Aero modding question

[Christ]

I wanna turn my 96 Passat sedan into a 'fastback' style car... but I suck at angles.

I know the average best rearward angle is 12* tapered, and I can set a solid 12* angle, but how do I measure the taper properly?

I planned on making a removable rear 'housing' with brake line [or framing wire or something] and then stretching cloth over it and soaking that in resin to solidify the shape... if I decide it's worth while, I'll make a full FG version and probably set it up with hinges and seals so i can remove the back window entirely and mold it to the lower portion of the trunk lid.

[freebeard]

I'm confuse. A join date in 2008 and closing in on 9,000 posts? Where's the real Christ and what did you do with him? Alternatively, how did you avoid this popular a question that long?

Let's start with Mair:


Are you going to increase the overall length? How much? I was just showing this to aerohead:



With the geodesic form you scale it to whatever you want and then pick the dimension of each edge from the 3D model. I was thinking of welding rod and doped dacron.

[aerohead]

Here's a link to a technical drawing for your car.You could use the Aerodynamic Streamlining Template tool near the top of the Aero.Forum page to see the comparison.
If you're not ever going to elongate the car I don't know if the fast-back conversion would show a significant advantage over VW's optimized aero-back.
http://carblueprints.info/blueprints...-passat-b5.gif

[some_other_dave]

You could also pick up an Angle Finder from your local hardware store. The digital ones would be more precise, but the weight style can work reasonably.

Or just get a protractor, a weight, and a string...

-soD

[YeahPete]

I'd use cardboard.

[Christ]

See, I can do the plain angle, I just can't figure in my head how to make an appropriate curve for the angle.

I don't really plan on extending the vehicle at all, I basically want to sharp end it right at the lip on deck lid, similar to the 90's Corrado.

Phil - Thanks for that, but unfortunately that's the generation just after mine. Mine's a '96, B4 Passat TDi sedan.

[Christ]

Freebeard - Like I said, I can do the plain angle, and I can form the taper and curve on paper without a problem... but when it comes to real world application, that's where the problem starts. I can't visualize in my head exactly what is needed or how to make the curve ideal to maintain the 12* taper.

[freebeard]

Like this?


A simple curve can be defined by 3 points. Any arbitarary curve can be defined by four points (B-spline curve). Your problem is going to the third dimension. Here's what I did: Use any limber, uniform material PVC pipe, bamboo, etc. I used redwood bender-board, 'cause I had lots of it.



Make a scaled down [in x and y] outline of the cabin rear profile and place it on the rear deck and bend your material over it, and then slide it back and forth.* It's like the street rodders say, "If it looks right, it is right."

For what gain you will get, like aerohead says, you might want to focus on a front diverter and front wheel well air management. Or rear skirts.

Edit: *Start at least a foot ahead of the point of separation to get an accurate tangent. I started at the stagnation point in the front to explore what happens across the roof after the rope at the highest/widest point.

[aerohead]

Originally Posted by Christ

Freebeard - Like I said, I can do the plain angle, and I can form the taper and curve on paper without a problem... but when it comes to real world application, that's where the problem starts. I can't visualize in my head exactly what is needed or how to make the curve ideal to maintain the 12* taper.

If you have defined your 'end' point you can use a flexible 'french curve' device to create a nice curve.
I've been using a 20-ft length of schedule-40,1-inch PVC pipe.I tape it to the vehicle roof and pull the trailing portion down to my 'point' and then clamp it.If it's done alongside something to scribe on,you can capture the curve for creating a pattern from which to fabricate from.It's dirt simple low tech,cheap,and renders an organic curve within an acceptable limit of perfection.

[Christ]

Sounds a lot like I should probably focus my efforts on the front end right now... lol

I need to get my front end put back together [get the nose bar and grille block in place] and add the airdam/bumper cover, both of those are fairly easy mods. I looked around under the car, don't really think a full belly pan is gonna do me as much good as the trouble to install it [other than rust management, so I may do it anyway], but I def need a skid plate under the motor area, that's terrible dirty and needs more protection anyway.

Currently, I'm riding on 185/65R15 tires, which are about 6.5% larger than the OE tire sizes. I wanna also get some pizza pans and put them over the [relatively smooth] wheels and possibly also make some rear skirts.

It's a junkyard car.. I don't really care what it ends up looking like. With 305k on it, it's due for plenty of pop riveting and scary looking mods anyway.