Aerodynamic Mods to Reduce Drag

[JoeS]

:!: :?: :!: Why is there an i-MiEV laying on its side? :shock: :?

Didn't you know, it's much easier to work on the underside of the car when it's on its side! Looks like they forgot to put down the obligatory mattress. :roll:

Yes, I had seen that emergency response training photo - they evidently had a hard time getting the iMiEV to stay on its side in the first place.

With drag force proportional to the product of Cd and cross-section area, in the example Mart provided the Dart still has a lower drag force than the i-MiEV for any given speed.

The NA iMiEV appears to have a fairly-clean underside until we get to the drivetrain area. When I was lying under the car yesterday I considered perhaps a flexible (heavy fabric?) cover under everything and connecting to the aft bumper; however, such a cover would unfortunately trap everything from snow to water to pebbles…

We're planning a long (>200-mile) trip on New Year's Day with the iMiEV - the fender skirts have survived about 50 miles so I'll leave them on - probably it's my imagination, but the wind noise in the back seems a bit more muted.

 

[NeilBlanchard]

The rear wheel skirts should help noticeably. You might want to use better tape like 3M white/translucent duct tape: http://www.amazon.com/3M-Transparent-1-88-Inch-20-Yard-2120-A/dp/B0000DH8I8 which will also look better. And I think you will have less noise as the drag is lowered - the turbulence is what produces the drag and the noise.

And yes, a full belly pan would be a big help. Bridging the gap from the rocker panels to the battery pan would be good, and the front and rear knarly open portions, too.

 

[PV1]

Did they have to get a blue one? I remember hearing about it. They called it "Weeble", as in "Weebles wobble but they don't fall down (over)."
I guess sacrifice one to save us.

The fender skirts definitely make the i-MiEV look different. Haven't decided if it's for better or worse, but it should help with noise and range. Perhaps the steel wheels have the advantage here being mostly closed. Nice work, either way.

 

[JoeS]

The rear wheel skirts should help noticeably. ..

Drove 80 miles to Monterey with the skirts on and then took them off for the return trip. No noticeable difference. Mileage was worse than normal in both directions because car was loaded with two people and about 250# of 'stuff'', and we had a significant (1600ft) climb. Fender skirt experiment inconclusive. I happen to think they looked 'cool'

 

[NeilBlanchard]

Yeah, you need to keep running them and see how it goes. Smooth and flat wheel covers (stretch cloth over the plastic ones, and glue it with a waterproof glue, and then spray paint 'em.

 

[NeilBlanchard]

I am in the midst of remaking my rear wheel skirts / strakes for my Scion xA, which is relatively close to the i MiEV. I am using 1/4" Dow XPS fanfold foam and 3oz fiberglass - here are some pictures:









I have them back off the car, to fiberglass the back of the skirt, and I need to put in a panel "roof" inside the strake (last picture) to keep the water out. I also have to fiberglass the skirt panels, and the rest of the Kamm panels, and work out a joint between the side Kamm panels on the hatch with the panels at the taillights. I will probably be replacing the arcing slots with acrylic panels; for seeing the taillights from the side.

 

[JoeS]

Neil, looking good! Exactly what scheme do you plan on using to attach the (removable) skirts to the body panel?

 

[NeilBlanchard]

I am going to pop rivet the main peices to the car. I bought some #10 stainless steel screws and nuts that I was planning on fiberglassing the nuts onto the back of the lip, to hold the skirt panels in place. But if I can fine some threaded clips that are not going to rust, and that spread out the pressure a bit, I will use those.

Some more pictures:

 

[jray3]

Neil, I realize I'm addressing a master Ecomodder, but why not use rivnuts rather than rivets? They seem like the best of both worlds- install 'em blind and re-install the panel as many times as desired.

 

[NeilBlanchard]

Ray, that's a new one on me - I have not heard of rivnuts. I like the sound of them.

Edit: I looked 'em up and found a short video on installing them. They look like they would be fine for all metal situations, but they may require more strength / crush resistance than a pop rivet; though maybe it's the other way around? I also already own a decent pop rivet tool. I'll have to test rivets with my fiberglass / foam / metal and plastic, and see if they work.

 

[JoeS]

There are many schemes for captive nuts; however, they presume the skirt is screwed on from the outside leaving an exposed screwhead. I was thinking something more along the lines of an inside clamp onto an inserted lip that would be invisible from the outside for those who worry about looks. I agree that using rivnuts or something similar is much easier, and I also personally prefer simply screwing in from the outside (use SS) as function trumps aesthetics in my book.

