-   Aerodynamics (
-   -   Partial Kammback prototype tested - preliminary results (

MetroMPG 11-28-2007 08:45 AM

Partial Kammback prototype tested - preliminary results
Originally posted: 07-16-2006, 07:13 PM

Spent about 45 minutes playing this afternoon. No, it's not tested yet, and was more of a design exercise than anything else for the moment. I may reinforce and test it if the right weather comes up in the next day or 3 (before it falls off/gets wet). Otherwise, it'll come off having served its purpose as a learning tool for how V 2.0 should look.

Constructive feedback welcome!

Metro95 was right: it doesn't seriously obscure rear vision. The boat-tail cuts of 1/5 of the rear window view, and though you can't tell from this photo, the horizon on level ground is just below the "roof", so full visibility is retained of following traffic.

MetroMPG 11-28-2007 08:46 AM

My thoughts while making this:

- compound curves are hard to get right (I got the "roof" of the tail eventually, but not the sides right).

- I need to round the transition from roof to sides; right now it's a sharp angle (unlike the car's shape ahead of it).

- The side-of-hatch to side-of-tail transition is messy. I really need to start the boat-tail sides at the forward edge of the hatch sides. Also, you can't tell in the photos, but the right side is tapered further than the left side (at first I was focussed entirely on roof taper, and didn't taper the right side as much as it looks like I could have).

- I tried to project the existing tapering lines of the bodywork into the tail without any big changes, but I could possibly be a little more aggressive. Only tuft-testing would say for sure (looking for clean flow, right to the back of the tail)

- Ryland: I had the same thought about getting a junk hatch. I know where there's a '95 sitting in a farmer's field about 20 minutes away.

- Materials? If this is worth making a "good" copy, I was thinking: fibreglass over high density foam (messy, complicated, time consuming, little experience with the materials); junk hatch with attached sheet metal, faired & painted (also time-consuming, but I'm more familiar with the materials & methods); lightweight plastic or aluminum frame covered with sheet plastic/sheet metal... Plexiglass? Might be hard to form into compound curves (the stuff I've used in the past was fairly brittle. Also, how to join the top & sides in a rounded transition?)

- Not sure about dimples or VGs though. Since I'm an amateur, I'm just trying to stick to the basics: reducing the size of the trailing wake by tapering the shape as much as possible without causing flow separation. Also, I recall reading that dimples don't work for automotive scale/shapes.

- a finished version which I leave permanently on the car could serve as an attachment point for an extended version for highway use. :eek:

MetroMPG 11-28-2007 08:46 AM

Public reaction...

I've never had too much unsolicited public response to the wheel skirts, but the cardboard partial boat tail drew a small crowd. A question and answer session ensued in which at one point my sanity was debated.

MetroMPG 11-28-2007 08:47 AM

I'd prefer a more rounded top-side transition if possible. Sharp angles can trip smooth airflow into unwanted vortices, according to people smarter about aero than myself.

Fiberglass... I did some searching on how people make custom body kit add-ons, and it's a pretty complicated process:

- Build the prototype on the car out of whatever combo of materials you want (wood, metal, high density foam)

- make a mold by coating the prototype with fiberglass

- use the mold to make your good copy...

Those are the basics, as far as I can tell, anyway.

MetroMPG 11-28-2007 08:48 AM

Possible construction techniques:

One-Off Construction Using GRP/Urethane Foam Composite
(for making the car - trike - body shown below)


MetroMPG 11-28-2007 08:48 AM

Did a non-rigorous test - A-B only. (Cardboard/duct tape prototypes don't lend themselves to removal & reinstallation, so sorry, no A-B-A.)

That said, conditions were PERFECT. Next to no wind.

Results @ 55 mph (88.5 km/h)

- with partial boattail: 61.23 mpg (US) avg.
- without boattail: 59.85 mpg (US) avg.
- partial boattail gain: 1.38 mpg (US) / 2.3% above "stock"

Observed at: Grenadier Island 18 July 2006 4:00 PM EDT
Temperature 28°CPressure/ Tendency 101.5 kPa↑Humidity 63 %Humidex 36 Dewpoint 20°CWind calm DRLs, alternator ON, fan ON @ 3


Partial boattail
59.8 61.6
60.2 63.3
59.4 63.1

61.23333333 avg

57.2 61.7
58.7 61.8

59.85 avg.

This is about in line with what I predicted, based on the gains seen from grille block & wheel skirts (5.7% combined).

The difference between this mod and those, is this is a complex shape which can be optimised with tuft testing. The others were essentially no-brainer gap-fillers.

MetroMPG 11-28-2007 08:50 AM

For testing, I run alternator ON.

And today I ran DRLs (2-lane stretch of highway, oncoming holiday traffic), and the interior fan at "3".

Also, I normally test with cruise control to remove my bias & right foot from the experiment and leave it all up to an impartial and more consistent microprocessor. All of which is to say: no DWL. My cruise sucks at getting good FE, even on mostly level roads (speed corrections, even minor ones, are not "gentle").

MetroMPG 11-28-2007 08:50 AM

I should have tested at 85 km./h, not 55 mph.

Why? Because all of my other tests have used metric speeds, and I can't compare today's numbers with past tests done at 85. (55 = 88.5 km/h).

I did it at 55 mph because I was barrelling towards the "start" line when I suddenly remembered I hadn't reset the SG to report in metric. So I just set the cruise to the nearest multiple of 5 and kept going. Lesson learned.

MetroMPG 11-28-2007 08:51 AM

I posted a fresh article at about the "Kardboard Kammback" described in this thread.

(And yes, I realize it's not technically a Kammback :) )In it, among other things, I outline the reasons for grafting cardboard on the *** end of the Firefly in an attempt to mimic the shapes of these slick hatchbacks:

FYI, you are forgiven for not recognizing the car on the right. It's the 99-05 Audi A2, which of course was never available in North America. With its at the time world-leading Cd for a production car equal to that of the Insight, the TDI version of the A2 was rated at 3L/100 km, or about 78 mpg (US).

All times are GMT -4. The time now is 11:24 PM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2019, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright