Designing a partial kammback on a sedan - Fuel Economy, Hypermiling, EcoModding News and Forum

AeroModder · 05-11-2010, 08:42 PM

I took out the measuring tape and a scrap of coroplast and mocked up what a partial kamm might look like in the center of my roof. Rough measuring puts the angle around 15 degrees, but it looks a bit steep to me. At this angle, a 14 inch extension goes to the second defroster wire past the stickers, which leaves plenty of area open for the mirror. Maybe a 20" extension with a few degrees less angle?

The window angle is at about 35 degrees, so I'd be reducing drag, but increasing lift. Since I'm rarely over 70 on the freeway, it shouldn't be too much of a problem, especially with the diffuser on the back and the kamm being short.

I did some additional measuring and trimwork, and the angle in the pictures is a little too steep. I cut a scrap of coroplast into a 12.5 degree wedge and took another look. If I get some time, I'll see about making a rough design and see what it looks like.

Thanks to Piwoslaw providing some handy charts here.

EDIT: After dragging my fiancee outside to hold the strip, I found that I could get away with a 2 foot kamm at a 12.5 degree angle, and it would go as far down as the third wire in the defroster under my stickers. This still leaves enough space to see out the back, but that's as far as I'll go.

aerohead · 05-13-2010, 07:23 PM

Quote:

Originally Posted by AeroModder

I took out the measuring tape and a scrap of coroplast and mocked up what a partial kamm might look like in the center of my roof. Rough measuring puts the angle around 15 degrees, but it looks a bit steep to me. At this angle, a 14 inch extension goes to the second defroster wire past the stickers, which leaves plenty of area open for the mirror. Maybe a 20" extension with a few degrees less angle?

The window angle is at about 35 degrees, so I'd be reducing drag, but increasing lift. Since I'm rarely over 70 on the freeway, it shouldn't be too much of a problem, especially with the diffuser on the back and the kamm being short.

I did some additional measuring and trimwork, and the angle in the pictures is a little too steep. I cut a scrap of coroplast into a 12.5 degree wedge and took another look. If I get some time, I'll see about making a rough design and see what it looks like.

Thanks to Piwoslaw providing some handy charts here.

EDIT: After dragging my fiancee outside to hold the strip, I found that I could get away with a 2 foot kamm at a 12.5 degree angle, and it would go as far down as the third wire in the defroster under my stickers. This still leaves enough space to see out the back, but that's as far as I'll go.

Aero,you should see the kind of benefit The new Honda Insight Kammback has over the CIVIC sedan and coupe.

AeroModder · 05-13-2010, 11:51 PM

I hope so. Maybe I can get regular tanks over 30. My last one was 28 with the K&N filter installed, but I'm attributing most of the gain to the warmer weather.

I got a sheet cut to about the right size, and I'll be working on designing supports. The sides of the C-pillars actually turn inward at about a 15 degree angle, so hopefully that will add to the effect. Of course, I'll need to remove the vortex generators to do this.

My sheet goes to the end of the window and as far down as the third wire past the stickers in my mirror.

wyatt · 05-14-2010, 01:20 AM

I built one over a month ago and have over 2000 miles on it. It has been good for a 12.5% reduction in Cd and a 6% increase in FE at highway speeds (over 60mph). It is considerably longer than what I see you going for, mine is 4' at the longest point, and extends past the end of my car. I think MetroMPG had about 3% gain on his initial kammback for his Firefly (about 2' long). Expect a 3% (estimate) gain in FE, a 6% drop in Cd, and a 600% increase in attention paid to your car.

AeroModder · 05-14-2010, 11:31 AM

Quote:

Originally Posted by wyatt

and a 600% increase in attention paid to your car.

I already get people checking it out and asking me things. XD I parked next to a couple sports cars (a track-ready Miata and a Toyota MR-2) and my Tempo was the one people were looking at.

AeroModder · 05-15-2010, 12:08 AM

I got the bug, and finished it this afternoon. It actually didn't come out half bad. Passers-by came and asked me what it was, and liked it.

The side supports actually come inward at about 15 degrees, and the kamm is between 12 and 13 degrees. It reduced my rear window view by about 1/3, but it's still enough to see out of. It extends 25" past the roofline.

We'll see how it holds up, and what kind of improvements it has. I already used a quarter tank driving about 80 miles, but it didn't fill all the way to F when I filled up (last tank was 28.6 MPG, surprisingly!). There was a round trip without the vortex generators, so I don't know what effect that had.

Hopefully this will bring me to tank averages at 30 or more!

AeroModder · 05-17-2010, 09:13 PM

Coastdown testing proves this definitely has an effect. Where I normally hit 35, I was coasting at 37, and it looks like the fuel gauge isn't dropping as far after the usual commutes. I've gone about 200 miles so far on this tank, and I still have a little under half a tank. Before, I was lucky to reach 300 miles before E. I usually refill the tank when it's about 1/8 on the dial, and it takes 10 gallons to fill it. To get a ballpark estimate, I just move the decimal place over one place to get my tank MPGs. I still have a ways to go, but I'm guessing this will come close to 30 MPG.

AeroModder · 05-26-2010, 12:44 AM

My last 2 tanks were around 25 and 28 MPG, but with wind and rain, and some hill climbing and deadlocked city traffic on the 25 tank. The tank before I installed this was 28, with warmer temps and clear weather, with only a little wind and rain. I'm pretty sure the kamm is having a positive effect if I can match a good weather tank in poor weather.

However, I'm not sure how the overhang is affecting things, and whether or not it's altering airflow along the sides. Other kamms I've seen have the top and sides end together. I'm considering shortening the top to meet the sides, which would take 9 inches off of the length and I'd lose 2 vertical inches. Cutting it off would give me slightly more use of my rear window and allow my trunk to lift higher.

I'm thinking of making it more like this: http://ecomodder.com/forum/showthrea...html#post63182

Thoughts?

foghorn · 05-26-2010, 10:04 AM

Hi Aeromodder, did you find much benefit from the vortex generators?

AeroModder · 05-26-2010, 10:58 AM

MPG wise, I can't say. Vortex generators don't reduce drag much at all. However, they are effective in reducing rear lift by raising the pressure over the trunk and rear glass, where there is normally a vaccum. I can only say my car felt more stable in high-speed corners.

On the kammback: I'm wondering about meeting in the middle. In it's current form, it looks like vorticies will be made that will tumble over the trunk. I'm not sure if this is benefitial in helping to fill a void or not.
I'm considering adding to the sides to make the ends vertical to eliminate any vorticies formed by the angled edge, and trimming off the top plane to meet the new edge. I would only be losing a few inches of the kamm, so it wouldn't be too much. I also need to redo the supports...