Cracked Wheels ???

[sphanna]

I have two wheels that have cracks in them. One is more serious because it runs from the lug nut hole almost to the hub cap. the other has a barely visible crack around a lug nut hole. A friend tells me that radial tires are the cause. He says that radials put too much stress on the rims. I find this hard to believe.
could this be true?