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Old 08-25-2010, 12:02 AM   #1 (permalink)
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Kammback doesn't work on my sedan

I made a clear Kammback for my 94 Mazda Protege.
( two pieces of plexi glass joined in the middle )

Unfortunately ( or is it luckily?) my coast down tests confirmed that I should take it off the car because it made it worse!

I have an established coast down section near my house where at a certain
spot cresting a small hill at 59 MPH I pop the car in neutral and about .6 of a mile later I have another landmark ( I call it checkpoint A ) where I consistently get a coasting speed of 39 MPH.

Did 3 runs with the Kammback on and I got:

1st run: 38 MPH
2nd run: 37 MPH
3rd run: 38 MPH

Then I took it off and did a final run and you guessed it,got my 39 MPH without the Kammback on.

Did tuft testing to see what's going on and I got the wiggles on the sides.

The top side appeared to have better flow.

Short video:



Did I go too aggressive with the angles? Perhaps I did even though they are
not exceeding 15 degrees on either top or the sides as of my last measuring attempt.

How do you guys post pics within the post? ( not thumbnails )
And how do you embed a youtube video? (not link)
Thanks!
Barna

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Old 08-25-2010, 12:26 AM   #2 (permalink)
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To me it looks rather short. I know you want to open your trunk, can you make a seem and have the Kammback hinge up with the trunk lid? It also looks like the transition from sides to the top is rather sharp. This might be causing some issues.

What is the angle from the roof to the back of the trunk? you might be making your wake longer with the Kammback (the air leaves at a shorter angle).

A thought on you testing, can you come to a complete stop? Like cost from the hill to where you stop. On my testing I could not make a good comparison until my speed was down to about 10MPH (but the real data was the differences in total coast length).
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Old 08-25-2010, 12:50 AM   #3 (permalink)
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Ok I see what you mean by the shorter/sharper new angle for the air to leave.

I never measured the angle between the edge of the rear roof and the trunk lid.

I thought my original wake would be as tall as the roof is and as wide as the width of my car but apparently that's not the case.

My "checkpoint B" is another .6 of a mile away ( normally doing 27 MPH ) and after that there is an uphill that I would never make so I could come to a stop but I'm afraid by then the test would be a crawl, easily effected by wind.?
Maybe I will give that a try if traffic allows. Thanks! : )
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Old 08-25-2010, 02:44 AM   #4 (permalink)
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3dplane -

*IF* my center-of-camber selection is correct, then I would make the claim that the angle is too steep and the airflow is not staying attached :



I agree with Weather Spotter that a longer kamm-back would be better, but I also think that if you get the angles right, it should still be a net benefit. I think that aerohead had a table of "diminishing returns" of a boat-tail. Obviously a full boat-tail would be best, but pragmatic concerns make it necessary for ecomodders to figure out how far they want to go.

But I'll let the real aero-experts chime in.

CarloSW2
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Old 08-25-2010, 09:49 AM   #5 (permalink)
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Can you find a back road near you that is flat (or close to it) for about 1 mile. Then set cruse to 55MPH (or the speed limit) go till you hit a known point (drive way, power poll.etc) does not mater what, just so it stays the same. At the mark put it into N and coast to a stop. Mark the spot with some chalk (use one color for A runs, a different one for B). when all the tests are done measure the distance from start to the first mark. then measure from there to each other mark. this will give you ABA data which is way better in terms of useful data.

What slope is your kammback?
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Old 08-25-2010, 08:35 PM   #6 (permalink)
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-Weather Spotter!

The slope on the top part is about 12 degrees. On the sides even less but
it was challenging to measure because even though the car is boxy looking, it becomes quite curvy when trying to measure angles.

In one of the pix,I made sure to snap the photo with the edge of the roof on the house in the background to be aligned with the top of the Kammback.
Although the front of the car leans down about 1 degree compared to the house in the background.

