A pump suitable for the tires on the bike. By which I mean
- It can pump to a high enough pressure
- It has the right valve attachment.
Pump should be mounted on the bike. Basically, I buy a pump for every bike I own. And I leave it with the bike. Think about it... you don't move your jack from car to car, why would you move your pump? Plus, its one less thing to forget when you are headed out and are packing a bag.
The corollary to this: the pump should not interfere with waterbottles, and should be easy to get on and off. Some are really hard and have really odd mounting setups.
An underseat bag. I like the small ones that strap under the seat. As with the pump, I just leave it attached to the bike. I always get the small ones that are just big enough for a few things, such as...
Tire fixin' stuff.
- A tube, make sure its the right size.
- A patch kit. Learn how to use this before you need to. It is a small insurance policy in case you mess up the tube installing it, or get another flat.
- Plastic tire levers, you'll probably need three for tight tires. Make sure they are plastic - you still see some metal ones out there, but these will scratch the rim. Don't use them.
- A twenty dollar bill. To get home if you can't fix your flat.
- Not related to tires, but I usually have a fold-up allen wrench for random repairs and adjustments.
Some other random thoughts on pumps:
- I like to have a floor pump for the house. Much easier to use than a handpump. And I always need to air up my tires before I head out.
- MAKE SURE YOU PRACTICE FIXING A FLAT. Fixing a flat is simple, but aggravating if you don't know how. And doing it on the side of the road in the rain, when you're tired, late and hungry is REALLY aggravating. Practice it a few times at home and make sure you have the hang of it. You'll thank yourself later. When we first got our bikes several years ago I forced my wife to practice it several times at home. I wouldn't necessarily recommend this as a formula for marital bliss, but she does now know how to change a tire.
- I don't like CO2. Lots of people do, I don't. I don't like having to carry the little bottles; I find them hard to control the airflow (and if you blow it on the road, you walk home); they are expensive. Probably some other things too. But like I said, many people like them so maybe they work for you.
2021 post script
Lots of bikes now have tubeless tires and this is amazing. But they have their own way of fixing flats which I do not cover here. That said, a lot of people also still carry tubes as a backup so all the same things above still apply as a Plan B.