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.