How Do Bike Gears Work?

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Whether you’re a beginner cyclist or a seasoned racer, at some point you’ve probably caught yourself wondering: “Just how do bike gears work?”

The way in which gears function is something that seems pretty complex, but in reality, is a very simple mechanism.

Gears are essential to your bike (unless you ride a single-speed), and it can be interesting and helpful to know exactly how bike gears work

To get you up to speed on your bike’s gears, we’ll be covering:

  • Why Do Bikes Need Gears?
  • How To Operate Your Bike’s Gears
  • How Do Bike Gears Work?

Ready to kick into gear?

Let’s get started!

How Do Bike Gears Work: Title Image

Why Do bikes need gears?

This may sound like a silly question to many – but given the existence of single-speed bikes, it’s doubtless a valid one.

If some bikes don’t have gears at all, then why bother with them?

Well, it depends on what type of riding you’re doing. For flat and relatively smooth terrain, a single-speed bike will do just fine. You have to put in a relatively stable amount of power in order to maintain a constant speed.

However, as soon as you chuck some hills or rough terrain into the mix, it’s a different story.

One of the golden rules of cycling, particularly road cycling, is to try and keep your cadence roughly constant throughout your ride, regardless of the terrain.

The rear end of a mountain bike's drivetrain.

Remember that cadence is the number of times you rotate the pedals in a given time period (the rpm of your pedaling).

On a single-speed bike on hilly terrain, this is impossible. There’s no way you can sustain the same cadence going up as you can on a descent.

However, with gears, you can shift into a lower gear when climbing, and a higher one when descending. Given that you have a wide enough range of gear ratios, this should mean that you can sustain a roughly constant cadence throughout your ride.

Of course, there are exceptions to this. If you’re tackling some 30% ramps on a brutal climb, it’s pretty difficult to sustain a high cadence without instantly blowing your doors.

An important thing to note here is that changing gear should only change your speed, not your cadence.

Gear shifters on a mountain bike.

How To Operate your bike’s Gears

If you’re the owner of a bicycle, then it’s likely you’re already clued up on how to use the gears on your own ride.

The answer is clear – you change gear through the action of the shift levers. Though, how do you operate the shift levers?

The slightly awkward part about this question is that there is no one answer – it depends on what type of gearing you have, and further on the brand.

In general, your shift levers will be located in one of two places on your bike. On the handlebars – often integrated with the brake levers – which are called handlebar shifters, or on the downtube, called downtube shifters.

If you have handlebar shifters, then it depends on which brand of shifters you have. Shimano, for example, uses a separate lever to change gears in one direction, and then you can push the whole shifter system inwards to change in the other direction.

Brifters on a black road bike.

SRAM shifters function as the action of a single lever. In this case, you push the paddle inwards a large amount to go shift in one direction and push the same paddle inwards a small amount to change gear in the other direction.

SRAM also pioneered “twist-grip” shifters, which remain popular with mountain bikers.

Campagnolo is different again – push the lever inwards to change in one direction, and push the separated “thumb paddle” to shift in the other direction.

If, on the other hand, you have downtube shifters, then there is essentially one way to operate them – though there are some nuances.

Indexed downtube shifters mean that the shifters “click” into place when you change gear. In this case, just push the lever “up” towards the handlebars to go into a harder gear, and “down” towards the bottom bracket to go into an easier gear.

If they are unindexed, then the operation is the same, but you have to push slowly until you feel the gear shift and then release the lever.

Close-up of the front derailleur on a navy road bike.

How do bike gears work?

Shifting Gear

Okay, so now the big question – how do bike gears work specifically?

Bike gears are essentially made up of seven parts. The cassette (rear gearing), the chainset (front gearing), the chain, the shifters, the cables, and the front and rear derailleurs.

The derailleurs have springs pushing them in one direction, but are held in place by a cable connected to the shift levers.

The action of the shift levers applies or releases tension to the cable, allowing the derailleur to move either inwards or outwards.

The chain, which runs through both derailleurs and then sits on the chainrings and sprockets of the cassette and chainset, is then pulled by the “shifted” derailleur onto a new sprocket or chainring, depending on if you shifted the rear or front gears.

  • Want to know more? Check out our Complete Guides To Adjusting Front and Rear Derailleurs here!
A cyclist climbs a steep hill on a white road bike.

Gear Ratios & Mechanical Advantage

But how does changing the sprocket or chainring that the chain sits on affect the force needed to rotate the pedals?

This happens through a phenomenon called mechanical advantage. This is essentially the rotational equivalent of using a lever. The force required to turn the pedals varies depending on which gear you’re in.

This works by using sprockets or chainrings (“cogs”) of different sizes in conjunction with one another. In order to illuminate exactly the mechanism through which this works, we’ll choose an example.

Say you’re in a gear of an 11-tooth sprocket at the back and a 53-tooth chainring at the front (this is usually the maximum possible gear ratio on a road bike).

Now, remember, that the chain is essentially rigid, and the teeth are the same size. When the chainring moves one tooth forward, the sprocket at the back also moves one tooth forward.

When you turn the pedals a full rotation, the chainring does the same. This means that, in our example, the chain moves forward 53 teeth for each rotation of the pedals.

Close-up of the cassette of a mountain bike.

Since, as we know, the chain is rigid, and the teeth are all the same width, the rear sprocket must rotate a full 53 teeth.

This, on an 11-tooth sprocket, is nearly five full rotations of the sprocket. The cassette, which houses the sprockets, is connected rigidly to the wheel (in one direction, in the other the wheel can move freely while the cassette remains stationary).

So, when the cassette turns nearly five full times, the rear wheel (and, by extension, the front wheel) also rotates nearly five full times.

Let’s say that you shift down at the front to a 39-tooth sprocket. Now, when you turn the pedals a full rotation, the chainset still rotates once. However, now, the chain only moves forward 39 teeth.

This is only just under 3.5 full rotations of the cassette, and thus the wheels.

Let’s say you’re using 700c wheels. Then, five rotations would be around 3.5 m traveled, but 3.5 rotations would be just 2.45 m traveled.

For one rotation of the pedals, you’ve moved along a different distance – you’re now going at a slower speed for the same cadence.

Black and white close-up of a bike cassette.

Why Does A Different Gear Ratio Make it Easier or Harder To Pedal?

We’ve established that changing gears will alter your speed for a given cadence.

But why does moving a different distance, with the same rate of rotation of the pedals, make it feel more difficult to turn the pedals?

Well, the extreme oversimplification of the situation is that you’re “dragging” the same weight along a longer distance in the same amount of time, and so it requires more force.

However, in reality, it’s not quite so simple.

In order to accelerate, you need to put force into the pedals. But, in a perfect situation, with zero drag, you actually would remain at a constant velocity without the need to input power into the pedals.

Obviously, though, you always experience drag. So you need to put force into the pedals to counteract the drag you experience from air resistance. Air resistance, in general, depends on your speed.

When you’re traveling faster after shifting up a gear, you then experience more air resistance, and so you are required to put in more force to the pedals in order to counteract that resistive force.

The “need to put more force into the pedals” is essentially equivalent to “the pedals feel more difficult to turn”, and so that’s why, when you shift up into a harder gear, the pedals feel harder to turn.

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Jack is an experienced cycling writer based in San Diego, California. Though he loves group rides on a road bike, his true passion is backcountry bikepacking trips. His greatest adventure so far has been cycling the length of the Carretera Austral in Chilean Patagonia, and the next bucket-list trip is already in the works. Jack has a collection of vintage steel racing bikes that he rides and painstakingly restores. The jewel in the crown is his Colnago Master X-Light.

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