Boat tail on a normal car? (3-box sedan)
 

Hey everyone,

First time post and its gonna be a tough one, but coach said go big or go home...if i have to go home i want to use as little gas as possible!.

I would like to build a boat tail for a 2003 Jetta GLS 2.0L gas engine. I know little to nothing about boat tails other than what they look like.

My plan is to use a trunk mounted bike rack and modify it accordingly.

I want to use plexiglass at least on the top so i can still see out of the back.

I will eventually use trailer lights for brakes and blinkers.



So my question/s:

How long do i need to make it?
What things do i need to keep in mind?
What angles do i need to maintain?
What am i forgetting that will cause worse MPGs?
What do i think i am doing right that is actually wrong?

Any help would be great!

http://ecomodder.com/forum/showthrea...rix-14954.html

this should be able to help you out with all of your questions

Sedan
 

The problem is that all the boat tails are on on hatch backs not many if any that i can find are done on sedans. But i think that post answered most of my questions,

What angle? 15d

How long? 4-6ft

Jetta tail
 

cliffy,I'll try a go at your questions:
* The length will lock in the drag reduction,so you you might want to target a specific mpg improvement,and from there we'll know how far to go.If you look at the 1st-gen 1999 Prius,at Cd 0.29,and the latest 2011 3rd-gen Prius,you'll notice that by adding only 7.3" length,moving the rear body up to near the boat-tail curvature,along with a few tweaks,the drag was cut to Cd 0.25.Since 'cars' have achieved Cd 0.10,you can see that there is substantial room for improvement.
* As far as 'angles',it's more about degree of curvature.There is an Aerodynamic Streamlining Template here at the aero forum which you could use as a guide for designing your tail.All you need is a decent photograph of the Jetta taken from the side,taken at enough distance from the car to accurately show the current roofline.Also,it's a good idea to get two friends to sit in the front seats when you take the photo,so it will be 'settled.'
* Since the tail needs really good airflow to it in order to perform to top level,I would recommend rear wheel skirts to smooth that lower flow.

Right now the way it sits, i would say im getting 26-28 mixed driving. Soon i will be commuting 80+ miles a day and would love to get near 40MPG. I think that should be reachable for a 2.0L manual Jetta...

mpg
 

It would be really good to find out what the car will do if you were to top off the tank( so full that you can actually see the gas in the filler neck just sitting there motionless),get directly up to a constant highway speed,and then just hold it at that speed long enough to get an mpg reading.
Later on,these kinds of numbers are exactly what you may use to calculate your progress.
Mixed-driving mpg figures may make it virtually impossible to reduce your data to a meaningfull level.The only variable you can have is the shape of the car!

I realize this is an aero question, but you might want to also look at some drive train mods such as cleaning up the air intake. Most OEM intakes strangle the engine and then there is the matter of the exhaust. You can usually get some benefit of a cat back reduction in back pressure by replacing the muffler and/or some of the rest of the exhaust without getting too loud. That and replace the lubricants with good quality synthetic lubes and don't forget to give it a good tuneup and clean out the injectors and intake valves with an additive such as 44K.

intake/exhaust/lubes
 

The only point I would make,is that should you do any of these mods you must completely redo your baseline mpg with these new mods in place or you'll never be able to determine what did what when aero-modding later on.

Wasn't there somebody with a Chevy celebrity that tailed it? Argh, can't think of who...

*off to use search function*

EDIT
Found him! Wagonman76 did one on a sedan:

http://ecomodder.com/forum/showthrea...deas-6715.html

I would say that my matrix (linked to above) is a good starting place. I went 4' past the back bumper and tapered it to the size I wanted the back to be ~15 degrees. I also used a large piece of Plexiglas to keep my rear visibility.

I think your idea of a bike rack might work, or just buy a hitch for the car and use that (which is what I did).

I am happy to answer questions if needed :)

Oh, also look thought the wiki page on boat tails:
Full boat tail - EcoModder

Jetta MODs in place/ ideas
 

I traded with my brother-in-law for this car, when i got it it already had high/cold air intake and performance exhaust. It also has aftermarket wheels on it which will be coming off soon.

