Like many parts on a bike, axles are vulnerable to improper maintenance and certain types of impact. The rear axle on your bike passes through a freehub and a set of bearings in the hub of the wheel. When an axle breaks, a variety of things can occur. Not all axles and freehubs are alike. Identifying these components and understanding how they function is the first step toward repairing a broken axle.
The freehub is an elongated metallic cylinder and depending on the type, screws into the wheel hub or is secured by cone nuts that thread onto the axle. All freehubs have metal cogs, or tabs, that fit into notches inside the wheel hub. Freehubs on many fixed-gear bikes, such as BMX and trials bikes, have dual-directional cogs that don't allow you to coast with your feet on the pedals because the pedals always rotate when the bike is rolling. Freehubs on single-speed and multispeed bikes have one-directional cogs on the freewheel body that allow you to coast with your feet on the pedals. Broken axles differ with fixed-gear, single-speed and multispeed bikes.
Axles on BMX and trials bikes are designed to withstand impact from wheel bounces, catching air and landing or riding on rough terrain. Typically, these axles break on the outside of the wheel hub. As a result, the rear wheel might jam between the frame arms or collapse under the bike. With the possible exception of mountain bikes, incidents of broken axles on single-speed or multispeed bikes are rare and often not apparent. With few exceptions, breakage can be attributed to damaged bearings that have not been properly cleaned and lubricated. Dry bearings can weaken from excessive abrasion caused by dirt. When the bearings fail, the axle loses its support. If you continue to ride the bike, the axle could crack or break inside the freehub under impact. In this case, you might not discover the problem until the rear wheel locks up.
Repairing a broken rear axle begins with removing a fixed-gear or single-speed sprocket or a multispeed cassette to access the freehub and axle. The removal tools are specific to the particular sprocket or cassette. A sprocket or cassette with a uniform recessed center is removed using a lock ring tool and an adjustable wrench. A sprocket with a notched center is removed using a spline wrench. Once the sprocket is removed, the extent of the repairs can be established. If the freehub isn't visibly damaged and the freewheel body rotates by hand, repairs might be limited to replacing the broken axle and bearings. Otherwise, all the components must be replaced. As for any damages to the bike, such as bent frame arms or a damaged wheel hub, these repairs are determined at the time.
Axles, freehubs and exposed ball bearings must be cleaned and lubricated periodically to ensure reliable performance. Sealed bearings on many newer bikes do not require servicing and must be replaced when damaged. The interior of the rear wheel hub should be cleaned and lubricated before reinstalling the freehub, bearings and axle. Use a nonresidue cleaner and the recommended lubricant that's specified in your service manual. Keeping the necessary tools, cleaners and lubricants on hand saves time and the cost of a repair shop.