The following is a somewhat edited excerpt from a Facebook conversation we were having with a track driver looking for some input on adding aero components.
Everything being discusses here is pretty well universal in application. “A car is nothing more than a box on wheels with a slight variation of how the wheels are mounted to the box.” -Brian Matsuda/BMSPEC
As much as possible you want to balance front and rear downforce. If your aero kit is matched up properly, removing any one component should/will naturally upset the balance of the entire vehicle.
You should have a fairly well sorted car at low speeds below 80-100km/h, and your handling at below these speeds is *almost* independent of your aero setup. I say almost because every little pound of downforce accounts for added grip, and downforce begins at 1km/h... So.....
Power level only becomes a concern at very high speed/open style tracks. Take a look at the BMSPEC Circuit Heart Time Attack car that was running ~120hp Mazda3 with full aero producing ~1000lbs of downforce at 160mph. We’ve worked with BMSPEC to get it to a much higher power level for the larger tracks they intend to race at.
Before you can really begin to add aero, you probably need to address your suspension set up. I'm not sure what spring rates you're running on those xxxxxxx’s [brand name redacted for obviously reasons], but I can bet they're not stiff enough for a proper aero package. To be completely honest, I'd be hesitant running xxxxxxx’s on an aero'd car whatsoever... The lateral g's that you will eventually be pulling will likely break them. That's slightly less of an issue, and more something to be address if/when they actually do break. My recommendation is Bilstein or BC Racing. Zero affiliation or sponsorship by either, just hear great things about both, and I happen to currently run Bilsteins as well. BC’s are extremely customizable and modular, and will allow you to easily make changes to you setup over time.
The following is spelled out with zero regard for where you may be in this process. If you're well advanced into the driver mod portion and setting up the car, then you can skip right to the end and bolt on some aero's! But regardless, hopefully there is some value to what I've said below:
Depending on what your goals are, I would fix the handling problems without aero FIRST. Adding aero is otherwise just masking inherent setup problems. Set your baseline lap times, get some consistency in your racing line and your lap times. Compare your lap times to other drivers in similar cars, or have a skilled driver take your car out and see what they can put down, and any setup suggestions they have. Their lap time becomes your new baseline. Get out and aim for that. Invest in a lap timer app like Harry's lap timer, or one of the other ones that allows for video overlay and segment timing.
Once you start maxing out what the car can do without aero, THEN put together a design plan and a proper aero package. You now have a well developed chassis, with a consistent benchmark time, and you can constantly compare your overall lap time and your segment lap times to that benchmark. If you're faster great, if not analyze why and make appropriate changes. Take tire data all along the way. Know your ideal hot and cold pressures, your camber and toe adjustments for good heat spread across the tires. Watch how that changes when you add aero, and make adjustments as appropriate.
Find someone who is willing to invest some time into coaching you with your setup and your driving. Experienced drivers are often the best place to start asking questions, regardless of what they drive. Find yourself a mentor for every aspect of racing. If one guy is great with tires and alignments, talk tire data with him. If another is awesome with driving lines, do some ride alongs with him and get him into your passenger seat.
And then there are loads of people like myself who just get excited about other people racing at grassroots 'entry' levels, and will happily give advice and recommendations provided you're doing the homework and understand the questions being asked.