For years, freewheels were the standard way of fitting sprockets to a back wheel, but they have now been superseded by the cassette design. You only come across a freewheel type on an older bike or on a very basic new one. Axles on the freewheel type are more prone to bending and breaking, because the bearings that support the axle are much nearer to the centre of the wheel.
Notches, splines and dogs
The most difficult thing here is choosing the correct tool. When freewheels were standard, there were a plethora of different designs, involving splines and dogs. Luckily, there are now fewer designs, and the only ones you’re likely to come across commonly are the Shimano splined freewheel and the SunTour 4dog freewheel. (A splined tool has ridges that fit into matching ridges in the component running along the tool. A dogged tool has pegs, called ’dogs’, that fit into matching notches on the component.)
Additional advice on freewheels
Fit the wheel back on the bike and change into the lowest gear (largest back sprocket, smallest front chainring). It’s important to make sure that the freewheel is firmly screwed on. With the bike on the ground, turn the pedals round so that the cranks are horizontal, then hold on the back brake and push down hard on the front pedal. You should feel the pedal move down a little, then stop when the freewheel is screwed fully home. Finally, check the gear adjustment and tune if necessary.
Freewheels contain a ratcheting device. It’s this which allows that rear wheel to continue spinning after you stop pedalling, to ’freewheel’. Pawls inside the freewheel are sprung so that they are pushed constantly outward against a toothed ring. The pawls point away from the direction of the wheel rotation, so that as you pedal they catch on the toothed ring and force the wheel around. When you stop pedalling, the teeth on the spring push the pawls inwards, so that they flap out of the way, springing back to be pushed out of the way again by the next tooth and creating a ticking noise as you pedal.
When the pawls become gummed up by mud or old oil, they fail to spring back and engage with the toothed wheel. If this happens when you pedal, the pawls won’t catch, and your pedals will spin forwards uselessly. You can often revive freewheels without opening them up. Remove the freewheel from the bike and look at the back surface. The central part stays still while the outer part with the sprockets attached rotates. Hold the freewheel flat and squirt plenty of thin oil into the gap between the two parts – WD40, GT85 or similar. This will flush dirt out of the front of the freewheel. Re-lubricate with thicker oil – chain lubricant is exactly the right thickness. Hold the central part of the freewheel still and rotate the outer part to work the oil into the pawls. Replace the freewheel on the bike.
Step 1: To remove a freewheel, first choose the correct tool. If possible, take your wheel along to your local bike shop and ask them to help you identify the tool you need. Remove any nuts or the quick-release skewer, and locate the tool in the freewheel (A). Splined tools hold themselves in place, but dogged tools need to be held in place with the nut or quick-release skewer. The tool in the picture is a 4dog freewheel remover (B).
Step 2: Freewheels screw themselves into place as you cycle and are often very tight. You need a large adjustable spanner to turn the tool. Stand the wheel up, with the spanner horizontal in your right hand, hold the wheel steady and push down hard on the spanner. If you’re using a dogged tool, you need to loosen the nut/quick-release a little when the freewheel starts to turn. Loosen the nut, unwind the freewheel a little and repeat as necessary.
Step 3: Refitting is much easier! Grease the threads thoroughly, to make removal next time as easy as possible. Carefully line up the freewheel
on the wheel – the threads are very fine, so it’s easy to cross them accidentally (which means the freewheel starts to go on crooked and gets stuck). Screw on firmly by hand.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Bicycle Cassette”