Flip-flop hubs, also called double-sided hubs, are rear bicycle hubs that are
threaded to accept fixed cogs and/or freewheels on both sides.
There are several different types of flip flop hubs available for different
applications. Their main purpose is to allow changing between two different
size gear ratios on one rear wheel without the added complications of a
derailleur or internal hub gear, or between a fixed and freewheeling option.
By removing the rear wheel and flipping it around, the rider can switch between
the two options. They are traditionally found on track bicycles, but can also be
found on other single speed bicycles. Types
= Fixed gear/track bicycles = Usually, one side has a fixed type
stepped threading, and the other side is threaded for a standard freewheel.
Occasionally you may find double sided fixed hubs with stepped threading on
both sides, but these are rare and only a few companies offer them. The fixed
side has a stepped down diameter thread to accept a lockring. The larger
diameter thread on the fixed side accepts a standard threaded cog, and
uses the same size threads as the freewheel side of the hub. The most
common standard I.S.O. thread size is 1.375" x 24 tpi, but there are other
less common older sizes. The smaller diameter thread on the fixed
side has reverse/left hand threads to accept a lockring. The lock ring is
reverse threaded to prevent the cog from coming loose when applying
reverse/braking power to the pedals. There are also different sized threads
for the left hand lockring, Campagnolo/Phil Wood and Old French. The
first two are the most common, the French threading is older, and a little
less common. Always check with the manufacturer of the hub for lock ring
compatibility. Often, the wrong thread type lockring will easily thread onto
the wrong hub, but with use over time, the poorly mating thread interface will
cause the threads on the hub to become damaged or destroyed.
This allows the installation of a cog on one side of the hub, and another on the
other side of the same hub. Reversing the rear wheel allows switching between
Fixed/Free drivetrain, between two different size gears, or both.
Generally, the freewheel is selected to be the same size, or slightly larger
than the fixed cog. Same size allows the same gearing, but with coasting option,
while a slightly larger freewheel gear allows coasting and easier pedaling for
longer or hilly rides. Both could also be freewheels for two different coasting
gear selections. Any two different size cogs/freewheels can be used as long as
they are within the limits of adjustibility allowed by the length of
the frame's dropout slots, derailleur cage and chain length. Large differences
in the number of teeth may require adding or subtracting chain links to
achieve proper fit. Also, if rear brakes are used brake pad reach to the rear rim
may need to be adjusted. The ISO threading for freewheels is the
same as for track/fixed cogs. A standard single speed BMX style freewheel can be
used on the track threads the hub. Because of the stepped down lock ring
threads, not as many threads of the freewheel engage the hub threads as they
would if it were threaded to a freewheel threads, however, this is not a problem
since the same number of threads are sufficient to secure a track cog. The
force applied to a freewheel is the same as that applied to a fixed cog of the
same size, so the missing threads cause no durability issue. Also, there is
never any reverse torque on the threads with a freewheel, so a freewheel
actually puts less strain on the threads than a fixed cog does.
If a track hub is not available, or if a beginner wishes to try a fixed gear
bicycle without buying a new hub or rear wheel, track cogs can be installed on a
standard Freewheel hub using a right hand threaded ISO English bottom bracket
lockring as the lockring for the cog. This may work for some lighter, or
delicate riders, but any repeated reverse or braking pressure on the
pedals will eventually cause the cog and lockring to come loose. It is
recommended that anyone using a freewheel hub with a track cog in this
manner do so with caution and with both front and rear brakes mounted to the
bicycle. Track cogs are generally available in a
range of sizes from 12T to 23T and two different widths - 3/32" and 1/8". The
3/32" size is made to accommodate narrower 3/32" chains, but is also
compatible with 1/8" and wider bmx/single speed chains. The thicker
1/8" cogs are ONLY compatible with 1/8" or wider single speed chains. There may
be larger sizes available as well, either custom manufactured or adapted
from chainwheels or front sprockets using a number of methods or devices.
There are some heavy duty 3/16" chains designed for BMX use which are
compatible with most cogs. However, because these chains usually have taller
thicker side plates, they may not work properly with some cogs smaller than 11T
or freewheels smaller than 16T. Freewheels are available in sizes 13T to
28T and in the same two different widths as the cogs. ACS makes a special "FAT"
heavy duty BMX freewheel in 14T and 16T with 3/16" teeth compatible only with
3/16" chains. ACS also makes a 15T freewheel that accepts a 3/32" chain.
= BMX/Single Speed Bicycles = There are a few different variations of
flip-flop hubs for BMX bikes that are different from the track style. The most
common type of BMX flip flop hub has standard ISO freewheel threads on one
side and smaller metric BMX threads on the other side that are designed to work
with smaller 14T to 15T BMX freewheels. In recent years a few companies have
started making 13T freewheels compatible with this thread as well, allowing more
gearing options. In recent years, an innovation in BMX
freestyle riding spawned "Left Side Drive". This component option switches
the drivetrain to the left side of the bike in order to move the sprocket and
chain out of harms way for riders who prefer to do grinding tricks using the
pegs or pedal on the right side of the bike. Moving the drivetrain to the left
side required modification to the design of three components.
A freewheel with left hand threads which threads on and ratchets in the opposite
direction for use on the left side. A rear hub with left hand threads to
accept the LH freewheel. A left side drive crank arm. Although
you could simply reverse a normal set of crank arms, the left side and right side
specific pedal bosses would be reversed, causing them to tend to come loose and
spin out with normal riding. Also, directional pedals would look backwards
unless the pedal spindles were also reversed. Many companies now provide
cranksets in which both arms are designed to be drive arms with the
appropriate sprocket drive bosses. While this application to BMX is new, this is
not a new idea. For decades, tandem bicycles have used left side drive
cranks to drive the timing chain. Left side drive introduced a different
type of flip flop hub for BMX. This hub is a Right Hand/Left Hand flip flop hub.
One side of the hub allows the use of normal right hand freewheels, while the
other side is threaded with left hand threads and accepts the new left hand
drive freewheels. The first/most common was the ACS South Paw Claw model,
available in 16T. Later, a version of this hub was made with small metric BMX
threads, left hand on one side and right hand on the other side. These hubs
accepted either Left or right side drive freewheels in the smaller 13T-15T sizes.
DK Bicycles introduced the Dual Drive hub back in the early 2000s which is a
BMX Left/Right drive flip flop hub with the smaller sized metric BMX threads for
13T-15T freewheels, but also includes two threaded adapters, one left and one
right, threaded inside to match the smaller metric threads and threaded on
the outside with standard ISO threads. This allows the hub to use any size or
combination of left and right drive freewheels available. Other companies
like Wethepeople also made these dual-threaded metric hubs for a short
time before cassette hubs enabled the use of much smaller rear cogs.
= Drum/Disk brake Tandem Hubs = A third type of flip-flop hub is rather
rare and not truly a flip flop hub by design, although it can be used in that
way. This type of hub has standard ISO 1.375" x 24 tpi on BOTH sides, but it
was not intended for two freewheels. These were designed for the use of a
drum or disc hand brake on the left side, and a multi speed freewheel on the
right. The drum and disc brakes designed for early tandems used standard ISO
1.375" x 24 freewheel threads to attach the disc or drum to the hub. These
tandem hubs can easily be adapted for use as single speed flip flop hubs using
any combination of two ISO English threaded freewheels.
See also Fixed-gear bicycle
References