ALL ABOUT TYRES

Presta or Schräder?

Presta valves (skinny) fit all rims, while Schrader valves (fat) only fit Schrader-drilled rims. Schrader valves are supported at most garages’ compressors and work better with the insertion of a slime system. Schrader valves are more robust, but tend to leak marginally more. Valves should always be kept clean, and Presta (skinny) valves’ bead occasionally tightened!

Correct tyre pressure

As a starting point try the tyre manufacturer's recommended pressure - which you'll find printed on the sidewall - but experiment to find which pressure works and feels best for your individual setup. (The maximum figure stamped on the tyre’s sidewall is simply half the rated pressure the tyre could possibly take, before exploding off the rim.) The tyre’s ‘perfect pressure’ is wholly dependant on the weight it has to carry (that be you!) and cannot really be ‘standardised’. Higher tyre pressures often help reduce the chances of puncture, especially snake-bite, but a ‘softer’ tyre’s potential lower rolling resistance and better traction must be off-set against it. Find your preference. Harder, thicker tyres - particularly in the side-walls - are less prone to punctures, but weigh more. Lighter tyres puncture more.

Road rubber more, knobbies less

The most common mistakes are riding with too little pressure on-road or too much pressure off-road. Road tyres are pumped up to high pressures, but - because they're “skinny” - 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 / volume equations within the tyre are greatly reduced. To prevent this, check tyre pressure before every ride. If you don't, you might be riding on too soft tyres, which could invite trouble.

Go low  

When riding off road, use lower pressures in the 35- to 45psi range, (depending on the terrain and your weight). This will increase your control and comfort over routes while improving traction and handling. If you've been riding off-road on 50 to 60 psi, you'll find a difference in comfort / fatigue.

Don't go too low, though

It will increase the risk of a puncture (this holds true for road and off-road rubber) as softer tyres pick up more debris which may work into the tyres (piercing the tube), or even pinch the tube (between the rim and an obstacle) cutting it in two places (a  “pinch flat” or “snakebite” puncture). Besides damaging the tube, such impact can bend the rim that is an expensive repair. Under-inflated tyres also lack the sidewall rigidity needed for hard cornering, and wear quicker as a result of increased rolling resistance.

MTB

Yes, it is possible to ride with knobblys on-road, but it’s not recommended. Rather fit slicks, as the rolling resistance of knobbies over slicks are quite dramatic and will increase with speed. In addition, knobbies have a vibrating effect on-road, which will aid fatigue.   If you thinking of doing some long distance on road on your MTB, definitely fit some slicks, but accept that even if the tyre is a bit smoother the bike is still much heavier, and the rider’s positioning less aerodynamic to be really competitive.
Mud

As a general principle, a softer, broader tyre will ensure both better grip and greater speed off-road, as the patch / footprint is actually more effective in countering rolling resistance (an over inflated tyre has to “lift” the bicycle and rider over every imperfection in the road, whereas a properly inflated ‘pliable’ tyre will smoothen the effect of debris, and increase the speed obtained). In mud – however - a relatively thinner tyre will slice through the muck and reach the firmer ground more easily, and have the benefit of allowing more clearance between the tyre, frame and shock not clogging up as quick. Go for a 1.95 in the wet, and feel free to use a cooking spray on your tyres prior to racing, as the mud tends to cling less to the tyre.