Aerodynamic Mods to Reduce Drag

[NeilBlanchard]

I used the Large Thumbnail link provided for Messageboards.

Is this curved downward at the back? That is the key thing for improving the aero. And the sides need to be angled in, as well. Having it this long may or may not be better than a shorter piece - have you noticed an improvement?

 

[jray3]

Thanks for helping with the photo, Neil. No curves added, just extending the roofline taper. Since the roofline is already near the template ideal, I think that's fine, but I'd like Version 2 to have some sides. Besides my inability to mold compound curves in plastic, going past 12 degrees of downslope could easily be counterproductive by generating a large vortex rather than the letting the air slipping cleanly off of a sharp termination.

 

[NeilBlanchard]

Extending the roof line is good because it then has a crisp edge and forms part of a "cavity". But if it extends too far, it works against you because instead of getting things closer to the template, it might be adding turbulence. Maybe you can tuft test it? If you tape some 6"-8" lengths of yarn about 1' apart in 2 rows. The harder part is to get a picture(s) or a video of the tufts while the car is moving at say at least 35MPH - and see what they can tell us!

If they are stable and laying back straight, then all is well. If they are fluttering wildly, then that may tell us what is not right.

 

[mitchell]

jray, is there a way to post your photos so that someone without a ShutterFly account could view them?

Dunno- I'm not a geek, just look like one, and haven't been successful in posting photos to this forum or my weblog for many months now..
:?
So, per others' suggestion, I'm resorting to IMGUR.com
http://imgur.com/39VJ5PA

Love this info - I actually ended up here from ecomodder.com but I've been part of this forum for a long time so it is pretty cool to see you guys do this with your iMievs!

You could start a blog on MyEVBlog.com and upload your images there... plus if you do it soon, you can win an EVSE or other prizes!
http://www.myevblog.com/electric-summer

 

[BlueLightning]

When it is sunny, instead of using the car air conditioner, I prefer to let slightly open the car front windows.
Following a Chicago Tribune article, the crossover point would be around 40 mph.

I also prefer to stay 10 to 20 yards behind trucks rather than overtaking them and the drag is reduced but it might not very safe.

 

[Llecentaur]

I tried the plexiglass mod (extension of roof as pictured earlier in this thread) and can not report any improvement at 85 km/h over a 65km trip.

I made another test, freewheeling along the highway as we approach a speed radar @60km. The car basically goes from 95 to 60 precisely at the right moment before the radar, Well, I am fairly sure to have started the free wheeling at the same place and speed as previous days and again no sign of any improvement, maybe even worse than without.

I think the car is just too high and too short to manage a real improvement without making a large protuberance.

Not sure i will keep it there if there is no gain. Will measure again in coming days.

 

[jray3]

Sorry you don't see any benefit, Llecentaur, but 85 kmh is pretty s l o o w, eh? Traffic flow on the highway around here is more like 113 kmh (70+ mph). Hypermiling has it's time, but I've got places to go! Any aero mods will matter exponentially more with speed. Probably explains why I've yet to hear of an iMiEV in Montana!

 

[Llecentaur]

Sure, 85 is slow, even slightly slower than lorries

Interesting you mentioned, higher speeds because on the way back I followed a truck to profit from its pull, there , by the way I can confirm massive gain maybe as much as 25%.

Having saved a lot of juice, I did later go faster than 110 and was surprised that the car did not suck up as much as I anticipated. Unfortunately I can not compare to similar trips at 110 before installing the spoiler.

Another possible benefit seems to be less wind noise at these higher speeds.

 

[NeilBlanchard]

It is hard to measure this quickly, and I think that either you keep track of charges (this is why a meter in an ESVE would be awesome) or you do a coastdown test. This requires at least 3 coasts in opposite directions (6 in total) and you can use a video camera to record the speed over the 70 second coast. A spreadsheet and instructions are here:

http://www.instructables.com/id/Measure-the-drag-coefficient-of-your-car/#step1

And you need to do a baseline test, and then the modded test - since we know the Cd of the stock car (0.35 right?) this is needed to calibrate the rolling resistance of the tires, etc. I think I figured what the frontal area is, or someone found it?

You need a straight and flat bit of road long enough to coast from 70+kph for 70 seconds. The spreadsheet (on the 3rd page I think) has all the formulas set up for you to enter in the speeds at 10s intervals, and add several other constants, and it calculates the Cd.

