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Old 08-07-2017, 07:09 PM   #1 (permalink)
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Exclamation Used AST. Boat Tail Must be 11 Feet Long. Is this normal?/Need Help!

Greetings,

So I used the AST and placed it over my RAV4 EV. Then I went ahead and used mspaint to measure the pixels within the image and recorded the placements for Length. The RAV4 EV is 180 inches in length, so I used that as a guide line.

In the picture the front end of the bumper ends at about 1.23.

In the picture the rear end of the rear bumper ends at about 6.09.

That is a 4.86 pixel difference.

Rav4 Ev Length/Pixel Value=Value per Pixel in "inches".
180/486=0.37037037"

Once I had this value then I measured the space between the end of the rear bumper to the very tip edge of the aero stream template.

The tip of the aero stream template was at 9.65.

Tip of Aero Template-End of rear bumper=Pixel space between said points
965-609=356

Pixel Space Value for Boat Tail Length*Pixel Point Value for Rav4 Ev= Possible Inches in Length for Boat Tail
356*0.37037037=131.85185172"

Boat Tail Inches/Inches Per Foot=Boat Tail Length in Feet.
131.85185172/12=10.98765431'

So, basically about 11 feet long unless I made a mistake. That seems like an awfully long boat tail! I'm not sure how I would be able to drive it around town. The longest vehicle I've owned was a 2004 Chevrolet Express Normal Wheel Base which was 224 inches long. I never really have any trouble parking it. However, there was a long wheel base version of 244" which is what I originally wanted.

If I were to only make the boat tail half the size it would be 5 feet 6 inches. In other words... 66 inches in length. Rav4 EV is 180 inches plus 66 inches = 246". So, it would be 2 inches longer than a 2004 Chevrolet Express Long Wheel Base. Which I think is cool! But, that is only half the size of a full size boat tail, and I really want a full size boat tail. I do feel that the length of a 2004 Chevrolet Express Long Wheel Base would be the longest I'd want to drive around in the city.

Also, isn't there a legal limit to how long a vehicle can be before it needs some sort of special permit? Vehicle Lengths It appears to be 40 feet total vehicle length? Which is 480 inches?

So I'd guess I will still be in the green if I do decide to make the boat tail 11 feet long? 180"RAv4 Length+ 132" Full Boat Tail =312 Inches long or a 26 feet long vehicle lol?

Can I even make safe lane changes on the freeway with something like this? If something happens... it'll be my fault isn't? Yeah, no pressure driving around that submarine on the hwy LOL.

So basically the boat tail needs to be 73.333333333% the length of the Rav4 EV. Is that the common average % for a full size boat tail around here?

180/100=1.8
132/1.8=73.333333333

Quote:
Originally Posted by aerohead View Post
Most modern cars will achieve Cd0.13 with a full tail.
So this beast of a boat tail should bring down my Rav4 EV's drag to Cd0.13 from Cd0.30?

Quote:
Originally Posted by aerohead View Post
I ran the profile out with some minimum constants for cooling system drag ( 0.011 ),'features drag'( gaps,cut lines,door handles,antenna,wheel openings,etc.) ( 0.007 ),and then subtracted out 0.015 for a complete wheel fairing set using data from the Sunraycer.
It appears that if we do everything 'right',we could get a car down to a conservative Cd 0.126,at 80 % of the template length.
The last bit of drag would be insignificant,residing within the 'phantom' tail.
Running the tail out to 100 % would only get the Cd down to 0.123.
Reading this bit... does this mean 80% of the template length would be just as good? I don't need to go full size? (Or retard :P)

If I made a mistake on the template please let me know! Or a mistake anywhere for that matter. Hopefully that could make the boat tail shorter! Here is the picture with template:


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Old 08-07-2017, 08:05 PM   #2 (permalink)
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Yes, that's why you don't see many boat tails getting about even on EM.

To be remotely practical, a full boat tail would have to be mounted on some sort of trailer.

