My brother reads a lot more about cars than I do, and he says that so many brands are going to glass roofs because of this plus the ease of access to the interior by robots, as a glass roof goes on almost at the end of assembly. He also tells me that Tesla, in particular, uses glass roofs to augment torsional rigidity, as Tesla's are not noted for their rigidity. With early Model X's, though, the glass was absorbing so much torsional flexion that some of the windshields were cracking. (I don't know now they resolved the issue, but I read that they did.) I have a 2015 Model S with a metal roof, and I have never had cowl shake or any sense of low torsional rigidity. In fact, the car feels rock solid. I once looked up the numbers and was surprised to find that many other four-door sedans had higher rigidity than Tesla. I had owned a couple of those brands and never thought the Tesla had more flexion . . . until I test drove a Model X, that is. I took it over a railroad crossing at low speed and could hear the car frame creak. Rivian is going to be a body-on-frame vehicle instead of the more common unibody construction. I wonder how that is going to affect the sense of solidity and whether they're going to have to use firmer suspension settings than they otherwise would because of body flexion.