Hitched Detachable Boat Tail finished. With tests!

[brucey]

I measured the car today with the tail on.

It's 235 inches. Just under 20 feet. Thats also a foot longer than a current suburban or excursion, the longest 'normal cars' I could think of. I think I could park it in a normal spot in a bind, but it wouldn't be pretty. This will probably remain a road trip only accessory.

Testing soon, I promise! It's gotten cold here and I havent had a lot of incentive to lay on the garage floor.

Wagonman76: Garage is completely unheaated/uninsulated (except a few leaky spots)/detached. Really more of a large shed. But I love it! With the door closed and a kreosene heater its bearable down to about 20 degrees.

[wagonman76]

Didn't realize a Suburban or Excursion was that short. But I guess a lot of vehicles got downsized over time, at least in length. A guy I work with has a Chevy pickup, extended cab, long bed. It needs 2 parking spots here. Only thing longer would be a crew cab.

Large shed would be really nice. Sure would beat working in pitch black, +20 to -20F, snow and ice and wind, and not even a level surface. Then in the summer, dealing with rain and mud and wind and laying down sheets of steel so the jack doesn't sink. And still running out of daylight.

One of these years I hope to have something. Maybe if I win the lottery, but then I'd move somewhere nicer anyway.

[Christ]

I got a friend that's building a quad cab dually 73-87 Chevy with a 10 foot work body on it. That's long.

[COcyclist]

Brucey, I can't wait to see more scientific testing but your short trial run looks promising. This design is very similar to what I have been wanting to do (hitch mount removable and keeping the factory taillights). Kudos for jumping in and trying something.

[brucey]

I took a round through the mountains again at 70 mph, did two runs each about 20 miles. (Actually had to go somewhere)

The average was 26.5 both ways.

It might be the cold/rain/winter gas hit but thats pretty low. I didnt get a A-B-A run just yet. I also havent closed the bottom up. I'll do that tomorrow and hopefully get my science runs in this weekend.

[brucey]

Did a similar run (night, freezing weather, slight drizzle, same road) tonight and got 25.6 at 70 mph.

I think that Its catching in some places and doesent have attached flow in others. Definately could be better. I was willing to compromise for the indicator lights but 4 feet for 1 mpg isn't good enough.

Can some of the aero guys tell me if I made it too steep? The always classic Pythagorean Theorem tells me its 13 degrees top and bottom. SO I don't know.

[RobertSmalls]

Here's a picture of three aerodynamic vehicles in profile:



And Les Baru:



Note how the Insight, Prius, and CRX have roofs with gradual transitions. I wonder whether flow remains attached across the corner at the back of Lesbaru, where it transitions from nearly -0° to -15°.

You need to determine where or whether you're getting a turbulent wake. Tuft testing is best, but you can also study the behaviour of road spray. When I'm traveling at highway speeds, rain doesn't land on my upper hatch glass, but the back of my car gets covered in road spray, just like the back of a Dodge Caravan. The outer surfaces of Chang's homemade roof extensions stayed clean while the inner surfaces got dirty.

So if you start with a clean boat tail and drive on wet roads, the portions that are dirty when you finish are probably turbulent.

[aerohead]

Originally Posted by brucey

Did a similar run (night, freezing weather, slight drizzle, same road) tonight and got 25.6 at 70 mph.

I think that Its catching in some places and doesent have attached flow in others. Definately could be better. I was willing to compromise for the indicator lights but 4 feet for 1 mpg isn't good enough.

Can some of the aero guys tell me if I made it too steep? The always classic Pythagorean Theorem tells me its 13 degrees top and bottom. SO I don't know.

All I can recommend,is that you analyze the form under the "aerodynamic template" you'll find at the forum here.
If the tail falls 'outside' the template,you'll lose a tad to extra skin friction.
If it falls "below" the curve,you're probably looking at separated flow and no chance of re-attachment.
This will be true for the top/sides and bottom.
Don't go below a 10-degree angle up from the back of the rear tires,or she'll crash into the road on driveway ramps and such.
Also,and this comes from many a frustrating testing "sessions".You may be easily capable of a couple more mpg than you're indicating,but it won't show up until next spring,when it warms back up.
I installed the bellypan on the T-100 during a winter cool down period and the lower temps canceled any "improvement" until the following May/June.Very frustrating!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
The tail may be doing better than you think.
Thanks for doing all that!

[brucey]

I drew that thing onto the car as in other pictures, it looks like i just missed the right angle.

Anyone got any ideas on how to fix this without redoing the entire thing? Most of the thing is put together with rivets so I don't know how much I could actually salvage.

[COcyclist]

Brucey, I think Aerohead has a good point. Don't give up on this yet. Before you change anything do a tuft test to see if you have attached flow. (Look at Craig Vetter's site for good photos of tuft testing) If you find your angles are too steep, as the template may suggest, is there a way to tilt the whole thing upward and then extend the bottom if necessary? In post #47 the CRX only has a top and sides. The airflow under the bottom is less clean and may not be worth going after. Some experts state the maximum angle for the bottom cannot exceed 4 degrees. This is simply not practical for the real world. Good luck.