The tail wouldn't be nearly as long if you start with a more reasonable vehicle - a Prius for example.

You can truncate the tail at a point that gives you your desired length, but you won't get quite the full benefit.
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Old 08-07-2017, 10:37 PM   #3 (permalink)
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Quote:
Originally Posted by oldtamiyaphile View Post
Yes, that's why you don't see many boat tails getting about even on EM.

To be remotely practical, a full boat tail would have to be mounted on some sort of trailer.

The tail wouldn't be nearly as long if you start with a more reasonable vehicle - a Prius for example.

You can truncate the tail at a point that gives you your desired length, but you won't get quite the full benefit.
I like the boat tail its just the rear wheels really need to be as close to the end as possible for best driving experience!

The trailer idea is nice, but I don't like the extra weight. Also more wheels. Plus lost space between the trailer and car, and also more weight! Driving on the high way would be limited to 55 mph I believe.

The prius is too slow and too small. Plus it still eats gas which means paying for gas. I had a nissan leaf for over a year and never paid for gas. My rav4 ev is over a year now and I still haven't paid for gas. Even with prius efficiency I'm saving almost 1k a year, so I'm getting close to 3k saved. I really only want an electric car at this point. I liked my leaf, but it felt too small. I was thinking on getting it stretched by 4 feet, but I got quoted 35k for the job, so I said no.

I think the longest would definitely be 5'6" which is 50% of the full boat tail length. That would place me 2" longer than the 2004 Chevrolet Express Long Wheel Base. I think that would be good for City Driving. Now, for high way driving I'm thinking perhaps keeping the extra 30% length (which I think would be about 3'3") in the vehicle and develop a system to attach it on the fly to the end of the boat tail so it can stretch to 80% total in the FWY. At least it seems that past 80% length in the temple just doesn't have as great a benefit at least from what I read in the above quotes?

I much rather just buy a car with a reverse wedge shape . Too bad no one is interested in those type of vehicles .

Oh, I think I could also get the boat tail to be shorter if I were to lower the SUV closer to the ground. SUVs stand pretty tall. Lowering it would make the template smaller when overlaying it. If I were to lift the SUV it would be worse. Anyway, lowering always has nice benefits.

I still wonder if the 80% of full boat tail length is just as Ideal as 100%?
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Old 08-09-2017, 07:40 AM   #4 (permalink)
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Welcome RAV4. I believe your are using the template correctly. It can be impractical for normal road going vehicles. Despite all the other mods that I have done to the Golf TDi this one is still in the dreaming stage. Most of the ones you will see on this site are done on pickups or small vehicles that already have half of the boat tail built into the car, like the gen 1 Insight. How much of your driving is highway/high speed? You may see zero benefit in town driving or even lose mpg-e due to increased weight. Many users report better handling at high speeds with a boat tail or Kamm cover. If you search for "phantom boat tail" you should find some data Phil has provided about the benefit of an 80% boat tail having nearly the same Cd as a full length one.
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Old 08-09-2017, 09:48 AM   #5 (permalink)
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Even a 12-18" long Kamm back (a truncated boat tail) would help a lot, to lower the Cd.
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Old 08-09-2017, 02:27 PM   #6 (permalink)
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Quote:
If I made a mistake on the template please let me know!
Yes. It's a common mistake. Here's an example:



Same as yours, but it also ignores the cross section. The template cross section is a half circle, your RAV4 is not (does it have the Tesla drivetrain?)

Here is a 3D version on the Beetle:



The 'rudder' is at 4ft (from stock length) to accommodate a reflex curve. The solid former is at three feet, for a 45 gathering angle. The hollow former is at the location of a stock rear bumper. All shorter than the Template.

