To really understand and appreciate the stress the valvetrain encounters, you need to see it for yourself. On the below video you’ll see a cutaway BMW S1000RR valve train hit the redline at 14,200 rpm engine speed, which translates to 118 revolutions per second on the camshaft.
This BMW valvetrain is a typical for a modern motorcycle engine.Valve springs are used to keep the valves closed, when not opened by the camshaft. Camshafts push finger followers, which again push valves to open them.
The valve springs need to be stiff enough to prevent valve float, i.e. to keep the cam lobe – follower – valve path solid at all times, that is to make sure the valve closes as soon as the cam lobe lets it. This would be easily achieved with extremely stiff springs, which would however cause significant internal power loss, since overcoming stiff springs take more power, that could be used to rotate the rear wheel instead. For minimising the internal power loss the springs need to be as close to float limit as practically possible.
After seeing (and hearing) the mesmerising high-speed operation of a motorcycle valvetrain, you’re sure to appreciate yours, especially your valve springs, a great deal more.