Bicycle Drivetrains 101
This entry was posted on February 8, 2012.
Your bicycle's drivetrain is comprised of all of the parts that power the bicycle and make it go. Bike pedals move the cranks, which turn the bottom bracket and the chainrings, which drive the chain that spins the rear sprockets, turning the rear hub, which moves the wheel and propels the bicycle forward. The crisp movement of these parts and the ability to minimize effort while maximizing speed is the heart of cycling. It is essential that your drivetrain be clean, true and well-maintained in order to more fully enjoy riding your bike. When one of these parts gets out of whack, the rest soon follow.
It is also important to get individual components that are compatible within the system (e.g. typically Campagnolo and Shimano parts are not inter-compatible). Many times the difference between 9- and 10-speed parts is enough to make them incompatible as well. The collective term for a complete bike drivetrain kit is "group" or "gruppo." Multi-speed drivetrains are classified by the number of cogs in the rear cassette. A modern day 10-speed bicycle can have as many as 30 "possible gears," with 10 in the rear and three in the front, but still have a 10-speed drivetrain.
Your pedals are the power transfer points between you and your bike. As you pump your legs and turn your feet, you cause the cranks to turn. Proper pedal choice is important for comfort, control and safety on the bike. Many recreational and enthusiast cyclists prefer the adaptability that platform pedals offer. They allow you to ride in any type of shoe and give a sense of security to a novice rider who is cautious of locking the feet into clipless pedals. Many platform pedals can be adapted with a toe cage to give more control and power while retaining the sense of security associated with platforms.
Racers and more advanced riders often prefer "clipless" pedals. These pedals are used in conjunction with matching cleats that are attached to special shoes. The cleats lock into place in the pedal, offering a secure attachment of rider to bicycle. Most cleats easily release the shoe by a simple twist of the ankle, and they are quite easy to use with some initial practice.
The cranks connect the pedals to the bottom bracket and hold the front chainrings. They connect to the axis around which your feet spin and transmit your power to the bike. Cranksets come in many different lengths based upon bicycle frame size and the rider's leg length. They are generally made out of aluminum, but some low-end cranks are made of steel, and some high-end cranks are made of carbon fiber. These cranksets are marketed to top-level racers and generally carry the appropriate price tag.
Cranks attach to the bicycle at the bottom bracket. There are several types of cranks/bottom bracket interfaces, and it is important to know their differences.
Chainrings are the forward chain interface with the bicycle. They contain teeth that hold the chain and drive it forward. Chainrings come in a wide range of sizes, identified by the number of teeth and application.
Typical mountain bikes use three chainrings; road bikes generally have two to three. Single-speed and fixed-gear bicycles use only one chainring. Chainrings are typically made of aluminum, with some being made of steel and a handful of boutique brands making them out of carbon fiber.
What makes a proper chainring is dependent upon many factors: number of teeth, single/double/triple configuration, 8/9/10/11 speed use and bolt circle diameter. Once all of those factors have been determined, you can choose the group level or brand that best suits your needs.
The bottom bracket is the cranks' interface with the bike. It contains bearings around which an axle rotates; this axle is connected to the cranks. Current bottom brackets come in varying interface types. Traditional bottom brackets have loose ball bearings with their accompanying cups and cones and an axle locked in place with locknuts, typically on the left side of the BB shell. More recent cartridge bottom brackets are built as one piece with cartridge bearings press-fit onto the axle and fit into the frame with "cups" that thread into opposite sides of the shell.
Many bike parts companies have most recently been developing bottom brackets without an axle. The axle is built into the cranks and runs through bearings that are press-fit into cups that thread into the bottom bracket shell. The other end of the axle then attaches to the opposite crankarm. Different brands have different bearing sizes and only work with their respective cranks.
Bottom brackets come in many different interfaces developed by the different component manufacturers. Cup and cone BBs are generally "square taper." Cartridge bottom brackets are one of two versions of square taper (International Standard or Japanese Standard) or one of the many versions of splined BB developed by the several companies.
Another important aspect of bottom bracket compatibility is whether your bicycle has an Italian or English threaded bottom bracket. The shell diameter of an Italian bottom bracket is larger, threaded 36 mm x 24 tpi, and its threads are both normal (or right-handed). English bottom brackets are threaded 1.370-inch x 24tpi, with the right cup having a reverse thread and the left cup having a regular thread.
