An objective fact is that when slowing a bike from high speed,pressing metal on metal isn't going to work well,while the tyres might work to provide more force of friction,but if your tyres has been worn to the limit,they will not be capable of providing safe brakings especially in wet condition ,as

Do you know why you need to adjust or repair ,even replace the valves after a certain amount of kilometers riding? It is owing to how the valves work!It is very easy to explain how the valves work theoretically: the cam pushes the valves down into the cylinder against the spring,so gases can flow in