Each have their own preferences, but generally: Lower air-pressure will make the wheel softer and cause for more friction as well as suspension. These features are best suited for a route that has uneven or soft paths with small obstacles.Higher air-pressure will make the wheel harder and cause for less friction as well as suspension. These features are best suited for a route that has even, asphalted (or similar) paths with no obstacles.We recommend look up the manufacturers recommended air-pressure which is typically described on the side of your tires.