This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: David Keppel <firstname.lastname@example.org>
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
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.
;-D oN ( A hubalaboo ) Pardo