Doing some flat undertrays for my car, amongst other things...
VT commodore
It's about 9 foot tall and a mile wide, but I'm looking at it from an economy point of view. It already gets a creditable 8l/100km on the highway (factory claim is 6.5, but that's in the rarely reached lean cruise mode), but I want better.
Data on the aerodynamically damn-near-identical VX car points to a Cd of 0.329. The later model VY commodore achieves a 0.319 due to a sharp cutoff of the rear, a 3% improvement for something that can be done (DIY style) in about 5 minutes with some plastic. It also gets 33% less lift at the rear.
Autospeed - Aerodynamic Development of the VY Commodore
I'm thinking about an undertray arrangment as well. This would work well with the independent rear end (cutouts for adequate suspension travel, not a massive gap). Flat wheel covers will have next to no effect (the factory hubcaps are so flat it's not funny), however modified guards or rear wheel covers might. I'm not entirely sure, but given the mega-smooth transitions of the windscreen, roof, and rear window, airflow will probably stay attached. However for my purposes I can also make up vortex generators if wool tuft testing proves otherwise.
I have a very long drive coming up (1800km of smooth, hill free outback roads with no traffic, or rain) so I can trial various things. My fuel consumption display shows down to 0.1l/hr, so I can see with reasonable accuracy.
What do you think I should do?
*Edit*
I looked under the car and was confronted with this.
Uploaded with
ImageShack.us
Behind it is the radiator exit and sump. Like so.
Uploaded with
ImageShack.us
In front of it is a hole below the entry to the radiator (this pic taken between the front bumper and that flat plate)
Uploaded with
ImageShack.us
And finally the rear of the car. Could deal with improvement. Other side is the same, but less exhaust.
Uploaded with
ImageShack.us
So now I need to think about the reason that front deflector is there. I think it's to cause a low pressure after the radiator and a slightly higher pressure in front. A flat cover between the front *should* give less resistance and have a similar effect through sealing the front of the radiator. It may also lower front lift (yay).
The rear, well that's just messy coz it's cheap, but the rough aero shape is there. Time for plastic sheet and duct tape. Oh zip ties. Can't forget zip ties...
** Edit: Front undertray***
Centre piece
Side Piece
"Complete" (taped and tied) front piece.
Yes it has a downwards slope. If I was to retain the function of that plate, there had to be. However I don't think that will cause lift, as the air coming through the front is now trapped, airflow should be a lot higher on the bottom of that piece, sucking it downwards enough to cover the slope... It would have been better if I was starting with an 'S Pack' front bar, as they are lower to start with.
Rear
I'll have to do something about that lip. The rear section is fine but the transition to the fuel tank is a step. And the RH side needs some kind of cover, it's acting as a scoop.
Preliminary testing (i.e. I couldn't wait and wanted a number) indicates 6.5-7.5l/100km for the short flat sections we have around warwick, and it seems a little quieter. So it's *slightly* better than last time I went on a trip. However more to the point, nothing fell off at 130...
Much revised right-rear section
