Hood splitter for my vw

[Varn]

I made the splitter the way I did because that is the piece of coroplast that I had. I was thinking I wanted a wider one but also wanted it deeper (fore/aft) I think of it as a way of spreading the pressure wave at the windshield out a bit and reducing the peak pressure. My hood scoop is different enough from the other tests that the results may be better or worse than theirs. I generally don't try and pee on others but have been provoked before. I just consider the source.

Good point of safety catches on the hinge. If do proceed that way I will make an alternative catch to prevent a potential blade like effect on the passenger compartment. I have also thought about making a wire frame for the back raised the amount desired and then stretching a layer of dacron fabric and using it for a mold coating it with composite and replace the metal hood entirely. The coroplast is just a quick and dirty test. If the results don't lend themselves to improvement then the idea can be dropped.

It was weird to see the air move the rain drops to the center but at the center they went straight up.

Thanks to those who posted.

[JasonG]

Originally Posted by BamZipPow

Nope...not mine. Yer thinking of Aerohead's T-100...

Right truck, wrong owner. Do I get partial credit ?


Frank, doing the same thing, repeatedly, with variations is called proving a theorum in the scientific community. If an event is not repeatable across thed applied range, it cannot be peer reviewed and entered as valid.

If we are going to convince the general public and manufacturers that aero works, we need multiple test cases. They are finally sloping the rear of 'wagons', and adding small Kamms/wings to them as well.

Keep it up Varn (and everybody else) !

[ECONORAM]

Originally Posted by Varn

Since I bought my VW last fall I have not done much to it. Changed tires, put a grill block on it. The weather had gotten colder mileage was still good but had dropped off to about 40 mpg.

Last week we had some weather approaching 70 degrees and I decided to take my last piece of election sign and make a hood splitter. I had also pulled the bottom of my grill cover out and attached it to the front of the bumper.

I carved it into a D shape and heat molded it with a propane torch. Right now it is duct taped on.

Earlier this week I was driving in the rain and really noticed a change in the pattern of the drops on the windshield. The yellow lines mimic the pattern they made on the windshield.

I can't say specifically the effect on the mileage. It may be good. I have 280 miles and it is just at 3/4 of a tank. Which is further than usual but not quantifiable.

Any comments on the air flow toward the center. None of the rain drops went past the center line of the windshield.

I may make some brackets and raise the back of the hood.

I am wondering how I could do this on my truck in a tasteful manner. I tend to think going all the way across the hood would be better also. This looks like a good start.
Would it be worth sticking a manometer at the windshield base to do some before and after pressure checks?

[Varn]

I bet that a manometer might be interesting but I bet the dial would be bouncing all over. I don't say that from experience so if you do run a test, please post it.

Perhaps a wider lip might be better but it might spread the interference out to the edges of our rather square vehicles. It might be that a narrower bump might help as well.

My car is so old that I have to rely on fills for economy data so changes come slow.

Years ago (2004) I tested a 1.5" wide fin on the front of my 17" wide recumbent low racer bicycle on a large hill. I started at a paint mark on a hill and measured the top speed coasting with my feet in the same position. After 4 back to back runs I was consistently about a mph faster on that hill with the rather ugly blue board and duct tape shark fin.

In my mind a splitter is similar to a turbulator

Originally Posted by ECONORAM

I am wondering how I could do this on my truck in a tasteful manner. I tend to think going all the way across the hood would be better also. This looks like a good start.
Would it be worth sticking a manometer at the windshield base to do some before and after pressure checks?

[instarx]

Originally Posted by BamZipPow

From a safety aspect...don't do that. There are specific safety catches on the fenders that are there to prevent the hood from flying over the vehicle in a front end collision.

Or worse yet, being pushed back through the windshield to decapitate you.

[Frank Lee]

Originally Posted by JasonG

Frank, doing the same thing, repeatedly, with variations is called proving a theorum in the scientific community. If an event is not repeatable across thed applied range, it cannot be peer reviewed and entered as valid.

