I've been looking at hatchback / squareback rear roof extensions. Two approaches are taken:
1. Near-horizontal extension designed to give clean separation edge.
2. Angled downwards extension designed to give clean separation and also reduce wake size.
As I've described elsewhere, for #2 above to reduce drag, the drag force developed by attached flow on the angled extension needs to be less than the reduction in drag achieved by the reduced wake. (On my Roomster I juggled the extension angle until I had achieved that.)
Today I was looking at the rear roof extension on a Mitsubishi Mirage. I've looked at these before, but perhaps the pink colour of the car made its design features stand out more.
It appears to have:
- downwards angle (I measured it at 13 degrees) to reduce wake size
- small spoiler to increase pressure on the sloping part of spoiler ahead of it (so addressing the point above about the attached flow causing drag)
- small side fins to improve spoiler efficiency
Impressive!