Should I use cassette or freewheel hubs?

From: David Keppel

People often ask "should I use a freewheel or a freehub?" The answer is usually "yes."

The hub is the center of a wheel and is composed of an axle, bolted to the bike frame, a hub shell or hub body, where the spokes attatch, and bearings to let the shell rotate around the axle.

Freewheels screw onto threads on the rear hub's shell, and cogs attatch to the freewheel. The freewheel's job is to provide a ratchet between the cogs and the hub shell, so that you can coast. Freehubs are similar but combine parts of the freewheel with parts of the hub shell. Freehubs are also sometimes called "cassettes"

The usual problem with rear hubs is that axles bend and break. This is because the axle diameter was chosen when single cogs were used and the hub bearing was positioned close to the frame. Since then, wider cog clusters have become the norm, the bearings and frame have moved further apart and leverage on the axle has increased. But since the axle has not gotten any stronger, it now has a tendency to fail.

Cassettes fix the problem by incorporating one hub bearing in to the freewheel mechanism, so that the bearing is once again outboard and the axle is carrying its load under less leverage. Some freewheel hubs solve the problem by using fatter axles. Since increasing the axle diameter dramatically improves axle strength, this is an effective solution and it is possible to use a fat axle that is aluminum and thus lighter than a standard skinny (weaker) steel axle.

Neither solution is perfect -- cassette hubs let you use standard replacement axles, cones, washers, etc., but force you to use cogs and spacers and whatnot by a particular manufacturer (and possibly derailleurs and shifters -- e.g. XTR uses 4.9mm cog-to-cog spacing instead of the normal 5.0mm). On the other hand, fat axles are nonstandard as are some other replacement parts.

As an aside, the cassette solution leaves a fairly long unsupported axle stub on the left side, and this is sometimes a source of more bending problems. Fatter axles solve the problem on both sides.

Note also that many cassette systems allow you to remove the cogs using a lightweight tool and thus give you ready access to the spokes in case of breakage. Freewheels attatch with a fine thread (another historical artifact, I believe) and are thus more difficult to remove on the road, making spoke replacement harder.

In principle, freehubs have all cogs attatch using the same size and shape of spline, so, e.g., a 20T cog can be used as both a large cog for a corncob cluster and as a middle cog for wide-range cluster. However, Shimano's marketing is just the opposite and is directed at selling whole clusters, without letting you replace individual cogs. (Shimano's policy is relevant here since they sell 90+% of such hubs.) Freewheels have several spline diameters in order to clear the bearings and ratchet. Further, small cogs typically screw on to the freewheel body or special cogs with extra threads. This introduces stocking problems and may make it hard to build some cog combinations.

I'm not a fan of freehubs for the simple reason that they lock me in to one maker's choices about cogs and cog spacing. For example, I had a 1988 Shimano 6-speed freehub and by 1991 Shimano had, according to my local bike store, discontinued 6-speed replacement cogs. Thus, simply replacing one worn cog meant upgrading to a 7-speed system, which in turn requires all new cogs, a new freehub body (lucky me -- for some it requires a new hub and thus new wheel), and, if I wanted to keep index shifting, new thumbshifters. Had this been a freewheel-equipped bicycle, I could have easily switched to another maker's 6-speed freewheels.

Fortunately, the market is stablizing, with a growing number of makers producing hubs and cogs using a spline pattern like the more recent Shimano 7-speed freehubs. However, it hasn't settled entirely, yet.