Cassettes

In the late nineteenth century, people rode bikes called velocipedes and highwheelers. The former was a little like today's bikes except that the crankset was attached to the front wheel. The highwheeler, which was invented after the veloci pede, shared this trait, however, the front wheel was huge (up to 60 inches in diameter) because inven tors realized that with an increased wheel size, riders could go faster.

These amazing bikes are quite collectible today, but in their day, they had two big inconveniences: They had only one gear; and if the wheels were spinning, so were the pedals, and your feet. Though this "fixed gear" system allowed you to contribute the strength in your legs to the braking process by resisting the forward movement of the pedals, it never made for very comfortable or very safe riding. Such a system is, however, still used on present-day track bikes (one-speed road bikes raced on an oval track called a velodrome) because of the added con trol that it offers track racers, whose machines do not have brakes.

With the exception of the track bike, modern bikes are equipped with a ratcheting mechanism and cogs that connect the rear hub, via the chain, to the pedals, but allow you to coast when you stop ped aling. This makes bikes safer and much more com fortable to ride, thanks to the various gearing it provides. This device is called a freewheel because it frees the rear wheel from the connection to the crankarms.

The ratcheting mechanism is made up of bearings, a gear, and pawls. The pawls are angled one way and rest on springs. When coasting, the gear spins past the pawls, which trip over the gear teeth, making the fast clicking sound you hear. When you pedal, however, the pawls engage the gear teeth, driving the bike.

The cogs-or sprockets, as they are sometimes called-are disks with teeth on them. The number of teeth on the cog, combined with the number of teeth on the chainring, determines the gear ratio of the bicycle's drive system at any given time. The larger the cog, the easier it is to pedal, but the larger the chainring, the harder it is to pedal. Cogs and chainrings are commonly referred to by the number of teeth on them (they're usually stamped with the number, so you don't have to count to determine size). The more cog and chainring combinations you have, the greater the number of available gear ratios and the greater the variety of terrain on which you can ride. A cluster of different-size cogs and the spacers that divide them is known as a freewheel or a cassette, depending on the type of ratch eting mechanism to which it is connected.

The Difference between Freewheels and Cassettes

When derailleur drivetrains were first invented there had to be a way to attach multiple cogs to the rear wheel of a bike to provide different gears. For the gearing, inventors came up with the freewheel, which is an integral unit comprising the cogs (usually five, six, or seven) and spacers attached to a ratcheting mechanism in the center that drives when you pedal but allows coasting (freewheeling) when you stop pedaling. This center section that includes the bearings, gear and pawls is called the freewheel body. To attach the freewheel to the wheel, there are threads in the center so that it can be screwed onto the rear wheel's hub.

To remove the freewheel, a special tool is needed called (naturally) a freewheel remover. Each brand of freewheel has its own remover. Freewheels can be difficult to remove because pedaling pressure tightens the freewheel on the hub. If it's not adequately lubricated or if a strong, powerful rider has been racing the bike, the freewheel can become very tight. Another tricky thing with freewheels is cog removal. It varies from brand to brand. But on most freewheels, there are cogs that thread on and those that slide on. It's important, however to know what you're dealing with before attempting to replace a cog.

Another interesting thing about freewheels is how they affect the design of the hub. Because of the spacing requirements of the freewheel, the hub bearings on either side of the hub can only be a certain distance apart. development of the cassette system. You can still get freewheels today on certain bikes, however the vast majority of new models are equipped with cassettes and have been since the late 1980s.

At a glance, cassettes look just like freewheels but there are significant differences. The basic one is that the ratcheting mechanism on a cassette system is part of the hub (called a freehub or cassette body). It's not screwed onto the hub, it's built into the hub. (Remember that on a freewheel system, the freewheel is threaded onto the hub, not built into it.)

This provides one of the main advantages of cassette hubs: it allows the bearings on either side of the hub to be placed further apart, which in creases support for the axle, strengthening it and preventing it from breaking in most cases.

Another big advantage is simple cog removal. All that's required is unscrewing a lock ring or the bottom cog or cogs (depending on model) and the cogs can be lifted off the hub as a unit or as a cassette. Then you can simply slide on a replacement cassette or one of a different size if you're changing cogs to make the gearing easier, for instance.

It's important to understand the differences between cassettes and then determine which type is on the bike you're working on. Cassettes are found on most modern bikes and can be identified by looking for a flat lockring on top of the smallest cog. The lockring is splined in its center. Freewheels are mostly on older bikes. Usually freewheels have two notches built into the freewheel body or a splined pattern just visible inside of the freewheel body. There is no lockring. If you're in doubt, a shop mechanic could quickly identify the type if you bring in the wheel. Once you know what you have, both are easy enough to work with.