An object weighing 1 kilo (2.2 lbs) hitting the helmet at 170mph would result in the instantaneous release of 548 ft.lbs of energy. it would have been like being hit on the head by a 1/4 ton weight. The actual energy released would be in excess of that because the spring was travelling towards the car at an unknown speed.
With this sort of thing in mind, can one imagine the mayhem if one was hit by a wheel coming off a car coming the other way at, say, 60mph. Because the wheel is unrestrained, it travels forward a twice the road speed of the car, i.e. 120mph. In this case, a wheel weighing, say, 40 lbs has an energy on release of 7040 ft.lbs. It is the equivalent to being hit by a three ton+ weight. If you are travelling towards it at 60mph, the relative velocity is doubled so the striking force is doubled. The cusioning effect of the tyre deforming on contact is of academic interest only. Frightening!
On the question of springs being made of steel rather than titanium, it sticks in my mind that titanium is basically a hard, brittle metal although this can be changed by the addition of various other materials such as manganese to increase ductility for sheet manufacture. The elasticity of steel is greater than that of titanium, hence the reason for using steel for springs even although titanium has the same strength as steel at roughly a third of the density and in racing cars, where weight is everything, would seem to make sense. Titanium is widely used in the aircraft industry for nuts and bolts and other components such as sheet metal, covers around engines, compressor blades in jet engines and other areas where the temperature does not exceed 400deg C because of its strength and lightness. If the designers can save 2/3rds of the weight of the nuts and bolts in an aircraft by using titanium, it amounts to a huge saving in weight therefore a reduction in the amount of power required to fly the aircraft at design speed. As a side effect, there is a corresponding reduction in fuel consumption. A reduction in fuel consumption results in a reduction in weight of fuel carried which in turn....... and so on. It really is one huge compromise balancing the design against laid down specifications of carrying capacity, distance without refuelling, operating costs etc. Oh what fun!