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The crankset, or chainset depending on where you’re from, is a vital part of bike specification that radically affects the rider’s experience.
Why is it so important?
Before the mid-2000s, road bikes always came with a “standard” crankset. The biggest gear was big, and the lowest gear was … still quite big. While it was easy to be confident on flat terrain, many cyclists suffered epically on hills.
Today’s road bikes usually have a “compact” crankset fitted, which changes everything from pedaling technique to mindset.
This article compares compact versus standard cranksets and shows you how to choose between them.
What Is A Crankset?
If you think of a drivetrain as the set of components that propels a bike forward, the crankset is the part that does the actual driving (i.e., it drives the cassette sprockets via the chain.) A bicycle is a rear-wheel-drive machine, but the mechanical impetus comes from the crankset, powered through the cranks by your legs.
On a typical road bike, the crankset comprises one big chainring, one small chainring and two cranks. In a standard crankset, the big chainring has 53 teeth (abbreviated as “t”) and the small ring has 39. A compact crankset has 50t and 34t rings.
For purposes of smooth gear shifting, the difference between the two rings in any crankset is not normally more than 16 teeth. Indeed, manufacturers have lately been working to reduce this gap.
How the Crankset is Put Together
Each chainring is bolted to the crankset “spider”, which in turn is attached to the right-hand crank.
Standard and compact chainrings have differing bolt circle diameters of 130 and 110mm, respectively, but they are often identified by the center-to-center distance between adjacent bolt holes.
Ease of Pedaling
The size of each chainring in a crankset and the number of teeth it has affects gearing and how easy it is to pedal.
Why? At any given speed, assuming you don’t change gear at the back, your cadence will be higher on a smaller chainring than a big one.
And that means you’re applying less force with each pedal stroke.
Gear Ratios & Their Meaning
A gear ratio is found by dividing the number of teeth in the front ring by the number of teeth on the rear cassette sprocket.
A compact 34t chainring at the front and a large 34t sprocket at the back gives a 1:1 ratio, for instance, meaning the wheels turn a full revolution for every turn of the pedals. On most climbs, this would be an extremely easy gear.
It is possible to install extreme MTB cassettes up to 11-42t on road bikes.
In decades gone by, a standard 39/25t gear combination was typical for climbing (1.56:1 ratio). Jumping to a bigger gear ratio makes you go faster, but only if you can hold the same cadence by delivering more power.
Video: Gears and Gear Ratios Explained
The Pro Influence
In professional cycling, there has long been a trend towards high-cadence “spinning” at 90 to 110 rpm, particularly during climbing attacks.
Although this means less force is put through the pedals per revolution, maintaining a high cadence uphill at professional speeds demands impressive power (a measure of torque x cadence).
Pro racing cyclists sometimes use a compact crankset on big climbing stages to avoid being overgeared.
When Chris Froome won a Giro d’Italia stage on Monte Zoncolan in 2018, he rode a 34/32 lowest gear. Most amateurs would need the same, but mere mortals would still be reduced to a slow cadence on such a climb.
Video: Chris Froome winning on the Zoncolan
Standard Crankset – 53/39
With all that’s been said in this article, you’d be forgiven for thinking that a standard 53/39 crankset has little going for it. But that’s not the case. Some riders have good reasons for choosing big chainrings.
Time trialists unfailingly use them and might even go bigger than a 53t.
On a standard crankset, larger chain rings and larger rear sprockets at any given speed reduce frictional losses. Drivetrain efficiency is also helped by a good chainline.
This is easier for many to achieve with a 50t compact chainring than a big 53t ring, but it’s a different matter on the 39t small ring.
The middle four sprockets on a 10-speed cassette create a good chainline with either of the front chainrings.
A standard crankset is ideal for endurance training on the 39t chainring, where moderately fast speeds of 17 to 20+ mph are sustainable without needing to switch to the bigger ring.
This is achievable at a high 90 rpm cadence, which would likely cause “cross chaining” on a compact 34t ring.
Standard Cranksets Are Suited To…
Standard cranksets make sense for strong and muscular riders, people who prefer flat terrain, or racing cyclists who might feel undergeared with a compact crankset.
Whether racers are literally undergeared or not with a compact crankset is immaterial in the end. They may just prefer to keep their cadence in check during hard efforts or among a fast group.
Compact Crankset – 50/34
Many off-the-peg bikes these days come with a compact crankset by default, with cranks that are sized in proportion to the frame. It’s all about the law of averages, but there are various things to consider before deciding on an appropriate crankset.
Does a compact crankset make you slower? No. In theory, you’ll “spin out” smaller chainrings at a lesser speed than larger ones, but this speed is so fast that most people needn’t worry about it.
For example, a typical top 50/11 gear ridden at a cadence of 90 rpm will move you along at 32+ mph (or over 51 km/h).
Video: The argument in favor of a compact crankset.
Racers may have a genuine need for a standard crankset, especially if they race flat crits, but many do race on compact cranksets. The choice must suit a rider’s strengths and riding preferences.
Some feel more in control of a bike and the pedal stroke if their legs are not spinning furiously.
A flipside for stronger riders is that a compact crankset allows a tighter gear range on the cassette. For instance, they could install an 11-23t cassette, which is almost equal to an 11-28t on a standard crankset.
This makes for smoother gear changes, just as a narrower difference between chainring sizes does.
Compact Cranksets Are Suited To…
Compact cranksets are ideal for anyone that often climbs steep hills. The ease with which you pedal can be moderated by cassette choice, so there’s plenty of potential to calibrate gearing.
Cyclists with “big engines” don’t always have tree-trunk legs, so they might need a crankset that emphasizes fitness over muscle.
Mid-Compact / Semi-Compact Crankset – 52/36
Many new bikes come with a “semi-compact” crankset fitted as standard. What’s one of those? The semi-compact crankset includes 52t and 36t chainrings, so it looks at first glance like an in-between choice.
But it doesn’t give much away to a 53/39 crankset in terms of a top gear.
A handy feature of the semi-compact crankset is its 110mm bolt circle diameter, which matches that of a typical compact crankset.
That means you can switch to a semi-compact configuration merely by swapping the chainrings. If you were changing from standard to semi-compact, you’d need to buy a complete crankset.
One reason that alternative chainring sizes have become popular in recent years is the wider range of cassette sprockets available.
At the easy end of a cassette, a 34-tooth sprocket is now possible, while SRAM’s 10-tooth sprocket makes smaller chainrings feasible at the front without forfeiting anything at the top end.
A disadvantage of the standard crankset with an old-fashioned gear range is that it often forced cyclists out of the saddle on steeper climbs, placing a significant strain on their core.
It should be possible to stay seated with a semi-compact crankset, but it’s not going to mollycoddle you up a hill quite like the smallest chainrings do.
Semi-Compact Cranksets Are Suited To…
A semi-compact crankset is typically found on performance road bikes, so that tells you something about the target user.
Anyone that likes to ride fast over flat terrain and perhaps climb sitting down will enjoy the semi-compact. It’s not for anyone whose chief priority is effortless climbing. In many ways, it’s an updated “standard”.
Final Word on Road Bike Chainsets
In summary, go for compact (or even sub-compact) if you want knee-friendly climbing; semi-compact as a fast, modern all-rounder; and standard if you train/race on flat terrain or have ultra-strong legs.
You can learn a lot just by looking at the gear choices you naturally make. If you find yourself often on the biggest or smallest rear sprockets, it may be worth trying a different crankset.
Please feel free to leave comments, add your own experiences or share this article.