05' focus zx3 partial kamm

[beatr911]

Please gauge your MPG changes over at least two tanks. With the amount of change you are likely to see, other factors (such as the tailwind) can impact your numbers and not give you true results.

Maybe an undertray would give better results and your girlfriend would love you even more.

[brucepick]

Tru,

I've also been working on a not-yet-successful back end aero project (http://ecomodder.com/forum/showthrea...upe-18906.html). I feel your pain in a good idea that didn't yet give good results.

If you have a gauge or a GPS that will read in tenths of a mile per hour, you could use this method...

I've been testing it with A-B testing. As aerohead (aka Phil Knox) suggested, build your mod as best you can, test it, then remove it and test again. I try for 2-3 good runs before I take it off, then another 2-3 good runs.

I've been doing coastdown tests. Find a stretch of road with a bit of a downgrade so your coast has some distance. Coasting between two consistent points, note your speed at the first and 2nd points. Start-point speed won't be identical every time but I think if you keep them within 1-2 mph of each other its good enough. Afterwards, calculate the mph speed lost for each run, and compare them. If all runs of each type (A + B) are pretty close in speed loss I'd consider it a good test. I've had cases where I threw out a run that was very different, because it was influenced by the traffic around me.

Check the "Details" weather forecast on weather.com to find a period when the wind speed and direction will be consistent through your test. And, do it when there are few cars on the road to eliminate their aero influence as much as possible.

This kind of testing isn't quick but I think it's faster and more informative than trying to just drive with it long term and see if you like the results. Adding a third group of runs so you have an A-B-A test is even better. In my case, each time I reviewed after running the A's and B's, I saw immediately that the mod hurt the coast, so I didn't continue testing. I went back to upgrade the mod instead. Still no "joy" but that hasn't kept me from doggedly pursuing this thing.

[tru]

Originally Posted by beatr911

Please gauge your MPG changes over at least two tanks. With the amount of change you are likely to see, other factors (such as the tailwind) can impact your numbers and not give you true results.

Maybe an undertray would give better results and your girlfriend would love you even more.

two tanks even with a scan gauge? that seems excessive?

i figure over the course of 2 or 3 days of my commute which is about 180-270 miles is plenty. i had to take it off early due to rain today, but i will add it back slightly modified when the weather clears some.

[tru]

heres my thoughts on the re-design, please critique.





this shape would compliment the inward slope of the body instead of oppose it like the previous design.

[brucepick]

This gets you out of covering over the rear lights, but I think it won't help much.

The curvature of the rear lights is a much smaller radius than the template. That is, if you use the template laid on its side to design a boattail in plan view. You would get this gently tapering tail shape. I'm convinced the air can't follow the inward curvature of the rear lights. So the flow detaches there. If you start your Kamm sides after that, the air is already doing its own thing, not following your side panel.

I believe the side panels would have to start where the lights begin and go rearward, at an angle that tapers inward only slightly. Just like the template tapers downward (or inward if you turn it horizontal) at only a very gentle curving slope.

You could try clear plastic, and use non-residue duct tape to attach it. 3M makes that tape, I got mine in a hardware store. Costs 2x to 3x the price of regular tape but I didn't want to ick up my pristine car (haha, you should see it!). For testing you can use cheap plexiglass. It will scratch but will be good enough for testing. Wrap the tape over the edges to protect the car's finish from the plastic. Then tape over that to stick it to the car.

Actually you'd need to start the side panels about an inch forward of the rear lights. Because you will cover about an inch of plastic with duct tape, to stick it to the car.

[aerohead]

Originally Posted by tru

interesting, i removed the extension last night, we are expecting some heavy weather the next day or so. im going to modify the extensions to flow towards the center of the car with the tail lights and perhaps extend it longer if i like the results.

Also wanted to add that on my return trip home i noticed higher mileage across the board with a bit of a tailwind, its possible the wind skewed my results in the morning on the way into work. I didnt notice much wind but its possible.

Thanks for the replies so far

Tailwind can dramatically alter your results.If you can,keep a weather radio close by when testing to record current weather conditions.It will really help later on as you ponder your data.
On my latest trip,one segment had a 26 mph tailwind and that tank registered 50% higher mpg than the trip average.

[NeilBlanchard]

Maybe add polycarbonate angled panels that smooth the sides into the Kamm back? This would let the light through, and it would help the air flow on the sides. The plastic could also extend down to the bumper, giving the proper departure angle for the sides, too.

It is not obvious that the Focus hatchback has rather *average* Cd -- about 0.36, and much of the drag has to be coming from the back. I drove a 2002 ZX-5 for 3+ years, so I am familiar with the shape.

[TXwaterdog]

This is looking good but I'm partial to the whale tail... whale tail

[brucepick]

Originally Posted by NeilBlanchard

Maybe add polycarbonate angled panels that smooth the sides into the Kamm back? This would let the light through, and it would help the air flow on the sides. The plastic could also extend down to the bumper, giving the proper departure angle for the sides, too.

It is not obvious that the Focus hatchback has rather *average* Cd -- about 0.36, and much of the drag has to be coming from the back. I drove a 2002 ZX-5 for 3+ years, so I am familiar with the shape.

Yes, clear panels on the sides. Angle them inwards at a gently angle, in my humble opinion. Various people have suggested 10-12 degrees as what you need, so that the air flow will follow the surface.