Quote:
Originally Posted by dogbone
I have a 67" GT250, along with the APR front splitter that extends 5" from the bumper. The 5" length was chosen for time attack rules. (I also have a generic lip filling in the gaps between the bumper and splitter.)
May I ask why you chose the air dam over the splitter? The 67" wing provides significant downforce. Since you went with such a large wing, I'm assuming you wanted solid downforce in the rear, but I'm concerned that you won't use all of the wing's capabilities because you can't balance out the front.
For awhile, I drove on track with just the rear wing without any front aero at all and while driving on rounders at speeds over 80mph, the understeer was incredible. Literally turning the steering wheel had no affect. The splitter did a nice job to balance it out overall, but I still find myself wishing for even more front grip in fast rounders. So, I'm concerned that the air dam just won't balance out with such a large wing.
It is possible to dial the wing back to some extent by reducing the angle of attack (AOA), but once you get a taste for that rear being planted, you just don't want to give that up.
Measuring the AOA has to take into account that there's a Gurney Flap sticking up on the rear of the wing that will affect all measurements. APR said to lay some kind of straight edge from the rear of the wing to the front and measure that angle and then subtract 4º to account for the Gurney.
Here's how I measure it:
-try to have the car sitting on as flat ground as possible
-I use the top part of my floor jack handle as the straight edge, and place that from front to back on the wing as perpendicular as possible.
-The iPhone can measure angle with the built in Compass app. If you go to the Compass app and swipe to page 2, there's an inclinometer. I place the phone on the jack handle and measure the degree of angle of attack. Whatever the number is, I subtract by 4 and that's the angle of attack.
It's been a long time since I adjusted mine, but I believe on my car, the iPhone shows somewhere between 8-9º. Subtracting 4 for the Gurney means I'm running around a net of 4-5º AOA.
But again, I have a sizable front splitter. If you end up running only an air dam, you may have to run less AOA to get it to balance out.
Ultimately I love the APR aero setup. The car sticks like crazy compared to non-aero. My times instantly dropped by 2-3 seconds at every track once the front aero went on. To me, that is an amazing result. I was very worried that with the full front splitter that I would rip it off the first time I slid off the track into the desert. I am happy to say that two years later, I have never damaged the splitter even with at least 2 dozen offs into Southern California deserts. I even slid into the rocks at Laguna Seca at the Turn 2 hairpin and the splitter lived to tell the tale. Of course, splitters that run longer and sit lower are more likely to rip off during landscaping excursions.
If you don't have a relationship with a shop that can give you specific aero advice, Racewerkz Engineering gave me very solid advice. Casey @ Racewerkz designed the cutout shape for the APR splitter that sits under the E9x M3.
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This explanation is amazing, this should help me out a ton as well for this weekend at SOW. I just installed a 61" GT-250 and had APR build be a custom splitter that extends past my dtm style air dam. Only difference is this is on my e30 but hopefully the rules still somewhat apply
