A commercially made hypercracker is now available, see: www.jastein.com/Tools_for_Wheels.htm
WARNING - If the lock ring is tight this tool will bend your dropout!
The Hypercracker is a tool for removing the lock-ring that secures Shimano cassette sprockets. This is necessary if a spoke breaks on the cassette side of the rear wheel, or if the freehub needs to be removed for maintenance. The 'official' tools for the job are a Shimano cassette lock-ring remover, a big spanner and a chain whip - not the most practical toolset for an extended tour!
Hypercrackers were invented by Pamir Engineering, who used to make them. Sadly they no longer do, indeed I think the company was wound up when its founder retired. To use a Hypercracker you remove the rear wheel and fit the Hypercracker into the spline of the lock-ring. Then you replace the rear wheel in such a way that the frame or dropout prevents the Hypercracker rotating. All it takes then is a pull on the pedals (by hand should be sufficient) which rotates the cassette and rear wheel while the Hypercracker and lock-ring remain stationary. Voila! the lock-ring is loosened and when you remove the wheel from the bike you will be able to unscrew the lock-ring by hand and remove the cassette.
I first saw the idea of making your own Hypercracker on another cycle touring website: Ed and Jeanne Downing's. Their version involves some welding which mine doesn't, but then mine puts more load on the dropout and I did bend Juliette's dropout trying to release a very tight lock ring. I was not popular until it had been hammered and bent back into shape!
I took a standard Shimano cassette lock-ring remover and ground most of it away so this much was left:
The stub sits against the dropout. I should think that if the lock-ring was really tight there is a risk of bending the dropout. It fits onto the lock-ring like this:
In use it looks like this:
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