What is a semi-compact chainset?
A semi-compact chainset (the crank arms and chainrings combined) consists of two chainrings. The big (outer) ring typically has 52 teeth and the small (inner) ring most commonly has 36 teeth although it may sometimes have 38.
A semi-compact chainset offers a middle ground between standard road double chainsets, typically with larger 53/39 chainrings, and the more common compact chainset which typically have 50/34 teeth chainrings.
With two chainrings rather than three, there is also the advantage that the chainline (how straight the chain runs from the chainrings to the cogs on the rear) will be straighter more of the time which, is mechanically more efficient. The chain on a road triple chainset, typically with 50/39/30 tooth chainrings, is forced to cover a wider area which is mechanically less efficient. A triple will also be heavier with that third chainring, but does offer more gears - ideal for less fit riders, those riding consistently hilly terrain and cyclists carrying heavy loads such as on touring trips.
What bikes do semi-compact chainsets typically feature on?
Like standard road double chainsets, semi-compact chainsets typically feature on bikes that are designed to go fast such as road racing or time trial / triathlon bikes. The larger chainrings on a semi-compact chainset (typically 52 big outer and 36 small inner chainring) are better suited to riding at higher average and top speeds. Conversely larger chainrings are less efficient on long arduous ascents such as those found in the Alps where compact or triple chainsets are usually a better choice, especially for leisure cyclists.
Click here to see all bikes that feature semi-compact chainsets.
What are the limitations of semi-compact chainsets?
For amateur racers and professional road cyclists a semi-compact or standard road double will make sense most of the time. For those less concerned with going as fast as possible, a compact (the most common choice) or road triple chainset is likely to be an ideal alternative. Compact chainsets are very popular as they offer a broad range of gears and are covered in more detail here. Similarly road triple chainsets are well suited to hilly terrain and those carrying heavy loads whilst touring - covered here. On seriously hilly stages in races such as the Tour de France even professionals will opt for compact chainsets to help them climb long arduous ascents.
Compared to semi-compact and standard road double chainset, a compact chainset will allow you to spin your cranks faster uphill (thanks to the easier gears available) which will make climbs feel easier to pedal up. This is especially useful for long or steep inclines where spinning up hills with a higher cadence (revolutions per minute) is generally considered to be more efficient, although perhaps slower. A higher cadence also means it is much easier to stay seated whilst going uphill.
Bike List Top Tip: it pays to look after your components
Keeping your chainset, cassette (the group of smaller cogs on your rear wheel) and chain clean and well-lubricated can make a big difference to how efficient your bike is. If your chain is dirty it can be up to 7 or 8% less efficient - the equivalent of being 5 or 6 kgs heavier on a climb. Source - The Obree Way, A training manual for cyclists, Graeme Obree.
Converting your semi-compact chainset to a compact or standard road double.
Changing your semi-compact chainset to a compact setup will typically only require new chainrings as 52/36 chainrings typically have the same BCD or Bolt Circle Diameter as 50/34 chainrings, see below for more details. As only new chainrings are required this is by far the simplest and cheapest change you can make to your chainset. If changing from a semi-compact or compact chainset to a standard road double a completely new chainset will be required as typically standard road double chainsets have a larger BCD.
What is BCD and why is it important?
BCD stands for Bolt Circle Diameter and is the diameter of the circle created by, in this case, the five holes at the end of the arms on the spider - the multi armed piece that connects the chainrings to the crankset.
BCD 110 And 130
The BCD number must be the same for both the crankset and the chainrings otherwise they won't be compatible.
The BCD on the majority of semi-compact and compact chainsets is 110mm whereas the BCD on the majority of standard road double chainsets is 130mm. This is significant as each BCD places restrictions on the maximum or minimum size of the small chainring. The small chainring must have 38 teeth or less on a 110mm BCD chainset and must have 38 teeth or more (usually 39) on a 130mm BCD chainset. This means you'll need a 130mm BCD if you want to run a 52/39 or 53/39 or 54/42 racing / triathlon / time trial specific chainring combination and if you want to run a 52/36, 52/38 or 50/34 etc you'll need to opt for the 110mm BCD crankset.
Converting your semi-compact to a road triple chainset.
Changing from either a compact, semi-compact or standard road double chainset to a road triple or vice versa will typically require a new left shifter, front derailleur and chain as well as the new chainset which means this type of change is usually not cost effective.
The Bike List Top tip: replace instead of converting
Price up a conversion from two to three or three to two chainrings, and include labour costs as well as the parts mentioned above, as in some cases it can be cheaper to replace the bike with one that has the setup you want and sell the one you have.
What cassettes are semi-compact chainsets typically paired with?
Semi-compact chainsets are typically paired with cassettes (the group of smaller cogs on your rear wheel) that have an 11 tooth small sprocket as the smallest cog giving you a 52 + 11 biggest gear.
They will also typically come with a cassette that has one of the following sprocket ranges: 11-21, 11-23, 11-25, 11-26.
To work out gear ratios, speed at cadence or cadence at speed visit BikeCalc.com for more information.
Changing A Cassette
These gear ranges are relatively narrow compared to others that are available as racers and time triallers typically prefer small incremental changes which allow for smooth gear changes with minimal difference in pedalling frequency or RPM (revolutions per minute).
That's not to say you can't change the cassette to one that has a broader range. Changing a cassette is a cost effective way of adapting your bike to make it more suitable for different terrain. A simple change to a cassette with a wider range of sprockets such as 11-26, 11-28, 12-28, 12-30 or 11-32 will make the same bike noticeably easier to pedal on hillier terrain.
If you shop around you can usually find a good cassette such as Shimano 105 (which strikes a good balance of value and quality) for around £25-£35. A like for like cassette replacement will usually be around 3-4 times cheaper than a like for like replacement chainset.
Read our guide to changing a Shimano 10 speed cassette here.
The Bike List Top tip:
Most rear derailleurs will be capable of running a number of different cassette ranges. If the largest sprocket on the cassette is changing significantly in size make sure your rear derailleur can accommodate the larger sprocket. Most road bike rear derailleurs are available in a short cage (small gear range), mid cage (middle gear range) and sometimes long cage (wide gear range) designs.
Summary of semi-compact chainset:
52/36 or 52/38 are the most common chainring teeth combinations.
Will have either 10, 12, 14, 16, 18, 20 or 22 gears.
Offers bigger (harder) gears than compact chainsets making it ideal for riding at high speeds such as when road racing or time trialling.
Weighs less than a triple chainset.
Not as efficient as a compact or road triple chainset when pedalling up hills.
The smallest cog on the cassette will typically have 11 teeth.
Well suited to pairing with a cassette with a small range to allow for small incremental gear changes and therefore smooth pedalling when changing gears.
The chainline will be straighter more of the time than on a road triple and therefore more efficient.
Much easier and cheaper to change to a compact chainset than it is to change to a road triple chainset.
Semi-compact and compact chainsets typically have a 110mm BCD where as standard road double chainsets typically have a 130mm BCD.
Changing from a semi-compact chainset to a compact typically only requires new chainrings.
Changing from a semi-compact chainset to a standard road double typically only requires a new chainset and chain.
Changing from a semi-compact to a road triple chainset is often not cost effective as it requires a new chainset, left shifter, front derailleur and chain. So, buying a new bike and selling your old one can be a better option.
To find out more about compact, standard road double and triple chainsets click below to read our comprehensive guides.
Compact Chainsets - The Complete Guide
Standard Road Double Chainsets - The Complete Guide
Road Triple Chainsets - The Complete Guide
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