I have attached a picture of kinda what im thinking.

I am thinking of having the "butt" of the boat tail rounded is there any aero gain to that?

Also i am in the process of getting one of those scan gauge things for testing my MPGs

Side note Someone suggested getting an air splitter for the front of the car you negate a belly pan...anyone ever heard of this?

I would square the back end up. It will make it easier to mount the licence plate and tail lghts.

good point
 

Yeah i think the flat "butt"" is the way to go. Something i should add, i don't mind this being a permanent boat tail, i want it to look like it came stock, i don't want to spend a million dollars either... any suggestions would help!

I haven't been able to find a blank aero streamline template, any links?

You are well on the way already.

I agree about the present wheel/tire combo. They are nice, but are not really what you want for an aero package. I am guessing that you are looking into some narrower low rolling resistance tires with either moon caps or something similar and rear wheel fairings.

You might give some thought to lowering the car. VW showed a concept car. They cleaned up the front end and lowered the car either an inch or inch and a half...nothing radical which meant the car should have kept a good ride in the real world. I don't recall whether there was a belly pan on the show car or not. It was a part of their planned Bluemotion (diesel) attempts to field a 70 MPG vehicle.

You might want to establish some baseline speed/RPM charts at the speeds you expect to travel at and then do some digging around to see what alternative gear sets or transmissions there may be to get the cruise RPMs down. That is one of the good things about VWs, there have been a lot of parts over the years. Lowering cruise RPMs is one of the ways that the manufacturers have been getting their EPA highway figures up. It also makes the cars quieter which is not all bad. I think there are also some pulleys to spin the alternator slower, but you have to be careful not to overdo it if you are using the AC in hot weather or driving at night a lot.

You might also give some thought to the influence of exhaust gases on the airflow.

You might take a look at the drawings of the Formula XL1 Concept for a little inspiration about the aero treatment of the rear wheel areas. It is an outgrowth of the original 1 Liter project. Wiki

You might want to first think about the materials you intend to use to fabricate the boat tail with as they may have a practical impact on what shape you can easily build.

body kit instead of lowering?
 

I figure i would not need to get a rear bumper (because of the tail), but the front and side skirts might not be a bad idea instead of lowering the car?

Link to front bumper: seems to have a front air splitter on it as well

Extreme Dimensions OTG Body Kit - Front Bumper for 99-04 Volkswagen Jetta at Andy's Auto Sport

As far as transmission gearing it could use some lower gears, it runs about 3500 RPM at 70MPH, which seems really high to me.

Welcome Cliffy, Here is the link to Part C with the template you asked about.

http://ecomodder.com/forum/showthrea...rt-c-9287.html

Part A has photos of various cars under the template, IIRC. I have found the VW to be low clearance already. I put on an aluminum skid plate under the engine right after I got the car. My stock bumper cover scrapes sometimes so unless your roads are totally flat I might hesitate to spend much on a different bumper. I have gotten good results from cleaning up the underside with a belly pan from just behind the cat to the back bumper.

Yea, 3,500 RPM is a lot at cruise speeds these days. I am not up on just what parts swap out on VWs these days (I used to play with Honda stuff, but the VW guys in the club were always playing with gearing), but here is a link to a comparison of the gas and diesel transmissions. I don't know if they are bolt up swaps so check carefully. I would look for some VW specialty shop or start looking in forums.

Just as an example though, swapping these transmissions out should get you down to a bit over 2,500 RPM if my calculations are close. (Obviously, do your own calculations before making any sort of decision/purchase.) There may even be other options.

[Edit] P.S. I forgot to ask about the wheel/tire package on the car. Have you compared them to the stock setup for revolutions per mile?

Streamline?
 

Did i do this right (the streamline)?


I did not compare the new wheel/tire package to the stock one, the new ones have low profile tires, and I believe they are slightly wider. I have the stock tires at home I'll get the specifics off of each. I think the diameters are the same.

