Roof spoiler effectiveness
 

This has been touched on before, but I'd like a recap if possible :) .

At what speeds should one start considering this? 130mph+? What about a car that may only see street/strip? Can anyone attest to its effectiveness?


I'm building a ~250hp subaru 1302 and enjoy accelerating more than high speeds but I may not be able to help reaching 120-130mph on occasion. Would a roof spoiler be effective in this case? I don't want a wing or a front spoiler. The car will sit as low as possible. Would a roof spoiler make me that much safer at 120mph that it would justify me buying one? If I was unsure I would just go ahead and get one but I would rather not disrupt the nice roof lines if I don't have to. I also don't want my back tires to pass my front tires at high speed (low speed that's ok ;) :D ) Help? :)

Eaton,

Good question and this is a topic I am wrestling with. At the moment my car is in pieces and I am working on my chasis, but end state I am curious how the car will handle at 100+ mph on a track (road course) and determine what kind of aerodynamic aids will be needed to maintain stability, both straight line and going around curves...

Similarly, I have a 1303 with a 5 speed, and a 2.7 (in pieces) that will drive my car. Based on my experiece at the track, I would like to drive the car to about 120-130. This objective time, while not the fastest, should make the car fun and still competitive in HPDE..against some of those higher hp cars. That top speed coupled with good handling, high trackout speed.. should keep me on pace.

I am not sure if this is asking for trouble, but if I can get to 115-120 safely, and maintain stability at those high speeds down the straights, and good track out speeds, I will have meet my goals.

Now in order to do that I have two front spoilers that I will play with..
I have seen a few products that have caught my attention, from V-force in Australia such as there rear wing and early light conversion fenders that have an extention on the bottom of the fender to help with air deflection and reduce lift. Also there are many rear wings that could be adapted to the back of the bug... from some of the modern cars.. Lastly I have eyed those roof spoilers, that while simple, might be effective?

None the less, I would be curious to hear what some folks think..
I know there are a few that have gone into this unchartered territory, usually reserved for other cars.. so all ears
VR
Alex

Glad to see I'm not alone in my wonderings. Now I know the roof spoiler only reduces lift at high speeds and I know how it does it, but I I guess the question is, how effective is it at reducing lift and at what speeds.

Mike

I have a pretty clear idea of what I want to do in the front of my car, but at the back, I've been toying with the idea of a rear window mounted lip, similar to the new beetle turbo rear window flip up lip. I think one could be fabricated to actually glue to the rear window with 3M double sided tap. It's not going to be a big + downforce tool, just reduce lift, so I don't think the rear window would have any trouble dealing with the downforce, especially spread over the entire top surface of the window. Lots of stuff to do before then, front airdam first, but I think I'll probably get there, bugs just want to take flight with the shape of the rear end!

Jason

Driving a bug that regularly sees over 100mph, a rear spoiler is not what you need to worry about. The rear of my car stays nicely planted. but the front end starts to feel light. you can start to feel the steering loose it's responsiveness (very slightly, but still a loss) Not having run a front, or rear spoiler i cant say how effective they would be. I think a small "lip" over the rear window would help, but personally i'd concentrate on keeping the front planted.

Alex

Thanks Alex, that's interesting to hear. So if no front air dam is used then the best one can do is have the car as low as possible to the ground and possibly the front a little lower than the back? Is your car fairly low?

i have done some testing on track with a few bits and i can tell you the biggest differance in time was a deep front spoiler and splitter, just under 8 seconds at prescott which is under a mile long :shocked:
didn't feel a lot different. the rear wing i run makes approx 80kg of downforce at 90mph and makes a huge difference to how the car feels at speed but not a lot in time, just feels a lot more stable.
if you want to make a big differance but not have it on show then try and make the under side as flat and smooth as poss with a slight front rear rake. front lower. vented backs to all wings makes a differance.
what ever you do make it strong, made the mistake of making the front splitter mounts out of 1.5mm alloy plate, at 100mph it bent:shocked: now you can stand on the splitter.
cheers rob

Good info Rob, thanks. :)

I have a porsche "whale tail" on my car. it works from about 60mph.
Drove it once without the tail and declid, and it felt unstable over 65mph.

Get a big spoiler.

Holy:shocked: , is your car really light or something? That speeds seems pretty slow compared to many others experiences with hi-perf suspension :confused:

Rob good info what size holes do you run on the rear of your fenders and how far up do you go? thanks JR

from what i have heard. the reason the front end feels light at high speeds is due the air being trapped under the front fenders. so if you want to open the rear of the fenders with louvers or some other way to vent the air the front remain planted better. this was a study done in the 80's i beleive .

front view
http://farm3.static.flickr.com/2288/...bcf80aaedc.jpg
rear view
http://farm3.static.flickr.com/2016/...9c92da56b6.jpg
cheers rob

This is just a thought but if the problem is with air gathering under the wings (fenders) and causing lift, then could wheel arch liners not be used like in modern cars that would have the effect of minimising the volume of the fenders and therefore reducing lift? Keep the liner as close to the wheel as possible to ensure the smallest volume of trapped air. Especially if you wanted to keep the car looking stock from the outside (i.e. no holes in the fenders)

would work but you would be hard pressed for any suspension travel if it sits close to the wheels.

