The chainrings are one or more sprockets that transfer power from the crankset to the cassette via the chain.
More gears helps cyclists keep the same cadence at many different speeds. Depending on the type of bike, it may need one, two, or three chainrings. Road and mountain bikes use a range of speeds and have a need for many gear combos. Some genres like BMX, downhill, and hardtails have just one chainring. They save on weight, and have lighter profiles for neat tricks and stuff.
Chainrings can range from about 20 to 55 teeth, and can be constructed from aluminum alloys, steel, titanium, or carbon fiber.
The teeth on chainrings with multiple sprockets are usually cut in a way that helps move the chain shift. The middle chainring shifts up and down, so it usually has ramps on either side, while the highest chainring will only shift down. Its teeth only have ramps to one side. The chain will be set down on and picked up from the smallest cog, so it usually has the least amount of shape on its teeth.
Chainrings attach to the crankset by what is called a spider (arms extending from the drive side crank arm). On cheaper bikes, the crankset is one piece, and sometimes the chainrings are welded directly to the crank arm! When the chainring wears out, the whole crankset needs to be replaced. Other scenarios have a pin on the drive side crank arm that inserts into the chainring, providing the power.
With more expensive bikes however, the chaining bolts to the spider, making replacements easier and obviously more affordable. These can be either four or five bolt. Why two systems you ask? Mountain bikes have big scary chainrings that might flex under heavy load, so the thicker frame and rigidity is needed. Other bikes conserve weight and opt for four bolt chainrings.
Inexpensive chainrings are either just welded directly to the spider (the arms with which the chainring attaches to the bottom bracket) or have a hole where a pin on the crank arm powers the ring. One-piece cranksets are very interchangeable and therefore very cheap. They are compatible with many sizes of chainrings you can find for dirt cheap at your local bike shop. One reason this design isn't so popular is because it applies loads differently while riding, and can be very hard on bottom bracket cups.
Multi-piece cranksets are heavier, but are easier on the bottom bracket.
Most good quality chainrings are between $100 and $200, while some can be several hundreds of dollars more. High-end chainrings have bolt holes which they attach to the spider. Chainrings with 4 or 5 bolts are the most common.
Chainrings can really take a beating. If it isn't rocks and debris smashing or chipping the teeth on the cogs, they could snap if the chain shifts under heavy load.
If you find a bent or broken tooth on the chainring, don't sweat. Repairs can be quick and easy. To get the bolts off to detach the crank from the spider, it's likely only a hex wrench or screwdriver is needed. If replacing any of the middle chainrings, the crank arms will need to come off. Fetch yourself a crank puller.
Chainrings have a lot of statistics. To save time, tears, and money, order the right chainring. How many bolts does it use? What's the bolt center diameter? Do you want the same number of teeth on the new sprocket?
If it's only the big chainring that needs to be replaced, thank your lucky stars. Sometimes, the big sprocket can be replaced without even taking the crank arms off.
If you're replacing a double or triple chainring, the crank arms need to come off.
Some chainrings have tension requirements on their bolts, which might require a torque wrench. Securely fasten all bolts on the chainring and reattach the chain.
Chainrings vary in size from 20 teeth to 55 teeth. On bikes with more than one chainring, the sprockets are ordered largest on the outside to smallest on the inside. This helps the chain from falling off while riding. Chainrings have several key measurements:
Width - The thickness of the chainring.
BCD (Bolt Circle Diameter) - The diameter of the circle made by the stack bolts on the inner chainring. Normally you would measure from the center of one hole across the center of the chainring to another hole. The problem is that so many common chainrings have odd numbered bolts and therefore the bolt holes are not aligned for this measurement to be so easy. To find the BCD, we measure C-C, and make a calculation to account for the number of bolts.
C-C (Distance between adjacent bolts) - Measured from the center of one bolt hole to the center of the next.
NOTE: Because the "center" of the hole isn't there, you may decide to use the edge of the bolt hole for this measurement. Just be consistent.
The center to center distance is different between chainrings that use 3, 4, 5 and 6 bolts. To finally determine the bolt circle diameter (BCD), multiply the C-C as such:
|Common 5 bolt chainring sizes and applications|
|BCD (mm)||Smallest Ring||Center-Center (mm)||Application|
|151||44||88.8||Pre-1967 Campagnolo standard (now obsolete)|
|144||41||84.6||Old Campagnolo standard, still used in track bikes|
|135||39||79.5||Current Campagnolo standard|
|130||38||76.4||Standard outer chainring on road bikes|
|122||38||71.7||Stronglight 93, 101, 103, 104, 105 (obsolete)|
|118||36||69.4||Ofmega, SR (obsolete)|
|116||35||68.2||Old Campagnolo (obsolete)|
|110/112||34||64.7||Campagnolo CT inner/middle/outer with one bolt at larger diameter behind crank|
|110||33||64.7||Touring double, standard triple outer|
|100||31;36||73.6||Merz adapter; Campagnolo triple (obsolete)|
|94||29||55.4||Compact triple outer|
|92||30||53.3||Shimano Dura-Ace triple inner|
|90||32||52.9||Edco, Mavic triple inner|
|86||28||50.5||Stronglight 80,00,100, some SR triples (obsolete)|
|74||24||43.5||Standard ("full-sized") triple inner. Used with 110mm, 130mm or 135mm rings|
|58||20||34.3||Compact granny gear|
|56||20||32.9||Sun Tour compact granny gear|
|Common 4 bolt chainring sizes and applications|
|BCD (mm)||Smallest Ring||Center-Center (mm)||Application|
|146||44||103.2||Shimano XTR M960 outer|
|112||34||79.2||Shimano XTR M950, M952 middle/outer|
|104||32||73.6||Shimano XTR M960, XT, LX outer|
|102||32||72.1||Shimano 2003 XTR middle|
|68||22||48.1||Shimano XTR M950, M952 inner|
|64||22||45.3||Shimano XTR M960, XT, LX inner|
|58||20||41.0||Sugino MX350 inner|