# Post-race analysis: aero

At the last Lemons race I did a lot of lightening, which is good, but I also turned the car into a parachute and destroyed our top speed. This race we made two aero improvements.

## 1. A-pillar deflectors

This is simply a piece of plastic that bulges out a bit. The intent is to have the air go around the window instead of into it. Randy Pobst signed it.

Let’s take a look at the top speed on the main straight before and after the new “aero package”. The top speed went up from 92.0 to 95.4. It’s even more dramatic when you look at the speed trace.

## 2. Rear wing

Normally, all you have to say is “wing” but I need to emphasize “rear” here because we made an aero joke at the last Lemons race.

Note that we didn’t actually run this on track.Â  It was just to give Mario and Daniel a laugh when they drove in and saw the wing on the wrong end of the car. Here’s a picture of the same wing installed on the roof. Also, there’s a note I wrote to try to persuade Randy to take a stint in the Yaris.

So what did the rear wing do? The #1 place I felt it did something was in T1. I usually have to brake in T1, but with the wing holding down the rear end, I was able to do a brief lift and then go right back to throttle. You can see how much faster I can do T1 in the graph below (blue is with wing).

The wing also turned T8 into a straight. It’s normally a corner that gives some people a fright, but with the wing on there was no drama at all.

## Conclusions

I think the new aero worked. I guess the next question is if we can make it work better.

## 5 thoughts on “Post-race analysis: aero”

1. Let’s run some numbers in OptimumLap. I’m using your Yaris with 1.1g and 100 hp. To get a top speed of 92 mph at TH3 I need to make your Yaris a .7 Cd and -.5 lift. These are truly miserable numbers! I’ll make a guess at drag and lift with the new window covers and wing, and give your Yaris a .5 Cd and -.3 lift. And that was a good guess because I got 95.4 mph. But those are still miserable numbers. So… imagine if you did some actual aero, rather than a \$50 wing and a couple pieces of plastic over the quarter windows? With improved airflow, splitter, and wing a Cd of .4 and Cl of 1.0 should be achievable. This would net 101 mph on the front straight, and five seconds on the 3 mile track.

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1. 92 mph isn’t the top speed of the car. It’s the top speed on the main straight. Given a longer straight, certainly it would go higher. In stock form, with a CoD of 0.27 (but windows open) it would get to 95 mph on the main straight. Actual top speed is listed as 120 on some website somewhere. Not sure what that would be with windows open.

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1. Those figures were top speed on TH3, not top speed on any straight.

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2. My numbers are a little different. I put the Yaris at 2400 lbs, 1.1G, 100 hp. With 0.53 CoD and -0.3 CoL, I get 95.48 mph top speed. Changing to 0.63 CoD gives me 92.14 mph.

Agreed that 0.53 and 0.63 are both miserable, but the numbers might not be that bad in reality. Optimum Lap doesn’t simulate a human driving. Shifting takes zero time and doesn’t upset the car in the middle of a turn. So while I drive through T14/15 in 3rd gear and take 1 second to shift from 3rd to 4th, Optimum Lap takes T14/15 in a combination of 2nd and 3rd and loses nothing to the switch there or on the main straight. Looking at my data, I lose about 1.5 mph from the shift from 3rd to 4th. The 2nd to 3rd shift I don’t take is even more advantageous because the tractive force at the top of 2nd is so much higher than the bottom of 3rd. And then there’s the power I can’t get down because of the open diff. The total mph lost is probably on the order of 4-5 mph. So if I could get down to 0.4 CoD the maximum speed I can actually get to might be 96-97 rather than the theoretical 101.

While Optimum Lap is a useful tool, it doesn’t take into account real world subtleties. IIRC, Optimum Lap showed that your 1.6 Miata would be faster at Pineview with the big wing. But in fact you went slower because it affected the handling. So which aero modifications are wins and which are losses? And how much of a win are they in reality vs. theory? The only way to know is to build and test. That sounds like fun, but effortful fun. So that’s a definite maybe.

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