The Wheels of Chance

There are three basic ways of transferring the motion of the chain to your bike’s rear wheel; the fixed sprocket, the freewheel and the cassette.

The Fixed Sprocket

This is as simple as it gets, just a sprocket screwed onto a hub. If you pedal forwards it rotates anti clockwise, and if you pedal backwards it goes clockwise – there’s no ratchet so you can pedal both forwards and backwards. This “fixed gear” style of riding is how bicycles started out, it’s the mechanism used on track bikes and it’s also become popular with a sub-culture of urban riders who’ve discovered it’s simplicity. To stop the fixed sprocket from unscrewing when you ride backwards it’s retained by a lock ring that screws onto a slightly smaller diameter left handed threading that all fixed hubs have. To install this puppy you need a chain whip……….it’s not what you’re thinking, it’s just a handle with a couple of lengths of chain that wrap around the sprocket so you can get a good grip on it to tighten it. The end of the handle will often be a lock ring spanner too. Remember when looking at the sprocket turn it clockwise to tighten and anti-clockwise to untighten, do the opposite for the lock ring and always use plenty of lube on those threads.

The Freewheel

This sprocket has some smarts. There’s a mechanism inside with a ratchet. If you pedal forwards the ratchet engages and it works like a fixed sprocket and the motion of the chain is transferred to the hub and the bike moves forward. However, if you pedal backwards the freewheel spins freely. The useful upshot of all this is that on down hills you can coast, unless you decided to go down the hill backwards and then the wheel’s motion would be transmitted to the pedals, but no one would do that.

Freewheels are notoriously difficult to remove because the act of pedaling thightens them onto the hub. There are a couple of different sorts of freewheels that need different tools and techniques to remove. Make sure you know what type of freewheel you have and necessary tools. They only cost a few dollars. Shimano type freewheels require spline tool (park FR-1). This spline tool fits down over the axle inside the freewheel and it is retained by reinstalling the skewer. The you grip it with a big wrench and unscrew it by turning the wrench anticlockwise. If it’s tough slip a bit of pipe over the wrench handle to give you some extra leverage.

The second type of freewheel uses a 2 or 4 pronged removal tool (FR-6). This fits into slots on the freewheel body and then you again use the skewer to lock it in place. If you don’t do this it will simply slip out when you apply any force. Now take your courage, a big wrench and maybe that bit of pipe and unscrew the freewheel.

The Cassette

The cassette is what comes on most modern bikes, but it’s actually similar to the fixed sprocket as it has no mechanism in it. It’s just a series of sprockets that instead of having a thread on the inside diameter have a series of splines. There’s still a ratcheting mechanism, but now it’s in the hub. The cassette’s splines slide over mating splines on the hub mechanism and it’s kept in place by a lock ring that on Shimano cassettes is tightened using the FR-5 Shimano spline tool. This arrangement makes removal of the cassette easy as there’s no thread that is tightened by pedaling. Place the FR-5 tool in to the cassettes splines, retain it with the skewer, grip the cassette with the chain whip to stop it from rotating and use a wrench to turn the spline tool anticlockwise until the retaining ring comes loose. Then simply pull the cassette off. Some of the cogs might be individual and others locked together on a carrier, don’t worry, just don’t loose any of the single cogs.

Filed under: Bicycle, Bike, Wrenching Tagged: | Wrenching