So all fastback shapes have a lot of lift -
but does that actually matter?
After all, even in the case of the relatively high rear lift Porsche Taycan described above (well, high lift before the rear spoiler deploys), the lift is still only a small proportion of the car's mass - especially at the speed limits in most countries.
But my experience with the Insight, and also based on some of the SAE paper research I did for my aero book, is that yes, lift does matter. In both cases, there seems to be evidence that the aero lift is
disportionately felt.
I quote some papers in the book where lift - especially rear lift - appears to be oscillatory in nature. (This would not be shown in a wind tunnel that uses averaged Cl figures.) Oscillatory forces (ie they fluctuate rapidly) have the real potential to excite resonances in car suspension. Pretty well all car suspension have natural frequencies in the range of 1-2Hz (it's easy to measure suspension natural frequency with a smartphone and an app, but off topic here) and it appears that the oscillatory aerodynamic forces are within this range. (Perhaps they are caused by Karman vortex shedding?) Thus this appears to be a mechanism by which the car may become disproportionately upset by aero lift - especially at the back.
Another (anecdotal) example is a guy who follows my YouTube channel - Nate - who put a really good undertray on his Passat wagon and immediately commented that the car felt "just planted to the road and incredibly smooth" - see
https://youtu.be/jKRrhhk5zj4.
That's also my experience with the Insight (that now develops measurable downforce) - the car is just
so different from 80 km/h (50 mph) and upwards. In my car, the ride quality
is literally smoother (as the effective weight of the car increases) and stability is much better.
But what if you're chasing just low drag, or you drive only at low speeds (sub 80 km/h - 50 mph)? In those cases I don't think that lift matters much. I would have once said that because lift causes induced drag, you don't want any lift at all for lowest drag, but I note the lowest drag production cars in the world (eg the Taycan) achieve that with lots of lift (although the solar race cars, still on another planet in terms of drag, chase zero lift).
So, TL;DR:
- Fastback cars develop lots of lift
- Lift - and especially rear lift - really matters
- But if you drive really slowly all the time, it doesn't matter much