Bicycle Gears Jammed

It's my bicycles turn to have bad luck today. On a straight road I use the big gear ring on the fron and the small on the back, it really can spped up to 30 mph in that gear. The trouble is the las part fo the route take me uphill to the house. As I tried to change down gear the system jammed and I came to halt 20 metres up the hill due to this. I had to walk the last 1/2 kilometre up the hill before pedalling in slow motion on the last straight. It was out with the tool box when arriving home.

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