Roof spoiler effectiveness

[Eatoniashoprat]

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 ) Help?

[Bug@5speed(US)]

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

[Eatoniashoprat]

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

[volkdent]

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

[gonebuggy]

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

[Eatoniashoprat]

Originally Posted by gonebuggy

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?

[bigguy]

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 .

[judgie]

front view

rear view

cheers rob

[trevorbrady]

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)

[bigguy]

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

[Wally]

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:

[beetle1303]

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

[Wally]

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

[Wally]

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

[Eatoniashoprat]

Originally Posted by Wally

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.