Roadholder 364 - April 2018

41 exhaust valve being forced open by cam control), • induction - inlet valve being pushed open by cam control, the exhaust valve will therefore have hardly any suction force imposed upon it, The 'influences' working on the valve train are not dissimilar to those for the early four-stroke motors that utilised the 'atmospheric' inlet valve, whereby the inlet valve was not opened via a cam but simply 'sucked' open by the descending piston on its inlet stroke; the valve was fitted with a light spring to assist the valve to close upon the compression stroke. Today (and in recent years) problems as you allow enough room, the opening valve has nothing in its way, thus the opening cam can push, within reason, as far as it likes, without coming up against any solid resistance (piston crown, the other valve(s) etc.). The closing cam, however, is pulling the valve shut; what happens if, due to wear, maladjustment, machining tolerances, heat expansion etc., the cam 'overpulls' the closing valves? I.e. the valve has reached its seat before the cam has stopped pulling - something's gotta' give: probably the valve head will snap off the stem, the cam drive train will be badly damaged etc. (see figure 1). Upon closer inspection, it is evident that almost all desmo systems do deploy a small 'helper spring' to gently close the valve over its last few thou' to reach its seat, thus protecting the drive train. I've referred to the springs as 'helper-springs' in my diagrams but they are also referred to as 'Lash' or 'Return' springs in other documents. In the '50s, Mercedes Benz used desmo operation in their Mercedes W196 three-litre racing engine and discovered that by leaving a tiny residual clearance of 0.03mm in the mechanism, the valves returned and maintained the closed position, the inertia of the valve and the gas pressure in the cylinder being sufficient to reach and maintain the closure. When you think about it, apart from during the induction stroke, both valves are always under positive closing pressure i.e. (see figure 3): • compression - both valves being pushed towards the ‘closed’ position, • ignition - both valves being pushed hard towards the ‘closed’ position, • exhaust - both valves being pushed towards the ‘closed’ position (but