If we are going to convince the general public and manufacturers that aero works, we need multiple test cases. They are finally sloping the rear of 'wagons', and adding small Kamms/wings to them as well.

Keep it up Varn (and everybody else) !

Re: doing the same thing ad nauseum: it is true... and can be applied to, oh, HHO too, right? Certainly one is free to do whatever experiments one wants; it's their time. The first thing that always comes up is, is the experiment properly designed and executed so as to deliver trustworthy, repeatable results such that erroneous claims are not being made? That's really a lot tougher than the vast majority of us think. Testing something with, say, a potential 1% gain when the test itself has 2% undefined variability is problematic...

Mfgs don't need to be convinced aero works; hell, they're the ones that have decades of experience and billions of dollars invested in the tech, not us. What they need to be convinced of is aero sells and it appears at this time radical aero may not, while aero tweaking of the conventional may. The unfortunate reality is that oftentimes ultimate aero is achieved at the expense of some other desired quality (space efficiency, manufacturability, cost, compliance to current trends, etc.) so it becomes a balancing act. They are driven mainly by "the business case" that can be made for any of their decisions. Were the consumers in droves to give feedback that says aero and fuel efficiency is a top priority to them, we'd see a low Cd competition amongst the mfgs pretty quickly, and the motoring press would jump on board too.

Then it seems the trick would be to get customers to demand high fe and low Cd. My take on it is the customer pretty much doesn't care as long as their fuel expenses are a certain percentage of their budget or less.

[euromodder]

Originally Posted by Varn

It was weird to see the air move the rain drops to the center

The idea behind such a hood fairing is to more gently lift the air over the windshield.
It very likely means you're redirecting the air either way of the fairing, and now have higher pressure on the left and right parts of the windshield, which then partly bleeds off over the sides, and partly towards the center.

but at the center they went straight up.

The higher pressures from both sides converge, combine with the uplifted airstream going over the center of the hood fairing, and blow off to the area with the lowest pressure : over the roof.


So basically, most of the air still seems to be hitting the windshield hard.

See if you can find some more coroplast, and widen the fairing in the center.
The moment you're no longer seeing concentrations of air drops being spread out, it's wide enough to either lift the air that carries the drops up & over, or move it off to the sides.

[Varn]

So the rain was moving from higher pressure to lower pressure?

The air flow toward the midline of the car was air that was moving toward a lower pressure zone when compared to air at the base of the windshield?

This could mean that I have reduced the pressure on my windshield over 3/4ths of its width.

The hood slope on my car is 7 degrees from horizontal.

Originally Posted by euromodder

The idea behind such a hood fairing is to more gently lift the air over the windshield.
It very likely means you're redirecting the air either way of the fairing, and now have higher pressure on the left and right parts of the windshield, which then partly bleeds off over the sides, and partly towards the center.


The higher pressures from both sides converge, combine with the uplifted airstream going over the center of the hood fairing, and blow off to the area with the lowest pressure : over the roof.


So basically, most of the air still seems to be hitting the windshield hard.

See if you can find some more coroplast, and widen the fairing in the center.
The moment you're no longer seeing concentrations of air drops being spread out, it's wide enough to either lift the air that carries the drops up & over, or move it off to the sides.

[Varn]

The more I consider what has been said The more I believe that a full width hood bulge is not what is needed. Looking from above most automotive windshields are relatively flat with very little 3D curve. Looking at a 2D template can lead one astray on to what may be really needed.

Consider what the windshield of almost any streamliner looks like.

Obviously my Varna has excellent aerodynamics runs 50 mph on 300 watts ... my diesel does not.

What I think might be needed to simulate a swept shape is to make some sort of shape similar to what I have but maybe 6" wider and should start further foward on the hood.

[Frank Lee]

I think now would be a good time to recall the radiussed leading edges theory... you know, the one that says any radius larger than that necessary to promote attached flow downstream doesn't do anything more to reduce Cd.