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Old 08-09-2017, 03:23 PM   #7 (permalink)
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Since the back end of the car is most important for aero, you need to pull the template forward so the top and bottom of the back end of the template matches the top and bottom of the back of your vehicle. You don't need to be concerned that the front end of the template is sitting at the front of your hood. In your example above the bottom of the template is well below the bottom of your RAV4. Pull the template forward and you will get a much more manageable boattail. The trick will be melding the fairly straight rear roofline of your vehicle with the curve of the template.
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Old 08-09-2017, 06:29 PM   #8 (permalink)
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So, in that sense you did nothing wrong, except to follow the instruction.

basjoos' knowledge base is prolly equal to PK, aka aerohead; who originated the Template itself. Maybe come the weekend, we'll find out if this is a bug or a feature.

Meanwhile, for entertainment purposes, the question of why the Template is a half-body of revolution:

Why does The Template have to be a half body of revolution?

If 'twern't obvious; the reason the Beetle gets away with a 3ft boat tail is the plan taper starts at the B-pillar, not the rear axle line.
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Old 08-11-2017, 05:31 AM   #9 (permalink)
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Quote:
Originally Posted by COcyclist View Post
Welcome RAV4. I believe your are using the template correctly. It can be impractical for normal road going vehicles. Despite all the other mods that I have done to the Golf TDi this one is still in the dreaming stage. Most of the ones you will see on this site are done on pickups or small vehicles that already have half of the boat tail built into the car, like the gen 1 Insight. How much of your driving is highway/high speed? You may see zero benefit in town driving or even lose mpg-e due to increased weight. Many users report better handling at high speeds with a boat tail or Kamm cover. If you search for "phantom boat tail" you should find some data Phil has provided about the benefit of an 80% boat tail having nearly the same Cd as a full length one.
Truth be told I probably do less than 50% high way driving. I was thinking of making the boat tail from card board or something light that will hopefully not weigh much. I also thought benefits from a boat tail could be had at around 30mph + ? That is still within city driving speeds. I generally see 35 to 40s.


Quote:
Originally Posted by freebeard View Post
Yes. It's a common mistake. Here's an example:

Same as yours, but it also ignores the cross section. The template cross section is a half circle, your RAV4 is not (does it have the Tesla drivetrain?)

Here is a 3D version on the Beetle:

The 'rudder' is at 4ft (from stock length) to accommodate a reflex curve. The solid former is at three feet, for a 45 gathering angle. The hollow former is at the location of a stock rear bumper. All shorter than the Template.
Hi Freebeard,

The rav4 ev does have a tesla power train unfortunately Toyota did a pretty poor job at adapting the power train to the rav4 ev. I don't think it even receive the same sway bar as the rav4 sport! It is FWD do it loses traction very quickly unfortunately, and you can only charge it from a lv2 charger so no CHAdeMO or CCS combo and definitely no super charger. Cruise Control sometiems crashes if you turn it on too fast @ speed. Other than that its a great car. Sits too high though and so has a high roll over risk, but if you look up the RAV4 EV that ran the pikes peak challenge it was lowered and looked great!

I will try again to see if I can fix the lining of the template and vehicle. It really is a challenge now that I notice how flat the roof line is. In fact, after some tries it almost seems like the kam on the back isn't helping? That and almost like I have to chop the top rear end of the rav4 ev. Of course the other option is just to have a ridiciously long boat tail. It seems that with a boat tail you can't go wrong with long!





Quote:
Originally Posted by basjoos View Post
Since the back end of the car is most important for aero, you need to pull the template forward so the top and bottom of the back end of the template matches the top and bottom of the back of your vehicle. You don't need to be concerned that the front end of the template is sitting at the front of your hood. In your example above the bottom of the template is well below the bottom of your RAV4. Pull the template forward and you will get a much more manageable boattail. The trick will be melding the fairly straight rear roofline of your vehicle with the curve of the template.


Still not sure if it is right. I didn't try to move the template on to the hood since it made the tail too short. Unless I am approaching this completely wrong!sorry
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Old 08-11-2017, 10:40 AM   #10 (permalink)
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I think you're finding out that the Template is suitable for land speed record attempts and a small number of other scenarios.

Morelli's 'Fluid Tail'

Morelli's fluid tail is an approach more suited to box-shaped vehicles.

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