How does gear ratio affect power output?

You may have heard this before, but power output is the single most important factor that determines your speed. Your gearing, and the resistance it presents, is what allows you to transfer the power generated at your legs to the wheels and the road.

“Put simply, power is a derived number made up from the multiplication of torque and cadence,” says Brian Taylor, president of Verve Cycling, leaders in power meter technology. ”Both torque (the force you apply to the cranks to turn them) and cadence (the rate at which you turn the cranks) are critically important for the cyclist to ride the way that their body is designed or trained to.”

In short, your gears should allow you to get the most from the power you’re able to produce, but not every rider is the same in how they generate power.

What kind of riding do you want to do?

Experience and your fitness level are key, too, and if you’re a new or inexperienced rider then it’s likely you will benefit from an easier set of gears so you can continue to turn the pedals as the road rises.

Indeed, the terrain you ride is intrinsically linked to the gear setup you’ll need. Are you planning on riding lots of hills because you live in an especially lumpy area, or will you tend to stick to the flatlands?

Ian Jenner, of Rule5Coaching, points out that a setup with smaller gear ratios will favour those people who intend to do lots of climbing, while larger ratios will allow greater speeds to be attained on the flat.

“If you’re going to do lots of hills in your riding, aren’t very fit or you’re new to cycling, a ‘compact’ chainset gives you a lot more options for small gears that you can spin.

“However, if you know you’re a strong, fit rider, perhaps looking to race, a ‘standard’ setup will help you achieve the higher speeds, provided you can turn the gear.

“In the middle is the ‘semi-compact’, which is usually ideally suited to the fit club and stronger sportive rider who may occasionally get a little competitive – a middle ground that gives you a lot of the benefits of both a compact and standard.”

Of course, this will often come down choosing the right gear as you’re riding – after all, you can potentially solve this particularly issue by clicking into an easier gear – but if can also be an indicator of having an over-ambitious gear setup for your current ability or the terrain, particularly if you’ve run out of gears sooner than you’d expect.

“Gearing and cadence are married together,” says Jenner. “In order to ride at an optimal cadence, you need to be able to ride in the correct gear.”

What are your choices when it comes to gear ratios?

There are two areas in which gearing can be customised: at the chainset (which we’ve already alluded to), and the cassette.

– The evolution of the chainset and the rise of the semi-compact –

Fundamentally, a lower number of teeth on the chainrings results in an easier gear, while conversely a lower number of teeth on the cassette provides more resistance, and therefore a bigger gear. Let’s take a closer look at the various chainset and cassette options.

The chainset

Standard: 53-39t
Semi-compact: 52-36t
Compact 50-34t

As we’ve already described, a compact provides the smallest gearing on a double chainset, and is ideal for new or inexperienced cyclists, less-fit riders, those who like to spin a high gear, or people who ride in a hilly area.

A standard chainset is at the other end of the spectrum, allowing for bigger gears which have the potential to let you generate more power or speed, but you need to be able to turn those gears to make the most of them. A standard is traditionally the favoured setup of racers, strong cyclists, or riders who live in largely flat areas.

A relatively new – and increasingly popular – option is the semi-compact, which straddles the divide between the two. It’s not uncommon to spot these on bikes belonging to strong climbers, sportive riders, and even racers who find a standard setup outside their optimum range for general riding. You can find out more about the evolution of the semi-compact in our guide here.

It’s also possible to find other options, such the 46-36t chainset often found on cyclo-cross bikes, while some chainset manufacturers are now offering even smaller chainrings (for example, 48-32t), sometimes found on gravel bikes, where the smaller gears are appreciated for steep, off-road climbs.

You can also get a ‘triple’, which adds an extra chain ring to the setup, normally in a 50-39-30t configuration. This used to be a common option for roadies wanting a wide range of gearing, but has slowly been phased out in preference of a double chainset and larger cassette. A triple chainset will require a specific front derailleur and levers in order to shift properly between all three chainrings.

Let’s take a closer look at two of those cassette: the 11-23t and 11-32t. The former will be ideally suited to flat rides or races,  and will offer smaller step-changes between gears, while the latter will allow for wider ratios (by virtue of the larger sprockets) and is better suited for riders in search of easier gears for hilly rides. The majority of bikes are now supplied with an 11-28t cassette, as this offers a sensible spread of gears for flat riding and climbing, with relatively equal shifts between each sprocket.

Changing the cassette is the easiest way to introduce lower gears to your bike, and it’s only in recent years that cassettes with a largest sprocket of 30 or 32-tooth have been introduced to give road riders more options than ever. However, stretching the chain across a wide-ranging cassette usually requires a long rear hanger to allow the movement required to actuate the shifts across to the largest sprockets.

Using your chosen gearing efficiently

Gears, however, don’t change by themselves, and if you’re confident you have the right gears on your bike, making the most of your ratios is vital to avoid stalling and losing momentum.

As Taylor explains: “There are two things that we want as a cyclist from our chainring and rear cassette combination.  The first is seamless shifting without really large steps, and the second is to have gears for virtually every eventuality.”

Jenner expands, explaining that it’s also important to understand the impact of your gear selection on the efficiency of your drivetrain.

“Let’s assume you’ve picked the perfect gear setup for you – for example, a 52-36t chainset married to a 11-28t cassette, which is an increasingly common combination on new bikes – but you may not understand how to use those gears efficiently,” he says.

“Firstly, you want to aim for a good chain line, where the aim is to run the chain in as straight line as possible. Twists in the chain, where it’s forced into a diagonal line because you’re using, for example, the 52-tooth chainring and 28-tooth cassette sprocket, isn’t efficient for you or your drivetrain.”

As a result, and as a general rule, you should use your big chainring in combination with the smaller sprockets on your cassette, and the small chainring with the bigger sprockets. This will help keep your chain in a straight line and avoid ‘cross-chaining’, where you’re in a big-big or small-small combination between the chainset and cassette.

Jenner also advises caution when shifting between chainrings. He explains: “When people shift between chainrings, I tend to see them then move up or down the cassette to regain their cadence.”

– Beginner’s guide: how to use road bike gears –

This can briefly impact on momentum, Jenner says, and instead advises smoothing this out by shifting on the cassette first – in effect getting the adjustment done early – then quicker moving across the chainrings to achieve an easier transition.

And there you have it – everything you need to know about ensuring you have the right gears on your bike. Ultimately, as Jenner points out, the bottom line is to remember what you want to get out of your cycling.

“Go back to what you need the bike to do,” he explains. “If in doubt, it’s best to go with a setup that gives you options without over-challenging you.”