Quote:
Originally Posted by basjoos
The underside of my boattail is steeper than I would have liked it, but I have a sharp approach to my driveway and didn't want to risk dragging the bottom of the boattail. I haven't tuff tested the underside of my boattail. I can't see it when I'm driving and don't have any aeronut neighbors to follow me, observe, and make sense of what they were seeing. But I have noticed that sections on the rear of vehicles that have turbulent flow tend to collect dust and dried deposits of dirt from water/salt spray. The underside of my boattail doesn't collect get dirty, so it can't be too turbulent under there. The side taper helps in allowing air to fill in the space between the road the underside of the boattail.
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If the underside of the tail were a little dirty it might have an advantage with high speed stability.
Some race car designs have purposely slanted tails which communicate the low base pressure under the shelf of the tail to the cars underside.
The distance out from the rear wheels acts as a moment arm creating a torque at speed,creating useful downforce albeit with some drag penalty.