Commodore’s Garage #23 – Sideforce & The Gen6 Update
March 31st, 2017 by Matt Holden
I’m not sure if you guys have noticed, but lurking in the shadows of the dirt update was a small update to the NASCAR Gen 6 Cup cars. In addition to a reworking of the aerodynamic behavior of the car itself (2017 butter-knife spoiler!!), the suspension was updated with new, tighter restrictions for the numbers in the garage. These all seem like updates to bring everyone closer together in terms of setup, but they actually eliminate the massive setup exploits that have been going on in these cars for years. The spring limitations eliminate the coil-binding that effectively eliminated bump-stop setups in lieu of something less complex. The sway bar limitations eliminate the pre-binding that became a cool fad in 2016. Superspeedways now have the ride height requirements that exist in the real-world at Daytona and Talladega. The track bar and toe limits? Well that’s a little more complex, and has possibly made the most drastic change to the car above all the other changes.
I’ve worked with Nick Ottinger in what is now the NASCAR PEAK Antifreeze Series since 2012. It should be no surprise to anyone that the setups necessary to win one of these races (let alone lead the race) required a bit of trickery. In my eyes, there are two types of setup “tricks”: 1) Tricks that get around a garage-page tech failure, 2) Tricks that aren’t governed by a garage-page tech failure.
An example of #1 is the NASCAR Truck stagger trick back when the truck was first released. Early in the iRacing days, you could turn your steering wheel in the garage and see how it would affect the car. By working with the truck’s stagger adjustment (which was promptly removed), you could turn the steering wheel so that a car that was too low would clear the garage ride heights, save the setup, and start the race. This allowed the car to be extremely low on-track and didn’t follow the ride height rules set in the garage.
The second trick is what was going on to warrant the track bar and rear toe limitations. Before the update this week, you could set the track bar to 6” on the left side and 15” on the right side and milk a few extra 1/16” of rear toe. Back in the COT days, it wasn’t uncommon for Nick’s car to have 5 or 6/16” of rear toe for qualifying. With the Gen 6 cars, he ran 3/16” everywhere. The speed was easy to add into the car, but the car was unbelievably difficult to drive. The problem was that there was no pass/fail check on any of these settings in the garage.
Do you remember the High-Drag package that was used in 2015 at Indianapolis and Michigan? It included a 9” spoiler and a very long splitter, which generated (what NASCAR assumed) to be a large amount of downforce, a huge wake, and should have produced more passing. It was a failure…at first. It wasn’t until NASCAR got the Indianapolis cars back to the R&D center that they discovered these cars produced more downforce in traffic due to the lack of air going over the lift-generating greenhouse. Still, the drivers couldn’t pass at all despite the large wake and massive amounts of downforce.
The reasoning behind the lack of passing was sideforce, something NASCAR hadn’t really looked at but teams knew a lot about. While downforce is applied downward on the car, sideforce is applied in the direction towards the inside of the corner (left, in this case). More sideforce means a higher cornering speed, and the taller spoiler produced a ton of sideforce due to the rear-end offset. Another way to get more is through rear-end skew, either achieved by extra rear toe or a higher track bar angle. In our case, we did both.
NASCAR has since implemented a track bar rule in the tech inspection area. All cars must pass through tech with the track bar parallel to the ground, or even on both sides. If I ran a car through tech with the track bar at 7.5” on the left side, I have to lower the right side to 7.5” to go through tech. To meet NASCAR’s zero-toe rule, I’d also need to change the length of the track bar to center the rear end housing. NASCAR will then tell me I cannot change the length of the bar once I’m out of the tech line, meaning the most skew I can get out of the car will be with the track bar at even heights since the body will shift over to the right as the track bar flattens out. This reduces the amount of track bar rake (difference in bar end heights) I can put in the car and still achieve an ideal amount of rear end skew. In the sim, running the 6-15 track bar produced a heavy amount of skew, and when combined with the extra toe, a ton of sideforce and a massive drop in laptimes…if you could hang onto it.
