What do bike gear ratios mean?

Knowing what bike gear ratios mean is vital knowledge for any progressing cyclist or triathlete.

Once you understand the interaction between your chainrings and cassette you’ll select the right gear more of the time. This enables you to pedal more efficiently and prevent premature component wear.

In this article, we’ll break down what bike gear ratios mean, the difference between brands’ gearing, and how to choose the right gearing for training and racing.

What do bike gear ratios mean?

Before getting bogged down, we need to discuss teeth. You have your chainrings at the front – the rings up by the pedals – and the sprockets at the back – the ones attached to the hub of your rear wheel. And they all have different numbers of teeth.

Chainring teeth

Chances are that you have two chainrings attached to your spider and cranks. Together these form a crankset.

On gravel bikes, a single chainring, known as a 1x drivetrain, is more popular.

What is traditionally known as a standard crankset consists of a 53-tooth big chainring and a smaller 39-tooth inner ring, often written as 53-39T or 39/53T.

The more teeth a chainring has the further it propels the bike with one rotation. This also makes it harder to turn. As a result, a standard crankset isn’t suitable for all riders.

These days mid-compact (52/36T), compact (50/34T) and sub-compact (48/32T) are more common on non-professionals’ road bikes.

These chainring configurations refer to Shimano and Campagnolo groupsets. SRAM offers 50/37, 48/35 and 46/33 tooth chainring combos.

Cassette teeth

Then you’ll probably have somewhere between 10 and 13 sprockets at the back, depending on the groupset fitted to your bike.

On road bikes, Shimano’s 11- and 12-speed cassettes start with an 11-tooth smallest sprocket and go up to a 36-tooth largest cog.

SRAM also has a 36-tooth largest cog but starts with a 10-tooth cog for a harder gear.

Campagnolo’s 11- and 12-speed cassettes run from 11 teeth to 29, 32 or 34 teeth.

So the cassette (the group of sprockets) might range from a 11-tooth small sprocket to a 32-tooth large sprocket.

That would be described as an 11-28T cassette. The fewer teeth on the sprocket, the higher (harder) the gear.

It’s the ratio between the number of teeth on the chainring and the number of teeth on the sprocket that tells you how hard the gear is.

If the chain is on the 39-tooth chainring and a 13-tooth sprocket (a 3:1 ratio), you’re in a bigger gear than if you’re in the 52-tooth chainring and a 21-tooth sprocket (roughly 2.5:1).

Bear in mind that using a small-small or big-big chainring (eg, the front ring and a large cassette cog) causes cross-chaining. This is okay from time to time but if done excessively the friction caused by an angled chain will wear out drivetrain parts.

Which gear ratios are best for triathlon?

There are few hard and fast rules about what gear ratio you should use at any particular time. The right choice comes down to many factors, including terrain, conditions, your strength and fitness, and how your body works.

Speaking of which, now is the ideal time to experiment with your gear selection and bike cadence.

Spend some time trying out different approaches – small gears with a high cadence; big gears with a slower cadence – to work out what’s most comfortable and effective for you.

It is possible to generalise a bit though.

Start low

You’ll almost certainly want easier gears as a beginner triathlete or cyclist.

Otherwise you could become overgeared, ie not have a low enough gear to maintain cadence on climbs. Smaller chainrings or a larger cassette should help.

Being undergeared is generally better than being overgeared.

Build up

But if your chainrings, particularly the front, are too small you’ll spend a lot of time at the bottom of your cassette block.

This is bad for chainline efficiency because less energy is lost from your drivetrain when the chain is straight.

When extremely undergeared, you might ‘spin out’ (be forced to turn the pedals excessively quickly) in fast group rides and on descents.

Other than a marginal weight increase and fractionally bigger jumps between cogs, there are no drawbacks to having a large cassette.

Indeed even pro cyclists and triathletes have caught on to the benefits of sprockets with as many as 34 teeth.

Adapt for training and racing

Another thing you can generally say is that you’ll want harder gears for racing than training.

Triathlon training plans will prescribe plenty of steady riding, especially in the base phase. During these rides, lower gears will help you maximise time spent in zone 2 to build endurance.

By contrast, when you’re pushing in a race, larger chainrings are more efficient and make it easier to find a comfortable cadence at higher speeds and power outputs.

Armed with knowledge, you’ll be able to pick an appropriate cassette if you upgrade to a pair of the best triathlon bike wheels.