Bicycle tires lose air slowly. It’s just their nature. Because they don’t hold a lot of volume of air and because a significant amount of that air can seep out over a relatively short period of time (a week for a road bike tire and about two weeks for a MTB knobby), there’s a risk if you just ride without checking the tire pressure. If you bike on soft tires and you hit a pothole, rock or other obstacle, it’s possible to damage or ruin the tire, tube and worst of all, the rim. A too-soft tire also means that you’re working a lot harder and on a mountain bike, it can make for a wobbly, hard-to-handle ride. So, be smart and check your tire pressure regularly: every week during the season for mountain bikes and before every ride for roadsters.