Replacing a Glass Windshield With Plastic | Project LS-Powered 350Z

Tom
Update by Tom Suddard to the Nissan 350Z project car
Mar 8, 2021

Our LS-swapped 350Z was rapidly transitioning from a street car to a track car, and after removing the front windshield for better access, we finished and painted the interior. Next step? Put a windshield back in and go racing!

Of course, we weren’t going to just glue the stock windshield back in. It was badly sandblasted and chipped even before we removed it, which meant we were, uh, less than careful during that extraction process. (Don’t worry, we vacuumed up the broken glass afterward.)

Since we needed to order a new windshield anyway, we figured it made sense to use this opportunity to save some weight and add safety—two things we’ve never turned down when building race cars. Bye-bye, glass windshield. It’s time to enter the world of plastic.

Why Do Cars Have a Glass Windshield?

If you’ve ever held a drink in your hand, then you’ve probably noticed that glass cups are heavier than plastic ones. And if you’ve ever held a few too many drinks in your hand, you’ve probably noticed that the plastic cups don’t shatter when dropped on the floor like glass. Congratulations, you’re now an expert on the weight and safety benefits of plastic windshields over their glass counterparts.

Okay, so that’s a bit of an extreme oversimplification, but the bottom line is that glass is heavy and prone to breaking. Then why is it the right choice for a street car?

There are a few reasons: First and foremost, glass is extremely durable. And we know that we just said glass is prone to breaking, but that's on track, where it might end up deflecting loose car parts. For a street car, most of the abuse a windshield faces comes from rubbing windshield wipers and dirt kicked up by other cars. Glass is extremely hard, which makes it extremely abrasion-resistant.

Second: clarity. Factories have perfected large-scale production of glass that's optically correct—a term for which we don’t have the advanced degrees necessary to really define. We’ll put it this way: If you’ve ever looked through the windowpane of an old house, or through a cheap pair of glasses, then you’ve probably noticed unwanted color, refraction or distortion. Optically correct glass doesn’t have any of these unwanted features, and it's about as close to looking through a perfectly clear window as possible.

And that brings us to the third reason glass is the default answer for cars: cost. All this expertise at making good glass at scale, along with the low cost of its primary ingredients (sand and heat), mean that glass is the least expensive way for drivers to easily see out of the 10,000,000 Corollas a carmaker builds.

But what if you’re only building one car, and it’s destined for a life on track? In our scenario, we realized that glass wasn’t actually the best choice. Welcome to the world of replacement windshields made not of glass, but of more advanced materials. The benefits are obvious: improved strength and roughly half the weight of glass.

What’s the Difference Between Acrylic and Polycarbonate Windshields?

We’ll start with the most common question we’ve heard: “Why can’t I drive down to the home improvement store and buy a 4x8-foot sheet of plastic for $200, screw it onto my car, and call it done?”

Well, you probably can’t, actually. Look in the rulebook: The inexpensive sheets of clear plastic stocked on store shelves are acrylic, while almost every rulebook requires polycarbonate.

Acrylic can be found under these names and types: Plexiglas, plexiglass, Acrylite, Lucite and Perspex.

You’ll find polycarbonate sold under names like Lexan and Makrolon.

What’s the difference? Just like an LS3 and a Coyote are both American V8 engines, acrylic and polycarbonate are both clear plastics. But as with the V8s, that’s pretty much where the similarities end.

Polycarbonate and acrylic are completely different materials, and just like the LS3, polycarbonate is a far better choice for your race car–V8 humor there, folks.

Those rules aren’t around because Big Polycarbonate is sending lobbyists to the paddock. They exist because polycarbonate is much more resistant to chips, cracks and impacts. While acrylic is about 17 times more resistant to impact than glass, polycarbonate is 250 times more.

Rulebooks require polycarbonate because it’s less likely to break and hurt the driver. Unfortunately, it’s also a bit more expensive.

So you can go down to your local store and buy a sheet of polycarbonate for your car, but be warned: You’ll probably need more than one.

Polycarbonate turns windshields into consumables. Why? Every material has tradeoffs, and one of polycarbonate's is its softness. In exchange for that high resilience to cracking, polycarbonate is much softer than acrylic. That means it’s very easy to scratch. You have to be extremely careful with how you clean it (even paper towels are too rough), and don’t even think about turning your windshield wipers on unless you want permanent streaks. Polycarbonate also has poorer optical clarity than acrylic and will yellow over time with UV exposure.

What Kind of Plastic Windshield Should Be Used in a Race Car?

All that glass found in production cars is starting to make sense, right? So if acrylic isn’t safe and polycarbonate is a terrible choice for a windshield, how do you replace glass with plastic to save weight and add safety to your race car?

Simple: You throw science at the problem, which brings us to SHIELDS® Windshields. They make formed and coated optical polycarbonate for a lot of different cars, including our 350Z. They claim to be “the only manufacturer in the world that both forms and post-coats polycarbonate to the most demanding standards.” Sounds like we’re on the right track.

