Of all the parts of a bicycle — from the frame and wheels, to the derailleurs, gear levers, and even brake calipers — the chainset is possibly the least exciting. It’s really just a few relatively simple bits of metal, attached together, which do nothing more than spin. Yet, in terms of the mechanics of how you interact with your bike, it’s a surprisingly crucial component.
The chainset is made up of three elements. There’s the spider, which is the drive side crank arm to which you attach chainrings and the right pedal; the non-drive side crank arm to which you attach the left pedal; and the chainrings, on which the chain runs.
One very obvious personal consideration — although one many cyclists might not think about enough — is the length of the crank arms. The relationship between crank arm length and the rider’s leg length will determine the range of motion that the rider’s leg will travel through during each pedal stroke. If a crank arm is too short, it was thought (as we shall see, erroneously) that the rider wouldn’t be able to generate the same amount of pedalling power. If it is too long, the rider’s legs will experience a potentially uncomfortable pedalling action.
On modern road bikes most manufacturers have decided upon a range of crank arm lengths between 170 to 175mm. Very generally speaking, you find 170mm cranks on small bikes, you might find crank arms of 172.5mm on medium bikes, and there’ll be 175mm cranks on large bikes. On custom order bicycles you may be able to specify a different crank arm size, but it will still probably be limited to between 165mm at the very least and 180mm at the most.
Why this choice of lengths? They’ve actually been settled on fairly arbitrarily, although the general idea is that they offer a decent compromise between leverage force and range of leg movement for average sized riders. In truth, though, that notion of bigger cranks helping transfer leverage force is misplaced, and there is a growing body of opinion that suggests smaller crank arms offering a smaller range of leg movement may be considerably more efficient.
Lord of the rings
The second aspect of chainsets that affects every rider is the choice of chainrings. In days past, road riders had just two options: a triple chainset with three rings of something like 52, 42 and 30 teeth; or a double chainset with two chainrings, normally of 53 and 39 teeth. The triple chainset offers a broad range of gears, including very low ones — so is great for applications such as cycle touring — while the double is lighter and quicker, so great for racing.
But just as in the last few years road bikes with drop bars have subtly veered away from being purely race bikes so, too, chainsets have adapted and the most popular option on new bikes is the compact chainset. Like the double this has just two rings — normally 50 and 34 teeth — but if combined with a suitable cassette at the back — say one ranging from 11-28 teeth — you can achieve a spread of gear ratios to almost rival a triple. So ‘compacts’ offer easier gears than an old-school ‘double’, very similar top gears to a ‘double’, and have essentially the same weight.
Another development in cycling over the last 10 or 15 years has been the accepted efficiency of higher cadences — pedalling faster. This puts less strain on the leg muscles, meaning less build up of lactic acid. Components such as ‘compacts’, which allow for easier, higher-speed pedalling are a perfect complement to this approach.
Compact chainsets aren’t for everybody, but for most leisure road riders — especially those who take on hilly sportives — they are a great compromise. In fact, team a 34-tooth inner ring with a 32-tooth biggest sprocket on a wide-range cassette at the rear, and you should be able to climb a brick wall!