For the most common breakdown, a flat tire, carry a spare tube and tire levers (these make it easy to remove the tire and they usually come in sets of two or three). A patch kit for repairing one of your tubes if you have a second flat. A tire boot (a 1- x 2-inch patch or an old section of tire) for tire cuts (the boot is placed between the tube and tire to cover the hole). A chain tool (illustration) allows you to fix a broken or damaged chain. And, a mini-tool (some include chain tools) with 4, 5 and 6mm Allen wrenches and screwdrivers will allow you to adjust most of the bike's bolts. And, always carry cash for food and to call someone in case of the rare failure that you can't fix.
Grease Be Gone
Other handy things to carry include a little hand cleaner (paste types work well) and a small rag to scrub your hands after completing a repair. A great way to carry cleaner is to pack it into an empty 35-mm film container and stow it in your seatbag.
Make Sure Your Mini-Tool Has What You Need
When selecting your all-in-one mini-tool, test it on your bike. Bicycles and components vary and not all tools work well on all bikes. Check to see if yours can access the brake and derailleur adjustments; if the chain tool looks sensible; if the all-in-one includes the right tools for your bike. Chain tools can turn out to be great ride savers should your or a friend's links fail, but some tools don't include them.
Carrying Tools Helps Others Help You
You might think that it's senseless to carry tools and supplies if you don't know how to fix your bike. But, if you carry the right stuff, you'll at least have what you need and can try to repair things. Also, you'll have what's needed in case another cyclist stops to help.
Pack A Manual, Too
If you're a novice mechanic, consider tucking a small repair book in your kit with your tools. This might require carrying everything in a slightly larger bag, but it'll pay dividends if the advice in the book helps you successfully repair your bike.
Don't leave that repair book in your kit, either. If you read it before the ride, you'll have a better idea where to begin when things go wrong. You can also learn about repair by watching friends fix their bikes. Bicycles are fairly user friendly and with a little know-how, experience and the right tools, minor repairs are easy to fix.