Cheap aero mods for AX

Okay, so my local AX has a very loose classing system that only cares about cylinder count and treadwear.

I've been thinking about cheap aero mods. Is there anything I could do that would be cheap/effective on my 5.0 convertible?

Would a plywood splitter woodscrewed to the bottom of my airdam do me any good?

Is there such a thing as a cheap rear spoiler, and if so how high in the air would it have to be to actually give me downforce? I know on the hatchback bodystyle, you need to get the wing up to the same height as the roof, would I be looking at something similar? What about the big ducktail spoilers I see on the back of Miatas?

What about diffusers? Someone was saying they do even more than a wing does in terms of rear downforce, but I don't really get what it is a diffuser is doing. Don't I also need a full belly pan to really take advantage of one?

And FWIW, I'm a function > form kinda guy. I really don't care what the end results look like so long as they're effective, and I have no qualms with putting a park bench on the back of my car if it puts me in the running, lol.

I hear you can build a complete sucker car for less than $2K.

A diffuser is trying to pull air out from under the car, dropping the under-car pressure. Even if you only make a difference of 0.1 psi, calculate how many square inches there are under your car!

The splitter will probably work, just don't start mowing cones down. The ducktail spoilers certainly work, especially big ones. Make the angle adjustable so you can play with it.

You can make a big ol' wing really cheap if you're interested. We did ours for under $100 I think. It was made, mostly by one guy in about a week (working in the evenings).

You'll want to balance that with some front aero too though.

So, if I wanted to do a redneck diffuser, where would I even start?

Would I need to do ground effects around the side of the car to keep outside air out and inside air in?

Would I need to do a belly pan?

For the front splitter, are there angles I need to consider, or is a 4-6" flat bit of plywood that's level to the ground enough to do the trick?

Does "aero" really do anything at autoX speeds?

I would think one would need to be at freeway speeds or higher to get much help from aero tack-ons.

Yes? No?

The key to success is going to be testing.

I am suspicious that the splitter we kludged onto the MR2 I was codriving was not effective at all... We never did devise a low cost way of measuring its effectiveness. I wanted to block off the radiator inlet in order to see if helped...

Due to the banning of aero in STS we have had to remove the wing and splitter, and to be honest I think it has actually improved our performance. So making a long story short no aero might be better than poorly sorted aero.

In reply to ReverendDexter:

Would I need to do ground effects around the side of the car to keep outside air out and inside air in? -Yes

Would I need to do a belly pan? -Yes (to make the most out of it)

For the front splitter, are there angles I need to consider, or is a 4-6" flat bit of plywood that's level to the ground enough to do the trick? - That's good enough for the front splitter tray, BUT keep in mind that if it's designed correctly and sees high speeds, that piece of plywood needs to support several hundred lbs of downforce without ripping off.

For the diffuser the angle is pretty critical - it needs to be something like 14 degrees or less.

Also, the diffuser won't really give you downforce it's actually to reduce drag. What you want is a small, smooth tunnel under the car, enclosed by side skirts.

I'll see if I can dig up some better sources of info on this.

In reply to DavidinDurango:

Truth. You're going to need very aggressive aero to make it worth anything in autox and it will probably not be suitable for highway driving.

I would start by looking at what the championship cars are using and figure out what you can fit on your car/fabricate. Keep in mind, you can always make your parts bigger, lower, higher, (whatever makes it better) than they can if you don't need to meet any rules.

Doesn't mean it won't work, though. Check out pictures of the Nationals. Just make it removable.

And keep in mind that the K&K Insurance speed limit for autox is 70 mph. Interstate speed limits may not be that high

If you're interested, I have a good article on practical racing aerodynamics, which covers the basics of DIY splitters, spoilers, air dams, wings, etc... Shoot me a PM and I can send it to you.

I think it's fair to say that many of us want to see that article.

Keith wrote: I think it's fair to say that many of us want to see that article.

+1

Keith wrote: I think it's fair to say that many of us want to see that article.

Um, yeah. PM on the way.

DavidinDurango wrote: Does "aero" really do anything at autoX speeds? I would think one would need to be at freeway speeds or higher to get much help from aero tack-ons. Yes? No?

Yes, if the wings are big enough and the car is light enough. That's the entire point of A-mod http://www.crowmountainhillclimb.org/entries/GeorgeBowland.htm

I can say for sure that areo made a difference with my autox only car,much better grip overall and far more stable.Before the wings when the rear stepped out you needed to deal with asap and it didn't recover grip very quickly,with the wings you can now stay on the throttle pretty hard still drive it. On board examples,

Pre wings; http://youtu.be/r_opHgKNnfU

After wings; http://youtu.be/HNpzN9iLb2E

Keith wrote: I think it's fair to say that many of us want to see that article.

Anyone who is interested, just shoot me a PM and I'll be happy to email it to you. I'd rather not post it, just in case there's any IP/reproduction issues.

PM sent. I am working on Aero for my Stalker. This is very timely. Thanks Johny

Duct tape and cardboard. Experiment and see what seems to work. More than strong enough for the job at ax speeds.

At ax speeds, I think you lose more due to friction than you will gain from downward force.

Look at some of the ecomod pictures. Slippery is faster.

Actually, Fox, I don't think either of those two statements are true. Cardboard and duct tape won't be stiff enough to provide any benefit, and power easily overcomes drag. I think thousands of aero-equipped autocross cars have proven the concept, even at relatively low speeds. Look again at Kevlar's videos above; the difference is obvious.

Totally non scientific but seat of the pants logic is that I think that anything that'll give down force on the rear and improve grip on my car has to help. My Stalker has 300+hp in a 1600 lb car and rear wheel grip has been hard come by. 13x10 race slicks have helped a lot but even a 5% improvement will translate into 1-2/10ths. Well worth it in my view. The HP can easily overcome the drag, something that might not be true in a momentum car.

Depends entirely on the cardboard you use.

Keith wrote: Doesn't mean it won't work, though. Check out pictures of the Nationals. Just make it removable. And keep in mind that the K&K Insurance speed limit for autox is 70 mph. Interstate speed limits may not be that high

I've heard different "rules" about the speed limit at a-x's ... this one and, supposedly from the SCCA rule book... speed limit: for a prepared car ( I'm assuming they mean one of the prepared classes) the suggested top speed to be held to under 100k/h.... ( ~ 62mph) ... in my STS prepped CRX I've topped 70mph... several seconds of 3rd gear

so question... is there an actual speed limit ...?

wbjones wrote: so question... is there an actual speed limit ...?

I believe that's a guideline for the course designers. Doesn't matter how much power you have, you still can't beat V^2/R.