Why Do Bike Gears Make Hills Feel Smaller? Count the Teeth!
Summary
A hill can't shrink, so how can clicking a little lever make it feel smaller? Become a gear detective and count the teeth on a bicycle's cogs to discover the clever trade between pedaling speed and pushing power.
Transcript
0:00The Hill Puzzle
0:00Here is a puzzle: a hill cannot shrink. It's made of dirt and rock, and it stays exactly as tall as it is. So why do bike riders say that clicking a little lever makes a steep hill feel smaller? Something on that bike is pulling off a clever trick, and today you 0:15are the detective who cracks it. Picture a rider at the bottom of a big, steep hill. Their legs push, push, push, and the bike barely creeps along. Then click! 0:25Suddenly the pedals spin round easily, and up they go. Here's a wow: some bikes have more than twenty different gears to choose from! But wait until you see where the first clue is hiding. It's small, shiny, and covered in teeth.
0:41Meet the Cogs
0:41Clue number one: the cogs. A cog is a metal wheel with bumps around its edge called teeth. Don't worry, they don't bite! Look by the pedals and you'll find one or more big cogs. 0:52Now look at the middle of the back wheel. There's a whole stack of cogs, from little to large, like a pile of pancakes tipped on its side. A chain loops around one front cog and one back cog, linking them together. 1:04When you push the pedals, the front cog turns, the chain pulls the back cog around, and that spins the wheel. Here's a wow: a bike chain is made of more than a hundred tiny links, all working as a team! 1:18But here's the strange part. Why would a bike need so many different cogs? To find out, we have to start counting.
1:25Counting the Teeth
1:25Let's count teeth! Imagine a front cog with thirty-two teeth, and a back cog with thirty-two teeth too. Same size, same number. Every time the chain moves one tooth forward at the front, 1:37it moves one tooth at the back. So when you turn the pedals around once, all thirty-two front teeth go by, and the back cog turns around exactly once. One pedal turn, one wheel turn. 1:48Fair and square! Here's a wow: the gaps between teeth are the same size on every cog, big or small, so they all fit the same chain. That's why the chain can hop from one cog to another. 2:00But what happens if the back cog is much smaller? Get ready, because that's when the bike starts zooming.
2:08The Speed Gear
2:08Now swap in a back cog with only sixteen teeth. That's half as many as the front. Turn the pedals once, and thirty-two teeth of chain get pulled along. But the little back cog only has sixteen teeth, so it has to spin around twice to keep up! 2:23One pedal turn, two wheel turns. Whoosh! You travel twice as far for every push of your feet. Here's a wow: on a long, flat road, a big front cog and a tiny back cog let riders go really 2:36fast without their legs spinning in a blur. So small back cogs are the speed setting. But speed comes with a catch, and you'll feel it the moment the road tips uphill.
2:48The Uphill Catch
2:48Here's the catch. When one pedal turn carries you twice as far, each push has to lift you twice as far up the hill. On flat ground, no problem. 2:57But on a steep hill, gravity tugs you backward the whole way. Now every push feels heavy, like your pedals are stuck in thick mud. You push and push, and the pedals barely creep around. Here's a wow: gravity is the same pull that makes a dropped ball fall, 3:13and on a hill it pulls you and your bike back down every single second! Your legs are strong, but they can only push so hard at once. So what if we flipped the trick around? Let's find the biggest cog at the back and see what it does.
3:28The Climbing Gear
3:28Now flip it. Imagine a small front cog with twenty teeth and a big back cog with forty teeth. Turn the pedals once, and twenty teeth of chain go by. But the big back cog has forty teeth, so it only turns halfway around! 3:43It takes two whole pedal turns to spin the wheel once. That sounds slow, and it is. But feel the difference! Each push only has to lift you a little way up the hill, so the pedals go round easily. 3:56Here's a wow: this is called a low gear, and it's the one riders click into when the road gets steep. Spinning legs, steady climb. Okay, detective, here's where the case takes a twist. 4:09Did the low gear really make the hill any easier?
4:12Two Staircases
4:12Here's the twist: no, it didn't! The hill is just as tall, and your legs still have to lift you and your bike all the way to the top. Gears don't shrink the climb. 4:22They chop it into smaller pieces! Picture two staircases up to the very same door. One has giant steps, each as tall as your knees, so you have to heave yourself up every single one. 4:33The other has lots of tiny steps, close together. Same door, same height. But the tiny steps feel easy! You just take more of them. 4:42Here's a wow: the total work of lifting you is the same either way. That's the clever trade: more turns, easier pushes. So how does a rider pick the right staircase?
4:54Click to Shift
4:54That's what the little lever on the handlebar is for. Click it, and a clever part called a derailleur gently nudges the chain sideways, so it hops onto a bigger or smaller cog. Say it with me: dih-RAY-ler! 5:08Smart riders shift into a low gear just before the hill gets steep, while the pedals are still turning smoothly. Then, at the top, click, click, back to a higher gear for the flat ride home. Here's a wow: on most bikes, shifting only works while you pedal, because the chain 5:26has to be moving to climb onto a new cog! So gears give you a choice: speed, or easy pushing. But here's one last clue, and it's hiding in your own legs.
5:37Your Legs Know
5:37Think about walking up a really steep hill. What do you do? Most people take shorter, quicker steps, without even thinking about it! Your body is shifting into its very own low gear. 5:48On flat ground, your steps stretch out long. Uphill, they shrink. A bike's cogs do the same trick, just with metal teeth instead of feet. Here's a wow: mountain trails often zigzag back and forth, turning one 6:01steep climb into a longer, gentler path. It's the very same trade! Every gear on a bike is really a choice of step size: giant steps for speed, tiny steps for climbing. 6:14Now, detective, you have every clue you need. Let's go back to that steep hill and solve the case.
6:21Case Solved
6:21Remember our puzzle? The hill never shrank one bit. Gears simply split one big, heavy push into lots of small, easy ones, so the climb feels smaller. 6:31And here's a final wow: some mountain bikes have a back cog with around fifty teeth, just for the steepest hills! Tomorrow, find a parked bicycle at school or near home. With a grown-up's okay, and without touching the chain, 6:44look at the stack of cogs on the back wheel. Notice that the biggest cog sits closest to the spokes. That's the hill-climbing gear, the one with the tiny staircase steps. Try counting the teeth on the biggest and the smallest cog, and see how different they are.
Also in other languages
- हिन्दी: साइकिल के गियर पहाड़ी को छोटा कैसे बना देते हैं? दाँत गिनकर जानिए!
- संस्कृतम्: दन्तान् गणय! द्विचक्रिका पर्वतं कथं लघुं करोति?
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Series: science (English)