VN #3: Aero Balance Musings

I’m going to aero the shit out of Veloster N. This will require serious head scratching at first, and then some DIY fabrication, followed by a lot of testing at low- and high-speed tracks. I expect this process to take the entire summer, with alternating gains and setbacks along the way. But I’ll eventually nail a satisfactory aero package, it’s just going to be a process to get there. So let’s start with the head scratching.

First the stats: I figure my VN weighs about 3250 lbs with me in it, and has about 64% of its weight on the front tires (2080 lbs). The rule of thumb is that aero balance should match chassis balance, and that means a 64/36 split on aerodynamic downforce. This should preserve the car’s natural handling characteristics at all speeds. As a practical matter, this means going after as much front downforce as possible, and balancing that with about half as much in the rear.

I want to figure out what that will do for the total grip of the car. I’ll start with a body that has 0.3 coefficient of lift, and assume that’s evenly distributed (in reality, it’s mostly in the rear, but I’m just doing some rough figuring here). I’ll estimate a splitter gives about 40 points of front downforce (that’s -0.4 Cl) and balance that with 20 points in the rear. That’s not much rear downforce, and I could achieve that with a very small wing, or more likely a spoiler.

Splitter and spoiler add downforce while keeping similar chassis balance.

I want to see what this aero package will do for downforce and grip at different speeds, so I’ll do some rough calculations based on tire grip being proportionate to weight (it isn’t exactly, but these are rough calculations).

MPHFront DF Rear DFFront weightRear weightFront aero balanceTotal grip
Aero balance same as chassis balance

The results show that the car will gain grip at speed and that the chassis balance will stay pretty much the same. In a 80 mph corner, this would give about 3.4% more grip, which is great.

But I don’t subscribe to the theory that I should balance the aero the same as the chassis. I’ve implemented exactly the opposite of that on my brothers Yaris race car, and had great results. Let’s see how that works out mathematically.

Dynamic rear aero balance

I like a FWD car that is loose in slow corners and tight in fast corners. This makes the car easy to turn in hairpins, neutral in the majority of corners, and stable in high speed kinks. This handling magic isn’t difficult to do, just set up the car to oversteer and use a big wing to shift aerodynamic balance rearward at speed.

There are numerous ways to get a FWD car to rotate easier.

  • Reduce front roll couple – Softer front springs and/or softer front sway bar; Harder rear springs and/or stiffer rear sway bar.
  • Tire grip – Reduce front tire pressure or use wider front tires or a softer tire compound. Or increase rear tire pressure, narrower rear tires, reduce rear, use harder tires.
  • Add rake – Make the car lower in the front or higher in the rear.
  • Alignment – There are numerous ways to use camber and toe to increase front grip and/or reduce rear grip.

So assuming that I can get the car handling the way I like it in low-speed corners, let’s see what happens when I spec a properly sized wing. In this scenario, I’m going to add twice as much downforce on the rear as the front, and this is about what a typical splitter and wing setup will do.

Wing makes a lot more downforce and shifts balance rearward at speed.

Just like before, I want to see how much grip the car has at different speeds. More importantly, I want to see the aero balance of the car, how much weight moves rearward, and how quickly it shifts.

MPHFront DFRear DFFront weightRear weightFront aero balanceTotal grip
Rear-biased downforce.

What the math says is that the aero balance will change about 2% between slow corners and fast corners. That’s like putting 200 lbs of luggage in the trunk, and you’d think that would be noticeable. But the weight shift doesn’t happen that suddenly, and for most corners, I doubt I could feel the difference. In an 80 mph corner, total grip has gone up to 8.1% with the wing, which is a lot better than with the spoiler. But can I use all that grip, or will the car just push like crazy?

Hopefully it works out like this:

  • In slow corners, the car rotates easily; there should be no noticeable change in rear grip or front aero balance.
  • In medium-speed corners (80 mph), aero balance should shift about 2% to the rear. This is where I want the car to feel neutral.
  • In fast corners, aero balance should shift about 3% rearward. I’d like the front tires to be pushing a bit, although I doubt a 1% increase will do that. Maybe I need a bigger wing for that to happen.
  • At top speed, aero balance should shift about 5% to the rear. I won’t be cornering at this speed, but moving weight, drag, and the center of pressure rearward should help stability when braking.

Now that’s a lot of shoulds, and I don’t usually go in for this kind of rampant speculation. But FWD hatchback aero is mostly unfamiliar territory, and I want to explore the numbers before I build anything. Also, nerding out on this shit is fun.

No splitter, just a wing?

Splitters are a pain in the ass on a street car. In order to make downforce, they need to be close to the ground, and since they also protrude forward, they run aground. The fact is, splitters are just not practical for a daily driver.

So what about using all of this previous conjecture (set up a car to rotate in slow corners and move aero balance rearwards at speed) and applying it to a street car? Could this be as simple as not using a splitter and simply adding a smaller rear wing?

Yeah, it should be. In fact my first project is a DIY spoiler and small wing that attach on the same brackets. I’ll test them back-to-back vs the OEM wing/spoiler and see how they perform.

I was hoping to do my first real-world test this weekend at Grid Life, but it’s going to be wet, and I’m behind schedule on DIY projects. Not a huge loss, I have a lot of dates at WGI this year, and I can do the high-speed testing at the end of May.

Before that I’ve got a track day at Waterford, and I should be able to bring some rear aero options. It’ll be interesting to see if a small amount of rear downforce alone helps at a small track.

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