The XLR is on the next generation Corvette frame, so it is different than any Corvette currently on the road. I quote from the big brochure:
Composed of aluminum, magnesium and other advanced meterials, these elements help maintain XLR's low weight while providing resistence to torsional and bending forces.
Two 170 inch hydroformed steel perimeter rails run the length of XLR's frame. Each provides exceptional strength in its simplicity, deftly doing its task with 93% fewer components.
I hope that helps.