Daihatsu Mira aerodynamic modifications
 

These modifications result in an improvement of Cd from 0.37 (estimated based on similar car shape) to 0.245 (measured via coast down tests) .
Note rear end of boattail is near horizontal, and beginning of boattail has same slope as rear at junction. Note also how about half of the rear ends as a 180 degree ledge. I believe this helps, but have not tested. Note also how minimally drag in a crosswind should be affected. You should be able to see the rear wheel skirt is a mini-teardrop shape. Wheel covers have clear pvc screwed on and heatshrunk a little, cheaper than racing rims (ship quicker too).
http://img153.imageshack.us/img153/8...x480ya2.th.jpg

http://img225.imageshack.us/img225/8...x480bp0.th.jpg

Note smooth contour of grille blocking:
http://img505.imageshack.us/img505/6...x480ef0.th.jpg

Front deflector is as low as possible to the ground without bumping anywhere on my commute. Yes, I'd probably do a nice compound curve if I knew how to do so ala Burt Rutan, but I only know how to work aluminium at present.
http://img505.imageshack.us/img505/8...x480iv9.th.jpg

Undertray consists of two 0.6mm aluminium sheets (same as all other shiny bits) screwed into the chassis, the panels under the door, each other, and the two bumpers. Joined by duct tape. Segments are cut out for suspension bits, tow ring, sump plug. (Not sure what to do about oil draining, maybe remove some of the duct tape join for draining. Should really have some heat proof insulation between panels and exhaust to stop rattling at idle. Note rear deflectors.
http://img225.imageshack.us/img225/9...x480nb8.th.jpg

Note extension beyond for purpose of ease of flow separation. Should cut the bolts to a less ugly size, oh well.
http://img153.imageshack.us/img153/5...x480hk8.th.jpg

LHS mirror replacement bits. A bit of rubber from Clark Rubber, and some aluminium. Yes, I know it should smoothly contour back, might fix it someday. Should be a good improvement anyhow.
http://img505.imageshack.us/img505/1...x480sq8.th.jpg

Only planned mods to go are front wheel skirts and some gap sealing. Maybe bigger tires in lieu of a taller fifth gear. Maybe clear mini-boattails for the side indicators.

Tuft Testing of Partial Boattail
 

I decided to add some expanding foam under the bonnet. Since I was taking the car out for a spin, I decided to tuft test the boattail, to see whether my design looked up to scratch or not.

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http://img410.imageshack.us/img410/8...x480zc9.th.jpg

http://img472.imageshack.us/img472/5...x480vc1.th.jpg

I could see much better in the car than my camera could (due to the motion of the tufts). My impression was that every tuft along the boattail was travelling parallel to the plane of the plastic it was attached to, in the general direction to the back of the car. There was one that was oscillating about 180 degrees in that direction, but was still parallel to that plane at all times.

I'm not sure exactly the reason for why that last tuft acted that way, but even then the average direction it pointed was the correct way, and it wasn't away from the surface, so it doesn't look like a separation bubble would (looping back and around). My wife's car does that (tested, no photos), and the angle of the hatchback is steeper.

I stuck a tuft right on the end, and that tended to curl back around due to the lower pressure under there. Which is what we expect - an area of low pressure but smaller than it otherwise would be.

Of course, the tuft testing is only a guide, the proof is in drag coefficient, which seems pretty good. Of course, I haven't tested it with various angles etc. I only have so much time.

Do you think adding some sort of curve to the boattail would lower the drag a bit or are you getting attached flow even with that angle between the roof and the boattail?

I'm not sure what you mean... the boattail is already curved, and if there is an angle between the roof and boattail it is very slight. See the second of the original photos, and this other one.

http://img225.imageshack.us/img225/8...x480bp0.th.jpg

http://img402.imageshack.us/img402/6...x480rd6.th.jpg

The reason that in some photos it appears as if there is an angle is because the top two lengths of aluminium also point inwards somewhat. This only happens with photos that aren't taken directly from the side, e.g. in front a bit.

The tuft testing in that region confirmed that flow is most likely attached everywhere, the only possible hazy area would be right near the end, and even then I'm doubtful (no looping back observed, just wild oscillations from side to side, never towards the front of the vehicle and never away from the plane of the boattail).

There is a curve to it, An initial curve to break away from the roofline, and a curve at the end to get the flow to approach horizontal again. Most of the curving happens in the end thirds if you look closely.

