Dear Solar Guy,

        I'm somewhat familiar with solar cars, having seen some of the
results of recent races.  I understand that these cars are capable of going
pretty fast - maybe 50 or 60 miles per hour.  However, in the recent races
the average speeds of some of the better cars are typically 25 or 30 miles
per hour.  Why the low performance ?  

 I. M. Otto, Wink, TX.

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Dear Mr. Otto,

        These cars can achieve speeds of even higher than the range you
mention - above 60 mph.  However, the aerodynamic and other losses increase
greatly with velocity.  Also, since the cars run solely on solar energy they
can collect a limited amount of energy in any day.  Therefore, since the
objective in these races is to traverse some distance (say 150 miles in a
day) and do it in the least time, the most effective way to achieve that
objective is to keep the car 's average speed as close to its maximum speed
as possible.  For typical car weights and solar array areas this turns out
to be in the 25 to 30 mph range for the better cars.  [[Note that in the
last World Solar Challenge in Australia the Honda car averaged a little over
50 mph, but this was a very high performance and expensive vehicle.]]  Also,
because of unpredictable weather, it is imperative that some battery
capacity be maintained for possibly poor weather the next day.  In this
regard, one will note that the average speeds of these vehicles for the last
day is always much higher than for other days of the race, i.e. at the end
of the last day the battery can be drawn fully down when one gets the
checkered flag.    

 The Solar Guy.