18-speed bike vs 21-speed – Differences Explained

The difference between an 18-speed bike vs 21-speed is often small. Many of these bicycles share similar parts and only differ in the extra gears of the 21-speed bicycle.

But does that mean that they are essentially the same bike?

And are 21-speed bikes always better?

Or are there 18-speed bikes that are (much) better than 21-speeds?

Let’s find out.

18-speed bike vs 21-speed – What’s the difference?

18-speed vs. 21-speed bikes are very similar. Both have 3 chainrings in the front. But the 18-speed has 6 cogs in the rear while the 21-speed has 7. An 18-speed can also have 2 chainrings in the front and 9 cogs in the rear. These 2×9’s are much better bikes.

But there is a little more to it than just that.

So let’s take a deeper look into all the differences between 18- and 21-speed bikes.

The Difference in gear ranges

Obviously, there are 3 additional gears on a 21-speed bike compared to an 18-speed bike.

That means that there are 21 steps on a 21-speed bike between the lowest to the highest gear.

It does not mean that a 21-speed bike has any higher gears than an 18-speed bike. Or that a 21-speed is any faster.

The more important question is if a 21-speed bike has a wider range of gears.

So, is the lowest gear lower and the highest gear higher on a 21-speed bike?

HOW TO MEASURE THE GEAR RANGE on an 18-speed bike vs 21-speed

The easiest way to measure the gear range is to count the teeth on the smallest and largest cogs (or gears) in the back.

Then, divide the biggest cogs by the smallest.

So for instance, a 12T-32T has a small cog with 12 teeth and a large cog with 32 teeth.

That works out to 32/12 = 2.66. We would say that this cassette has a gear range of 266%.

We can do the same for the chainrings in the front.

The chainrings in the front of an 18-speed bike vs 21-speed are often very similar. So this has less of an impact than the cassette in the rear.

The 3x drivetrain on 18-speed and 21-speed bikes

Most 18-speed and 21-speed bikes have a 3x drivetrain.

That means that they have 3 chainrings in the front.

• The 18-speed has 6 cogs in the back (3×6 = 18 speeds)
• The 21-speed bike has 7 cogs in the back (3×7 = 21 speeds)

The benefit of a 3x drivetrain is that you have one chainring for accelerating and uphill, one chainring for cruising and flats, and one chainring for high speed and downhill.

The downside of a 3x drivetrain is that you have some duplicate gears.

Practically speaking, a 21-speed bike has around 12 unique speeds.

The remaining 9 speeds are duplicates or so similar to others that you can’t feel the difference.

That is not necessarily a bad thing in usage since it makes shifting between chainrings much smoother.

But it is an inherent characteristic of a 3x drivetrain that it only provides a little more than 50% unique speeds.

The 2x drivetrain on an 18-speed bike

This is the new kid on the block that can be found on many modern bikes.

Instead of 3 chainrings, it only has 2 chainrings in the front. But it makes up for it with 9 cogs in the back.

A typical cassette is 11T-34T with a gear range of 309%.

That does not sound all that impressive compared to our 266% gear range of the 3x drivetrain example above.

But it has 9 unique steps in that range, giving you a little more climbing power and top speed with smoother transitions between gears.

It also has fewer duplicate speeds than a 3x drivetrain.

Overall, it is a superior drivetrain with much more range.

But it also comes at a cost.

New bikes with 2x drivetrains are usually found in the mid-to-upper price segment. 3x drivetrains on the other hand are plentiful in the lowest price bracket.

So the 2x may not make sense for everyone.

But it’s really nice to have.

Can I use 18-speed bike parts on a 21-speed bike (and vice versa)?

Cassettes

Cassettes compatible with Shimano and SRAM use different spacing on an 18-speed vs 21-speed bike.

6-cog cassettes found on 18-speed bikes usually have a 5.5 mm distance between cogs, and 7-cog cassettes (on 21-speed bikes) have a 5 mm distance.

That means you may be able to put a 6-cog cassette on a 21-speed bike.

But you will have to increase the shifting distance of the derailleur to allow for smooth gear changes and likely add a spacer onto the hub.

Unfortunately, a 7-cog cassette does not fit on an 18-speed bike without swapping in a new derailleur and shifters.

You will also usually not be able to add a 7th cog to a 6-cog cassette. The width of those 7 gears would be too great for most bike frames.

Be cautious of cassettes vs freewheels

Keep in mind that many 18 and 21-speed bikes use freewheels instead of cassettes.

A freewheel is a self-contained unit that has all cogs pinned together and mounted onto a central hub.

They are especially common in budget bikes.

Cassettes will not fit onto the bikes designed for cassettes without replacing the entire rear wheel or adding a different hub to your existing wheel.

Chains

Most chains are interchangeable between 18, 21, and 24-speed bikes.

Some chains, however, are made for 21, 24, and 27-speed bikes and don’t fit on 18-speed bikes.

So don’t assume that your existing chain will fit and be sure to read the product info before purchasing any new bicycle chains.

Shifters

For obvious reasons, 6-speed shifters found on 18-speed bikes are not compatible with 21-speed bikes.

A 6-speed shifter found is only going to result in a 6, 12, 18, or 24-speed bike (depending on the number of chainrings).

But 7-speed shifters (from 21-speed bikes) can be made to work on an 18-speed bike if you are okay with having an “empty” 7th gear.

However, switching to this empty 8th gear will pop the chain off the cassette unless you are able to adjust your derailleur and shifter just right.

Both scenarios are not great.

So converting the number of speeds on a bike should always be followed by upgrading shifters as well.

They are cheap enough to be a no-brainer.

DERAILLEURS

A rear derailleur is a small arm that physically moves the chain onto the next cog when shifting up or down.

Thin wires connect the derailleur to the shifter. By changing the wire tension, the shifter makes the derailleur derail the chain to jump into another gear.

Generally speaking, derailleurs are designed for a specific number of speeds.

So most 6-speed derailleurs (for 18-speed bikes) are incompatible with 21-speed bikes and vice versa.

Some derailleurs like this one from Shimano work on 6-speed (18-speed), 7-speed (21-speed), and even 8-speed bikes, however.

The front derailleur is the exact same part.

18-speed bike vs 21-speed – Which one is better?

18-speed and 21-speed bikes (with a 3x drivetrain) are too similar to say which one is better.

It is more important to pick the right bike for a particular situation (city, trails, racing, etc.) than to obsess over speeds and gear ranges.

That being said, a 21-speed bike is usually a little more versatile across the board and would be my first pick.

Those 3 extra speeds often translate to slightly better acceleration and higher speeds.

In fact, here are two posts about how fast an 18-speed bike can go and how fast a 21-speed bike can go.

But given the chance, I would pick a 2×9 18-speed bike every day of the week.

Even if it comes at a price.

It’s well worth it to me.