The Bicycle: A Marvel of Physics and Engineering

Oh, how I love riding my bicycle! It has gotten me all over Chicago, along the lakefront from the far south side to the north suburbs, through the college neighborhoods and the ethnic neighborhoods, residential ones and industrial ones, and through downtown. My bicycle has enabled me to explore parts of the city I don’t normally pass through or that I only stop in for specific reasons, with direct exposure to the sights, sounds, and smells of each local neighborhood.

I also love my bicycle as the beautiful machine it is: Its 24 speeds, its straight lines, its deep, black color. I bought it as a gift to myself after my last one was stolen while I was taking a final exam (talk about adding insult to injury). I just love how efficient it is. For the amount of energy I put into it, I get most of it back in the form of forward motion. In fact, in the right conditions, bicycles are over 98% efficient! I often think about when I’m stuck in traffic on the highway, because cars, guzzling their gasoline and releasing toxic gases, , don’t even come close: they’re only 20-40% efficient.

Think about all that extra weight you’re pulling every time you accelerate in a car. Recall the last time you burned your finger touching a running car engine. Where did that heat come from? It’s all the energy your car didn’t use to move you, but was instead wasted.

As I ride along Monroe Harbor on my bicycle, swiftly cruising down the Lakefront Trail, speeding past the traffic stuck on Lake Shore Drive, I think about how each car requires an external source of fuel and how each driver is not only transporting themselves but also a couple of tons of metal. I find myself turning away from the street as each car spews putrid gases into the air. Meanwhile, I rejoice in the fact that my bicycle doesn’t consume fossil fuels and it doesn’t have to waste energy sitting in traffic. The fuel for my bicycle comes from the food I eat.

Bicycles come in several varieties, so read below to figure out what kind of bike is right for you:

Fixed-Gear Bicycles

As I mentioned, I have a 24-speed bike. But if you’re looking to buy a bike for yourself, you have a lot more options. For instance, a lot of urban riders are buying fixed gear or “fixie” bikes these days. On these bikes, the chain wraps around two sprockets that are fixed to the pedals and the back wheel, respectively. Because the sprockets are fixed to the bike, the wheels and the pedals always revolve together. This is a simple and easy-to-manage design, but it also causes problems when you want to coast: You simply can’t. If you try to stop pedaling while you’re going downhill, your wheels will stop too. On a fixie, a revolving pedal always revolves with the wheel, and vice versa, no exception.

Single-Speed Bicycles

To solve this problem, most bicycles, such as the standard single-speed bike, have something fixed to the back wheel called a “freewheel.” A freewheel, as opposed to a fixed sprocket, allows you to stop pedaling, and even pedal backward, while your bike coasts forward.

When you’re pedaling forward with a single-speed bike, it acts much like a fixed-gear bicycle, in that pedaling will cause the wheel to revolve at a fixed speed compared to your pedals. But when you stop pedaling or pedal backwards, the pedal on a single-speed bike disengages from the sprocket and the sprocket continues to revolve by its own momentum. We’re coasting now!!

[youtube https://www.youtube.com/watch?v=bAL_nWjuhOI]The video above shows how this works. The first half of the video shows what happens when you coast or pedal backward, while the second half of the video shows what happens when you pedal forward.

Some cyclists prefer a fixed-gear bicycle because of its simplicity. But personally, I would choose a single-speed bike any day. If I did happen to find a hill in Chicago (it does happen sometimes!!), I’d much rather keep my feet on the pedals as I ride down it, rather than be forced to either awkwardly raise my legs to get them out of the way or ride those pedals like a madman, trying to keep up with the momentum of the bike.

Multi-Speed Bicycles

For me, a single-speed isn’t even enough. I’m happy that a bike with a freewheel can coast, but when I’m pedaling forward, I want more flexibility. Since pedaling forward works the same on a single-speed bike as it does on a fixie, the mechanics make pedaling a challenge in certain situations. On solid ground with no wind, the amount of force I apply to the pedals will decide how fast I will go: the more force I apply, the faster the pedals revolve, and the faster the bike will go. But on hills and rough terrain, we run into problems. Whether it’s gravity pulling me down as I ride up an incline or it’s mud holding me back in a wet field of grass, I’ll struggle more than usual to move forward. It is going to take more of my energy to overcome these forces and keep my bike moving at the speed I want to go.

To circumvent this challenge, I opted to purchase a multi-speed bike, with three sprockets of successively smaller size fixed to the pedals, and eight sprockets in a similar arrangement fixed to the back wheel. On any given gear, like on any other type of bike, one full turn of the pedal makes the front sprocket revolve all the way around. But difference in size between the sprockets — called the “gear ratio” — affect the energy I need to put into pedaling.

Specifically, the gear ratio affects how far I go each time I revolve the pedals. The smaller the rear sprocket is (on higher gears), the fewer teeth it has, which means it will revolve more times every time I revolve the petals, and the bike will move farther. On a smaller sprocket, the amount of energy I’ll need to put into pedaling will be spread out across more revolutions. This means I’ll have less energy to put into each individual revolution, and pedaling will feel a lot harder.

Conversely, If I’m on a lower gear, where my chain wraps around a larger sprocket, all the energy I put into the pedals will translate into less motion, which means that the energy I put into pedaling is more concentrated over a shorter distance, which makes pedaling feel much easier.

Riding in high gear, with a small sprocket, is really great when I’m going down hills, when my momentum is doing most of the work and my daredevil side wants to add a bit more speed on top. But going up hills, this could be torture for the calves.

Gravity is working against me when I’m going up a hill. In this case, I will want to switch to a lower gear. I won’t get to the top as fast (because I’ll be covering less distance per pedal stroke), but at least I won’t have to push as hard on the pedals to move forward, which means I’ll be able to tolerate the ride!

What Else Impacts Your Riding Efficiency?

External factors such as wind resistance and gravity aren’t the only things that affect the amount of force you have to put into your riding to get where you want to go at a certain speed. A lot of it depends on you and the bike you’re riding. When it comes to internal factors that affect your efficiency, enemy number one is rolling resistance.

Rolling resistance is the dragging force you feel when your bike tires are underinflated or when you’re riding on soft ground. When your bike wheel is rolling along the ground, the part that touches the ground deforms, using up energy and causing your bike to lose momentum. Besides underinflated tires and riding on soft ground, a heavy bicycle frame and your own weight all increase rolling resistance on a bicycle, meaning you have to work harder to travel at the same speed.

Weight also puts more pressure on the joints in your bicycle, causing more internal friction that takes energy away from your forward motion. In addition, weight also affects acceleration – have you noticed that it’s harder to get a full shopping cart going from a complete stop than an empty one? That’s because it takes more energy to change the speed of something if it is heavier. The same goes for your bicycle.

Serious cyclists try to stay skinny and ride lightweight bicycles with well-greased joints and thinner, well-inflated tires to reduce rolling resistance and internal friction as much as possible.

Optimizing a bicycle to minimize the effects of rolling resistance, internal friction, and wind resistance can run you thousands of dollars, so unless you are a competitive cyclist, you’ll do just fine keeping your chain greased and your tires properly inflated.

The Joy of Cycling

Cycling is considered the most efficient form of transportation — even more efficient than walking! It’s also great exercise (coasting down hills doesn’t count), and great for the environment. If you have a choice between driving and bicycling, leave the car in the garage and take your bicycle out for a spin. It might feel like more work, but your body and your planet will ultimately thank you for it.