"Bottom Kammback" shape
 

What kind of shapes are ideal for the bottom end of a kammback (sloping up from the bottom of a/the bumper)?

What is the angle relative to the top (e.g. is it simply an inversion of the shape of the usual top-end Kammback, or can/should it be a more dramatic curve/angle)?

bottom
 

If you have a complete and well executed belly on the car,the region aft of the rear axle can be made to form a diffuser of 2.5-degrees,to 4-degrees up-slope,depending on where the origin of the diffuser begins.
2.5-degrees is preferred for low drag.But you need to honor ramp and departure angles which can create a ground strike on driveway ramps,etc..

Angles for several body types, compared to drag values.

http://ecomodder.com/forum/member-sv...8-diffuser.jpg

This all started with one race car, for loading / unloading. The driver discovered the downforce, his competitors saw his advantage, and the very next race the entire field had done it. Better to copy than to whine. I guess it was worth the loss if a win to learn the lesson. Shame today's racers are all such losers.

Less dramatic, as aerohead says. The free air over the top of the car and the air in the tunnel between the belly and the road are different situations. But the side view angle is only part of the story, it's three-dimensional.

http://ecomodder.com/forum/member-fr...80-silver4.jpg

An overall rake is helpful.