Curiously enough, these frame modifications weren’t intended to add some ground clearance, as can be seen in my last overlay. If you stretch the forks upwards, and the front and saddle frame tubes and upper rear frame tubes downwards, all you get is a raised saddle and petrol tank, while the engine is still in its original position. Obviously the ground clearance isn’t affected… Makes you wonder why they took the trouble… Did the longer stroke (90 instead of 77mm) require a taller frame? Ron didn’t have much problems fitting the engine to a standard frame… but Ron’s engine has a steel cylinder and cylinder head. Was the aluminium head higher? But even if it is, the first LW (#101) was first delivered with a steel cylinder… Or was it just an easy way to make this small and light WD/D motorcycle a bit taller, without adding too much weight? It looks as if a low centre of gravity (“low” engine position, for increased stability) was considered more important than increased ground clearance…
Another thing that is worth studying now that we know that the frame has to be modified is the steering head geometry (rake and trail). And although the fork length will obviously affect the steering head geometry, this steering head geometry is completely independent of the geometry of the rest of the frame. I have added two lines to the WD/D and lightweight factory photographs to visualise these parameters, and although the accuracy of my sketch is not spot-on, these lines clearly show that Enfield “radically” changed both values (rake and trail) when they designed the lightweight! The rake decreases from 30° (WD/D) to 25° (LW), and the trail decreases from approximately 4” to approximately 2”. The effect of this modified geometry will be a more manoeuvrable and easier steering motorcycle, with less high speed stability. No doubt these are the handling characteristics that you would expect on a military motorcycle. In order to obtain this geometry, the steering head was tilted 5° and now makes a 40° angle with the downtube instead of 45° (WD/D).
Jan