21 Speed Bike Gears Explained (Understanding Your Bicycle)

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Sure, everyone knows what “21 speed gears” means on a bike… right?

Well, maybe not. Bicycles, and especially gear systems, can be quite perplexing.

With 21 speeds, how do you know when (and how!) to change gears on your bike?

When you’re going uphill, which gear should you use? How about when you’re going downhill or just riding along on flat pavement?

And, where are the gear shifters on a bike?

On a mountain bike or a hybrid bike, the shifters are easy to identify (they’re simply near the brake levers if you’re wondering!)

For road bikes though, it’s a little different: the gears are not controlled by separate shifters; they are actually built into the brake levers.

Even the phrase “21 speed” is a little deceptive. To be more accurate, for each “speed,” you could pedal along at the SAME miles/hour on your bike.

Isn’t it strange? Yep, well what it really means is that you have a choice of 21 different gears when cycling. You can pick any one of these gears at any time, and it will either make pedaling on the flat or up/down a slope easier or more difficult at that moment.

So, yes, bicycle gear systems are difficult to understand.

The aim of this article is to explain what 21-speed bike gears are. In plain language that, I hope, makes sense.

I’ll tell you how to shift gears on the most common styles of bicycles.

Then, I’ll explain some of the bike gears terminology that you need to know (e.g., “low gear” and “high gear”).

We’ll examine what gear you should use when ascending hills and what gear you should change to descend.

In addition, we’ll discuss certain gear combinations to stay away from while riding (commonly called “cross chaining”).

Let’s switch things up a notch and get into the nitty gritty.

What does 21 speed mean on a bike?

If you’re looking at a 21-speed bike, it’s important to realize that you won’t see 21 different gears.

Instead, you get three cogs (which are called “chain rings”) near the pedals at the front of the drivetrain*. At the rear wheel hub, there are seven additional cogs (which together are known as a “cassette”).

*All the components on a bike that make the wheels move are collectively called the “drivetrain”. These include the two pedals, all the cogs on the chain rings and cassette, the chain and front/rear derailleurs.

The image below demonstrates what this looks like from a side view.

The total number of gear combinations that you may get from all these cogs is equal to 21, when you multiply 3 cogs on the chain rings by 7 cogs on the rear cassette.

When you sit on the bike and look down at the drivetrain from above (see diagram below), you can see how all of these pieces work with each other more clearly: by moving the chain across to connect any two cogs, different gear combinations are chosen.

Top down: This is what ’21 gears’ looks like

So, on a 21-speed bike, how do you shift gears?

How do you change gears on a 21-speed bike?

Bear in mind, that you don’t ever have to change gears when you’re cycling if you don’t want to – the chain will connect a chain ring (one of the front cogs) and a cog from the rear cassette – and you can stay in that gear.

However, you’ll discover that it’s sometimes difficult to pedal (for example, when you’re going up hills), while at other times it will be too easy (going down steep hills, say), resulting in your legs spinning out excessively.

So, it’s time to make a (gear) change!

On the handlebars, there are two gear shifters that can be operated by hand. These are linked via cables to the front and rear derailleurs:

  • Left gear shifter controls the front derailleur.
  • Right gear shifter controls the rear derailleur.

In French, the word “derailleur” (more properly, “dérailleur”) means to de-rail. In bike terms, it’s the chain that is being derailed, as the derailleur knocks it off one cog and onto another.

At the crank, just on top of the chain rings, you’ll find the front derailleur.

At the rear wheel hub, where the rear cassette lives, you’ll find the rear derailleur (which sits just below the cassette).

Bikes manufactured in the last 20 years or so are likely to have one of just a couple variations of gear shifter:

On road bikes, the gear shifters are incorporated into the brake levers. Often these will operate be pushing the brake lever to one side to shift one way and pushing a separate button or lever to shift the other way.

It’s a little easier with electric bikes, fitness bikes, city bikes, hybrids and MTBs as the gear shifters are normally separate to the brake levers (although generally they’ll be positioned next to the brakes). Each gear shifter will comprise two levers – push one to change up a gear and the other to change down.

With either style of gear shifter, the left will control the front derailleur and the right controls the rear derailleur.

It’s important to remember that you need to keep pedalling whilst you’re shifting gears – the chain can only move from one cog to another when it’s going round the drivetrain.

Which gear on a 21-speed bike is best for going up hills?

To make pedalling uphill easier you need to shift into a ‘low gear’.

But, what exactly IS a ‘low gear’?

The graphic below should hopefully explain.

Low gears means the chain will be between any of these cogs

If you use the left gear shifter to move the chain across to the smallest of the front chain rings then you will be in one of the lowest seven gears that you have on the bike.

This will mean that you won’t have to push the pedals so hard as you climb hills, though you’ll find that your legs will be turning the pedals faster.

For the steepest climbs, you should shift to your lowest gear (“gear 1”). To find this, use the left gear shifter to move the chain to the smallest of the front chain rings. Then, use the right gear shifter to move the chain to the largest of the cogs in the rear cassette.

PRO TIP: always change gears one at a time. If you try and leap through lots of gears in one go, you will likely find that the chain can either get stuck, come off the cogs, or both.

Which gear on a 21-speed bike should you use for flat or going downhill?

At the top of a slope, you need to change gears from a low gear to a higher gear. This will stop your legs from spinning too fast and mean that you cover more ground with each turn of the pedals.

High gears on a 21-speed bike look like the graphic below:

High gears means the chain will be between any of these cogs

To move into the high gears, use the left gear shifter to move the chain to the largest of the three chain rings at the front. You’ll now be in one of the highest seven gears on your bike.

When you’re going down steep hills or cruising along at a steady speed on the flat, you should move into your highest gear of all (‘gear 21’ or your ‘top gear’).

To get into the highest gear, make sure you’re on the largest chain ring at the front and then use the right gear shifter to move the chain to the smallest cog at the rear wheel.

PRO TIP: Always look at the road or trail ahead so you can plan what gear you’ll need coming up.

If you’re in low gear and about to descend a steep slope, your legs will just spin (change to a higher gear!)

If you’re in high gear and about to go up a steep hill, your legs may not be able to turn the pedals (change to a lower gear!)

The dangers of cross-chaining

Sadly there are a couple of gears in the twenty-one that you shouldn’t use.

These are:

  • Biggest cog at the front and biggest cog at the back
  • Smallest cog at the front and smallest cog at the back

You can see them in this graphic:

Avoid this gear combination
…and this one too

Why is this? Well, you’ll see that each of these pulls the chain diagonally across the drivetrain.

This can cause undue wear to the chain and may also be quite noisy as the chain slips and clunks its way around.

And there’s really no need to use either of these gears because you have nineteen others to choose from that will all make your chain happier!


Bike gears can be confusing and, when you have twenty-one of them, even more so!

Keep the following rules in mind and you’ll soon have the hang of them though:

  • Left shifter for front derailleur / Right shifter for rear derailleur
  • For uphills, make sure the chain is on the smallest front chain ring
  • For downhills or cruising along the flat, make sure the chain is on the largest front chain ring

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