I made a cardboard kammback today and a friend helped me do some coast down testing.
Although I believe the thing was not structural enough, and therefore lifted up in the middle at freeway speeds, it did show a slight improvement. Possibly not outside the margin of error considering the conditions of our tests, but whatever. It's neat. I'll try to do another version that should work better. Ultimately working toward a permanent kammback.
Data. I did this test on Interstate 696 between Hoover Rd and Gratiot Rd, accelerating to 85mph then coasting to 45mph. Traffic was moderate so there may be remnants of slipstreams muddying the waters.
I did two runs each way, totaling four coasts with the kammback and four without. I am an artist, not an engineer, so if my math is off please correct me!
With Kammback
1:01.2
1:00.6
1:00.5
1:00.9
AVERAGE- 1:00.8
Without Kammback
1:03.4
0:57.1
0:57.2
0:58.6
AVERAGE- 0:59.1
That works out to be a 3% increase in coasting time with the kammback. Does that mean it's a 6% drag reduction?
I also put tufts on the car in preparation to get video of it, but we didn't have time nor a third person to drive while he filmed. I got this video, though, of the airflow rearward of my aero mirror... around 50mph. Good news is the mirror does not seem to affect flow on the side of the car.
pics of the kammback and all the terrible things I've been doing to this poor car.
not bad for just eyeballing it-
Kammback Test par
Tyler Linner, on ipernity
Kammback Test par
Tyler Linner, on ipernity
Kammback Test par
Tyler Linner, on ipernity
Kammback Test par
Tyler Linner, on ipernity
Also got a set of 15x7" Chevy Cobalt steelies for $10, so I can throw some of those orange oil tires on them. Sooner than later I'd like to get some Rota Slipstreams which are very light.
Future par
Tyler Linner, on ipernity