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wyatt 10-16-2009 12:57 AM

Suzuki Swift (1994 sedan) Coast Down Numbers
Suzi has been a great car. Small cross sectional area, lower Cd than the hatch (.32 vs .34), taller gearing, and a "larger" 4 cylinder engine (1.3 L). Suzi does great and has consistently returned me 50 mpg in the summer if I try. I have done some aerodynamic modifications to her, so I did a coast down test to see if the work had helped.

AeroSpike drivers side mirror
I don't think this helps at all, but boy does it get the questions going! It is actually really solid, and from the calculations I have done (on paper) it is about as effective as what I had originally. This one may get remade eventually, but people seem to like it just like it is.

Folded passengers side mirror
1/2 grill block (leaving the radiator side 100% open). Not too pretty, and not 100% smooth, but it works. This is blocking the half of the grill that the AC condenser would be on, but with no AC, it really doesn't matter. The radiator remains 100% open, and the lower block attempts to seal off and direct air flow toward the radiator.
Windshield wiper block
This one also isn't the prettiest. I think the gains from this one are relatively small, but it gets people to ask questions, so I will keep it (or make a better one). Right now it is coroplast and is held up by scrap 2x4 pieces. Since the picture was taken, I have trimmed it to the point where I can lift the driver's side wiper when cleaning the windshield.
Smooth undertray beneath the engine compartment and trunk (none in middle of car). Made out of coroplast (used campaign signs [like everything else]) and sheet metal (old light fixtrues). The sheet metal is used to cover the exhaust.
Front Window Suction Generators
These pieces of coroplast prevent air from trying to enter/exit the top of the window. The result is suction being created at the front of the window where there is a natural low pressure area. I think I will make another set that is considerably larger (allowing ~4-6 inches of roll down) and see if I can get the suction increased.

I had done a coast down test once before the aero mods(single coast down in a single direction) and determined my Cd,Crr to be 0.304,0.0147. On my trip to Wisconsin (where I got 48.4mpg@61.7mph) I did another coast down, again single coast down in a single direction. This time the Cd,Crr come out to be 0.265,0.160. Between the two tests I went from bald tires to new tires, which may explain the small discrepency on that number. I know the results are not rock solid, but even if it's close, the mods are a HUGE improvement. It puts Suzi in the sub 0.28 category! It puts me close to the Cd of my wife's "new" Prius II (Cd 0.26)! The biggest difference I noticed when adding the bellypan/wiper block/grill block was the difference in cabin noise at 70 mph. My wife and I could hold a conversation! I have been thinking for quite some time about building a partial kamm back (which I saw someone on here do and call it the "uglyback", but before I do, maybe some real coast down tests would nail down what I have now, and the uglyback effectiveness could be quantified with more certainty.

wyatt 11-08-2009 06:01 PM

Coast Down Numbers
I did some coast down testing today. Changes from the pictures above are:
1. A bigger grill block made of dark gray foam, it's harder to see and covers more of the grill.
2. More of the foam stuffed in around the radiator area to direct airflow through the radiator as best possible (the combination of the two left me with no overheating problems during testing)
3. An Uglyback/Kammback that still allows my trunk to open and close.

It was nice out today, mid 70's, light breeze every now and again, SUNNY.
My test track is flat with a bend about half way through. When I say flat, there is less than 1 meter of elevation change through the course. (Thanks go to bwilson4web for the tip on the location)

Coast down started at either 55 or 60 mph (depending on how fast I could get going before having to coast), and with no traffic encountered on the road, I was able to coast down to 15 or even 10 mph (depending on the start speed). I did 5 bi-directional runs, and the car was warmed up when I got to the test site. Total time (including getting to and from the site) was approximately 2 hours... almost as much time as making sure I had my data set up correctly!

Results (from excel) of the testing:
Cd = 0.277939368
Crr = 0.011853646

We can call that Cd,Crr 0.278,0.0119... and that is probably putting a lot of confidence in my timing abilities!

I also did some "smoothing of numbers" to correct for inaccuracies in my data collection, and the numbers were very close 0.286,0.0115... I guess this is why ASME recommends 10 bi-directional runs. If I had some good data collection software, I think that would help too. I guess you should be able to do this with a Garmin, since you can export the speed and time data, or a video camera and a digital or analog readout, but I had my iPod touch and a pad of paper.

I don't know what the individual benefits of each mod are, but collectively this is a 0.04 drop in Cd (given a stock Cd = 0.32). Not bad, but not finished either.

MadisonMPG 11-08-2009 06:34 PM

You may be able to redo all of the mods, smooth them out, and get a bigger drop in Cd.

winkosmosis 11-08-2009 09:38 PM

That's an impressive cD. Are you using that Prius parked on your right as a template? :D

I don't think you want to create suction out of the cabin. Vehicles usually maintain positive pressure with the HVAC system in order to prevent exhaust gas from getting into the cabin through any crevices.

wyatt 11-09-2009 01:27 AM

I could, and someday I will make a better version of what I have on the car now, but I keep looking at Basjoos car, and how good his Cd is and can't see the point in doing much more than caulking the gaps... I would guess that it will be another year before I need to do an oil change on Suzi, and at that time I may be looking at doing a bunch of other stuff too, like putting new gaskets on the engine to stop the SLOW leaks... (15,000 mile oil)
Thanks for the props. Yes, the Prius is one of the places I pull from, but lots of cars around this forum have also inspired me... and I'm using my Hucho book as a guide.
I would also agree with you that negative pressure in the cabin is a bad thing... but the suction only occurs when I am moving, and leaving the exhaust behind me. When I am sitting still, it's like having my windows cracked. With the wiper block, I need the suction to help pull air through the car without having to run my blower.

hypermiler01 11-09-2009 10:07 PM

In some states, the side mirrors are not required, so you can take them off and get a wide angle center mirror.

wyatt 11-10-2009 10:39 AM

Alabama is one of those states... but I find that I use the driver's side mirror too much to get rid of it, and I like having the option of just folding the other mirror open (I have used it a couple times, not so much to use as a mirror, but to support trim hanging out my window :) ). One of these days I will have to try making a smooth mirror replacement. I also don't feel comfortable removing the mirrors (driver's anyhow) since I have to pass a security check point on my way to work in the morning... less questions is just better!

wyatt 11-10-2009 10:45 AM

I think I will be making a modification to the kamm back... I think it is just too steep of an angle, and may not have helped my Cd numbers much if at all. The tape is becoming lose and as it does the angle gets steeper and steeper, now around 17 at the center and 19 on the edges. I will be building supports for it that should help correct the problem.

MetroMPG 11-10-2009 11:12 AM

I agree with you on the angle of the roof extension - probably too steep. Aim for an initial angle of no more than 10 degrees from the preceding bodywork and you're likely OK for retaining attached flow. (Better to think in terms of curves than angles also.)

MetroMPG 11-10-2009 11:15 AM

PS - interesting idea with the front window.

Basically you just re-invented the "vent window" in function :)

Even 70's/80's VW Rabbit/Jettas had them:

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