Lower rear lip design question

[puddleglum]

I'm hoping to do a belly pan this summer and I'm wondering how to deal with the rear bumper. Here is a pic. of the stock bumper cover.
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Iv'e noticed a similar grooved design on several other hatchback and SUV type cars also. My question is, is this just a design element or is there some aero. advantage to it? Is this supposed to help reduce the wake behind the car by pulling some of the underbody air up to fill the wake? It looks too rounded to me to do any good. I'm wondering if it would be better to run the pan up to it and leave the rear as is, or try to extend the pan past this to create a lower rear spoiler? Any advise is appreciated.

[aerohead]

Puddle,you'll want to work your way back with the bellypan and as you near the back of the car,incorporate a diffuser of which the underside of the Rondo will determine its design ( 2.5-degrees or 4-degrees up-sweep depending on where it begins ).
Where the diffuser extends to, will 'fix' the bumper detail at the lower edge.I would not extend it beyond the back of the bumper for safety reasons ( tripping hazard ).
The back of the car should basically be all uniform,cut off like a loaf of bread.

[puddleglum]

Thanks for the response. Should it end where the rear bumper starts to roll up or should the diffuser extend to the back edge of the bumper and come up flat? Its sounds like your saying it should come up flat.

[aerohead]

flat is okay.It's what Kamm and others advocated.You could take some stylistic license with it.Keep people guessing.Performance wise,it's no biggy.