What you will need
Cash (or credit card)
Why it's important
If you get a flat
It's often said there are two certainties in life: death and taxes. If you ride, you can add another certainty: flats. There is no doubt that, if you ride a bike, you will at some point get a flat tire. Knowing how to change a flat tire is up there with knowing how to change gears. It's a must.
Having all the right tools will make this inevitable job easier and quicker, two things you want when you are on the side of the road with a flat.
Tire levers, tubes, a hand pump and / or CO2 cartridges and a CO2 inflator, are essential. You can also add a patch kit for extra security. When you are packing, make sure you include two tubes (at least). Multiple punctures are not uncommon, and if you do change a flat tire and have only taken one tube, you're only a piece of glass away from walking home or flagging down a taxi.
If you get a mechanical
Having a mechanical issue with your bike is less common than getting a flat tire, but may impact your ride to a far greater extent. Loose handlebars, a dropped seat post, rubbing brakes: these are just some of the common issues a multi-tool will be able to fix in the blink of an eye.
Multi-tools can contain a variety of Allen key sizes, screwdriver heads (flat and Phillips), a chain breaker, a torque wrench, spoke wrench, and some will even contain a bottle opener (perhaps to celebrate once the job's done?).
Whichever multi-tool you decide on, make sure you know how to use it. There's no point parting with lots of cash for a multi-tool with everything including the kitchen sink if you have no idea what to do with it.
In Case of Emergency
Hope for the best; plan for the worst. Simple philosophy. Great advice.
There are so many things that could potentially go wrong on a bike. You need to have a plan in place for all of them. It's not nice to think about, but what if you had a crash?
What if you ran out of tubes and puncture repair supplies, but were stranded 35 miles from any help? What if you bonked with no food and were nowhere near home?
Carrying your I.D. and some extra cash or a credit could get you out of all of these potential issues, or at least give you a helping hand. Carrying ID will help others identify you in case you are involved in a serious crash and become unconscious.
Another handy tip if you are involved in a serious crash is to create an 'I.C.E.' - In Case of Emergency contact in your phone. That way if you have a crash, someone will be able to look up your 'In Case of Emergency' contact and let them know what has happened.
Having cash with you is another logical addition to your pre-ride pack. Take more than you think you will need, not just enough to cover the traditional post-ride-brew. Take enough money to cover a coffee, drink, food and perhaps even a cab or train ticket. The longer your ride is, the more important this becomes. If you run out of food and bonk you'll need to top up at the nearest servo or cafe (if you're lucky enough to pass one).
If you have a serious mechanical or the weather turns diabolical, a taxi or train ride home could be your only salvation - two things that are out of the question without cash or a credit card.
And make sure all of these things are stored within a waterproof case. Your phone will be pretty useless if water gets into it, and most cafes or servos won't be too happy if you hand over a soaking wet note.
In Case You BONK!
The same philosophy applies with nutrition and money, take more than you think you need. And again, the longer your ride is, the more important this becomes.
Bonking or running out of fuel is a terrible feeling. Your legs feel hollow, your stomach hurts, you have a killer headache and you're kind of angry but so tired and flat you can't express it. The best way to avoid it is to plan ahead and take enough food to cover your journey.
However, even with the best planning we all know things don't always go to plan. You might get lost, it could be hotter than you thought, you might spend longer out on the road thanks to a flat or mechanical, countless reasons why you need to take more nutrition than you think you need.
A few extra dollars will also come in handy if you pass a servo or cafe and fancy something other than gels or energy bars.