Cogsets (Rear Gears)--Freewheels and Cassettes
Each individual rear gear is called a cog. The set of rear gears is called the cogset.
There are two types of cogsets (rear gear sets), depending on how they are connected to the hub:
1. Freewheels are threaded, or screwed, on to the hub.
2. Cassettes are inserted on to a slotted freehub body and connected with a lock ring.
Note that when you are not pedaling, your pedals don't move. You can coast without moving the pedals. This is called "freewheeling." Freewheeling is made possible by small pawls buried either in the cogset or the hub. The pawls engage when you are pedaling making a solid connection for your power to transfer into motion. When you are not pedaling, the pawls disengage allowing the wheels to turn without the rest of the drivetrain moving. The rapid clicking you hear when freewheeling is caused by the pawls.
Freewheels have the pawls in the cogset. Cassettes have the pawls in the hub.
Freewheels
Freewheels are on old bikes, new bikes that are inexpensive, and single speed bikes.
Freewheels are screwed to the hub. The hub has the male threads, and the freewheel has the female threads. The pawls are in the freewheel. Freewheels may have 5 to 7 gears. (Single speed bikes usually have freewheels, too.) If you have a freewheel hub, you must use a freewheel--you cannot upgrade to 8 or more speeds without getting a new wheel.
Replacement wheels with freewheel hubs are usually low end. For high end, custom made freewheel wheels see www.discoverycycle.com.
Replace a Freewheel
To remove a freewheel you need a freewheel removal tool. The most common is the Park FR-1C, for Shimano, Sram, Sunrace and other freewheels. Suntour freewheels after 1986 require the Park FR-3C tool. Most BMX single speed freewheels use the Park 6C tool. Old freewheels may require exotic, hard-to-find tools.
Tools needed:
freewheel removal tool, most commonly Park FR-1C
vise or large adjustable wrench
grease
The easiest procedure to replace a freewheel is:
1. Remove the rear wheel from the bike and the quick release skewer from the hub.
2. Place the freewheel removal tool in a vise with the business end pointed up.
3. Put the freewheel on the tool.
4. Rotate the wheel counterclockwise--push the right side of the wheel away from you. It may take a lot of strength. It is common to break the tool. Some freewheels are impossible to remove.
5. Clean everything you can reach.
6. Lightly grease the threads of the new freewheel.
7. Place the new freewheel in the tool.
8. Put the hub on the freewheel and rotate the wheel clockwise. Be careful not to cross the threads. Tighten snugly, but don't worry about the snugness. Pedaling tightens the freewheel.
9. Install the wheel. Check your rear derailleur adjustment.
The harder procedure to remove a freewheel is:
1. Remove the rear wheel from the bike and the quick release skewer from the hub.
2. You will use the quick release skewer to hold the freewheel removal tool on the freewheel.
a. Remove the springs from the quick release skewer.
b. Put the removal tool in the freewheel.
c. Install the quick release skewer back in the hub, with the bolt over the freewheel removal tool. Snug the bolt loosely over the tool, allowing a little jiggle room.
3. Supporting the wheel between your body and the floor, with the adjustable wrench rotate the removal tool counterclockwise. Substantial force may be required. It is common to break the tool, especially on old Suntour freewheels. Some freewheels are impossible to remove.
4. Once the freewheel removal tool starts to move, continuously loosen the quick release axle bolt to allow further movement.
Cassettes
Some 6 speed, most 7 speed, and all 8+ speed cogsets are cassettes.
A cogset is not screwed on to the hub. It is slipped on to a tightly fitting protrusion from the hub called a freehub body. The cogset is held to the freehub body by a lock ring. The pawls are in the freehub body.
Notice the difference between the hubs from freewheels and cassettes.
Seven speed freehub bodies are shorter than 8, 9 and 10 speed freehub bodies. With a spacer, a 7 speed cassette can go on 8+ speed freehub bodies. But 8+ speed cassettes cannot fit on a 7 speed freehub body. Campagnolo cassettes need Campagnolo freehub bodies. Other manufacturers are usually compatible.
Replace a Cassette
To remove a cassette the correct tool is usually the Shimano compatible Park FR-5C. Campagnolo requires a Campagnolo tool.
Tools needed:
cassette removal tool, usually Park FR-5C
chain whip
large adjustable wrench
grease
To replace a cassette:
1. Remove the rear wheel.
2. Hold the wheel firmly on a work bench with the cassette pointed up.
3. Insert the cassette removal tool in the cogset lock ring.
4. You will be rotating the removal tool counterclockwise, or if holding the wrench on the right side of the wheel in your right hand you will be pushing it away. Rotate the tool counterclockwise with your fingers. Notice the cassette moves freely in this direction, so you have to prevent this movement with the chain whip. Position the chain whip as shown in the photograph to prevent this movement.
5. Rotate the cassette removal tool counterclockwise with the adjustable wrench. Substantial force may be required. Try to wedge the wheel between your stomach and the wall to get leverage and resistance.
6. Remove the cassette.
7. Clean everything you can reach.
8. Lightly grease the threads of the new lock ring.
9. Install the new cassette. Differentiated grooves on the freehub body insure that you can only do it right way.
10. Being careful not to cross thread the lock ring, tighten very snugly with the removal tool and adjustable wrench. Lock rings do wiggle loose, and when they do it is a mess, so tighten with authority.
11. Reinstall the wheel. Check your derailleur adjustment.
Replacing a freehub body is covered in the section on rear hubs.
Rear Gear Wear
Freewheels and cassettes wear out.
The chain is the weakest part of a bike. With use, the chain stretches. As the chain stretches, the gears are worn to match the stretch. Eventually the chain starts skipping because the gears don't fit. It is hard to tell by looking when gears are too worn out to work. If you suspect your freewheel or cogset is worn out, install a new chain. If you still have a problem, install a friend's compatible rear wheel on your bike. If the problem persists, it's probably your chainrings that need replacing. If your friend's rear wheel cures the problem, its probably your cogset that needs replacing. (This presumes no other glaring problem, like loose wheel axles or bottom bracket or bad chain line.)