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A sedan has a center post (basically the window is fully framed), the hard top has nothing in the middle of the car to reinforce the roof. so it has to have a heavier frame to compensate for the missing structural member. You can think of the hard top as a convertible with the top stuck up. Looks a lot more sporty, as a lot of folks who like the convertible don't like the leaks and maintenance required of a rag top, so they buy up hard tops instead. Sedans are relegated to second class citizens because of this sporty look envy, even though racers chose sedans because they are lighter and stronger.