Well, sounds like (d) is out at least. This option is used on solar cars etc, but then they have fairly high ground clearances (in proportion to body thickness) and negligible high pressure zones at the nose, so they wouldn't get the compression effect...
I'm not concerned with downforce at all (though I'd rather avoid generating *too* much extra lift
) - just acknowledging the 'conventional wisdom' that 'air dams are for drag and splitters are for downforce', which is what I'm questioning here, based on a few studies I've seen (including the one linked).
The vertical air dams shown all extend very far down, which is not an option - a vertical air dam which ends 10-15 cm (4-6 in) above the ground (as per local road-worthy regulations) will also force air under the vehicle, simply by means of the high pressure zone at the front of the vehicle - a situation a splitter or vertical dam with an angled lip *may* improve.
On the speed record cars, downforce is a huge concern (hence the full-height dams), but there may also be other reasons to avoid a splitter (salt tracks can apparently be rougher on the small scale than bitumen). Plus the speeds are quite a bit different, I'm looking at 60-110 kph (45-65mph) not the 300kph / 200mph range. I suppose what I'm saying here is that building a salt-flats car that goes over 300kph doesn't imply you've done careful comparisons of air dams vs splitters and then optimised it for the highway at 100kph... there will no doubt be similarities, but the best solution may not be the same. By the same token, studies I've read showing drag improvements from adding a splitter to a vertical dam are also from race car studies, though at lower speeds. Which is why I'm looking for "passenger car" experience...
One big factor, as pointed out, is that splitters do "look" dangerous (I imagine the plywood or aluminium ones referenced elsewhere on the 'net would be quite deadly, in fact) - one reason option (c) was included.
Does anyone have links to FE comparisons of, say, vertical vs vertical with splitter on this or similar sites, at realistic on-road clearances? I did look, but couldn't find any references to actual results from anyone trialling a splitter or angled lip in a FE context.