A question of Aerodynamics

[Davemk1]

So as some of you might recall I own a Birkin S3. I use it for general fun driving, running errands and lots of autocross. My car has not so much horsepower (145-ish) and my budget is limited.

 

So I've been thinking about trying to make the aerodynamics a bit smoother to help in some of the high speed autocross events we have here. I don't expect that I can get any real downforce but I was thinking that if I could minimize lift and make it a bit more slippery that would be cool.

 

 

Spring is only months away here in Montana so it's time for the work to be done before the race season starts. My idea of getting the airflow a bit better are three fold -

 

1) a front air-damn/splitter - this is very simple and I've mocked it up with cardboard and it doesn't even look that stupid.

 

2) a rear diffuser - much like the front splitter this is simple. I've made one form cardboard and think that both this and the front splitter could be made from sheets of 1/8" ABS.

 

3) a cover that would bridge the undertray under the rear axle. Bridging from the seat base area to the gas tank area. - This one would be very difficult and in the end wouldn't be too effective. Just too much stuff moving up and down and hanging low to make a good clean flowing cover.

 

 

All this said I wonder if I might ask some questions and get some opinions.

 

A) IF the whole thing was done well do you think it might do what I want it to do?

 

B) knowing that autocross is for the most part a low speed deal do you feel that the extra weight (I'm guessing about 25 pounds) would be worth it? In other words would the weight hurt more than the aero helps?

 

C) the hard part design and construction-wise would be the rear axle undertray. Laying on my back and staring underneath the car makes me think that WITHOUT the rear axle undertray that the rear diffuser would be worthless. The air flow under the car would be pretty smooth at the front and once it gets to the rear axle area is must swirl around in a big way and make a high pressure area and therefore drag and maybe lift. Does this make sense to you?

 

D) does it seem that just the front splitter alone would be worth anything?

 

E) can anyone think of anything or any other way to make the car slip through the air a bit easier? I'll be rigging up a way to easily remove the windshield as that seems like a big deal. Other than that anyone have any ideas?

 

 

 

Thanks for reading and your time.

 

Dave

[yellowss7]

Dave, if you run in an SCCA sponsored autox, best read the regs. If it doesn't specifically say you can make a change you can't.

 

Lots of discussion on the aero mods in the D/Emod forum on the scca site. I haven't followed it that closely to give accurate advice, sorry.

 

I do seem to recall that the front dam/splitter details were very specific. As well as those for the rear spoiler. I don't think you can do a rear diffuser but again not definate about that.

 

Tom

[Kitcat]

Wouldn't removing the windshield and adding a brooklands windscreen accomplish the same thing? It would save weight as well.

 

Seems like you could just swap windshields at the events, which I know you drive your car to (and mite want the comfort/protection of a full windshield for).

 

I am toying with this for track-days as a cheap way of breaking the current drag-induced 105 mph top speed barrier of my mighty Crossflow powered Seven. I also drive my Seven to all events, sometimes hundreds of miles, one way, and would like the full windshield for the highway.

[Mandurath]

Carefully read the rules first. Or, as someone told me, you could build yourself right into A-Mod. They are pretty specific about size's there.