 

[NeilBlanchard]

Today was a perfect day, and I took as much advantage of it as I could - I got most of the fiberglassing done on the wheel skirts and strakes, including on the inside to try an keep water out.




I will hopefully will get the Kamm panels done (on the outside, at least) tomorrow, and I may decide to fasten it to the car.

I noticed the improved coasting and the improved mileage, too. It is good to have this close to done.

 

[JoeS]

Neil, thank you for posting. Looking forward to your finishing this and then publishing the before/after test results.

 

[JoeS]

A little over a year ago I tried an experiment using some foam-cored sheets and simply taped them on.

Now, with "my" i-MiEV (wife has her 'own'), I'd like to continue the experimentation. The material I'll use is a simple foam sandwich which I can epoxy sheathe with either fibreglass deckcloth or some very thin carbon cloth. Not a problem to produce a thin yet stiff fender skirt.

The issue is how to attach it -

After considering various cantilevered clamping techniques (which would only work in a couple of spots) I've come to the conclusion that a simple rivnut should work in both the sheet metal as well as the plastic bumper, with six rivnuts per side. The disadvantage is that I will now have six permanant holes in the wheelwell periphery. I could probably camouflage the fasteners (either with or without the fender skirt) with a dab of white paint.

Since both the sheet metal and bumper are very thin I think I need to use soft aluminum rivnuts, after appropriately coating the drilled hole to prevent galvanic corrosion. I was unable to find any plastic rivnuts which would do the same thing.

Questions:

1. I am open to suggestions for fender skirt attaching schemes
2. Anyone know of any plastic rivnuts?
3. Should I be concerned about corrosion between the aluminum rivnut and the car's sheet metal, recognizing that the rivnut is less noble?

 

[Phximiev]

What was the result of the experiment and how did you measure the difference in performance?

 

[JoeS]

What was the result of the experiment and how did you measure the difference in performance?

I think I mentioned here that the one-day test was inconclusive. Quite frankly, I don't think we would notice a difference while driving because many other variables have a greater influence. The way I look at it is that every little thing helps… besides, if it's good enough for my Gen1 Insight it's good enough for the i-MiEV.

I agree with NeilBlanchard that a full-length full-width smooth floorpan under the i-MiEV would be a significant help. Now, about that squared-off aft end...

 

[Don]

Since the initial tests were inconclusive, I think I'd stick with the tape idea until you determine they work well enough to justify drilling holes in the car. Since your car and the panel are both white, it should be easy enough to find some white tape which will closely match both. You might have to re-do the tape every six months or so, but hopefully by then you'll have an idea of whether they do anything for you - They don't do anything for the look of the car, so unless they prove really useful to extend range/efficiency, I don't think you'd want to drill holes . . . .

Don

 

[pbui19]

I am definitely going to do some aero mod, though I think the gain depends very much on your driving application. I believe highway speed would benefit most with aero mods, and not so much for around town.

A recent experiment with my '99 3-cyl Chevy Metro gave surprising & encouraging result. A cheap garden edging tie-wrapped for an air damp and partial grill block increased highway mpg from high 40s into low 50s. Further mods pushed mpg into the high 50s, flirting 60 mpg, with wheel covers, removal of passenger side mirror and windshield wipers. I also had similar improvement, though not as dramatic on the '95 Corolla, from 35 to 45 mpg.

It's more challenging on an EV to quantify the change though, as we "refill" daily and at fewer miles per refill.

I was planning to remove the iMiev windshield wiper since it usually doesn't rain in Calif after April, but this year weather is all funky

 

[jray3]

I am definitely going to do some aero mod...
It's more challenging on an EV to quantify the change though, as we "refill" daily and at fewer miles per refill.

But with CanIon we have much more precise consumption data that can be de-linked from the 'refill' intervals.
Go for it pbui- we need more i-modders. Turns out I've got too many projects, and have promised the wife to mess around with our newest, most reliable, most efficient car the least of all. Go Figure! :roll:

 

[Phximiev]

Aerodynamic improvements would seem to be worthwhile. Someone mentioned the Insight, so I found this:

http://www.insightcentral.net/forums/mpg-issues/10072-how-much-do-rear-wheel-covers-help-gas-mileage.html

All leads to the question of how you measure it?

I saw an older Insight the other day in the parking lot of the Changing Hands bookstore in Phoenix. It looked as if all of the drag areas had been eliminated and it had wheel 'skirts/covers'. I couldn't tell if it had a belly pan though. All red and looked great.

Definitly debatable, but wouldn't one need a wind tunnel to property measure it? Anyone here an engineer with that kind of experience?