Daihatsu
 

Like what you've done with the Mira.An almost 34% drag reduction is nothing to sneeze at! And no doubt,the 17% better mpg is repaying you and the rest of us for your efforts.I can't help wondering what fully enclosing your boattail area would do,however I realize how it could impact utility on a day today basis.Thanks for what you've done and keep us updated on any future mischief.60+ mpg is good medicine for whats going on around us right now.

Hi everyone. I have had a few busy years what with a baby, work and everything else. Sorry for not hanging out here. I should probably fill in everyone with how things have been since the mods. It has been nearly two years now!

I removed the rubber from the front skirts and just left the aluminium front part of the skirts, much like the compromise that the original Honda Insight did with their front wheels. Seems not to have made much difference.

In a mix of about 60-70% highway driving, I averaged about 3.7l/100km with pulse and glide. In the summer I got up to 3.3l/100km with one tank and maybe 5 3.5l/100km, in the winter several at 3.9l/100km.

Recently the clutch has started to go (from all the P&G). This has meant an end to P&G, and careful gear changes - e.g. from start to first, attempt to change in first while minimizing rpm, then accelerating, or matching rpm to gear while changing. It's winter and I get about 4.1l/100km, so it's probably around .2l/100km difference to not engine-off P&G.

My driving changed somewhat. I didn't limit myself to 80km/h after a while, as long as I had a large vehicle to follow. No real difference in fuel economy.

Although I have done some maintenance to it (regular oil and filter changes, repaired a leaking valve cover gasket), the number of km and the resale value of the car versus expense of required tyre, clutch changes, registration now about to run out, heater fan not working any more would be such that I am going to retire it.

There is also the convenience factor - basically I don't feel comfortable having the car anywhere but at work or at home off the street, because I'd fear it getting vandalized. I guess in hindsight this is a bonus as I'm now using my bike to get anywhere within an hour's biking distance. But that was also as a result of reading this forum. *

As basjoos would know, the rubbernecking gets old real fast. That means I try to leave work early on Friday so that I miss the peak hours of 18-25 year old male demographic driving around looking to impress their mates. I also try and avoid peak hours in general.

To avoid all this would be worth an extra 0.5-1.5l/100km, probably.

Now, I am the first to admit that while my modifications were functional, they did not look professional in the slightest. To make them look professional would have required more expense than I would have recouped. I maybe saved $500-750 in 2 years with the modifications. In the absence of fuel prices where they were in 2008 (or higher), I am skeptical whether a good business could have been made out of producing aerodynamic modifications for the used car market.

And if prices did get about 2008 levels for an extended period of time, car makers will start to produce properly streamlined 1-2 seat tandem automobiles like the VW 1 litre car. Either that or a business could be built to manufacture them.

So what now? Unfortunately, I will continue to need an enclosed vehicle for work purposes (I would prefer to ditch licensing costs entirely and go with a bicycle full time). I also refuse to compromise frugality for environmentalism (in practice there will not be very much compromise). I will do my best to achieve lowest yearly costs for any enclosed vehicle.

In practice this means waiting until 2nd Gen Prius comes down in price enough to be attractive to me, or there are VW 1 litre type cars at an appropriate price, or work circumstances allow eliminating the ICE from my life forever, or I build my own no-compromise VW 1 litre type car. For now, what I have calculated as being a combination of lowest cost per year and lowest cost (because I would prefer to have my money to invest), would be a 1995-97 Nissan Micra. Cheap, reputation for reliability, a supposed 4.4l/100km which I might be able to improve on. Also common enough to have a Haynes manual and a proper enthusiast community.

However, I would still attempt some subtle aerodynamic & other modifications because I know I wouldn't be able to stand the money floating away. Grille blocking, undertray, maybe mirror delete. The standard wheel covers are nearly flat anyway, so no need to modify them.


* Biking is cool. I follow the Japanese concept that a bike should be like an AK47 - cheap enough to be almost throwaway, and rugged. If you go European with expensive, nicer and built to last longer, it has to be either kept inside or it will probably be stolen. I'd rather not have to worry about that - with a cheap bike it can get stolen and is still probably cheaper than a couple year's worth of depreciation. Plus no one wants to steal an obviously cheap bike!

I have what would be known by cyclists as a "Bike Shaped Object" - a mass produced mountain bike that basically does the job. It doubles as an exercise bike because it is less efficient in getting me from A to B, so it keeps me fitter. ;) I probably cycle a minimum of 50km/week.

Accident avoidance is a high priority. I always wear bright (preferably reflective clothing), lights at night, and a helmet mounted mirror so that I can see what's behind me without having to turn and perhaps run into something through either not looking or changing direction unintentionally.