Replacing the sprocket

MagicMan over at Chinese Bike Forum was asking about changing the front sprocket, so here's some images to try and explain it. I'd imagine the process will be very similar for most chain driven bikes.

I fitted a 16 tooth JTF259 sprocket (cheap and easily available at all good fleaBays). There's also do a 17 tooth sprocket, which I intend to try as well; I think this engine would be well suited to it (spoiler: it is, I'd suggest going straight to the 17).

First, slacken off the rear axle nut, circled in yellow here. The hex head circled in red is one of the axle adjustment / chain tensioning bolts.




Then if you're fitting a larger sprocket, you'll need to slacken off the chain. You can do this now or later, once you get the old sprocket off.

This is done by moving the axle forwards, for which you'll need to loosen off both adjustment bolts, then nudging (kicking, rubber malleting...) the rear wheel forwards. Take care to count the number of turns that you make so that you keep them both even!



I think I had to loosen them about 20 'flats' (i.e. about 3 or 4 full revolutions) per side, but it may be more. It'll entirely depend on how tight your chain was to begin with; checking chain tension is something that you'll want to do regularly anyway.

Now remove the sprocket cover. This is located above the left footrest. There are 2 bolts to undo, one at the top, and one underneath, right next to the gear change shaft. A socket with a universal joint and extender bar is very handy here!



The cover then lifts off easily. Under it is the sprocket. It's held in place by a retaining shim and two bolts. Put the bike in gear to stop the engine turning and undo these bolts...



Then you'll need to rotate the shim so that the holes line up with the splines on the sprocket. It can take a little fingernail-fiddling to get it lined up, but once it's oriented correctly, it should come off without the need to lever it.

Then the sprocket just slides off the spline, and you can take it out and replace it.

Fitting is just the opposite of removal. Put your new sprocket on; if it's a larger one, then you'll have to slacken off the rear axle adjusting bolts and knock the wheel forwards until you can get the sprocket onto the spline, or the chain to go over the sprocket.

Then the shim slides back on and rotates around until you can get the bolts back in. Don't push it too far on, or it won't rotate; just slide it on until it does rotate, then get it aligned.

The cover should pop back on easily, then it's just a matter of checking the chain tension, which you'll likely have to increase: tighten the adjusting bolts, by the same number of turns each side, until the chain is tight. There should be about 1/2" (10-15mm) of vertical movement in the middle of its length. Once you're happy, tighten up the lock-nut at the front of each adjuster bolt until it's secure.

Then remember to tighten up the rear axle bolt!

Finally, adjust the tension in the rear brake. If the axle has had to come forward to fit a larger sprocket, then there will be some slack in the rear brake. Adjusting it is just a matter of turning the knurled nut on the back to take the slack out. It helps if you press the lever forwards to take all the tension out, then turn the nut until it's tight. Due to the shape of the nut, there will be some slack when you release the lever, just enough to stop the brake binding.



Blurry sloppy photography, but hopefully you get the idea.

Last check: you did tighten up the adjuster bolt lock-nuts and the rear axle bolt, right?