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There are a lot of trade-offs involved. A two-lump shape has more drag than a continuous curve, all else being equal, but by so little that the skin drag difference may matter more if you don't need all the volume. As for HPVs, You are dealing with problems such as solar gain, demisting access, vision through imperfect plastic, which is terrible at low angles with bad lighting and/or rain and so on, as well as doing trade-offs between frontal area, rider position, wheel placement and size, vision, cooling, surface area and so on.
A pure laminar shape tempted the HPV pioneers, but surface flaws and vibration generally defeated them, while adding considerably to frontal area in some examples. Lately, gains have been found by using small cranks to reduce frontal area, raising RPM 30% or so, until a 1 or 2% power loss wipes out further gains- until someone who has learned to spin a Hot Wheel at 300 RPM shows up.
We can change anything,
but we can never change just one thing.
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