Hood-Windshield Transition
 

any ideas on how i can improve air flow transition from the hood to the windshield on my Jeep? i'm looking for something relatively simple to do, and that doesn't look like a giant cowl induction hood scoop

...a clear "bug-deflector" that goes from the front of the hood right up to the top of the windshield?

...maybe not!

that's a little easier than i was looking for, and i don't know how well that would work anyway

What makes you think that is the problem?

decreased hood to windshield angle is on the 65+ mods list. i have no intention of chopping the top of my Jeep, so i'm looking for something that could possibly give similar results without a ton of fabrication

By golly it is on there... I don't think it should be :mad:

The Geo example given... is a good example of frontal area reduction and also better aero on the rear of the vehicle. I don't see a shred of evidence that the hood/windshield angle did anything good.

Look up aerohead's posts (he could very well be our best resident aero authority), in about 1/4 of them he states and restates that it is the rear end of the vehicle that benefits the most from aeromods.

yeah i've looked through his stuff quite a bit (hence the statement of "doesn't look like a giant cowl induction hood scoop", like on his pickup (not knocking him, i have no doubt it works, just saying i need something simpler)) but not sure of what i could really do on the backside of my Jeep

But He has also stated if you don't clean up the front, the back will not do much good. It is a total package, and I have seen multiple people suggest starting at the front and working your way back to realize the most benefit.

J

I'm going on a roadtrip this summer to California which will require me to put a hitch basket on the back for carrying extra gear. maybe i can design it into a sort of hitch mounted semi-boat tail??

Really? :confused:

Can you find that?

In fact there is such a product on the market.

i've seen the big enclosed hitch mounted pods, but i'm thinking more along the line of building it onto one of the hitch baskets, and have the entire assembly be able to slide back to access the spare tire.

Sure, but Phil also has a deflector on the hood of his T100 to reduce the (apparent) windshield angle.

http://ecomodder.com/forum/member-ae...od-blister.jpg

I think I saw this in a post somewhere - the suggestion that you could build an aero-optimized boattail that would attach to a hitch. So rather than drill and screw to the sheet metal, it mounts and dismounts. And if you get pulled over you say "It's a luggage carrier". Which it will be, because it has a big space inside and a hinged or removable access panel.

Or maybe I thought of it - - who knows. Beyond my modding skills, but it might be a workable idea. Needs a full set of working lights and a license plate mounting location.

how's this look??

^^^the giant cowl induction looking hood i'm wanting to avoid, just not my taste

Yep.

Oh for ****s sake, he's talking about a diffuser, of course in that location it needs something other than total turbulent chaos to work.

Look at the rest of aerohead's posts; THIS guy is talking about the TOPSIDE.

Thanks aeromodder, I couldn't remember where I had seen that.
Frank Lee, you are correct in a boattail will help, and the back end of almost every vehicle on the road today is in horrible need to fixing. From what I have gathered on this site though is you will not realize enough potential from a boattail/kammback until you have smoothed out most of the car starting at the front.

You won't get the full potential for improvement of a boattail until you sort out everything in front.
But you do gain a serious improvement even if the front of the vehicle is a brick.


Streamlining a brick : see what the air resistance does when you add fairings to a brick .
http://ecomodder.com/forum/attachmen...1&d=1259337975

Then why are we talking about boattails. Working from the front to the back is both cheaper before you get to the boat tail, but easier as well. This is about hood-windshield transition.

This is what was done to aerocivic. As far as the angle goes I am not sure, but I would think an angle meeting up with the top edge of the windshield where the roof begins would help and it somewhat bisects the difference of the hood-windshield transition.
Aerocivic - aerodynamic mods for maximum fuel economy - aerocivic.com boat tail storage space

J

Those are wiper deflectors, rather than an attempt to reduce the apparent windshield angle (which is already smaller than on trucks).

Headed in the right direction
 

It looks awefully steep on the top. You want a smooth transition into about a 15 degree angle. I did a 4 foot kamm back on my car and realized a 6% improvement in MPG (12% drag reduction). Also if you have roof racks, you will want them off when the boat tail is on. Removal of the roof racks can be a 3% (or there abouts) improvement in MPG, and the boat tail will have much smoother flow coming into it.
I also cleaned up the front end of the car with a grill block (over 50% blocked), and a belly pan that goes to the firewall. This was also good for about 6% MPG improvement.
If you remove the roof racks, and have attached flow on your roof (easy to test with tufts of yarn and some tape, window down and mirror in hand at 35 mph with NO TRAFFIC) you will be good to go on the boat tail, as long as you follow the above advise.

angle was due to size of cargo box and how tall i can make it without obstructing my window too much. i would have the box go to about 5-6 inches above the spare tire, which would only block about the bottom 1/4-1/3 of the rear window.

Making sure the flow attaches nicely to the front of the hood above the grill will help keep the airflow energized. Probably already good.

Venting the hood to allow used cooling air up through it (first third of the hood, probably, would require verification) would effectively increase the hood angle without being too garish (see Mitsubishi Evo line). Probably make hot air get sucked into the ventilation system, though.

Gains should be available by increasing the radius of the A pillars to reduce the vortices that form where they meet the roof (that little patch of swirling water on the side window when you drive in the rain). I've been trying to figure out a nice way to do this on my own project.

Mirrors, of course.

A little rounded hump right behind the windshield might help. It looks like the roofline slopes, so a small hump could smooth the transition, reduce pressure drop, improve pressure gradient on the roof, and leave you with more energy to make the curve onto the Kammback, without increasing the profile. I've seen something like it on the front of a bus, IIRC.

Alternatively, cab-over semis sometimes use turning vanes on sharp corners to help simulate a bigger radius. Like those station wagon rear-window wind deflectors, but on the front. Never seen it done here, but again, I've seen it on trucks, and probably wouldn't look out of place on a jeep.

If it's like most rigs, especially 4WD, the undercarriage is probably where you'll find the biggest easy gains, though. Air dam is quick and easy, belly pan allows for better diffuser performance and better ground clearance. Steep approach angle on the jeep should make the air dam less likely to scrape than on other rigs.

'course, it's all just book learnin' for me. Haven't gotten to test it yet.

There is a thread here but i can't link it due to only having 3 posts yet. I will quote it instead - it's in my bookmarks.

To quote....
1. Narrowing the body at the front and rear,
2. Side window angle (22° and 30°),
3. Length of front bumper,
4. Angle of grille (0°, 12.5°, 25°),
5. Angle of hood/bonnet (5°, 7°, 9°),
6. Angle of windshield (25°, 30°, 35°),
7. Angle of rear window (30°, 35°, 40°, 45°, 50°),
8. Inward angling of rear pillar,
9. Angle of trunk/boot (4°, 8°, 12°),
10. Geometry of rear spoiler.


The effect of each of these elements on the drag coefficient was tested, then the propotional change in Cx (Cd) was calculated. Here is an ordering of those effects:

1. 46.5% - Angle of hood/bonnet (element #5)
2. 17.0% - Angle of grille (element #4)
3. 11.7% - Narrowing the body at the front and rear (element #1)
4. 11.5% - Angle of rear window (element #7)
5. 3.9% - Inward angling of rear pillar (element #8)
6. 1.9% - Relationship between rear window angle and spoiler
7. 1.7% - Relationship between windshield and rear window angles


As the above list shows, the hood/bonnet angle has the largest effect on the drag coefficient Cx of a sedan shaped vehicle. Increasing it from 5° to 9° reduced Cd from 0.52 to 0.47.

What I was suggesting (a 15 or less degree slope) would allow you MORE visibility and LESS drag. You will find that pressure drag would indeed be less if done like your drawing, but total drag would be more. Total drag usually bottoms out with a slope around 15 degrees, at least for a "fastback" design as opposed to a full out boat tail.

hood-windshield
 

Sorry! Late catching up.
My impression on the Liberty, is that the Germans did a good job with the fore-body,the flow should be attached all the way to the rear, and as Frank has inferred,it's the huge turbulent wake you should be targeting.
If I had one,to begin with,I'd install a 2" receiver-hitch,buy a $59 cargo platform from Harbor Freight Tools and build a slide-on boat-tail,extending the body at least 24," with plan and elevational taper,with diffuser section.
If you do the first 24" real well,you can add on in the future for even lower drag."Plug-and-play."
If you get the tail "wrong," it will prevent maximum performance.
The reason for the blister fairing on the T-100 is to help reduce the pressure spike from the abrupt windshield eruption only properly cured with a $3,000,compound-curvature, GTP laminated safety glass replacement and all the attendant body work to integrate it.
Hucho informs us that once these hood/windshield areas are 'optimized' and we have attached flow,no further streamlining will reduce drag.And there are studies in his book which relate the validity of his comment.
I'm going for around Cd 0.10 with the truck and kinda going overboard.
PS,the mod your thinking about did nothing for my CRX at Bonneville.The boat tail did almost everything!

