Tyres Pressure

How much tire pressure should you run? Start by trying the manufacturer's recommended pressure, which you'll find printed on the tire sidewall (it's often on a small label but it might be molded into the casing, too, so look closely). This suggested inflation range is a good starting point. If it's a wide range, for example 40 to 60 psi, experiment to find which pressure works and feels best.

Pump Road Rubber More, Knobbies Less
The most common mistakes are riding with too little pressure in road tires and too much pressure in off-road rubber. The former happens because road treads don't have a lot of air volume. Sure, road tires are pumped up to high pressures. But, because they're skinny tires, there's hardly any air inside. Consequently, even if only a little leaks out (most bicycle tubes are made of butyl rubber, which is porous and naturally seeps air), the pressure and volume are greatly reduced. To prevent this, check tire pressure on a road bike before every ride. If you don't, you'll be riding on soft tires, which is asking for trouble. More about this in a minute.

Off-road rubber is inflated to lower pressures and because the tires are much wider than road models, there's considerably more air inside. These differences mean that fat tires don't seep air very quickly so they don't require frequent inflation the way skinny tires do. Unfortunately, the tendency is to over inflate off-road tires. By all means, if you're riding your fat tires exclusively on pavement and smooth surfaces, inflate them as hard as you like (don't exceed the manufacturer's maximum recommendation).

Go Low
If you're riding off road, however, seriously consider lower pressures -- in the 35- to 45-psi range, depending on the terrain and your weight. This will greatly increase your control and comfort over trails while improving traction and handling. Indeed, if you've been riding off-road on 50 to 60 psi, you'll be amazed at the difference.

How Low Is Too Low
Just, don't go too low. That'll increase the risk of a flat two ways (this holds true for road and off-road rubber): First, softer tires pick up more debris, which may work into the tires popping the tubes. Second, when you hit holes, ruts, rocks, etc, soft tires can deform to the point that the rim hits the ground or rock so hard that it pinches the tube (between the rim and obstacle) and cuts it in two places, which is what's known as a pinch flat or snakebite puncture (because the holes in the tube resemble a snakebite). Besides damaging the tube, this impact can bend the rim, leading to an expensive repair. Under-inflated tires also lack the sidewall rigidity needed for hard cornering. And, too-soft tires wear quicker.

Road Rating
But this doesn't mean you should always inflate road tires to the maximum pressure. Roads in the real world aren't billiard-table smooth. The jarring effect of bumpy pavement on over-inflated tires robs energy and makes for a bone-rattling ride. Properly inflated tires will roll over bumpy roads smoother and faster and get you home without shaking loose your dental work. On ultra-smooth roads, however, when rolling resistance is critical, such as in a time-trial or triathlon, go as high as 140 psi if your tires are rated to take it. Stay at the lower end of the pressure zone for comfort and rough roads.