I run this wing, and from track experience, it makes a big difference to stability from speads as low as 60mph...

http://www.aircooled.net/images/pphotos/drw0002.jpg
http://www.aircooled.net/new-bin/vie...RW0002&cartid=

My car isnt specially light. about 900kg ready to go.
But the whale tail is a big wing and make a big difference to aerodynamics.

and as we all know the beetle isn't very good aerodynamic from original design.

Talking about aerodynamics without wind tunnel testing is the same as talking about horsepower and never going near a dyno.

Some good reading.

Click

Click

Reading that and then going there are the only ways you'll know if anything you do is worth it. Otherwise you could be making things worse than they already are. And lets face it, is a Type4 powered Beetle ever going to handle or have a top speed warranting any sort of aerodynamic change?

Trogdor,

Take a look here under 'Feature Articles' for a reprint of 'Käfer-Cup Beetle vs Porsche Carrera RS' from Sept. 95 VW Trends magazine...

http://www.germanlook.com/Html/Main.php#

I think it just might open your eyes to the quickness/speed that the 'lowly VW' :eekno: can actually accomplish...


Take care,

Kelly (Cohibra45)

Agreed, however laptimes don't lie and you can't negate experienced driver feedback(I'm not in this group). For myself, I have a mechanical engineering degree and have studied a lot of the science behind it, but I can't attest to what is most effective on the track.

Yes?

If anyone has driven a super beetle in 80-100kmh wind without a kamei front spoiler and then bolted one on.........nuff said.
The wife fought the idea of a spoiler until we took our first trip on the highway with it. Huge difference even with cross winds and semi's passing.

Hi

I agree, I recently fitted one to my daily 1303, now I only hear the wind and don't feel it.

Steve

I agree with the above remarks about spoilers.
Front air dam made a huge difference. I have an original NOS Kamai one fitted now. A rear roof lip would work some I think. Anything that breaks the air wanting to follow the wing-liked shap of a rear beetle end, will work. I use a acn street-style rear wing for this:

http://www.keversite.nl/fotoalbum/fotos/48501.jpg

One thing to consider IMO is how much the added spoiler/wing will affect the amount of air going into the slotted area under the rear window. For example the wing in the pic posted by Chris Percival will reduce the amount of air going into the engine bay... and this is the cooling air for your engine.

I seriously believe that a combination of a roof lip and wing of suitable size and position will provide the best results while keeping airflow into the engine bay at a respectable volume. On the other hand you could use a wing like Chris Percival and find another way of allowing air into your engine bay. one way to do it is from the underside if you want to keep the original look of your bug. this though dictates the use of some kind of belly pan.

Ive been thinking about this for my 1303 for a while now trying to find mounting positions and different mounting mediums to avoid drilling the pan halves. if you start going this way it would be worth it to try NOT to make the belly pan flat. in addition you can prepare it for some sort of diffuser at a later stage.


Another thing to consider is the air spilage aroung the back surface of the fenders. only if you are looking to sqeeze everything out of aerodynamics aids. Unfortunately the rounded shape of uor fenders if pure eye candy, but they kind of suck in aerodynamic respect. I would be wise to open holes on the back of the fenders to allow escape routes for the air coming in from under the car. Air is NOT coming into the fender well area through the wheel. also place some kind of winglets not to allow the air coming from the top to merge with the air coming from the side. I need to investigate further on this to justify, but its an idea...
Chris

Chris,
The wing Chris Percival posted was the old version. Mine has quite a nice large gap between wing and body just to overcome the point you mentioned.
I run it and have head (4 of them) and oil temp sensors and none showed any difference...

Walter

Chris,
The wing Chris Percival posted was the old version. Mine has quite a nice large gap between wing and body just to overcome the point you mentioned.
I run it and have head (4 of them) and oil temp sensors and none showed any difference...

I do believe the lip works as it disrupts airflow and therefore reduces lift. The difference is (I think) that the wing style will probably give some extra downforce to boot ;)

Walter

You're right with that, the lip only reduces lift, and the wing provides downforce.

One can see how the wing produces downforce, but how the lip works is less intuitive (I think anyway). The lip acts as a sort of vortex generator to make the boundary layer (air flowing very close to the car surface) more turbulent. Turbulent boundary layers stay attached to the surface longer than laminar ones. This is why vortex generators are sometimes added to fighter jets, in order to increase the stall angle if the wings are designed poorly and the flow would normally seperate off the wing prematurely. Where the boundary layer is 'attached' (not seperated) it provides a pressure on the surface, so you can see that if the boundary layer seperates early this pressure is reduced and the car will start to lift. Make any sense? I'm not sure if the explanation was needed (or clear) but I just felt like explaining, for interests sake if anyone was interested.

