Reader Questions: Gen-6 Windshields on Passenger Cars?

PittCaleb asks:

If we used Lexan in passenger cars, would there be any benefit such as reason repelling our ice buildup? Scratching or other potential downsides? What would the price diff be?

The implementation of superstrength laminated polycarbonate (Lexan) in NASCAR windshields raises the obvious question:   “If it’s so good for NASCAR, why isn’t it in my car?”

A couple considerations:

  • Weight:  The less a car weighs, the less fuel it takes to run, so decreasing weight is not just good for going fast:  it’s good for saving gas as well.  That said, the weight differential replacing a windshield with Lexan is pretty small compared to other places on a car.  the Department of Transportation (DOT) must approve all windshields for use in passenger cars and they require 1/4″ hard-coated Lexan.  Since the density of Lexan is roughly half the density of glass, assuming equal thickness, a Lexan windshield would weigh about half of a laminated glass windshield.   Comparing that to a 3500-lb overall car weight of, you’re not saving much in terms of fuel.
  • Cost:  You can buy a sheet of 1/4″ thick Lexan big enough for a windshield for about a hundred bucks.  BUT:  Lexan (polycarb in general) is extremely susceptible to scratches.  A cheap piece of basic polycarb will be scratched the first time a bit of sand or dirt gets under your windshield wiper.  Manufacturers get around this by putting hard coatings over the polycarb that make it more scratch resistant – but those coatings are expensive and ramp up the cost pretty rapidly.  You’re probably talking 2-3 times the cost of polycarb vs. glass, which is probably a couple hundred dollars for a windshield.  Does that offset the lighter weight?  Depends on the person buying the car.  Polycarb is becoming more common for things like headlight covers, but the only cars using a lot of Lexan are in the class of the Bugatti Veyron – which costs more than most of us make in a decade.
  • Maintenance:  Americans are not very good at taking care of their cars.  Most people don’t check their tire pressure, change the oil regularly, or even pay heed when the engine warning light comes on.  Manufacturers won’t implement  Lexan windshields until they require absolutely no additional TLC relative to glass.
  • Icing:  Interesting point.  The thermal conductivity (how well a material transmits heat) of Lexan is about a quarter that of glass.  That means Lexan is a much better insulator than glass – one attractive point for Lexan from the perspective of environmental aspects is that Lexan would enable you to use less heating and air conditioning.  The thermal conductivity of polycarb can be changed by using glass fillers – but that also changes the strength.  We argued a bit about  this one.  It would take a longer time for the inside of the car to get cold, but since Lexan is a bad thermal conductor, that might not impact the outside of the windshield very much.  If you were using the defroster, it would take a long time for heat on one side of the Lexan to get to the other side, and thus longer to defrost.

The thing might make Lexan a feature of road cars is (in my opinion), if electric vehicles really take off.  Saving ten or twenty pounds on a race car is huge; but it won’t make much of a difference on an internal combustion engine-driven vehicle.  For an electric vehicle, however, every pound counts in terms of extending the car’s range.  Since range limitations are one of the big barriers for EV acceptance, using Lexan to reduce the weight without compromising safety could make a real difference.

1 Comment

  1. Thanks for diving into this question. My main concerns would be scratching, and if headlights are an example, it’s a big issue! IIRC, glass has a porous (maybe wrong word these) surface which is why rain doesn’t sheet off so easily. RainX fills in those voids, for a time, which makes driving easier. I was wondering if it would be so slick that rain would just slide off without ever using wipers.

    The ability to defrost the windows is not something I thought about. I wonder if manufacturers would ever consider creating a car that worked great in one area of the country but not in another? Let’s presume Lexan windows were cheaper & safer and maybe didn’t even require wipers (more savings and less parts, etc.) – but you could never drive it in a cold weather environment. Would that ever even be an option for them? Doubt it, but for someone who lives in the south and never drives up north, why not? No different than some of the limitations the current EV’s have. Not regional, just limitations between them and a standard vehicle.

    Thanks for leaving in all my smart-phone-induced typos 😉

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