Bicycles/Modifications/Fixed Gear Hub Conversion
Introduction
editTo create a fixed-gear bike, a screw-on freewheel hub (not freehub with cassette) may be converted to fixed gear use.
Supplies
editParts:
- Wheel with screw-on freewheel hub
- Track sprocket
- Bottom bracket lockring
- Axle spacers
Tools:
- Freewheel removal tool
- Adjustable wrench (large) 10"-12"
- Thread locker
- Chainwhip
- Lockring wrench
- Cone wrenches
- Straight edge
- Metric ruler
- Truing stand
- Spoke wrench
- Dishing tool
Hub selection
editThe preferred hub for this process is called a freewheel hub. In this type of hub, the freewheel mechanism is an independent mechanism that screws onto the hub proper. More importantly, the bearings for the axle are contained within the hub itself; the wheel does not rely on the structure, or presence, of the freewheel for operation as a wheel, so the freewheel mechanism can be removed entirely.
Not all hubs are suitable for conversion to a fixed gear, though. Modern hubs are an evolution of the freewheel: they split the gear cluster from the freewheeling mechanism in order to move the bearings further out from the bicycle's center plane. These hubs are called freehub hubs, and the cluster of gears used on such hubs is readily replaced. These hubs can not be converted using the method described below because the freewheeling mechanism (or body) is an integral part of the hub; the bearings that support the axle itself depend on the presence of the freehub body, so the freehub body cannot be removed and still leave a working wheel.
When considering a candidate hub, it will be necessary to identify the type of hub in hand. While the distinction between the two types of hub discussed above is not obvious if the hub has gears of the original type still installed, there is a simple method to distinguish freewheel hubs from freehub hubs: while holding the wheel and looking closely at the gears, spin the gears counter-clockwise, then note whether or not the circular metal band running around the axle is moving with the gears as they freewheel. If the ring remains stationary, the hub uses a freewheel; if the ring turns with the gears, the wheel is equipped with a freehub--and is not suitable for this conversion.
Dismantling
editOnce a proper hub is selected, it needs to be prepared for conversion. Principally, the freewheel needs to be removed from the hub. This is accomplished through the use of a freewheel removal tool. The tool has splines on one end and a hex-nut on the other. The splines fit into the freewheel, and the nut, through the use of a wrench, allows the freewheel to be unthreaded (in a normal counter-clockwise fashion) from the hub body. The freewheel must be held in position by a quick release or axle nut when it is turned by the wrench.
Many varieties of freewheel removal tools exist, so care should be taken that the correct tool is selected before trying to remove the hub.
Sprocket selection
editGear ratio and sprocket selection
editWith the freewheel removed from the hub body, the track sprocket can be installed. When selecting a sprocket for installation, some consideration should be made. The tooth count and sprocket width must be decided upon.
The number of teeth on the sprocket is a matter of user preference. The number of teeth affect both the ease of acceleration and the top attainable speed, but the two factors are inversely related. A lower gear (higher tooth count) will make starting from a dead-stop easier, but will require the cyclist to spin the cranks faster at high speeds, eventually at such a rate that a higher speed is not possible. With training, higher rotational speeds are possible, but there is definitely a limit.
A higher gear (lower tooth count) on the other hand will make standing starts more difficult but will make higher speeds possible before reaching the physical limit of how fast a rider can ride.
Regardless of the tooth count guidelines, the final gear ratio is dependent, also, on the tooth count of the front chainring. For the purposes of this instruction, it is assumed that the front chainring is a fixed variable. When deciding the desired gear ratio, it is often helpful to ride a multi-geared bike first, testing the ridability of each gear ratio, to decide on the most suitable ratio. If the chosen sprocket size later turns out to be unsuitable, it can be changed.
It is also important to select a sprockets of proper width. A traditional track cog, used on track bikes raced in velodromes, is 1/8" in width, however, it is often the case with fixed-gear conversions, that the wheel is used on a normal road bike where the chainring and chain require a cog that is 3/32" in width. A cog of proper width should be obtained depending on the application.
Sprocket installation
editAfter the sprocket is selected, it can be installed onto the hub. The design of a freewheel hub difers from a proper track hub, so special care must be taken with the installation. Because braking on a fixed gear bike can be accomplished by resisting the rotation of the cranks, it is possible to unthread the sprocket from the hub, which could prove disastrous under the wrong circumstance--even deadly. This problem is solved on a true track hub by having a set of left-hand (reverse) threads outboard of the regular sprocket thread. A special left-hand threaded lock ring is installed after the cog and prevents the cog from loosening, since the lockring would be tightened by any loosening of the sprocket.