So, what’s that mean in practice, and what sets apart a piece of plastic sourced from Shields from a piece of plastic bought at Home Depot? It starts in the Shields factory, which is designed to make windshields, not sheets of plastic for home improvement projects, which means there’s a focus on optically correct parts from the start. Put simply, a Shields windshield has better optical quality than garden-variety polycarbonate.

And because Shields makes only one windshield at a time, the product can be tailored to the application. The brand's polycarbonate is formed and trimmed in the shape and curve of the OEM windshield, meaning you don’t need to try to replicate a big, curved windshield with a big flat sheet, nor do you have to deal with the distortion that can often be a side effect of bending polycarbonate yourself.

But the key reason we chose a Shields product for our 350Z is the company’s proprietary SUPERCOAT coating.

Remember that part about throwing science at the problem of soft plastic windshields? SUPERCOAT is that science. The coating is applied to each side of the windshield after it's formed and creates an extremely hard protective layer. SUPERCOAT is abrasion-resistant, solvent-resistant, UV-resistant, and basically everything else-resistant. It means you don’t have to treat your new polycarbonate windshield as a consumable, or as something that can only be handled with kid gloves and a microfiber cloth.

How Much Does a Plastic Windshield Cost?

By using a Shields Supercoated polycarbonate windshield, we’d theoretically have all the upsides of a plastic windshield without the downsides that come with DIY options. And we know what you’re thinking: This is NASCAR-level tech with NASCAR-level pricing, right?

Actually, we were shocked at how affordable this project was. NASA Time Trial rules require front polycarbonate windshields to be at least 3/16-inch thick, which translates to about $250 worth of polycarbonate sourced from a big-box store and a few hours of tracing/cutting to fit the car.

We decided to go with a slightly thicker, 1/4-inch windshield. That thickness would guarantee that we wouldn't need additional bracing to keep the windshield from flexing while on track, and would future-proof the car if we ever decided to run with an organization requiring thicker polycarbonate. The Shields option designed to perfectly fit our 350Z using the stock windshield trim cost $547. You’ll spend a bit less if your organization allows thinner replacements or you’re comfortable ordering the windshield oversized and doing the final precise trimming yourself. (This option is the right choice if you’re planning to bolt the windshield in place.) If you want extra features, like an anti-fog coating designed for rain racing or built-in window tint, you’ll spend a bit more.

What about just replacing our glass with, well, glass? We found new OEM windshields listed online for $850; aftermarket glass was listed for as little as $116 on eBay Motors by a local glass shop. Generally, you’ll spend $300 to $500 to have a local glass company come out and put a fresh windshield in your car.

Homework done, we placed the order for our 350Z: a quarter-inch-thick Supercoated Shields windshield for the front and an eighth-inch-thick Supercoated rear windshield.

Why such a thin rear window? Because we couldn’t find a rule against it, and we’re trying to save weight at every opportunity. At $459, this wasn’t the least expensive way to replace the rear glass, but the Shields coating should mean we won't be replacing cheap plastic every other season.

How to Install the Shields Plastic Windshield

A few weeks later, the Shields plastics arrived, and our first stop was the scale: We knew we’d be saving weight, but we wanted to know exactly how much.

We weighed our broken aftermarket glass windshield at 25.5 pounds, while the Shields replacement weighed just 16.7 pounds. We’d saved 8.8 pounds while also making our car far, far safer. In the rear, the weight savings will be more drastic thanks to the thinner polycarbonate, but we’ll cover that installation in a future update.

Time for the fun part: installation. And after some time on the phone with Shields experts, we settled on what we figured would be the easiest, lightest and most durable method: gluing in the windshield, just like Nissan intended.

The 350Z, like most modern cars, secures the glass with a 1-inch bead of urethane windshield adhesive. This makes a completely waterproof seal that also stiffens the chassis and the windshield by permanently bonding them together. It’s also inexpensive and easy to do: We bought the tools (a special caulk gun and some suction cups are needed) and a few tubes of urethane for about $80 online.

One problem, though: SUPERCOAT is resistant to all sorts of things, including glue, which literally won’t stick to it.

Before we could glue in the polycarbonate, we needed a frit. If you’re not a glass connoisseur, you’d probably refer to the frit as “that black band around the windshield.” Its primary purpose is to give the urethane a good surface to bond to, so we needed to create one for our polycarbonate.

The first step? Figuring out where to put it. To do that, we set the windshield in the car, then traced around the sheet metal with a Sharpie. Leaving the protective shipping wrap applied gave us an convenient surface to mark.

Then, once we’d traced the car onto the windshield, we set the plastic on a stand and cleaned up our markings to create a smooth, tidy band of black around the windshield’s perimeter. We trimmed away a strip of that shipping wrap, leaving most of the windshield masked off so we wouldn’t damage it during the next step.

That next step? Sanding, of course. After all, SUPERCOAT doesn’t remove itself, and Shields said we could just sand it off.

