Hitch mounted boat tail idea, yes another one.

[drmiller100]

every vehicle made in the past 20 years has rounded rear corners. I think you round them in hard, and let the flow stay attached for however long it does, further reducing your rear area.

I don't understand why you would want them squared off. Squared off is just going to make more turbulence over a bigger area.

???????????????

[Sven7]

Rounding slowly is simply a styling feature and will create more turbulence and "dirty" air. Aerodynamic cars like the Prius and Volt have sharp edges to cleanly detach the air, and so do custom ecomodder boat tails such as MetroMPG's test and 3-Wheeler's Insight. It's not by accident.

[aerohead]

Originally Posted by drmiller100

every vehicle made in the past 20 years has rounded rear corners. I think you round them in hard, and let the flow stay attached for however long it does, further reducing your rear area.

I don't understand why you would want them squared off. Squared off is just going to make more turbulence over a bigger area.

???????????????

I'll try and have another pictorial drag table ready for next Saturday which has some wind tunnel data for a 12-meter bus of square and radiused rear vertical edges.
Rounding the rear actually drops the drag on the bus,from Cd 0.325,to 0.314,although interior space is compromised a bit.
Kamm's research pretty much convinced everyone just to truncate the body,chopping it off as with a loaf of bread.
This gives a perfect separation line,the added length allows a bit more pressure recovery, and makes a very stiff rear bulkhead which can be constructed lighter.

[Tesla]

I was thinking about this rounding vs sharp cutoff, and did some research, and yet again found myself reading about projectiles, specifically RBT's (rebated boat tail, bullets). The bullet is rebated, just before the boat tail, this causes the propulsion gasses to create a better cushion behind the bullet to push it out faster with less wear of the barrel.
Anyway it got me thinking about what drmiller100 said about the air wrapping around the vehicle, my consideration was primarily for the junction between the boattail and vehicle, just like the RBT. Rather than having to merge the tail with the body panels, which gives the boat tail a leading edge into the airflow, what if we actually mount it just attatched to the rear and deliberately set in from the body by say 1-2", give or take about half way around the body panel curve, then start boat tail from there.
This would make design and attachment easier and may even allow for a further reduction in the boat tail rear area. I imagine at the rebate as the air begins to wrap around hard it will form a small vortex in the rebate, but with an angled boat tail reattachment may be possible.
Possible or Crazy?

[Sven7]

Why not find out where the flow is detaching (via dirt road or tufts), build the boat tail up to that point, and put a line of painter's tape around the edge as a seal?

[Tesla]

Originally Posted by Sven7

Why not find out where the flow is detaching (via dirt road or tufts), build the boat tail up to that point, and put a line of painter's tape around the edge as a seal?

Don't think I havent been looking at the dust and dirt patterns, need to go for a bit of a run in the scrub, I was thinking moreso duct tape, but the thinking is the same.

[drmiller100]

thank you for the idea of looking at bullets. obviously there has been a LOT of research on bullets - more then cars by far.
Very-low-drag bullet - Wikipedia, the free encyclopedia

looking at the pic on the right, no rounding. but they cheated the tail diameter down as much as they had room for.

I went out and spent a couple of hours looking at the details of how to mount a tail to my Suburban.

One thing to keep in mind is if at the rear, any air separation takes a LOT of effort to get reattached.

Therefore, the tail needs to be pretty well attached to the body of the car, and I actually am thinking of making the first 3 inches or so of the tail parallel to the cars direction of travel to ENSURE air stays attached at that point.

if you make it smaller then the body of the car, or leave a gap, I doubt it will ever reattach unless you make the tail 10 degrees or something silly.

The math of wind resistance certainly is appealing. Math says if I am getting 17 mpg now, I may get up to 23 mpg with a good sized tail.

that is a significant savings.

[drmiller100]

so been doing some measuring, bought some parts, will start fabbing this week.

back of my suburban is roughly 4 feet tall and 6 feet wide.

I'm going to make a back of the car with 4 foot long sides (slightly less then 4 feet long).

so, top will be 1:3 drop ratio - about 19 degrees. Sides I think i will do the same.
Bottom willl be 1:4 rise - about 15 degrees.

my math says the suburban has a 3456 square inches of "frontal area" in the back, and the new "frontal area" will be 800.

any math to guess what reduction in drag will be? math? Found some info on the net which suggests the suburban is a .5 Cd, so there is some room for improvement.

[aerohead]

Originally Posted by Tesla

I was thinking about this rounding vs sharp cutoff, and did some research, and yet again found myself reading about projectiles, specifically RBT's (rebated boat tail, bullets). The bullet is rebated, just before the boat tail, this causes the propulsion gasses to create a better cushion behind the bullet to push it out faster with less wear of the barrel.
Anyway it got me thinking about what drmiller100 said about the air wrapping around the vehicle, my consideration was primarily for the junction between the boattail and vehicle, just like the RBT. Rather than having to merge the tail with the body panels, which gives the boat tail a leading edge into the airflow, what if we actually mount it just attatched to the rear and deliberately set in from the body by say 1-2", give or take about half way around the body panel curve, then start boat tail from there.
This would make design and attachment easier and may even allow for a further reduction in the boat tail rear area. I imagine at the rebate as the air begins to wrap around hard it will form a small vortex in the rebate, but with an angled boat tail reattachment may be possible.
Possible or Crazy?

It would work.There would be a penalty do to the circulation within the rebate,as fuel would be consumed to power the perpetual circulation when moving.
But yes,if the gross contour of the tail was otherwise proper,the flow would re-attach and flow on to the trailing edge.

[drmiller100]

i built the frame today for my suburban. used scrap metal I had laying around - some 1.6 inch round tubing and some 1 inch square.

had the sheet metal guy come over - he's going to make some sides for it.

will take a few days for that.