You guys are really giving me some good places to start and what information i need before i dig in Thanks

Just asking about the tires because that can have an effect on the cruise RPMs.

The link says that stock rims are 6x15J and the tires are 195/65 R 15 H, but if you have the stock ones check them to make sure.

Tire Size
 

The tires on there now say 225/40 ZR 18, 92w xl, i don't know what effect the bigger wheels will have, but i will do some tests today on my Speed to RPM ratio then swap the stock tires back on and see what happens.

According to this calculator, the 225/40x18 tires make 804 revolutions per mile as compared to 807 revolutions per mile for the 195/65x15 tires. In other words, not real difference. There are greater variations than that as tire tread wears. The 18 inchers appear to be a perfect "plus 3". As I said before, they look nice and if you decide to sell them should be worth something...or just keep them for those occasions when you want more grip for a day trip through the twisties. :)

The only reason I asked was that sometimes people go for some very low profile tires that wind up lowering the car and affect the speedometer as well as the cruise RPM. A friend recently had this happen by accident...it seems he dropped the car off to get some new tires and the shop accidentally put one size smaller tires on it. It corners like nobody's business, but tends to scrape on rough roads. His car gets good mileage anyway and so he did not notice it right away.

Cheers

Keeping an eye on this one as I also have a Mk4 Jetta... :)

Streamline
 

Did I do the streamline right?

I looked at the stock tires and they are bald, where as my brother-in-law just put new tires on the aftermarket wheels, would it be cost effective to buy four new for stock wheels? What would you do?

I am hesitant to say "do this", except to suggest that you prioritize what you would like to do and compare that with your budget, both short term and long term.

The car runs right now with the rims and tires you have, so I would say that changing tires is something that can be put on hold while you sort things out. I would look into the options for low rolling resistance tires, keeping in mind that all low rolling resistance tires have made compromises which have made the tire's performance in braking and wet weather traction less than many of the tires which are not marketed as low rolling resistance. That said, most of the tire manufacturers have been working to improve the rolling resistance of their 'regular' tires...and have been pushed by the auto manufacturers because it can improve their CAFE ratings. Only you know the conditions in which you will drive and only you can best balance the qualities you want in the tires.

Frankly, there is nothing "wrong" with the tires on the car at the moment. Sure, they are wider than one would choose for all out fuel economy and are probably a bit heavier than the mileage tire, but your brother-in-law made a good choice in the tire sizing to keep the speedo, gearing and so on correct which matters a lot when dealing with the modern electronically controlled systems. It is all too easy to get a car set up where the readings of some sensor or other are out of parameters and so the system thinks something is wrong.

Do some thinking before you spend money on anything other than maintenance that you would do anyway.

As far as the streamlining goes, it looks reasonable, but look at how some other people have constructed theirs and see how you can approach it on your Jetta. For one thing, do you use the trunk much? Easy access to the trunk would be very important to me, but might be less so to you. Remember to keep an area large enough for you to mount your license plate, tail lights and so on.

You can swap out fifth gear with the tranny in the car. No need to swap transmissions unless you are going to the six speed. Lots of diesel VW guys go to a taller fifth so you should be able to buy a used diesel fifth gear set in good shape. Search tdiclub.com for lots of how-tos and info.

Exactly, if the OP can swap out 5th gear and the final reduction gear he should be able to get a substantial improvement in the car's fuel economy and comparatively inexpensively. If he does need to swap out the transmission and can get a six speed that works out that would be the way to go.

Cheers

Good stuff guys
 

Thanks for all of the information guys. I think that deleting the trunk hatch might be the way to go. Plus i would have a ton of easy mounting points. Maybe put a hatch on the tail so i can still use the inside part of the trunk. I don't ever use the trunk and when i have something to put back there I usually put it in the back seat.

Does my high tech simulation seem about right or does it need to be steeper?

No, you want clean air separation, so use a sharp transition from the sides of the boat-tail to the rear butt plate.

You could even extend the shape of the boat-tail a few inches beyond the butt plate, i.e. make the butt plate recessed.


