Help me design my partial Kammback

[COcyclist]

Chaz, Nice drawing, looks pretty good to me. Thanks for the tips on Draw.

Weatherspotter, Good job on the wheel skirts on your other thread. You have steel rims and seem pretty handy, you could add screw-on racing aluminum wheel covers to the front wheels. 'Not sure you have enough clearance with the skirts on in back. I have a hatchback too and just added a coroplast belly pan. It seems to be helping.

[Weather Spotter]

Weatherspotter, Good job on the wheel skirts on your other thread. You have steel rims and seem pretty handy, you could add screw-on racing aluminum wheel covers to the front wheels. 'Not sure you have enough clearance with the skirts on in back. I have a hatchback too and just added a coroplast belly pan. It seems to be helping.[/QUOTE]

Thanks, I like reading how to's and thought that I might as well make my build into one.

I have a partial belly pan, I might make it into a full one, not sure yet. It will depend on if I can find some free election signs.

About the wheel disks, I have not seen an attachment method that I like. screws might work, but to keep the wheel balanced would be tricky. Ideas?

[COcyclist]

Here is a link to a source for screw-on aluminum racing disk wheel covers. These are typically used on Bonneville Salt Flats racers. I have seen a YouTube video somewhere of this type of installation. It is a little involved as you must remove at least one bead of the tire, drill and tap screw holes in the edge of the rim and re-mount the tire. A competent tire shop should have no trouble balancing the whole assembly for you. (You could mark the covers so you get them back the same way when you check tire pressures)

Racing Disc Hubcaps (Pop-On). Scion Hubcaps, Vintage, Kustom Wheel Covers

[MetroMPG]

Originally Posted by ChazInMT

If you add only 1 foot or less to it, the chances of you getting it right are pretty darn slim, at best you could equal the Cd, but it's a 80% chance you'd make it worse.

I'm curious how you came up with those conclusions?

I would have said it's not too difficult to extend the roof/sides of the hatch above the tail lights - continuing the existing plan & profile taper would be the conservative design approach.

The result will be a smaller base area where flow detaches and likely a lower Cd.

I'd agree there's probably a very small (~1-2% at 55 mph would be my guess) improvement in fuel economy with a short extension. The builder would have to decide how much effort is justified for a small gain.

[Weather Spotter]

they look nice, it would take some work to get them installed. the bigger thing though is the cost. $99 is a lot of gallons of gas to pay back the cost.

[aerohead]

WS,I compared your Matrix to the blue template line ( thanks Chaz! ) and it looks like your car has about 24% of the template aft-body.
At this length, with a comprehensive cleanup ( take a look at AeroCivic ) you might get the Matrix to Cd 0.28 at its present length.This number is based on a factory Cd of 0.34.I'm not sure about it but it's not that important.
If you extended the car to 30% aft-body you'd have a shot at Cd 0.25 and 12.5% better HWY mpg.

40%-Cd 0.20 and 20% mpg
50%-Cd 0.17 and 24% mpg
These numbers will give you a feel for what's doable.
If you decide that you will forever limit the cars length we can probably cheat a bit.
If your reserving the option to do later further elongation of the tail,you might want to stick close to the 'template' curvature as it is 'clean' all the way down to the tip and can be used on a 'modular' basis.

[Weather Spotter]

Originally Posted by aerohead

WS,I compared your Matrix to the blue template line ( thanks Chaz! ) and it looks like your car has about 24% of the template aft-body.
At this length, with a comprehensive cleanup ( take a look at AeroCivic ) you might get the Matrix to Cd 0.28 at its present length.This number is based on a factory Cd of 0.34.I'm not sure about it but it's not that important.
If you extended the car to 30% aft-body you'd have a shot at Cd 0.25 and 12.5% better HWY mpg.

40%-Cd 0.20 and 20% mpg
50%-Cd 0.17 and 24% mpg
These numbers will give you a feel for what's doable.
If you decide that you will forever limit the cars length we can probably cheat a bit.
If your reserving the option to do later further elongation of the tail,you might want to stick close to the 'template' curvature as it is 'clean' all the way down to the tip and can be used on a 'modular' basis.

When you say 24%, are you saying my car fills 24% of the ideal shape (blue line) or that there is 24% not filled?

The Matrix has a Cd of .321 stock.
you peak my interest in the % possible improvement, I am willing to try a tail, but I am not sure exactly how I want to do it. Ideas?

thanks

[ChazInMT]

Hey Metro, just common sense, Toyota knows aero and optimized this car, so anything small is just not likely to improve things. Contrary to some popular opinions in here, a large radius is better than a sharp cut off on the sides in back. If Toyota has these large radiuses designed in, and the modification takes that away, it could be worse. Also adding odd length extensions run the risk of forming vortex generators, and that would be bad. If the extension is long enough, as I said, it stands a better chance of substantially reducing the base area as you put it, this would be an improvement even if current aero design features get boogered up.

[aerohead]

Originally Posted by Weather Spotter

When you say 24%, are you saying my car fills 24% of the ideal shape (blue line) or that there is 24% not filled?

The Matrix has a Cd of .321 stock.
you peak my interest in the % possible improvement, I am willing to try a tail, but I am not sure exactly how I want to do it. Ideas?

thanks

The 'Aerodynamic Streamlining Template' is set up where any vehicle image is scaled to fit it's point of maximum roof height aligned with the point of maximum roof camber on the 'Template',which is at 0-% aft-body.
From there back,the 'boat tail' aft-body is divided into 10-sections of 10% aft-body length each.
It looks like your Matrix already occupies about 24% of that length.
Dr.Kamm and others recommended that cars be as long as to allow their wake to be only 50% of frontal area.
On the template,the frontal area occurs at 0-%, and 50 % frontal area occurs at 54% of aft-body length,where Kamm would recommend the truncation to occur.
If you extend the height of your ground clearance back,you'll see that you couldn't actually do the whole 100%,more like 90%.
Then,if you were going to chop some more away to allow for tailights and license plate,then only about 80% is useful.
Removing the last 20% increases drag only from Cd 0.13,to Cd 0.133,statistically insignificant.

[Weather Spotter]

So If I get it, I only need to go back to about 50% for maximum benefit while still leaving room for tail lights?

Also a Kammback or tail should follow the slope of the roof and the blue line (curve it not strait)?

How would I know what angle to bring the sides in at?

Thanks for the help!
If I can get a good mental picture of what I need to do I just might make one!