I am absolutely confident that my test results were true. I get to test my
coast down site every day coming home from work and today I got the same 39 MPH that I always get on a valid run! ( little or no wind, no traffic etc.)

Barna
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Old 08-25-2010, 09:36 PM   #7 (permalink)
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My aero eyeball says that the angle [12deg] in the picture is to great. although most people on this website us 10 deg as the bench mark I use the slope of 1 in 6 which is about 9.5 deg. you need to keep in mind that this is the maximum slope! So if you are at 12deg. then it is too steep. An other consideration is the lenght to width ratio of the object [car] the drag decreases as the ratio aproches 7 to 1 then increases as the object gets longer. I agree with those who have said that longer is better, and if you reduce the slope to something like 5 or 6 deg and then clear the open trunk, you may find some improvement. When in doubt keep testing...
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Old 08-25-2010, 10:29 PM   #8 (permalink)
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To measure the angle this is what I did. I took a 4 foot level and had it on the car top, I held it level and measured down off that. I would try a lesser angle (say 5 or 6 degrees) and see what happens.

As for your data, I trust that you are not messing the numbers, I just nodiced in my coast down tests that rolling speed was not a good indicator (we are looking for a very small change 1-4%). Where I drove on the road (crown, side, middle) messed with the rolling speed from moment to moment and run to run. This means that coasting to a stop was the only way I could really tell. I did have a good landmark (intersection) that I would roll through (I had right of way) at about 8 MPH (5 on the runs with out my Kammback, 8-10 on the runs with). any sooner than that and the data had too much variation.

If you can take the hour needed just try the full coast down. If you get the same results than all is good, if not you might have found a better testing method.
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Old 08-26-2010, 03:09 AM   #9 (permalink)
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Barna,

Man, That is a SWEET lookin Kamm, your fabrication is great on that. In looking at your car's basic design, it is decidedly unaero in the sense it has very little roundness to the roof line, and the transition at the top of the front windscreen to roof is quite abrupt by more modern standards as well as the rear window to roof, That is a cd killer.

I think you're definitely on the right track with the Kamm thought, but you either need to raise the rear edge a bit, or soften the transition from Roof to Kamm by making the shape of the Kamm rounder. You need something here to create a clean detachment for the air going over the roof. I think now your problem is you still have a rather abrupt change in the air from roof to Kamm and it isn't working.

OK, I wrote the above then went and messed around with drawing things over your pics and those of another Protege I found online.....I have to say I'm stumped, it looks all the world to me like you're at 10° on your Kamm, that is how I'd make it too. It could be a bit flatter, but I can't see that making a huge diff. I wonder if you aren't too aggressive on the sides? I don't know how you could modify it, but if you could make the sides less angled in by making the top trailing edge a few inches wider, it may help. I'm thinking you may have hit a "Sweet spot" (as in bad) for creating a large vortex generator. So by making the slope less steep a tad, maybe raise the trailing edge by 1.5" and wider by 3 inches or so, you'll get the clean air release your looking for without some weird pressure differentials creating a pair of horizontal tornados that you're dragging around.

Another thought is your speeds for testing. Is there someplace you can go to get up to 75 or so to start your test with? Since higher speeds really magnify aero problems / benefits, you may get different results starting at higher speeds. (Of course this is coming from a guy in Montana where I'm road kill if I drive 59 MPH where speed limits on our highways are 75 and cops don't even quit chewing their donuts if you pass them at 80)

And to answer your earlier question, seems like you got the You Tube thing worked out. To post pics without uploading them to EcoModder as attachments, I post them on a free web site called TinyPic. Then I copy the little box on there with the "IMG code for Forums & Message Boards" and paste it right into the post.

Like this



Hint, if you right click any pic then hit "Properties" at the bottom of the pop up menu, it tells you the source of the pic.

Great job again on your Kamm.

