction, either static or kinetic, and N is the normal force.

When one walks uphill the leg muscles mainly carry the body weight efficiently, the foot making a minimal contact and the body weight is transferred up hill. Pushing a bike uses the hand muscles to transfer a force , the wheels of the bike making the minimal contact on the road surface , rolling resistance

My take is ,(as I do not even bike, much less change gears on a bike) that the spending of more energy (harder) is because of more friction. I.e the sum of frictional forces "weight on feet"+"rolling bike", is smaller than friction on "body +bicycle weights" on rolling bike. The reason: the weight will flatten the wheel contact on the tarmac , making the "rolling" friction larger, thus in total more work needs to be done.

Answer 5

There's a major difference between riding a bicycle up a hill and pushing it: how fast you go.

The cruising speed of a bicycle on flat ground is about $5 m/s$. The cruising speed of a person walking is about $1 m/s$. Therefore, it's not fair to directly compare riding the bicycle up the hill & pushing it, without specifying the same speed.

Once we consider speed the cause is clear. If you ride a bicycle up a hill, you will slow down compared to cruising speed, but still travel at a speed of $\sim 2 m/s$ (Go slower than this, and you are likely to lose balance). Against that, nobody pushes a bicycle up a hill at $2 m/s$ - that would be tantamount to running the bicycle up the hill, which would indeed be more tiring than riding it up the hill.