The chain connects the chainring to the rear sprocket, transmitting power from the rider to the rear wheel. Current bicycles use roller chains (with few exceptions) that are categorized by pitch and width. The chain's pitch is the distance from roller to roller and is typically 1/2 inch on modern bicycles. The width is measured between chain plates and is either 1/8 inch for derailleur-less bikes and 3/32 inch for multi-speed, derailed bikes.
Another specification of a bicycle chain is based on outer width: the outside distance between chain-link plates. There are two main standards: Campagnolo and Shimano. Shimano uses the same width chain for all drive trains with eight or fewer gears in the rear cassette or freewheel and a different width each for its 9- and 10-speed groups. Campagnolo uses three widths presently: 9, 10 and 11. As the number of sprockets in the rear increase, the width of the chain must decrease, providing enough clearance for the extra gears. Other companies manufacture chains that are designed to work with the Campy and Shimano systems.
On multi-speed bicycles, the derailleur moves the chain from one sprocket to another. They are actuated by a cable pulled from the shifters, typically mounted onto the handlebars (and sometimes down tubes) of the bicycle.
The front derailleur moves the chain from chainring to chainring and acts as a chain guide, keeping the chain aligned. There are several chainring characteristics to consider when shopping for a front derailleur. The most important is double or triple (whether there are two or three chainrings attached to the crank). A triple front derailleur is designed with a deeper inner cage than a double derailleur to be able to push the chain up from the small ring.
Another key characteristic is chainring size. Certain derailleurs' cage diameters limit the maximum number of teeth possible on the large ring. The final characteristic is 8/9/10-speed compatibility. The width between the inner and outer cage varies between designs and may not work well when used with the wrong gearing.
The rear derailleur acts as a chain guide and chain tensioner. As the chain moves from a large sprocket to a smaller one, it needs less chain to cover the circumference. The rear derailleur has a spring mechanism that pulls in that extra length of chain, known as "chain wrap." The rear derailleur also moves the chain from cog to cog and is one of the more active components of the bicycle. As you pedal and drive the chain around the chainrings and rear sprockets, the chain continuously runs through the rear derailleur.
Rear derailleurs are classified in the same manner as front derailleurs--first by speed: 8/9/10, then by double or triple (it is actually the difference between the largest and smallest chainring added to the difference between your largest and smallest cog). Typically, with a double chainring you can use a short cage and with a triple you would need a long cage. The longer cage helps take up the extra slack that appears when in the small ring.
The rear sprocket is the chain's rear interface with the bicycle. On a single speed (fixed gear included) you only use one rear sprocket, or cog. Multi-speed bikes have a cluster of cogs called "cassettes" or "freewheels."
A freewheel is a group of cogs fixed to a "freewheel" mechanism that allows you to coast; there is a ratcheting mechanism that allows the rear hub to spin as you coast, or ride without pedaling. A bike cassette is designed to be used with a rear hub equipped with a freewheel mechanism to which you attach the cogs.
Typical freewheels come in 5- to 7-speed versions, and cassettes are 8/9/10/11-speed, with few exceptions. Cassette cogs are designed to be set up in a particular orientation to aid in shifting; any variation of this orientation can cause serious shifting and safety issues. Cassettes are designed to be used with the appropriate chain, and the wrong chain won't ride on the teeth properly, resulting in a very poor and annoying ride.
The rear sprocket is attached to the hub directly (in the case of a fixed gear), or via a freewheel mechanism. The power from the front chainring is transmitted via the chain to the rear sprocket and into the rear hub, the central point of the rear wheel. The two main characteristics of a rear hub are hub type (cassette, freewheel or fixed) and hub spacing (the width from axle end to axle end). Modern road bikes are spaced to 130mm, mountain bikes to 135mm and track bikes to 120mm. Older bicycles were spaced to 126mm. They were typically designed for use with freewheel-type rear hubs that carried fewer gears.
With the advent of the freewheel hub and more gears, the rear dropout spacing was increased to accommodate. Another thing to consider when choosing a hub (if it is already built as part of a wheel) is the wheel diameter. The rim must be the appropriate size to fit into the frame and line up with the brake pads.