Quote:
Originally Posted by yabert
Hi
I have a 1980 Vanagon electric powered by a 2017 Bolt EV powertrain.
Or if you prefer, I have a transgender Bolt EV 
I average 80 mpge (mpg equivalent) and I can easily do over 100-120 mpge in summer at lower speed.
Details here: https://www.thesamba.com/vw/forum/vi...737&highlight=
The pop top on our van is pure crap and I hate it so much. It's an old fiberglass piece of crap with holes, crack, etc.
To add, it is 3-4 inch's to low to stand up in the van without open it.
So I want to replace this pop top by a ridgid high roof top. 
You can see the problem/challenge. How to improve energy consumption with a bigger front area?
I have to improve aerodynamic for sure and it's not so complex to do better than a stock Vanagon, but where can I start? |
If you're set on a 'fixed' roof, by 'eye-balling' what you've drawn, there's nothing wrong that I can discern, except for the sharp, longitudinal edges. You've taken care of the tumblehome already, but without softening the intersection between the top and sides, you'd be looking at a 13% drag penalty. It would require around an 80mm radius to mitigate vortex formation, the entire length of the roof, however, complicate the build by about 1,000%, requiring compound curvature, the absolutely-most-difficult shape to fabricate. The edge rounding would also benefit roll-moment during crosswind driving and gusts.
DON-BUR TEARDROP TRAILERS claims a 35.7% drag reduction @ 90-km/h, in spite of the aggravated frontal area, and the faster you drive, the greater the effect.
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There's also a more efficient shape than this 'classic' teardrop bubble canopy/ blister, courtesy of Goro Tamai of MIT, who presented it in his 1999 book, THE LEADING EDGE, Figure 4.3.16, page- 195.
It's a reverse-bobbed, conjoined, double-canopy/blister, of Cd 0.045, instead of Cd 0.07, for the 'classic.'
It's pretty 'worthless' as far as outwards vision, but since that's not an issue up on the roof, it would be an option, and at 35% less drag, kinda compelling.
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The Vanagon chassis gives you almost 4569mm of length to work with. Keeping standing headroom clearance to a minimum would provide quite a bit of 'fairing'/ 'canopy'/ 'blister' to work with. You'd go from zero, up a 17.5-degree incline, top out at the new height, along an arc, then just descend down the same 17.5-degree incline, blending into the metal roof.
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Great project! Thanks for bringing attention to it here. My 1970 Transporter is now mumuring to me.
