The bit I don’t get is that he states the two springs (main spring and rebound spring) are in series so the initial total stiffness is the addition of both spring rates.
The answer to this is best understood by thinking about preload adjusters on the rear springs of motorcycles.
How do they work? By compressing the spring using a stepped or threaded adjuster so you can apply some force to the spring when the bike is at normal static ride height. What is the effect? It makes the spring harder and less compliant to road bumps and undulations.
Now think about a 140lb/inch main spring on a Morgan. How much force does it need to compress it by 1 inch? 140lb. How much force does it need to compress it by 2 inches? 280lb.
Now think about the effect the rebound spring has when it is pushing up against the bottom of the stub axle when the car is at static ride height. What is the effect? The rebound spring, being much stronger than the main spring, is going to compress the main spring so the stub axle is at the point where the forces balance out. The rebound spring won't be free as it is still pushing against the stub axle, and the main spring will be squashed too, being shorter than it would otherwise be. Remember the far ends of both springs are constrained by the same bit of metal, the Crosshead.
So we now have a situation where the main spring is preloaded by the force of the rebound spring, exactly like the motorcycle rear suspension is compressed by the adjuster. How much is it going to be compressed by depends on the relative poundage of the two springs. Let's pretend just to make the illustration easier, that the main spring is compressed by 1 inch. We'll ignore the weight of the car and the occupants as it's the same in any scenario.
Now let's go for a drive. We hit a bump. That bump is going to impart a force to the suspension. How much force do we need to compress the suspension by 1 inch so the car moves smoothly over the bump rather than jolting up into the air? 280 lb. We're using the second inch of the spring's movement as the first inch is already been taken up by the rebound spring.
So our main spring is behaving not as if it was a 140lb/inch spring, but rather as if it was a 280lb/inch spring. That's why having a rebound spring under compression makes your front suspension harder than it should be and causes the 'nodding dog' effect when you go down the road.