Very interesting to say the least!
Maybe an avionics engineer or student could chime for this aspect of aerodynamics?

I don't think a lip spoiler is that clever in our application. They just detach the laminar flow from the rear of the car, thereby eliminating any lift that flow might be generating, nothing more..

Well I am a helicopter engineer, and I say you cannot use them in a combination.
Just as individual spoilers, because the roof spoiler wil destroy the airflow to the tail, and you loose all effect of it.

My opinion based on experience is that the Tail spoiler gives you most downforce and makes the car more stable. bigger=better!!

Well I'm a mechanical engineer who studied some aerodynamics in school if that counts :p

:D
Ah, we are well surrounded by true professionals!
I should have known :)
Thanks @all!

And now it only feel stupid... :o ;)

well, i just finished my automotive engineering design degree and did my part on aerodynamics/ design as well ( straight A on design!!!! i just to say that):p

Regarding the afforementioned wings, i have no experience, or seen them in real life so i can be absolute. The fact that the cylinder temp with the new one hasnt changed is very positive.
Just to add something to what Eatoniashoprat said, the vorteces at the back of a car MUST EITHER merge to the back of the car OR hit the road and stay attached to it at some point. only then u have a higher efficiency. if they dont attach to either they just swirl around increasing drag/ allowing the one behind to get into slipstreaming...
For our racers out there this is what all aerodynamicists try to do. to provide a laminar flow behind the car such as to maximize efficiency AND minimize the distance that this happens or in other words how close the guy behind can come to take advantage of the slipstream phenomenon.

On the combined use part... i think that if u investigate/research/design a roof spoiler and a wing u can achieve maximum downforce by directing the vortices leaving the roof spoiler to be at a small angle to the wing. so u get rid of the laminar flow at he back, only to direct it onto the wing.

one would say that this wouldresult in a maximum pressure on the top of the wing, and no air flow below. if u know what u r doing, then the vortices will still be at an angle to the wing, but fall in front( cross the horizontal plane in front of the wing) of the wing, and with the wing a turbulence will result at the bottom wing surface. i hope u r still with me. if u need i can draw it. aerodynamics is a language spoken with pictures and sketches etc...

sorry for the long post guys. i just like explaining things ( they r still fresh):p
Chris

Nothing wrong with a long post as long as it is good, which it was!

Mike

If you're not concerned about slipstreaming the lip will still reduced lift though correct? It will not be as efficient if it doesn't meet the road and stay attached but you're still decreasing the area that the flow is not attached therefore decreasing lift.

Kind of like the 2007 STi? I can't find a good pic to post

One day
 

One day I'll have mine running. I hope it helps, my cay to hold the road.
http://i86.photobucket.com/albums/k9...2.jpg~original
I'm told the Remmele one does a good job, however I don't like the ideal of have something glued to my car, which add safety.

I feel the need to make smthng straight for the ease of everybody understanding:

Imagine, a wing mounted somewhere and air flowing through it. IF the wing's plate is flat (not the profile) ie is straight, like the ones in the pics in this post, the air leaving the trailing edge will flow straight, evenlt across the wing's width.

Vortices will be generated between the side plates and the wing's plate, and around the corners of the side plates. IE rule of thumb, vortices need edges, protrutions to be generated.

Eatoniashoprat,
you got it the other way round, lift is generated, by attached flow, or low pressure cavities...

Now, a lip may reduce local lift, and disrupt the attached air flow ( to the car's body), but how effective it will be depends on the angle of the lip, and its height,

effvee, am i silly asking or it is facing the front of your car???

Chris

Facing forward
 

Hi, its facing forward. I have seen many of us (including myself) trying to get it right. When I installed that lip (which is 1" or 25.4mm) height) my attempt is to force the air passing over the roof (heightest point on the roof) to not so much to cause drag, but rather push on the lip. I'm hoping for the force to aid in a down force, yes it will deflect upward, but push on the lip. I don't know what the out come will be, however I'm this works in con-junton with a front air dam. I did not like the ideal of have something glued on at 120mph weather it worked, worked good or just caused drag, go flying off and hit something. If I ever get that car running, I will post what it feels like at 120 going to Vegas.

Not only that, I heard somewhere that as air pass over the deck lid area at high speeds, there is lift of the rear deck lid, something like how a shirt lift up on the back of a motocycle rider. So I reversed louvered my deck lid to catch air and release it into the engine compartment and below the license plate.
http://i86.photobucket.com/albums/k9...3.jpg~original

I really like the deck-lid idea :) , but not so much the roof lip :o