Tires: solid rim, bicycle rim, spoked wheels

Question
   My son asked the following question.  Why do car wheels have holes?  He means why do the rims have spaces instead of being a uniform solid?  It is probably the same reason a bicycle rim has spokes.  Weight or material cost.  Is that why?  I am not really sure myself.  

Answer
Doug,

As it turns out, the amount of material needed to support the weight of a vehicle is pretty small, and the only problem is with side loading as would happen in cornering.  In a bicycle, there is very little side load on the wheel, the turning action is the result of the leaning action, so wire spoked wheels work well,  where a car has to generate the forces needed by causing the tire to slip relative to the direction of travel, so you need something more substantial.

But the reason there are holes in the wheels is to allow air to get to the brakes to cool them.