Opinions on Kammback needed

[paintme205]

I did the aerodynamic overlay tonight on my new kammback. Looks like the kammback is a little too steep. It currently extends out 22 inches beyond the Hatch. Angle was approximately 18 degrees when I measured it.

Any ideas on how to make it better? Does the template look correct?

[t vago]

Have you done any A-B-A testing on your installed Kammback? That'd be a start.

You can also try this design, perhaps.



I want to eventually try this design on the Karen-Mobile.

[kach22i]

Originally Posted by paintme205

Looks like the kammback is a little too steep.

I think that the conventional wisdom in the forum for this situation is that it need not fit the template, but should not exceed 22 degrees.

T-Vago, I believe that the inside body curve is as important as anything else in that diagram. The "G-G" location on the car far above is quite an abrupt angle.

Paintme205, can you do a tuft test?

[MetroMPG]

Originally Posted by kach22i

I think that the conventional wisdom in the forum for this situation is that it need not fit the template, but should not exceed 22 degrees.

I've had positive results with Kammbacks & tails I've built that are "faster" than (fall "beneath") the template, but with the important distinction that I am always careful to avoid introducing sharp transitions.

(Though I'll also admit I haven't ever re-built a suboptimal mod to raise it up to the template line and compare the effectiveness of the two.)

I see a sharp transition here at the roof/Kammback transition, and also a sharp transition from the side of the Kammback to its top surface. That's where I see red flags.

"Think radius, not corner."

[t vago]

Originally Posted by kach22i

T-Vago, I believe that the inside body curve is as important as anything else in that diagram. The "G-G" location on the car far above is quite an abrupt angle.

I am also tending toward the idea that the curve should not matter too much as long as it is not abrupt.

[kach22i]

When we look at the NASA Dryden test truck, there is a large radius transition as well.

I agree with you guys.

Automobile 2 - Odds And Ends Photos by kach22i | Photobucket

[paintme205]

Thanks for the insight guys.

So if I raise it up a couple degrees, that would make the roof to kammback transition smoother?

I Have some extra material for the sides tucked under the main sheet of sheetmetal. I am going to try and pull it out a little to make more of a radius from side to top.

On the A-B-A testing. What is a good way to do it since I don't have any instrumentation or a scangauge? Only thing I can think of is drive like 40 miles one way and then repeat over and over. Even then we are talking 1 gallon fills. Any ideas how you would do it?

[MetroMPG]

Yes, I would both raise it up and make the side/top transition rounded. Hard side/top corners encourage vortex formation (energy-sucking vampire vortices).

Since you don't have instrumentation, I don't think there's an easy way for you to test it. (Though I'd recommend getting an MPGuino anyway, for feedback while driving.)

Coast-down testing is another method, but a lot of members have found it quite difficult to get repeatable, quality measurements.

[paintme205]

Well I filled up yesterday and saw a 1 MPG increase. All I do is city driving so I don't think it really shows how it is helping the car.

I looked into the MPGuino but I would have to add a vehicle speed sensor (VSS) and not sure how to do that. Supposedly rockauto sells something.

I raised it up yesterday and I like it better already. I tried to fashion a little more curve into the transitions but not sure if it helped or not.

Blocking the lower gril is my next mod. Hoping it helps the car heat up a little faster.

[wmjinman]

I have found blocking the grille helps a LOT - up to 2 mpg in the case of my cars.

MetroMPG is the guru of testing, but for coastdown testing, I always try to minimize all variables other than what I'm testing. So for aerodynamic drag, pump the tires up as high as you dare, so rolling resistance is minimized, and remove all extra weight possible to remove as much "momentum factor" as possible (no "assistant" riding along). I also like to rig it for a longer duration so errors starting & stopping the stopwatch and reading the speedometer are less of an influence.

Then I try to MAXIMIZE the factor I AM testing. So since aerodynamic drag increases so much with speed, start the test at as high a speed as you dare. Also, for the same reason, try to end the test while you're still going at a fairly decent speed. This probably wouldn't be workable for you, but when I was testing aerodynamics on my salt flats race car, I found a desolate stretch of highway and tested from 110 mph down to 60 mph. I was fairly sure aerodynamics would be significant at these speeds, and it would take between 50 seconds and a minute to coast down those 50 mph). I also had my tires pumped up to 70 psi and tried to compensate for the gas I burned by putting a gallon in every couple sets of runs (with the gas can left alongside the road during the runs). Of course, that car probably burned gas at a much faster rate than your eco-modder car.

I had a GPS speedometer and took it a few mph above my "start speed", then threw it into neutral & let it coast until down to the target speed when I'd hit the stopwatch. If your regular speedo is steady, that should work ok, but mine was really bouncy. I also set out orange cones to mark the "start point" and "end point" a mile apart. The tests are done in both directions so wind and grade will cancel-out. I'd go a little over a mile past one end, then turn around & accelerate up to start speed (around 112 - 114 in my case) & try to time it so I was down to 110 at about the start cone. Then do the same thing going the other direction.

I like three "sets" of two way runs for a good average. Start without the modification, do three sets, then attach the mod & do three sets that way. Then finally, take the mod off & do at least one more set to confirm you're still getting similar results you did in the first group. That's the "A-B-A" technique. The first "A" is without the mod, the "B" is with it, then the second "A" is without the mod again.

To compensate for the weight of the mod (since extra weight helps it coast longer and could affect accuracy), I'd either carry the mod inside the car when it wasn't attached (if it would fit), or else carry an equal amount of ballast weight in the car if the mod was off and alongside the road with the gas can.

I was pleasantly surprised the results seemed to end up really good. I could definitely see enough consistency to tell which mods worked best (longer coastdown times mean less drag, right?)