Thanks Jon, font of knowledge as always.
Just going back to the diffuser topic.. I'm certainly far from an expert but doing a lot of research lately , the stability from the diffuser is offered by the downforce it produces
When a flat floor transitions into the diffuser, it's as that point where the most downforce is produced. Race car builders will want to start the transition as close to the middle of the car as possible. So a percentage of the downforce will have an effect to the front suspension as well as the rear.
But rear suspension positioning and the droop of the suspension on the inside of the car when turning on a bend will clout the diffuser, so clever car design benefits.
Formula 1 is a great example of popular misconception that if teams build aero such a way, it must be right. Wrong. Teams are faced with different rules and regulations season after season, and what they build is the best they can achieve without breaking the rules, doesn't mean necessarily it's the most optimum design. Us mere home brew car builders only have 2 restrictions, lack of resource and lack of knowledge and experience lol
The transition from the flat floor to the diffuser needs to be smooth with an optimum angle of 7 degrees. The main purpose is to allow high velocity low pressure air to gradually go back to low velocity high pressure air when it meets the air flowing over the top of the car.
If the angle of the diffuser is too steep or the transition is too abrupt you will get air separation which creates turbulence and a partial vacuum. The best way to envisage air separation it go to your kitchen sink, turn on the tap and hold a spoon under the flow of water. The bottom underside rounded end of the spoon needs to be in the flow of the water.
The flow of the water from the tap is the air travelling underneath the car, the rounded bottom of the spoon is the diffuser. Let the tap run and you'll get a constant stream of water in a straight line, then when holding the handle vertically down almost parallel with the water offer up the rounded bottom of the spoon into the stream of water.
The water clings to the contour of the spoon, changing direction of the water as it creates a boundary layer with the spoon. The air under the car does exactly the same. You can change angle in which the rounded bottom is presented in the flow of water and if it gets too steep, the water can't cling to the surface of the spoon and the water goes all crappy and broken up, that's exactly the same as air separation which is bad on cars.
Ideally the air on the underside needs to flow into the air from the top of the car and meet nicely at the rear without creating a vacuum.
Transit van on the m5 doing 100 mph, the air flow from underneath, over the top, down the sides of the van will very rarely get anywhere near the rear van doors, you'll get a vortex behind the van and some air will spill into that void. But by n large that void is a vacuum which actually slows the vehicle and tries to pull the vehicle backwards so the air from the flow can fill that void. Not good.
Lastly, ( as I need to get up n go in the shed, insomnia ruined my night ) another misconception about diffusers........ The vertical strakes that you often see dangling from the diffuser is not to channel air and keep it in a straight line.
Imagine a diffuser with no strakes at all, one big channel of air from under the car flowing nicely out the back. Soon to be disrupted by the turbulent air that's coming around the rear wheels as it spills into the area of the diffuser. It's called tyre squiffing or something like that I can't remember the name, anyway, the turbulent air, enters the diffuser and disrupts your nice air flow and negates all your hard efforts in producing downforce.
Add in some verical strakes, say 2, so the diffuser now effectively has 3 chambers LH, centre and RH. At the bottom edge of each strake, you will get a vortex, it always occurs to the side of the strake but adjacent to the bottom edge. This vortex helps to prevent the turbulent air from the rear tyres entering the next chamber along, so the LH and RH chamber will have a partial disruption from the tyres, leaving clean flowing air in the centre chamber from the underside of the car to create the downforce at the transition from the flat floor to the diffuser.
Add more strakes ( with a trade off of more weight) and you'll have more chambers in the centre of the diffuser that are uninterrupted by turbulent air.
Put a 90 degree bend at the bottom of the strake like an L with the bend pointing from the centre outwards and you can move the vortex closer to the ground without reducing ground clearance which is even more beneficial.
Right I got pins n needles in my left arm from holding my iPad in bed. Getting up.
I'm not an expert on this, and sorry for "ladybird booking" what I've learnt so far.
Just hope all these theories I've learnt work, I really don't want to upset the classic lines of the Rotus by putting a spoiler over the boot.