tested my kammback
 

I needed to retape my kammback so I decided to do a test with and without it from 55-45 mph average of 2 bi-directional runs each.

Here are some preliminary results of the coast down testing.

Same test mile used for mega airdam test, 95 f, 30.03" hg, winds from west around 5 mph, no airdam installed all other aero appliances in place , test mass 1981 lbs.

With kammback = 15.86 seconds.

Without kammback = 14.14 seconds.

from 55-50 mph

with = 7.97 sec.

without = 7.16 sec.

So kammback is a good add on :thumbup: .
I still need to clean my hatch and reattatch the thing.

Disclaimer...I didn't follow the best testing procedures due to time and space limitations (and some laziness). I should have posted this in the experiments section but it wasn't proper enough IMO.

Ooo, nice results. Did you put them in the coast-down calculator to see what kind of Cd reduction that implies?

Can you post a pic here of the current configuration? I think you just have the side pieces with a hollow center - correct?

I don't think I have the nerve to build one for my "brick" - it would be huge, and would have to have a clean join to the main body as well as to the liftgate. But I'd like to see yours again in this thread.

thanks bill

No, I haven't got that far yet. Too busy at work today.

jwxr7 - I like it. At first I thought it hindered you rearward vision, but I can see by your pics in the garage that it really doesn't block it too much.

brucepick - Your correct, it would be fairly large on a 245 hatch. But you might get away with just fastening it to the hatch alone. The d-pillars aren't very wide in the back so laminar sep. would be minimal. Just an idea.

I just put it back on.

Sorry, the best I can do right now is direct you to my garage pics :o . I'm not photo literate. It is one long piece bent to make the sides and a support was cut to fit the middle. BTW the airdam in some of those pics is the 7" version which is now trimmed to 4".
I would like a better more permanent way of attatchment, but the tape works for now. I may try different angles sometime too.

Thanks, yeah It really doesn't obstruct rear view (unless you want to see up behind you ). In fact it makes a nice sun shade while parked :).

I'd really like to make one up. When I take off my temporary coroplast air dam I'll see what kind of gunk the duct tape leaves behind and how much of a PITA to clean it off. My car's not gorgeous but it would be worse with tape gunk residue all over.

I had a full sheet coroplast, 4' x 8'. I thought that was big enough to build nearly anything but I used it all for the temporary air dam + the belly pan to go with it. No free coroplast till electon day, dang. Plywood??? Not with duct tape. Frustration!!

I just added a couple current pics (top two) to my garage. One shows 4" airdam.

Cool! In the first pic, the side view, it looks "factory"!

Now that's a compliment :) .

I did some more calculations. It appears the kammback improves Cd by about .05 :thumbup: . And average road load is reduced from 63.18 lbs to 56.75 lbs between 55-50 mph by using the kammback. (I love when i can use physics for real life stuff)

jwxr7 -

Ha ha. While I was reading your mega air dam thread I was admiring your kammback. I noticed the vertical center brace. Nice touch.

Thanks for the test. You made me add it to my todo list.

CarloSW2

Thanks,

The results seem almost too good to be true. I'm probably not an awesome tester but there was an obvious difference with the first coast after I removed it. I thought, man that was way shorter distance and time.

Anyone think .05 Cd improvement is feasable for a single aero appliance such as a kammback?

WD-40 cleans off tape goo real good. :)

Looking forward to grill blocks on the MZ3 this fall and winter.

psy

Oh yeah bruce, I forgot to mention that I use "goo remover" from menards to remove residue left by the packing tape. I've also used mineral sprits and wd-40 with success. Duct tape can be a bit more of a B*^%$ due to the reinforcing grid that sometimes gets left behind and sometimes to goo hardens. Packing tape varies alote, most tear lengthwise when you go to pull it off, but the "duck" high performance I've been using seems tougher.

your car starts to resemble a miniature version of the new renault senic
https://www.fuelly.com/attachments/fo...7a9e66c757.jpg
looks like you're inventing a new car rather than modding an existing one :)

there's been some positive report on kammbacks, yet i've noticed they're all with an open end, could the fact that they hold a pocket of non turbulent air somehow help them reduce the wake? has anyone ever rested open vs closed kammbacs? or for example, remove the rear hatch?

Reading up on Kammbacks in wikipedia, it said they are like a teardrop shape (or a boattail, I'd say), just not coming to a complete point. You get most of the benefit of a complete taper without going all the way to a point.

Many of the cars designed with a Kamm back are also partially hollow - have a concave rear end. It looks like the air moves along the surface and then just keeps going, it apparently doesn't really care much whether the surface continues or not. Just my half-educated take on it.

>>Anyone think .05 Cd improvement is feasable
>>for a single aero appliance such as a kammback?

Seems high to me, but depends on your starting Cd. For example, .30 to .25? Pretty hard to believe. But a square back truck at .5, I can see that. My guess is if you tighten up your testing methodology, the delta will close more towards half of what you are seeing now.... just a guess, but I have played around with this stuff some.

I saw a 5% improvement from my air dam, and sometimes I doubt that number, even though it is defensible.

.05 would be something like a 20% improvement, which seems pretty high. Still, it shows you are going the right direction. As always, running more trials with and without the mod helps you to be more sure of the results.

jwxr7,
I saw a VW with a nicely manufactured, shorter version of your Kamm back yesterday. Top lip was maybe 3-4 inches front to back. It had little triangle side pieces that were also about 3-4". Didn't need the center support since they were able to make it out of something pretty strong.

It was a hatchback type, sorry I didn't note which model. Maybe a Rabbit.

I had the kammback off and decided some pictures may help others. Here are some shots with it laying flat. BTW I am replacing this one because the plastic has degraded and is cracking. The middle pic has a yard stick for reference and there is still old tape ragged around the edges.