I thought for a long while on how to make my car faster for this year. At first I was considering adding more power, either with building up the compression, or adding a turbo/supercharger. But all those options were fairly costly, and required lots of time. I wanted to make sure my car would be ready and reliable for this season as it’s the last year our local autox venue will exist. At the end of the season the track will be bulldozed and redeveloped in to an industrial park with a little bit of residential mashed in there. So instead of power, I thought focusing on grip would be better. I don’t drive the car much on the street these days, and I have access to a trailer, so it opened up the possibilities a little.
The first and easiest way to make it faster was obvious, tires. I was pretty impressed with the BFG Rivals that I first had on the car. When they wore out, I opted for some Hankook RS4’s that I got a smoking deal on. I thought I would be OK with the drop in grip, since they would last longer and they were a great deal. I loved how they drove and thought they were great tires, but in the end I just couldn’t handle intentionally making the car slower. I considered A052’s, AR1’s, NT01’s, and RT660’s, but I figured if I’m going to trailer it and I want to see what the cars full potential is, then it has got to be Hoosier A7’s.
The second part of adding grip was aero. When I first built the car I had a pair of wings for it but they were just $100 ebay specials. The wing profiles were pretty horrible, the element spacing and overlap was all wrong, and they were pretty small as well. Maybe they had a small effect on the road course, although I doubt it, but they were certainly useless for autox. I started by looking at commercially available wings, that would actually produce some decent down force, but they were way outside my budget so then I started looking into building my own. I had planned to do fiberglass over a foam core, and settled on using an Eppler E423 profile to get decent performance while not being too difficult to make. The problem with this approach was it was going to be very time and labour intensive. One day I discovered 9LivesRacing (9LR) sells “mini-wangs”, and thought this would be a really good option. Their normal “wang” (wing) is a really nice aluminum extrusion with great performance at a reasonable weight. Because they get these extrusions in huge lengths, they end up with off-cuts up to 30” long and sell them super cheap. I pulled the trigger and ordered a dozen 30” off-cuts for less than the price of one single 6’ wing. I wasn’t exactly sure how I was going to use them but now I was committed.
I had a couple wing ideas that I wanted to explore, both through modeling and real world experimentation. I wanted to use JavaFoil (a free cfd program that can handle multi element wings) to compare the performance between a single 9LR wing, a biplane configuration, and a multi element configuration. I couldn’t blindly trust the software so I wanted to build a test rig and verify the general findings. I started by cutting a thin slice of a wing section, scanning it, and tracing it in AutoCAD.
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After running the numbers in JavaFoil, and taking them with a grain of salt, I built a test rig to mount to my truck and take some actual measurements. I would do a small scale test with just one 30” section in each configuration. I made some roller slides for the wing assembly to slide up and down on (or forward/backward for drag), purchased a 300lb digital luggage scale for the down force measurement, and used an old cell phone for GPS speed readout. My data acquisition unit was just a gopro camera filming both the scale and phone at the same time. Runs were made at a consistent speed, on the same stretch of road, same day, minimal wind, and averaged over both directions. After confirming the dual element was in fact the best configuration I decided to explore it a little more. I tried various gurney flaps and AoA up to the point where JavaFoil predicted stall. When it didn’t stall I drilled more adjustment holes and kept increasing it. At 12* beyond where the software said it would stall, the down force just kept increasing. At this point I figured I must be close, and needed to move on to the construction stage.
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