How much air is too little and you should add more for the safety and ease of your bike ride?
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Look on the sidewalls of your tires, and you should see a recommended pressure range. If your tires are outside that range, you need more air. The lower end of the pressure range is good for soft ground like dirt or gravel or snow, where you need more traction; the higher end is best for roads, and will help you avoid pinch flats.
The short answer is: it depends on the tires.
If the air pressure is lower, there is more contact between the ground and the tire. This has the advantage of more grip (especially in rainy and/or icy conditions this can be nice), but the disadvantage of more resistance, making the cycling harder.
If the air pressure is higher, the advantage is making it easier, but the disadvantage is less grip.
In addition, if the pressure is low, the wheel can damage the tire, if the pressure is high, the ground can damage the tire.
In general the thin tires (such as on racing bikes) do require higher pressure then wide tires (such as on mountain bikes).
If you see the bottom of the wheel as cutting in to the side of the tire, then the pressure is definitely too low.
Other looking at the sidewall, here are some clues that I use:
Simple answer is, they always need more air. Tires leak, slowly, but they all leak.
If you are riding every day, you probably want to top them up every 2-3 days. If you ride once a week, you need to top them up every ride.
There are 2 ways to determine this. One is to use an air pressure gauge. The other is to squeeze the tire with your fingers.
Like Neilfein says above, first check the inflation specs on your tires.
Ok, so here's something you can do. Using a pressure gauge, pump up your tires to the max pressure. Then, using your fingers, see how that feels. Now, let the air out, and pump up to the min pressure, and see how that feels. Also, do the same with a middle pressure. Now you have a range.
Once you are familiar with how that pressure range feels, you can either head off for your ride or add air. And of course, you can always use a gauge to check the pressure before each and every ride.
For casual short rides I just squeeze the tires. And it works. Beyond that, I get out the pump and top them off.