We attacked it with a dual-action sander and 80-grit sanding pads, sanding until we’d gotten through the SUPERCOAT and scuffed the raw polycarbonate underneath. We figured this step would take a few minutes, but we ended up sanding for hours.

We assumed SUPERCOAT was mostly marketing speak. Turns out that it’s so darn hard that you can hold a sander on it for minutes at a time and still not see visible scratches. After lots of patience, we finally got it off, then went over the band of bare plastic with 220-grit to provide a smooth surface for paint.

What exotic coating do you need to paint a windshield frit? Turns out the answer is simple: flat-black spray paint.

We picked up a few cans of Krylon and sprayed light, even coats until we couldn’t see the light of a flashlight through it. After letting everything dry overnight, the windshield was ready to install.

We cleaned the surface rust out of the 350Z’s windshield channel while we were waiting, then treated the channel to a coat of that same flat-black paint to ensure a good bond.

We also ordered some universal rubber windshield edging online and glued it to the top of the windshield to replace the OEM trim that would normally bridge the gap between windshield and roofline.

Prep work complete, it was time for the moment of truth: Glue Day.

We watched some YouTube videos, then carefully cut our urethane tube’s tip in a V-shape and spread a nice bead around the windshield frame. With our wife’s assistance and those suction cups, the windshield dropped right in with zero trimming or adjustment. Plus, it mirrored the curvature of the OEM glass perfectly. After letting the urethane dry overnight, we hopped in and went for a drive.

And that drive was super weird. Why? After only ever driving this car with a worn glass windshield, the Shields polycarbonate was basically invisible, and we felt more exposed than usual. We’ve never seen this level of clarity in a plastic windshield before, and we’re convinced it’s worth every bit of its $500 price tag.

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Comments
View comments on the GRM forums
trigun7469
trigun7469 SuperDork
3/3/21 12:58 p.m.

Great write up!

californiamilleghia
californiamilleghia SuperDork
3/3/21 1:11 p.m.

Great story , thanks , 

We are going over next week to get some top canopies made for a Messerschmitt kr200, 

we will check and see if they offer the SUPERCOAT™ type option .

 

Great story , thanks , 

We are going over next week to get some top canopies made for a Messerschmitt kr200, 

we will check and see if they offer the SUPERCOAT™ type option .

 

Tom1200
Tom1200 SuperDork
3/3/21 1:25 p.m.

Since the windshield and hatch window are up high, saving weight there makes a lot of sense.

I'd be curious if you know the weight of an aftermarket glass windshield? On my Datsun the aftermarket windshield is slightly thinner thereby making it almost 2lbs lighter.

Tom Suddard
Tom Suddard GRM+ Memberand Director of Marketing & Digital Assets
3/3/21 1:26 p.m.

Our windshield was aftermarket, and weighed 25.5 lbs. I couldn't find any hard data, but I'm pretty sure it was thinner than OEM.

maschinenbau (I live here)
maschinenbau (I live here) GRM+ Memberand UltraDork
3/3/21 1:36 p.m.

That is awesome. I'm amazed you had that much trouble trying to rough up the coating. Sounds like amazing stuff.

While buying a fitted windshield is ideal, how would one go about getting a sheet of the material so I can trim it to fit something really old, really obscure, that they probably don't have a mold for, like a Lotus Eur- Oh nevermind they have that too. Hmm...

Tom Suddard
Tom Suddard GRM+ Memberand Director of Marketing & Digital Assets
3/3/21 2:01 p.m.

In reply to maschinenbau (I live here) :

Yeah, so was I. I really didn't expect it to put up a fight, but it was like trying to sand modern ceramic clear coat off of a car. Super impressive.

I'm not sure if Shields sells sheets of the material, but your best bet is to call and ask them. They're super nice on the phone, and answered all of my dumb questions.

maschinenbau (I live here)
maschinenbau (I live here) GRM+ Memberand UltraDork
3/3/21 2:27 p.m.

Follow-up question: how would you do this on a Challenge budget? 

Stampie (FS)
Stampie (FS) GRM+ Memberand MegaDork
3/3/21 3:11 p.m.

In reply to maschinenbau (I live here) :

I got 4x8 sheets of lexan at Alro metals for $100 a few years ago. 

DeadSkunk  (Warren)
DeadSkunk (Warren) PowerDork
3/3/21 4:06 p.m.

In reply to maschinenbau (I live here) :

I picked up some 16"x84" Lexan for $20 each. That'll work for my side windows, but not the front . The rear is in the pondering stage , trying to figure out some two piece ,vented rear windscreen.

FMB42
FMB42 New Reader
3/3/21 4:51 p.m.

Thanks for posting a very informative article Tom. Gotta love that 350z too. And thanks to CA for posting that kr200 photo. Talk about a rare vehicle. Never a boring moment in the GRM forums.

malibuguy
malibuguy GRM+ Memberand HalfDork
3/3/21 5:31 p.m.

go figure i just made an appointment to get a new windshield on the 'racecar' for next week.

sheilds doesnt have a listing for it so it makes it better

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