Last night my bike broke, I was pedaling, but the bike wasn't moving!
We soon realized that the ratchet mechanism in the rear wheel was broken.
Time to learn how to replace a freehub body!
You will need:
- cassette spline tool
- suitable spanner for cassette spline tool
- chain whip (or vise whip)
- cone spanner (usually 15mm, 2 spanners make it easier.)
- spanner (usually 17mm, 2 spanners make it easier.)
- large Allen key to fit freehub (usually 10mm, sometimes 12mm or 14mm.)
- bicycle grease
- replacement freehub body
Step 1: Diagnose the Problem!
If you can spin the pedals, but the back wheel isn't moving, and the chain is all on correctly, then this means that the ratchet mechanism that allows you to freewheel is probably broken.
The action of freewheeling is when your bike is moving along but you don't have to spin the pedals.
There are two very different types of ratchet mechanisms on a bicycle.
There is a thread-on freewheel where the ratchet mechanism is within the sprockets.
There is also the more modern freehub and cassette where the ratchet mechanism is within the freehub body.
On my bike I have a freehub so I need to replace it to fix the problem. If you have a thread-on freewheel you just need to replace the freewheel unit.
(Second photograph stolen from sheldonbrown.com - excellent resource for bike mechanics.)
Step 2: Remove the Cassette.
Remove your rear wheel.
Remove your quick release skewer.
My bike is a single speed, but this process would be exactly the same if you have a cassette with lots of sprockets on it.
As we have to undo the cassette lock ring in the same direction as the freewheel, you need a tool to hold the sprockets as you do this. Most common would be a chain whip (as seen in photo 2.) or a vise whip (as seen in photo 1.)
Insert a cassette spline tool into the cassette lock ring. Sometimes it's useful to hold the spline tool in place with your quick release skewer to stop it slipping.
Whilst holding the sprockets use a suitable spanner to undo the cassette lock ring.
When the lock ring is removed, your cassette should slide off your freehub body.
Step 3: Remove the Axle.
To gain access to the freehub body, you need to remove the axle from the wheel.
Using the correct size cone spanner (usually 15mm) and spanner (usually 17mm) hold the cone and lock nut on the non-drive side of the wheel (the side without the freehub body.)
Unscrew the lock nut. When removed, there may be some spacers or washers. Remove these and keep note of the order.
Hold the lock nut on the drive side and unscrew the cone on the non-drive side.
You can now remove the axle from the drive side.
When you do this, the bearings may be left inside the hub or stuck to the cones. We always replace bearings with new ones, but if you want to use your old bearings make sure you catch them all!
A pair of needlenose pliers are handy for removing bearings from the hub.
Step 4: Remove the Broken Freehub Body.
There are quite a few different types of freehub bodies. Unless you've got an expensive rear wheel, it's most likely that your freehub body will be a fairly standard Shimano type as ours was.
Using the correct size Allen key, you should be able to engage a bolt inside the freehub body from the drive side. Most commonly this is a 10mm Allen key but sometimes it's a 12mm or a 14mm.
The bolt unscrews in the normal direction but it can be very tight so you may need extra leverage. Unscrew the bolt fully and the freehub body can be removed from the hub.
Step 5: Find a Replacement Freehub Body.
You can either buy a new freehub body or if you're lucky your local bike shop or bike recycling project may have a stash!
Your replacement freehub body needs to have the same interface as your old one (meaning the same spline pattern) so it can fit onto your old hub.
There are also two different sizes of freehub bodies. The smaller for up to 7 gears and the larger for 8 or more.
Step 6: Install New Freehub Body.
Grease the threads of the freehub body bolt.
Place the freehub body in position on the hub and using your Allen key tighten the freehub body bolt from the drive side.
Step 7: Replace the Axle.
Take time to clean the axle, cones and the insides of your hub where the new bearings will be placed. The cleaner the better, any grit or dirt will wear your bearings out fast.
Put a layer of grease inside your hub where the bearings will be placed. Then carefully insert your new bearings, these would normally be 9 x 1/4" bearings on each side of the hub for a rear wheel. A plastic biro lid is a good tool to nudge your bearings into place.
Once you've got your bearings in the hub, cover them in more grease to stop them falling out.
Check your cones for any wear (roughness on the surface), if they are worn this would be a great time to change them.
Push the axle (which still has the cones, spacers and lock nut on the drive side) through from the drive side.
Wind the cone on the non-drive side finger tight.
Replace any spacers or washers and the lock nut on the non-drive side, wind finger tight.
We need to tighten our cones so that there is no wobble or rocking in the axle, but not so tight that we crush our bearings. This can be difficult, don't worry if you don't get it right first time.
A nice trick for small adjustments is shown in the last two photos of this step.
Step 8: Install Cassette.
Slide your cassette back onto your freehub body.
The freehub body will have one spline smaller than all of the others, which means you can only fit your cassette in one position.
My bike is a single speed so it looks quite different to a normal geared cassette, but the same principle applies.
Grease the threads of your cassette lock ring and tighten with the lock ring tool and a suitable spanner. You don't need to hold the sprocket when tightening if your ratchet mechanism is working.
Install your quick release skewer and fit the wheel back into the bicycle.
Step 9: Finished!
Now when we spin the pedals, the back wheel spins!