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Testing grille blocking & wheel skirts: +5.7% improvement
Posted Tuesday, December 6/05 in Mods & Tests
The start of a conversation I had this weekend at a Christmas party:
Neighbour - "So, looks like you got a new Firefly."
Me - "Yup."
Neighbour - "Sort of looked like there was some cardboard taped on it."
Me - "Um... Yup."
I was just relieved he didn't notice last weekend, when I pulled into the driveway with wet newspaper plastered across the front of the car. Somehow that fact made it slightly less unusual to be explaining homebrew aerodynamic mods at a Christmas party. It was made even easier still by the fact that the modifications were a solid success.
Overview: testing DIY aerodynamics:
This all started when I came across an EV World article about a guy called Phil Knox, who aerodynamically modified his 1994 Toyota pickup and boosted his highway mileage by 28%. While his modifications were fairly extensive, they weren't difficult to make - with materials available from any home improvement store.
Tongue-in-cheek, the EV World article described Knox as a "preacher" spreading "the environmental gospel of aerodynamics".
And clearly, I'm not the only convert. Read the comments following that article and you'll see that Phil Knox has inspired more than a few folks to learn about aerodynamics and efficiency. Those extensive and enthusiastic comments apparently vaulted the story to "most-discussed" status at EV World. It even resulted in the launch of a Yahoo group to continue the dialogue in a more user-friendly forum.
And ultimately, it resulted in the article you're reading right now.
Keep on reading to find out how my first steps into DIY aerodynamics increased my highway mileage by nearly 6% under controlled conditions.
The mods: grille block...
If you read about the one-shot aerodynamic test that was an "appetizer" to this one, you'll recall my roadside modification took the form of a "papier mache" wet newspaper barrier to divert excess airflow around the car which would otherwise have gone through the grille, the radiator and the engine compartment. In addition to aero benefits, the grille block aids efficiency in cold weather by permitting the engine to reach operating temperature sooner, and the transaxle to run warmer also.
The results, while startlingly good (an apparent 3.1% mileage boost), were based on just a single run and so could hardly be called definitive.
But they were definitely enticing - I was encouraged to to have a second look, this time over repeated runs. So I remade the grille block using cardboard and duct tape. (I've graduated from papier mache!)
... and rear wheel skirts
The second modification under examination was also very quick and easy to make. Rear wheel skirts - cardboard cutouts that enclose the back wheels within the wheel arches. Think Honda Insight.
The aerodynamic principle is simple enough: an uninterrupted air flow along the side of the car past the wheel skirts will be cleaner than one that has to cross the gap between the body and wheel, plus interact with the turbulence generated by the spinning wheel itself.
These are easy mods: after 20 minutes with a utility knife, some cardboard and duct tape, I headed for the "test course".
Route, conditions & results
Again, this ScanGauge-enabled trial was run under constant speed-controlled conditions (using cruise control, set once), on a level stretch of road. Unless otherwise noted, results are averaged from bi-directional runs to cancel any effects of wind/grade. (For details of the course, see the air filter test, part 1.)
The weather was overcast and calm, with NE winds below 3 kts (5.6 km/h / 3.5 mph). The road was dry. The temperature was about 2 C / 35.6 F.
Just to review, here's the result from the one-run "appetizer" from before, testing the papier mache grille block:
I started this new set with a bi-directional run with the cardboard grille block; next was a series of 3 bi-directional runs with combination of grille block and rear wheel skirts:
Observations & conclusions
It looks like it's time to graduate again - from cardboard to something that won't turn to mush in the rain and snow!
Making a more durable grille block will be easy: either a vinyl bra, or semi-permanent plastic slats installed in the grille openings would do the job. I say semi-permanent because I still want the option of easily changing cooling flow as the season and other conditions demand.
It should go without saying that tinkering with grille blocking requires careful attention to the temperature gauge to avoid potential overheating.
Wheel skirts will be more challenging, as they need to look respectable and also be easy to remove (to facilitate tire changes or other repairs that require the wheel to come off). I'm also reluctant to drill any holes in the bodywork for fasteners, as they will be prone to rust.
One final observation: tinkering with aerodynamics is bound to draw attention - like comments at a Christmas party. Aero experiments will yield more than just efficiency data - you'll get psycho/social feedback as well:
"That look -- especially the distinctive 'skirt' that extends halfway down the rear wheels -- is not for everyone. When I took the Insight for a spin, one driver stopped me to call it a computer mouse on wheels. (Business Week article).
Why exactly people can have such strong reactions to an aerodynamically shaped car is a topic for another post.
Other possible future aero tests
Tuft testing; wheel disks; deflectors/dams ahead of the tires; ride height lowering; front wheel arch gap fillers; underbody panels (belly pans)...
Free Fuel Riding on the Wind - EV World
Papier mache aerodynamics: +1.5 MPG? - MetroMPG.com
Aerodynamics links - MetroMPG.com - (scroll down to Aerodynamics)
darin AT metrompg D-O-T com, or here