W.E.Lay 1933
 

Frank,wherever I said that is gonna come from a comment Hucho said,regarding Walter E.Lays research in 1933.
Lay did a bunch of scale model car wind-tunnel studies,got about five of 'em down to Cd 0.12 with full boat tails.
He found,as did Fachsenfeld and Kamm,that if the fore-body had crappy flow,the boat tail was a moot point,as flow would be un-attached coming to it,rendering it useless.
Hucho goes on to say that virtually all production cars circa 1986 and later will be clean enough to ignore fore-body streamlining and to basically concentrate on the back end.

front
 

It's tough attempting 'one size fits all" comments.Most modern cars,vans,SUVs,and especially 'cross-dresser' (cross-over) SUVs are plenty 'clean' enough,save for the likes of Wrangler,H2,H1-type vehicles.
For the exception of grille-blocking ( standard now on many late model cars ), bellypanswheel skirts,wheel fairings,diffusers,wheel covers,the front ends of the majority of vehicles are plenty 'clean.'
If I'd done nothing other than the boat-tail to the 1984 CRX I would have ended up just about as well off.
This is really where the big money is.

Template
 

Joenavy,what you've imaged is a good plan.I recommend the Aerodynamic Streamlining Template as a guide for your curvatures,top,sides,and bottom.
If you get this part right,you could always add more length in the future.

Lay
 

Walter E.Lay investigated all this and published in 1933.Hucho refers to it specifically in his 1986 book.
Lay found that for a pure pumpkinseed or one with a conventional windshield cut in ( with minimum radius respected) the drag remained un- changed.
From wind tunnel still photos of similar shapes,I'd be willing to bet a dollar to a donut that the Liberty's front end is already clean enough to ignore with respect to a boat-tail.Just my opinion.

hood windshield
 

I recommend you get a peek at Hucho's book.
The doctorate-level mechanical engineers who spend an entire career in and around the best full-scale wind tunnels in the world are the ones telling us to essentially ignore the front ends of modern vehicles and focus on the back.

1985
 

Bear in mind that all this was written in the mid-1980s.The Liberty already embodies virtually all these technologies!

the brick observation doesnt quite work so well with most vehicles, since they arent actually "brick shaped" your forgetting that most vehicles have a windshield and roof that is almost 24" above the rest of the body.

I think you could achieve some gain just by angling you hood up near the windshield. by raising your hood 1-2 inches at the hinges Im betting you would see a much better airflow transition. it doesnt take much, aerodynamics are very close to aqua-dynamics. air and water flow very similar over surfaces.

I didn't get a good look at the picture before, it looks like it already has a nice smooth windshield-roof transition. So nevermind.

If the undercarriage is very rough-exposed solid rear axle, big stuff hanging down into the airflow, I'd do belly pan first. Higher ground clearance vehicle like this, the belly pan will almost certainly work better than an air dam, plus it makes your tail treatments more effective. (Hucho)

If the belly is like on most new cars, reasonably flush and even, probably tail treatment will give the biggest gain.

5.Angle of hood/bonnet 46.5% > 4.65 %?
 

I looked at my SAE Paper# 860211,by Ford of Germany,at Cologne.They investigated these areas also.
Working with a baseline 'standard' 10-degree slope hood and 57-degree windshield,increasing the angle by either raising the cowl,or raking back the windshield netted only about 3% drag reduction.
I suspect that the report of 46.5% has the decimal point in the wrong place do to a slip-up on the part of a proof reader.I believe that 4.65% would better represent their actual findings.
I believe that Carr's research,cited in Hucho's book will concur.

rain water tracings reveal adequate front-end
 

I walked past a Liberty on the way over here.We've had some rain and the owner hadn't washed the Jeep yet.
From the dried rain water tracings left on the Liberty it would indicate mighty fine, energetic ,and fully-attached flow over the windshield header and around the A-pillars.
* On close inspection of the car I would remark that an add-on nose would allow you to seal the entire grille leaving only a shark-mouth slit at the bottom.
* A deeper,wrap-around airdam would prevent air colliding with the suspension members.
* minimizing the front wheel opening gaps would help.
* dropping the rocker panels down another 5-inches ( 125mm ) would be good.
* blending the rocker panels into full rear wheel skirts would guarantee better lower onset flow to the back.
* flush wheel covers.
* bellypan later with diffuser.
* slip-on tail of any length.( the Liberty has only a 2.425 L/H ratio and without additional length it\'s going to be a rough go getting the drag down appreciably.Hucho suggests L/H = 5.0 as a goal for low drag which equals L/D=2.5 in ground-reflection.The \'Template\' is modeled on this protocol.
You\'re at Cd 0.421 right now,about same as Grand Cherokee.Wrangler is Cd 0.55,down from Cd 0.8 in 1971.