If you look at the rear bumpers and / or taillights of ever more cars, especially eco versions and hybrids, they use a sharp edge rather than curving inward - or they use a sharp ridge somewhere near the start of the curve inward, keeping the curve for esthetics.


There's always going to be air underneath the car, so guiding it properly will improve drag.
An air dam and splitter reduce what goes underneath, but they don't eliminate it.

the Pics you put up are to small to see, try hosting them on photo bucked then linking them.

But the idea looks about right. The angle will change as you build it :)

I like the hatch removal idea, then the tail area just adds to your cargo space. If you hinge the glass piece (like I did) then you have a hatch back car :)

Better photos and test results
 

Side shot of possible design:

http://i1217.photobucket.com/albums/...s/Taillook.jpg

Streamline Template:

http://i1217.photobucket.com/albums/...Streamline.jpg

I did an RPM to speed comparison:

75MPH = 3800RPM
70MPH = 3500RPM
65MPH = 3000RPM
60MPH = 2800RPM
55MPH = 2600RPM

This was all in 5th gear

Then a 10mph coasting slow down test

70-60 11.3 second average
60-50 13.2 second average
50-40 16 second average

Taft test on the trunk coming next

Regarding the comment about the intake being a hindrance to fuel economy.

If you are truely driving for the best fuel mileage, with the engine running just past idle, then the intake would not be much of a factor.

On the other hand if you driving 75 mph on the highway and into a head wind, well there's not much that you can do to get good mileage at that rate. (hp= v^3 function)

Jim.

Tail Build idea
 

I was wondering if a rippled design would work. I was thinking of overlapping material down the length of the boat tail. So i guess from a side view it would look like stairs?? Any thoughts?

Here is the link to the amature taft testing video...i could only get up to about 50mph the actions starts at about 40 seconds

http://s1217.photobucket.com/albums/...-31-38_772.mp4

Not to make too fine a point of it, if for no other reason than, as it turns out, the OP's car has already had the work done by his brother-in-law (and a cat back exhaust, too), it is sometimes amazing how restrictive the stock intake systems are on many vehicles. You would think that the manufacturers would place more emphasis on this area of design because of the potential for fuel economy gains, but it does not often appear to be the case.

My earlier estimation was that a gearing change should get the RPM for a 70 MPH cruise speed down to 2,500 to 2,600 RPM as compared to the reported 3,500 RPM at present. With the planned aero mods the OP may be able to run somewhat fewer RPM, but that is still enough for there to be some benefit to freeing up the intake and exhaust.

On the other hand, it is true that this is not a max horsepower situation.

Cheers

You'd still get most of the benefits if the steps are quite small - allowing the air to easily re-attach.
The bigger you make them, the more you'll lose.

Grumman used overlapping plates on the fuselage of the Wildcat and Hellcat. ;)

From the video it looks like there is a lot of side turbulence on your car.

For the stepped design, If the overlaps are like siding (small, say under an inch) the air will reattach and you are fine. If they get bigger the air will not flow smoothly over the tail and you gains go away.

Photoshop structure ideas
 

That side turblance i think is coming from my mirrors, because rear of the car actually is tapered somewhat. I am planning on making new mirros to practice my fiberglass skills (i do not want to delete them completely).

Which leads me to a design question:

I am considering starting the boat tail on the rear of the car where the Jetta's curvature ends. The first picture is of the Jetta without the trunk hatch (photoshopped) and the white area is what the boat tail would cover from the back.

http://i1217.photobucket.com/albums/...ls/removal.jpg

The other two pictures are of structure and support ideas, crudly photoshopped and paint.

http://i1217.photobucket.com/albums/...withwindow.jpg

http://i1217.photobucket.com/albums/...idesupport.jpg

Thanks everyone!!

Both options look good to me. The bigger one will net you more gains but the smaller one will look less odd.

You might want to ask people who have actually used the longer boat tails whether they had problems with sensitivity to crosswinds, especially ones quartering from the rear. Just looking at the diagram it appears that the center of pressure would be moved much further to the rear than the present stock configuration, but I really don't know how much effect that might have in the real world.