Gears and Shifting 101: Intro to Gears and Shifting
3-speed, 8-speed, 21-speed; bikes come in a variety of gearing set-ups but they share a common goal, to make your bike-life better. In today’s blog we’ll look at why some bikes have multiple gears and how to get the most out of them during your rides!
Most folks are familiar with the gears in a car, so let’s use that analogy. In your car, the engine spins (and spins faster the more you push on the gas pedal). The gears live in the transmission and convert that engine rotation into some amount of rotation of the driveshaft and ultimately the wheels (so you move, yay!).
On your bike, you’re the engine and you spin the pedals. The faster you spin your legs, the faster those pedals spin and, like a car, that rotation is converted by the gears on your bike into rotation of the wheels.
The gears work by adjusting the amount of rotation being transmitted from the engine (on your bike, you) to the wheels. In a low-gear, it takes a lot of spinning to generate a little bit of wheel rotation. In a high-gear, the wheels turn over more for every revolution of the pedals. To use some totally made up numbers, imagine that in low gear the wheels will make 1 full rotation for every 4 rotations of the pedals; in high gear, the wheels make 4 full rotations for every 1 rotation of the pedals. In short, low-gear is a lot of pedaling for a little movement, high-gear is a little pedaling for a lot of movement.
How does that affect you? If you’re starting from a stop, for example, you’ll need to accelerate the bike up to riding speed. If you try to do that in our “high-gear” above, every pedal rotation makes the wheels go around 4 times, so your first pedal rotation will move the bike a bit over 7 feet (4 wheel rotations at 220cm/rotation). In low gear, your first pedal rotation will move the bike just under 2 feet (1/4 wheel rotation at 220cm/rotation). Because it takes more energy to move 150-200 pounds of bike and rider 7 feet than 2 feet, you’re asking your legs to do more work in the first scenario. It’s like when you went riding as a kid and every time you’d start up, you’d get off the saddle and really jump/stand on the pedals to get up to speed before finally sitting back down and cruising. Starting in a low gear means you need less force on the pedals initially, so you can get started more easily and get up to speed without tiring out your legs.
If we think back to the car analogy, it’s the difference between starting from a stop in 1st gear or 4th gear. In 1st gear, your feel the engine power transmitted to the wheels right away, the engine spins up quickly (you can watch the rpm jump up on the tacometer) and that gets all 3000 pounds of you rolling. If you try to start from a stop in 4th gear, you’re asking the engine to move all 3000lbs of you a lot further per engine rotation, so you’ll see the rpm rise really slowly as you apply gas and the engine strains to move all of that mass (you might not even be able to get the car rolling if the engine isn’t powerful enough).
Starting from a Stop
Back to our bike, if you’re starting out in low gear it’s easy to get those legs spinning, and thanks to momentum/inertia we’ll be able to get up to speed through the magic of shifting. As you accelerate, you’ll find your legs spinning faster and faster to remain effective and you can eventually spin out (where you can’t spin your legs fast enough to keep applying force to the pedals). Let’s say the fastest your little legs will spin is 120rpm (2 pedal rotations/second), in our made-up low gear, that’d be 30 revolutions for the wheels so you’d covering about 30ft/min. Once the bike is going that speed, pedaling any slower than 120rpm won’t have an effect as the wheels will be moving on their own at a faster rate than the pedals would move them, luckily for us though, we have gears!
Once you’ve spun out, you can shift up to the next gear (let’s imagine our next gear is 2:1, so 2 pedal revolutions make 1 wheel revolution). With our new ratio, it only takes 60rpm to maintain that pace, so your legs can still add energy to the system and make you accelerate. Eventually getting up to 120rpm again, you’ll now be rolling at 60ft/min, spinning out, and ready to shift.
Basically, you keep shifting through the gears as you accelerate until you find a gear that balances your ideal cruising speed with the cadence and effort you’re most comfortable with. If you kept going all the way to our imagined high gear of 1:4, at 120rpm you’d be covering 480ft/min, or 16 times faster than our lowest gear at the same cadence!
Again, it’s super similar to cars. You start out in 1st gear, accelerating until your engine starts to spin out (the rpms hit the redline), so you shift to second and if you spin out there, up to 3rd, etc… In your car, you probably you probably settle into 3rd gear for city cruising between 30-40mph at 2500rpm, and you’re likely all the way up in 5th gear doing 80-mph on the freeway at the same 2500rpm. In your car, being in the wrong gear can prematurely wear the engine, and the same thing happens on the bike. Being in a gear that’s too high makes your legs work too hard to accelerate you and prematurely tires you out, causes knee issues, and all that bad stuff that can be avoided by being in the right gear and spinning your legs.
You’ll want to find the right cadence for you, but most experienced cyclists will ride at 70-90rpm, with racers more often riding at 90-110rpm. It can feel silly at first to be spinning your legs that much when you’re not moving very quickly, but it gives your legs the benefit of momentum to keep everything moving and it’s a much lower impact workout to ride a bike 20mph at 90rpm than it is to ride the same bike 20mph at 45rpm, so your knees, quads, and hamstrings will thank you.
To recap, gears exist (on bikes and cars) to give us control over the relationship between power input and output. By manipulating how much of the rotation of the engine/pedals is transferred to the wheels, we can affect how much effort it takes to get us moving, maximize our efficiency, and reduce the impact and wear on our bodies.
In the next class, we’ll talk about the difference between our 3-speed and 8-speed gearing options and how to choose the one that's right for you!