And finally (Man, will I ever shut up?) How in the wide wide world of sports did you make the Kamm? I'm considering doing something similar on my 04 Civic. And how much did the plastic cost?
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Old 08-27-2010, 01:32 AM   #10 (permalink)
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ChazInMT!

Hey! Thanks for the tips on the pics and the complement on my dysfunctional Kammback : )

If anything is wrong with the Kammback itself,then it has to be the sides because that's where the yarn wiggled a lot. On the top they stayed put,straight back.

I could still relax the radius and decrease the angle on the sides but honestly I don't think I'm going to play with it because I think Weather Spotter hit the nail on the head when he pointed out that I might have just gave the airflow a new sharper angle/ larger wake as it leaves the back of the roof and now it has no chance to "touch off" on the trunk!

Sort of like if the trunk area was a boat tail and now it is chopped off!
( Just my speculation )

To answer your questions: The Plexi sheet was a 32"x44" sheet about 2.5mm thick ( .1" ). Cost $33 at Ace hardware. Had it been a longer piece,I believe I could've made it diagonally out of one piece not having to join two pieces.

How I made it:

I started with making a large enough piece of paper using hot glue to attach pieces together and also in two layers so it's not flimsy. If you have cardboard it's probably better!

I taped that to the back of the roof and propped up the back side on the rear window in a fashion that represented my Kammback.
( eyeballing the angle and let it droop down the on the sides )

Then I dug in with my finger nails around the perimeter of the rear glass and followed the outline with a marker so I can cut it out along the line later.

I also made lines with the marker at the radius of the bend so I know where the bend is going to be even when the paper is laying flat.

Chop chop,now I have a paper template. Next,transfer that over to styrofoam insulation sheet that I had laying around.( about 5/8" thick)
This is going to serve as a mold or plug actually for the plexi Kamm.

Cut the styrofoam to the shape and size of the paper template.

Draw the lines of where the bend radius is going to be on to the styrofoam.
( these were not parallel lines in my case because the radius at the roof
end was larger than the radius at the back end)


We are going to have to bend the foam without braking it!
- I used a dremel tool with a cutoff wheel to cut grooves into the inside of the foam along the lines for the radius. ( actually any kind of wheel works,
at 25 k rpm or what ever it turns anything will just melt that foam right out )

Only cut about half way into the thickness of the foam!

Here is a trick! Put tape on the other side of the foam ( outside of bend) opposite of the cut grooves where you removed material to allow bending!

Now you can bend the foam and it will not brake!



Try it on the car to get the idea of how much to bend and to keep it from springing back towards flat, I had my glue gun ready and glued a cross brace piece of foam along the back side ( right above the back window ) to lock the angles in place!



So now we have a foam Kammback! Could actually start coastdown testing with it but it's so white : )
I actually should've tested this before I made the actual plexi Kamm but I guess I just wanted to bend some plexi glass anyways.

-Cut plexi glass according to paper template! ( if you have big enough piece)

-Lay plexi glass over the styrofoam structure and keep it aligned at the edges in the next steps!



- Sneak past wife or girlfriend and unnoticeably borrow her hairdryer!

Actually I have a heat gun but two, or one that puts out some serious heat would work better. Mine was a little wimpy.

- Start heating the sheet where the radius begins on the inside (top of Kamm) Run back and forth along the line of bend until gravity starts bending it down,then continue heating the plexi where it now touches the tape actually that you put on the outside of the foam.

Don't stop moving the heat gun! There will be an ugly spot there!

-Work your way down and keep heating at the new touch point/lines as it
gradually bends conforming to the styrofoam structure.
It will need a little push help bending unless your heat source is blistering

- Repeat other side.

Now go try it on the car with a piece of clear tape and let out an audible "Yaaay"

- Put the hairdryer back where it was taken from : )

Well that's how I made it!

Barna

EDIT! I stuck some images in the story but the pictures were taken after all was done and not step by step as the building process went on. Gives a better idea though!

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