The crankset is the guts of the drivetrain. It consists of the pedals, crank arms, chainrings, and the bottom bracket working in concert transferring power from the rider's legs to the cassette and eventually the rear wheel.
The crank arms connect the bottom bracket axle with the pedals. They are manufactured from many materials, such as: aluminum alloy, titanium, carbon fiber, and some steel alloys. The Shimano "Hollowtech" crank arms are made by casting around a hard metal that when removed leave a hollow space, saving on weight.
Crank arms can vary in length, which affects the amount of torque a rider can apply to the drivetrain (in one gear). However, bicycles are built to have many gears and the length of the crank arms is only customized to fit the length of the rider's legs.
There are numerous interfaces which crank arms attach to the bottom bracket. In the old days, one piece cranksets were used. The crankset is threaded through the bottom bracket, and the pedals are attached when it's in the frame. This design is powered by a pin on the crankset sitting in a pinhole on the chainring. One-piece cranksets are hard on the bottom bracket because of the loads being applied differently as it rotates and rolls in-frame.
Most cranksets now are multi-piece, consisting of 2 or 3 pieces, depending if the spindle is independent, or attached to one of the cranks. These disperse the load applied on the spindle by the rider more evenly during use, and results in less maintenance.
Cranksets can be built for many purposes of riding, so there is a lot of diversity in the market. Big-name manufacturers such as Campagnolo, Shimano, SRAM, and FSA seem to have a crankset for every budget.
On the lower end, cranksets can be just a little more than $50. The only bottom bracket interface we could find for this price were square-tapered.
Middle of the road cranksets are around $150 on Amazon.com. Here you can start to find cranks made by the manufacturers we all know and love. The metal alloys can be trusted to be durable, and they come in a variety of interfaces.
The most expensive cranksets to be found from companies like SRAM, Campagnolo, and Shimano on Amazon are around a jaw-dropping $3,300. These are for the cycling junkies and competitive animals that make up the world-class circuits we can only watch on tv or dream about in our sleep. Obviously these interfaces are all Octalink or ISIS drive. What makes a crankset worth that kind of money? Generally, higher-end cranksets are much lighter weight than the cheaper ones. They are comprised of advanced alloys and carbon fiber designs.
Replacing a crankset is actually a really easy process, and aside from differences in the interfaces, the same with all multi-piece cranks.
Square taper cranks will have a hex bolt of some kind pinching the cranks together between the bottom bracket. Unscrew the bolts and the cranks can slide off the spindle. Remember that the bottom bracket system is a tight squeeze, so if the spindle has a hard time coming out, a rubber mallet can be very convincing.
Octalink and ISIS drive crank interfaces require a special tool called a crank puller(kinda like a freewheel remover) to fit in the interface grooves and the threads in the spindle. A nut on the crank puller threads in until it's snug. As the handle is then rotated, it drives a pin through the shaft of the crank puller, separating the crank arm from the spindle. Be very careful that all threads are snug, and not cross-threaded. With a crank puller tool, any Octalink or ISIS drive crankset can be unscrewed and removed.
Remember to put the chain on the bottom bracket shell before installing the crankset. Obviously the crank arms need to be 180 degrees opposite of one another. On the intricate Octalink or ISIS drive interfaces, improper installation can damage either the crank arms or the spindle.
When the cranks are aligned and fit into their respective interfaces, the bolts should be tightened to around 305-391 inch pounds (according to ParkTool.com)
Crank length is something often customized by the rider, and has a significant affect on the performance and comfort of the rider. Most standard crank arms can range in size from 165-180mm, but other specialized cranks exist. Riders have different sized legs, and differences in their muscle flexibility. In order to keep an easy cadence with the correct amount of travel each stroke, each rider needs a different setup. Saddle height can account for leg length, but the stroke is determined by the length of the crank arms. Some riders can use extremely low handlebars and bounce their quads on their torso as they ride. Others aren't so flexible but might have stronger muscles in different areas, so a shorter stroke is preferred for them. Another factor is if a rider has a longer foot, they can naturally extend more. If the length is too long, you run the risk of bottoming out the crank arms in the corners, so take care.
Experts have different opinions on how to calculate proper crank length. One formula sounds off:
Crank length in mm = inseam measurement (in inches) x 5.48
Inseam measurement is a fancy word for the length between the ground and the crotch of the rider.
A common theory is that a longer crank will give the rider more torque on the bottom bracket, providing a more powerful stroke. The Martin study shows that between 145mm and 170mm, there is no statistically significant difference in the amount of power a rider can provide. Even if proven, bicycles have multiple gears designed to spread the power band of the rider through a wide range of speeds.
Another theory is that crank arms should be 9.5% of the rider's height:
|Rider's height||Crank length|