Now for the window, and some other cool stuff.
Every time one of my design or fabricator buddies comes over to the shop the question about the GTX1 is "What are you going to do for a rear window?".
The concept car was supposed to have just a flat plane of glass, but it never got fabricated correctly. After getting into this car, it was clear that sealing was going to be tough, if we did go with just a flat plane of glass. The final straw was when a hot rod builder friend (Bill Jagenow of Brother's Customs) said "It would be too easy to just do a flat plane of glass." I take that as a dare, so no flate planes of glass...
Having my buddy Larry Erickson (of Cadzilla and Alumacoupe fame) come over to bless the design direction gave confidence too.
The more observant readers of this post will have noticed a blue fine line tape line in prior photos of the roof. This was "thinking in progress" for how I eventually changed the profile of the roof, to not have to just have a flat plane of glass.
After some pretty major surgery on the roof pattern, we came up with a pretty cool way of exectuting the rear window.
Here's the first step.
I wanted to bring the window all the way back to the joint at the clamshell.
This would allow for the rear window to essentially be shingled. Any water running off the window would get directed back to the clamshell and drain out past the sides of the seal.
Here's a photo of the mock-up, before we modified the roof pattern.
Once I was satisfied with the concept, I then started to make the window pattern while one of my crew started to modify the roof.
We need a consistent margin around the rear window structure to allow for some volume for the seal. We accomplish this with "sheet wax". This wax comes in calibrated thicknesses and is adhesive-backed. If you need to change the surface of a pattern by some measured thickness, you cover the area with the required thickness of wax. In this case, we are allowing for a 0.25" seal gap.
For the rear window pattern, I started with a substrait made up of "Home Depot" grade insulation foam. This stuff works fine for this sort of work. If we were making a precision model out of foam, we'd use urethane foam. We form the substrait to be smaller than the desired final shape. To match the surface of the window model we "squeeze" body filler between the foam substrait and the sheet wax. This gives us a form that is exactly 0.25" off the surface of the bodywork.
After matching the surface, we build up body filler on what will be the window surface and sculpt that to shape.
Now, I didn't take a lot of photos along the way (it was just a ton of noise and dust anyway) but here is what we ended up with.
Essentially, we sculpted the form of the head fairings to flow into the roof.
This allows us to resolve the surface of the rear window.
Right now the rear window looks pretty "square" in section. The radius is the last thing you model.
These photos don't really do justice to the surfacing of the roof.
It is very 'concept car' like. It makes the car look even more "mid-engined" with the roof window lining up on the window over the intake plenum (this one is twin turbo-ed!).
We'll finish the surfacing of the roof and make the production tool later this week.