I agree that longer is better, but in order to help, it has to curve down some. I understand that since the roof is curved in the other direction, it will be hard to get a compound curve into it.

 

[Llecentaur]

Wife hit her head against the invisible plexiglass the first time she took the i.

Therefore had to remove it to avoid further "sanctions" :mrgreen: .

 

[NeilBlanchard]

There is that...

Belly pans and smooth wheel covers don't protrude from the car; and shorter Kamm back panels are going to have benefits.

The best way to see if an aero mod is helping is to learn how your car coasts on familiar roads. Remembering that air temperature has a strong effect - colder air is denser than warm air, and aero mods will have a greater effect in the winter. Which is precisely when the i MiEV needs them most.

 

[jray3]

With winter here along with heavy air and reduced battery performance, I'm thinking again on how to re-attach my plexiglass duckbill. Also noticed the Chevy Volt sideview mirror aero mod, though it allegedly increases drag to reduce noise...
http://evworld.com/blogs.cfm?authorid=201&blogid=1201
Anybody looked at adding a fairing to the mirror or replacing the passenger side mirror with a camera?

 

[NeilBlanchard]

I have been driving my Scion xA with video mirrors for about 4 years. Here's the installation:

http://ecomodder.com/forum/showthread.php/scion-xa-aero-mods-2969-17.html#post106080
http://ecomodder.com/forum/showthread.php/scion-xa-aero-mods-2969-18.html#post106284
http://ecomodder.com/forum/showthread.php/scion-xa-aero-mods-2969-22.html#post122825

I have to put the optical mirrors back on for the safety inspection. Make sure you get a monitor(s) that turn on when you power the car. Usually this means it has a blue/green "trigger" wire that is meant for the backup light. Also be sure to get a 7"+ with LED backlighting. Cold cathode bulbs get darker and darker the hotter the car gets.

 

[jray3]

Thx for the tips- I did read your Scion writeup some months ago when I ordered my camera. Now to find the right monitor.

 

[NeilBlanchard]

These are the monitors I am currently using:

http://www.newegg.com/Product/Product.aspx?Item=9SIA2C50X40851

They are only 234 vertical lines, but it is hard to find a 7" monitor with more.

 

[JoeS]

It was a beautiful warm sunny afternoon today, so instead of catching up on end-of-year paperwork I got ambitious as we're considering a long trip in the iMiEV in the next few days and I thought it might be fun to experiment with some simple foam-cored sheets. Note the skirts don't protrude much. Took a brief drive and the right one is rubbing as it's cut a little lower than the left one. Sorry for the sloppy blue tape. Have no idea if they'll stay on or if there will be any noticeable effect.

 

[Kuuuurija]

Better be sure, that those things will stay on, before you use those at higher speeds. They may turn into lethal weapon when get released at high speed.

 

[Mart]

The international version of the i-MiEV supposedly has a Cd of .33 vs. the NA version's .35. The NA version has a larger cross section, and the bumpers are not part of the original design. The rear spoiler has disappeared, and the antenna got shortened and moved to the rear. Plus the NA version got the added wiper and its drag. All things considered, it doesn't seem like the short spoiler on the international version had much effect on Cd, though it might have helped styling.

Flat wheel covers are often avoided due to heat dissipation from the brakes being retarded, but with regenerative braking, it would seem to be less of a concern. The sideview mirrors probably account for 5-7% of the total drag, but digital cameras and screens don't meet meet US regulations drawn up in the 1930s. An under-tray, at least as far back as the battery pack, might be easier to install than a usable kammback-spoiler.

I wonder where Neil would suggest attaching the under-tray?

For comparison, the Dodge Dart with Cd of .285

EDIT to add the numbers.
i-Miev (NA spec)
H: 63.6"/1.61m
W: 62.4"/1.59m
Frontal Section: 3,968.6 sq. in./2.56 sq. meters
Cd: 0.35
Dart
H: 57.7"/1.47m
W: 72"/1.83m
Frontal Section: 4,154.4 sq. in./2.68 sq. meters
Cd: 0.285

 

[PV1]

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

 

[Mart]

Emergency response training...(It gets worse.)


However it does provide an excellent view of the underside for aerodynamic purposes.