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First, acknowledgements to Bruce Dunbar who originally started this thread years back. I can't find the original Bruce but it was always one of my favourites. In the comments below, tell us the item that you swear by on the bike... a tool, a component, an item of clothing or food, an accessory... so long as it's connected with cycling and is in your cycling 'Desert Island Disc' items to take with you, then it's fair game.
If you've done this before, feel free to submit again as many won't have seen the original which is lost in the depths of the forum!