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May 19, 2012 Fixed, Mechanics No Comments

Track cogs for fixed gear bicycles can be a real pain to get on and off. Ask anyone who’s done it and they probably remember losing skin off their knuckles at some point.

Chain whips are largely useless in this situation. Maybe with a bit of extra leverage, like a bit of pipe to extend the handle they could be useful, but there’s actually a much simpler and more effective way of putting track sprockets on and getting them off – even the really stubborn ones.

This trick is accredited to legendary Italian track racer and master mechanic Giovanni Pettenella. It’s called the Rotafix and is supposedly so effective the lockring is no longer needed. That bit is your own decision though – I still put a lockring on my wheels.

To put a cog on:

  • Screw the new cog on finger tight and install the wheel in the dropouts. Tighten the axle nuts.
  • Unhook the chain from the chainring at the front and hook it round the bottom bracket shell. It’s best to put some padding between the shell and the chain as you might damage the paintwork. Here you can see I put and old sock round the bottom bracket shell (I keep all my old socks instead of throwing them. I find them very useful for rags).
  • Now on the rear hub, wrap the chain around the cog so the chain traps itself. What you should have is a triple layer of chain on top of the cog.
  • The chain should now be taut between the bottom bracket shell and the cog.

  • If you grip the rear wheel and gently but firmly turn it anti-clockwise you will tighten the cog on the rear hub. Don’t overdo it or you will strip the threads on the hub. You don’t need to be forceful as the tightening torque is applied at a distance equal to the radius of the wheel instead of the smaller radius of the sprocket.
  • Job done. Put the lockring on with a lockring tool.

Reverse Rotafix

It’s also possible to undo a stubborn cog this way. You just reverse the process. What you need to do is take the lockring off and wrap the chain around the bottom side of the rear cog so when you turn the wheel clockwise it loosens up. I’ve had great success with this method.

Written by james

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