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In front, there are either two or three chainwheels. The innermost, smallest chainring is intended for spinning up steep hills. It’s also called “granny gear”, presumably to indicate who would need one. Me, I prefer two chainrings on road bikes because I don’t have to worry about overshifting, because I have to worry less about extreme chain lines, and because I haven’t yet met a mountain that was not easier to climb without a granny gear. The problem with the chainline is that a chain should not run from an inner cog to an outer chainwheel or vice versa.

There are cassettes with eight, nine, or ten cogs on the rear hub (for hub types, see the chapter on wheels). Cassettes with five to seven, and arguable eight, cogs are obsolete. At the time of this writing (2006), nine cogs are still available but fading, and people switch to ten. Ten-cog cassettes require a special narrow chain, and derailleurs and levers may not be compatible, so I recommend going with ten. In most cases, the number of cogs must agree with the type of shift levers; it cannot be upgraded without also replacing the shift lever. Usually cassettes from one vendor cannot be mixed with shift levers from another vendor. There are some exceptions, for example Sachs is said to work with Campagnolo, and Campagnolo sells replacement ratchets to convert levers.

Chains must be the single most neglected part of the typical bicycle. It seems that every other bicycle I see has not been lubricated for years. Unlubricated chains squeak pitifully, make pedaling much harder, and destroy chainwheels and cogs. The problem is that rust and other particles get between the bolts and sleeves of the chain and wear them down, so that the chain gets loose and hence very slightly longer. This means that the distance between two chain links no longer matches the distance between two chainwheel and cog teeth. When this happens only one link connects with a tooth, all others are loose because of the slack. This one tooth takes the entire force and gets worn into a “shark fin” shape. If this shape is pronounced enough, the chain won’t even roll smoothly off the cog or chainwheel; instead the chain will be grabbed by the shark fin.

The insidious part of this is that you don’t notice it until the cogs you use most are already destroyed. Replacing the chain will seem to make matters worse because now all links of the chain get hooked by all the shark fin teeth of the cog simultaneously, which will make the chain skip. This makes the cog unrideable. The only option at this point is to replace all the worn cogs together with the chain.

To prevent this from happening, it is important to lubricate the chain frequently, at least every three weeks or so, more often when riding in wet weather. Before lubricating the chain, it must first be cleaned with an old T-shirt. You can run the chain through the fabric, but it’s really better to clean each link individually. It will only take a few minutes. You should also clean the cogs, chainwheels, and especially both pulleys in the rear derailleur. Then, lubricate the inside liberally, and run the chain through at high speed for some time. Then, wipe off all excess lube. This is important because only the lube inside the chain will do any good, the rest just attracts dirt and increases wear. Ride around the block and wipe off excess lube again.

There is much debate about the best chain lube. You can’t go wrong with synthetic bicycle chain oil, perhaps containing suspended Teflon or other lubricating particles. Don’t use regular (WD-40 or other) machine oil because its viscosity is too low to stay in the chain for very long, and don’t use motor oil because motor oil is designed to work in certain (hot) environments only. Grease doesn’t work too well because it won’t reach the critical parts, you’ll wipe off most of what you apply. This is even more true of wax. Don’t use gasoline to clean your chain, it will remove the factory grease packing of the inner surfaces and you’ll never be able to restore them sufficiently. Chains are not that expensive.

To replace a chain, push a pin out of the old chain and measure the new chain against the old to make sure it has the same number of links. If you have to remove links, remove them at the end without the bolt sticking out. If you have to break a chain with the intent to reinstall it, push one bolt towards the outer side of the bicycle until the link just barely holds together. Bend the chain to disengage the broken link. It’s impossible to re-insert a bolt pushed out all the way. To install a chain, join the chain and push the bolt in until it is flush with the link plate, then push it back from the other side until it’s as flush with the plate on that side as all the other links. Then, bend the chain sideways until it moves freely. A stiff link makes the chain skip when riding. You need a special chaintool for pushing bolts.

The chain length should be chosen so that all gear combinations are possible: if the chain is on the largest chainring and the largest cog, the rear derailleur should still have some play, and if the chain is on the smallest chainwheel and the smallest cog, it should not touch itself near the upper pulley in the rear derailleur. Of course you won’t normally ride these extreme gears because of the poor chainline. If these conditions can’t be met, consider changing the pulleys or chainwheels, or the rear derailleur capacity (i.e. use one with a longer cage). As a rule of thumb, the rear derailleur should be roughly vertical (both its pulleys are exactly below the axle) when the chain is on the largest chainwheel and the smallest cog.

In case of doubt I prefer a shorter chain because this increases its tension and makes it less likely to bounce and hit the chainstays (the frame tubes that connect the bottom bracket and the rear dropouts). On mountainbikes “chain suck” can become a problem when the chain bounces so much that it gets caught between the chainstay and the chainwheels. You can bolt little gizmos to the chainstay to prevent this, but I have seen these gizmos to cause more chainsuck problems than they solve because if the chain gets stuck anyway it gets stuck so tightly that it requires tools to make the bicycle ridable again. I am generally wary of little add-on gadgets.