Chainrings (also called “chain rings”, “chainwheels” or “sprockets”, although sprocket is used this way mostly in the BMX community) engage the chain to transfer power to the (usually rear) wheel. They usually have teeth spaced to engage every link of the chain as it passes over; however, in the past, some designs (called skip-tooth or inch-pitch) have had one tooth for every other link of the chain.
By convention, the largest chainring is outboard and the smallest is inboard. Chainrings vary in size from as few as 20 teeth to as many as 60 and potentially more.
Chainrings also come in several nominal widths:
3/16″ (4.76 mm) for old-time bikes (especially skip-tooth or inch-pitch), heavy duty BMX, Worksman, and exercise bikes
1/8″ (3.18 mm) for track, BMX, cruiser bikes, one-speed, three-speeds, and the rare derailleur bike.
3/32″ (2.38 mm) for road, hybrid, mountain bikes, single-speed and 5-, 6-, 7-speed freewheels.
5/64″ (1.98 mm) for any bike with 9- or 10-speed cassettes
Chainrings are constructed of either an aluminum alloy, titanium, steel, or carbon fiber.
Cheaper cranksets may have the chainrings welded or riveted directly to the crank arm or spider. More expensive sets have the chainrings bolted on so that they can be replaced if worn or damaged, or to provide different gearing.
Replacement chainrings must be chosen with a bolt-hole count and spacing that matches the spider.
Chainrings designed for use with multi-chainring crank arms may have ramps or pins to aid in shifting. The middle chainring, in the case of a triple crankset, usually has the most shaping to aid in shifting up and down. The smallest chainring usually has the least, if any shaping.