On a bicycle, the cranks connect the pedals to the chainring, so you can deliver power to the wheels. Besides the wheels, the cranks are the bike parts that take the most abuse. Every time you pedal, you bring your entire weight plus all of your strength down onto the pedals. Typically, cranks come in two different varieties: aluminum and carbon.
Each material has advantages and disadvantages that you want to consider. As a general rule of thumb, bike parts can only fit into two out of the three following categories: lightweight, durable, and inexpensive. If a part is lightweight and durable, then it will not be cheap. If it is inexpensive and durable, then it will not be lightweight, and so on. When it comes to choosing between aluminum and carbon, you have to decide which two of these are most important.
Weight Bonded carbon parts are typically lighter weight than aluminum. Carbon cranksets tend to weigh between 700 and 800 grams (24 to 28 ounces). That is less than two pounds for both crank arms and the full set of chainrings. Aluminum cranksets, however, tend to weigh between 900 and 1000 grams (about two pounds). While the difference between 28 ounces and 35 ounces might not sound like very much, that is a 20% weight difference. Every time you pedal, you will be using 20% more energy. On a long ride or up a steep hill, that difference could quickly become exhausting.
Durability Where aluminum falls behind bonded carbon in weight, it is the definite winner when it comes to durability. Aluminum arms tend to be stiffer than carbon fiber because they tend to have a larger construction. Stiffer cranks are often preferable, especially for anyone who wants to sprint. This is because more of your kinetic energy is delivered to the wheels, and less is lost due to the arms flexing. The metal is also more durable because it is one solid construction as opposed to a blend of fibers such as carbon.
Price Bike parts tend to range in price from the standard consumer quality to the very expensive, professional grade. These parts can be as inexpensive as $50.00, and they can be as expensive as $2,100. The price is often the factor that determines the other two qualities. As the price of a particular part increases, the weight decreases. Carbon cranksets start at around $1,000, whereas metal ones run upwards from $50.