Sideforce is a fairly counter-intuitive aerodynamic principle, and I’m not ashamed to say that I misunderstood it when I was first learning about it. Common ideas say that, as you move the rear of the car towards the outside of a corner and put the rear spoiler into the air stream, it should try to rotate back in line and tighten the car. That makes sense, right? In reality, the more the car is yawed out, the looser it will get. To the right is a simplified pressure diagram of a NASCAR-style car in a yawed situation:
Here we would have the car traveling from the right of the image to the left, so airflow is moving from the left to the right. What is often unclear is the fact that a pressure change will occur at the left-front tire as well as the right-rear tire as the car is yawed out. Our net gains from sideforce will come from the two pressure areas applying a force towards the inside of the corner and pushing the car to the left as it moves through the corner. If the rumors are to be believed, 100lbs of downforce would be worth 0.1 seconds per lap. An extra 100lbs of sideforce, however, would be worth 0.5 seconds per lap.
The next thing to consider with sideforce is what’s known as the “crossover point”. To the left is a graph of the forces acting on the car as it yaws out, with the red trace representing the rear pressure zone and the blue trace representing the front pressure zone. As yaw increases, both pressure zones will increase in force acting on the car. These two will add together and result in a lower lap time, so more sideforce is always desirable for a faster lap. In my graph (which isn’t accurate to real-world, I just threw some numbers on it for this example), we see that the front pressure zone starts to exert more force beyond 3° of yaw. This point is known as the “crossover” point, and beyond that the car will change from relatively stable to aerodynamically loose.
In the case of the high-drag package (as well as the 2014 and possibly the 2015 packages), it’s believed that the cars were beyond the tire’s maximum slip angle in the corners, but were held in place by the high amount of sideforce. In a clean air stream, this meant the car was very fast. In traffic, where no air could cleanly get to the spoiler, the car generated very little sideforce and would start to spin out due to the tires being beyond their physical limit. This is why the cars couldn’t pass, let alone follow, in the two races where the high-drag package was used. This is also what has led to the rules taking away rear end skew in the hopes of lowering sideforce amounts on the cars.
iRacing’s Gen 6 Update
With all that known and understood we can now look at what has happened to the iRacing Gen 6 car with the rear toe removed. Here is the same pressure diagram from earlier modified in the way it may have looked with our 6-15 track bar and high rear toe values:
First, these diagrams aren’t drawn to scale. Regardless, we can now see that the left-front pressure zone is much larger than the rear pressure zone, so it would behave a lot looser than the standard 1/16” rear toe settings. This also produces a higher net sideforce with a much lower lap time. Sure, it was very loose, and we had to run a mechanically tight setup, but it was so fast out of the gate that we couldn’t leave it out of the car.
This is more like what we have now with zero toe. We have a net sideforce, but now the rear pressure zone is very large relative to the front pressure zone, leading the car to feel much tighter than it did on previous builds.
Chassis Setup Changes
These sideforce changes cause the chassis setups to shift focus in a different direction. For starters, we will have a much more aerodynamically tight car than previously, so we will need a mechanically looser car to compensate. This can be found primarily with lower crossweight, but springs will need to be slightly different due to the lower downforce from the 2017 aero package. Similarly, more focus will be placed on the track bar angle to get the best skew angle you can. Another change will be a shift towards aerodynamic downforce instead of sideforce. The setups we ran in the NASCAR PEAK Antifreeze Series were awful in terms of downforce but were tremendous in terms of sideforce. The gains we got from extra skew outweighed the losses we saw from downforce and mechanical sacrifices.
You may also find that the cars handle better in traffic now without the sideforce changes to ruin your momentum. We will likely see closer racing, more controllable cars, and a better experience driving the cars.
Normally, I don’t recommend completely starting over on setups following a build update, but in this case it will almost be a necessity. Everything was changed in the update, and almost everything is going to need to be rebuilt. If you had a spring package you liked in the previous build, give it a shot in the new build and see if you can make it work. If you could never find a spring package you were happy with, now’s the chance to get it figured out.
If this is the direction iRacing is heading in for its asphalt cars, the future looks very bright. Too many times in the past we’ve seen sims defined by a single setup exploit that takes everything as far from “simulation” as possible and the developers just let it happen. This is the first time I’m aware of developers actively eliminating setup exploits that were ruining the simulation aspect of the NASCAR vehicles, while also implementing changes to bring the cars more in-line with their real-world counterparts. I have driven the new Gen 6 cars quite a lot since their release and I’m more than pleased with the updates. The cars are much more pleasant to drive, they feel more “in” the track, and setup adjustments actually produce a result. The future looks bright, and I hope you feel the same way.
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