Air tyres provide a very efficient suspension system. To work well they need to be hard enough but not too hard. Hard tyres keep the wheels round so - in general - harder is better than softer, but if the tyres are too hard you loose their suspension effect.
every imperfection in the riding surface, every bump and ridge is like a tiny hill. The upward side slows you down but going down the other side gravity speeds you up. Gravity only pushes you forward if your tyres are touching the ramp that converts downward force into forward motion. If a bump launches you off the ground you don't get any benefit from the 'down'. If you're off the ground you're slowing down. The point of air tyres is to keep you on the ground. - The load is the weight of the bike, the rider and any luggage. The heavier the load the harder the tyres need to be.
- The smoother the riding surface the harder the tyres need to be.
- If the bike has some kind of suspension - apart from the air in the tyres - the tyres can be harder.
- A dynamic rider, willing and able to use energy to press the bike on the ground, can use harder tyres than a tired or passive rider.
- When conditions are wet or icy reducing tyre pressure slightly can increase the amount of tyre surface in contact with the ground.
- The more uneven the surface, the softer the tyres need to be.
- You need enough air to provide a permanent cushion between rim and the ground in the hardest impact expected.
- If you want to run tyres at low pressure you need fat tyres that provide a reservoir of air big enough to avoid a compression puncture.
- If the tyres are too soft you risk a compression puncture.
In some applications - for example carrying very heavy loads or on a tandem - you may want to run a tyre at a pressure harder than the manufacturer's recommendation. If you want to try this, blow the tyre 50% beyond the recommended maximum and leave it overnight. If it doesn't explode you are probably safe to run the tyre 10% over the maximum. Running over pressure may shorten the life of your tyres.
Check the Pressure of Your Tyre
Pinch the sidewalls of the tyre between thumb and forefinger to check the pressure. For more precise information connect a pressure gauge to the valve. If you are using a pump with a gauge, the gauge measures the pressure inside the pump. Give the pump a couple of strokes to open the valve and get a reading of the pressure inside the tyre.
If there is too much pressure your tyres, and are too hard, let some air out.