This feature does not exist on a freewheel hub. There is only a single set of normal, right-hand threads. A track hub lockring will not fit on the hub; however, the lockring from an adjustable bottom bracket has the same threads as the hub and can be used to secure the cog, to a degree. The caveat is that the lockring is threaded in the same direction as the cog and, theoretically, can loosen in the same way as the sprocket. An improvement to this combination--a threaded sprocket in combination with an identically-threaded lock ring--is judicious pinning of the sprocket to the hub. The downside to this method is that the pins used to secure the sprocket must be ground flush with the outside face of the sprocket, in order to accommodate the lock-ring, making it impossible to later remove the sprocket. However, this method is much more secure than simply threading the sprocket and lock-ring onto the hub.
Before installing the sprocket, apply threadlocker to the threads. Screw the sprocket all the way onto the hub. Use the chainwhip to tighten the sprocket. It is recommended that the wheel be supported upright while the rider's weight is applied to the end of the chainwhip to greatly tighten the sprocket. Caution should be taken not to strip the threads.
After the sprocket is installed, use the lockring wrench to install the lockring in the same fashion as the sprocket, using threadlocker and the rider's weight again. Allow the threadlocker to fully set before using the wheel.
Spacing
editOnce the track sprocket has been installed, the hub is still not ready for practical use. In its original freewheel form, the hub makes room for the sprocket cluster on the right by spacing the hub body toward the left side of the axle. With the track sprocket installed, the positions of the sprocket is too far to the left and leaves a disproportionate length of axle exposed on the right.
To correct this spacing problem, axle spacers are removed from the right side of the hub, the bearing cones (and thus the hub body) are move towards the right, and extra spacers are installed on the left. Technically, this is a straight forward process. The spacers are usually situated on the axle between the outer locknuts and the threaded cones. Use appropriately sized cone wrenches to loosen the locknuts. The spacers should be able to be removed by hand at this point. Depending on the hub, the existing spacers installed on the hub may be multiple, equally sized spacers or a single, specifically sized spacer on each size. In the former case, it may be possible to simply remove one or more spacers from the right side and install them directly on the left side to achieve the desired spacing. In the latter case, new spacers will need to be acquired before continuing. These are usually available at better equipped bike shops.
Besides the purely technical aspect of respacing the hubs, the chainline must also be considered. For a fixed-gear bike to be as safe and efficient as possible, the rear sprocket must be directly inline, from front to rear, with the front chainring. This alignment, called the chainline, may be adjusted in a number of ways. Varying length bottom bracket axles, cranks with differing spider depth, or chainring stackbolt spacers may all be used to affect the chainline. Assuming that the crank and bottom bracket, though, are not open to modification, the chainline must be adjusted through the use of rear hub spacers. Since the hub has already been dismantled for the purposes of converting it, it is reasonable to adjust the chainline at this point.
With the hub initially respaced, install the wheel in the frame and place a rigid straight-edge against the face of the chainring so that it is extending rearward toward the cog. If the straight-edge aligns perfectly with the face of the cog, no further spacing adjustments are required. Otherwise, using a metric (or other finely graduated) ruler, measure the distance from the straight edge to the face of the cog. Spacers can be moved around on the axle to account for the difference in the chainline. For example, if the distance from the straight-edge to the cog is 3mm, 3mm worth of spacers should be removed from the right side and installed on the left. This will move the cog 3mm to the right to bring it into alignment with the chainring. Be sure, however, to maintain the correct locknut-to-locknut length for your frame (120, 126, 130 or 135mm depending on type and vintage of frame).
After the desired spacing is achieved, the hub bearings should be adjusted in a normal fashion.
Dishing
editWith the hub properly spaced, the rim will be found to be too far to the right and must be redished to move it back inline with the center of the frame. Dishing is the process of adjusting spokes on each side of the wheel to bring the rim inline with the hub centerline. The centerline of the hub is not related to the hub flanges where the spokes fits into the hub, but rather to the locknut faces where the axle contacts the frame.
Dishing is a normal part of truing a wheel and should be carried out as such. At a minimum only a spoke wrench is needed, a bicycle frames can be used as a truing stand. However a truing stand and dishing gauge will make the job much more convenient and quicker. And if the wheel is in poor adjustment to begin with, a spoke tensiometer may be desired.
The only specific consideration that should be made is to the spoke length. There is usually a sufficient amount of spoke threads and length to account for the relatively large shift in dishing. There are times, however, when the process causes the spokes to protrude too far above the rim (more of a concern for a single-, rather than double-, walled rim), causes the spoke nipple to bottom out on the spoke threading, or causes the spoke nipple to disengage completely from the spoke threading. The former two symptoms occur on the right-(or drive-)side. The latter occurs on the left. If any of these problems occur, the wheel may need to be partially rebuilt with spokes of correct length.