Scott Harvey and co. recommended using double wheel spiders (centers) in the Rally Tips bulletin.

The only photos I'm aware of showing steel rim failure was the ralley style rims on one of the Chrysler France race cars in 1970 or 71 IIRC. It used to be on the web, but the website changed. Basically as Mitch described. My recollection was it looked like a single center. Depending on which FIA Group the car was in, a stock rim may have been required.

There is definately a point where lower weight is slower due to flex (specificly road racing where I know a shop that tested this) And of course some lightweigt rims may not flex much but will start to fracture. Got to finish to win a race...

Both the shape and material of the rim construction will determine potential to flex or fail under a given load.