Commodore’s Garage #27 – Related Adjustments

In the last article we looked at the process for initial chassis setup.  To recap, we’re trying to replicate what a real-world team would do on a pull-down rig in the shop to get the car essentially race-ready by the time it arrives at the track.  This involves cycling through various spring options to find a package we can use as the base for our chassis setup while leaving virtually everything else on the car alone.  The first stage of the setup process is not really a search for handling so much as it’s a search for the platform to work off of.  We want the car off the track, we want a good aero platform for the track we’re racing at, and we’re looking for responsiveness with adjustments.  In the best case scenario, a shock adjustment of just two “clicks” should be enough to produce some kind of handling change felt by the driver.  If you’re not at that point yet there’s no need to worry, we’ll eventually get there!

Beyond the initial spring package, there are a couple of adjustments that we can look at right now that can dial in your car a little bit beyond the baseline built from last week’s article, but these are going to be largely dependent on other characteristics in the car, and even the track itself.  These are relatively simple adjustments, but are almost always changed when other things in the setup are changed.

Ballast and Nose Weight

A few of the cars in iRacing’s road racing stable and the majority of the cars on the oval racing stable have an adjustment known as “Forward Ballast” which is a direct adjustment for the car’s nose-weight percentage. This adjustment is simply blocks of Lead or Tungsten that can be moved forward or rearward to tune the car’s handling. Every car builder has their own unique way to represent this in their chassis setup sheets, whether it is exact locations on the car in terms of a measurement or something as simple as drawing the location on an outline of the car.  Our garage represents the adjustment in a measurement of length relative to some arbitrary point in the car.  This point’s location is unique to the car, and largely unimportant in the grand scheme, so we’ll discuss any ballast adjustments in terms of the car’s Nose Weight percentage, or the amount of weight situated on the front tires, not the value of the ballast location itself.  Knowing the percentage instead of the location is extremely important when transferring knowledge from one car to the other or whenever a build update changes the car.  For example, if Nick Ottinger wants to borrow notes from my NASCAR Class B car for his NASCAR Class A car, we can’t use the ballast location adjustment at all since they’re totally different.  Nose Weight percentage, however, will transfer between the two cars.  Plus, the ballast location changed at the start of the Season 3 build for the Class B cars, so knowing the percentage was helpful in getting the car dialed in again following the update.

Nick Ottinger’s Pocono NPAS car used information from my Class B Xfinity car, especially the nose weight percentage. Since ballast values don’t transfer between the two cars, we needed the Nose Weight percentage to relate the two cars.

We looked at what the ballast does in the third Commodore’s Garage article, which can be found here:  https://www.iracing.com/commodores-garage-3-centers-of-mass/.  The quick refresher:

• Ballast location controls the front-to-rear location of the car’s Center of Mass. More forward ballast (higher Nose Weight percentage) shifts the CoM forward, and less forward ballast (lower Nose Weight percentage) shifts the CoM rearward.
• For most applications, higher Nose Weight percentages will increase the weight on the front tires and create more understeer. Lower Nose Weight percentages will increase the weight on the rear tires and create more oversteer.
• This effect flips at very low speeds, usually somewhere under 50mph or 80kph unless the car is very efficient with aerodynamics, but the transition is very mild if it happens at all.

Since we’ve looked at aerodynamics and spring packages by now, we can dive deeper into this setting and how it will affect the car in more detail.  For almost every track, our best performing car will be one that has very efficient aerodynamic properties.  It will create a lot of downforce without a lot of drag, causing mechanical grip and suspension settings to become an afterthought in some cases.  For many cars this results in a bias towards more rear downforce for traction and stability, but it can sometimes be the opposite depending on the driver.  Regardless of how the car is set up aerodynamically, the aero balance is never 50-50 and one end of the car will always have more downforce than the other.  During tuning for aerodynamic balance, the ballast is used to counter the inevitable aerodynamic imbalance by generating mechanical under- or oversteer to compliment the effects of aerodynamics.  Essentially, if we have a lot of front downforce the car will turn extremely well but could be unstable, so ballast can be moved forward to impart stability while keeping the aerodynamic handling effects present.  Similarly, a car with a lot of rear downforce may see the ballast moved rearward to impart some mechanical oversteer.  This allows the car to turn, mechanically speaking, while the aerodynamics will keep the rear tires stuck to the track.

All of this considered, the Nose Weight of the car has a direct correlation to how much rake is in the chassis and how high the splitter or valence is run above the track.  In one of the few “if this, do this” adjustments in the entire garage, Nose Weight should be increased with added front downforce and decreased with more rear downforce if the intent is to keep the handling characteristics the same.  Doing this results in a change of the car’s aerodynamic efficiency, not necessarily the handling itself.  When either downforce or Nose Weight is changed independently of the other, this will result in an immediate handling shift towards either under- or oversteer.  A common adjustment for oval cars for qualifying is to add grille tape, which increases front downforce, so Nose Weight would need to be increased to compensate and keep the handling from shifting too far towards oversteer as a result.  This combination of adjustments, if made together, would keep the car’s handling the same as when the tape was set in race trim, but would decrease the drag acting on the car.  Having less drag with no change in handling is free speed!

Track Bar Height/Angle

A more oval-specific adjustment, the track bar angle is usually set to agree with the amount of rear end travel.  Ideally, the track bar should be perfectly flat and parallel to the ground when cornering for aerodynamic reasons, so the bar needs to be set to wind up at that point by using a difference in height (often called “rake”).  Currently, all of iRacing’s oval cars that use a track bar have the left-side mounted to the rear end housing with the right-side mounted to the car’s frame.  If we’re going for a flat bar in the corners, this means the right-side will need to be set higher than the left-side since that will be the end that moves in travel.

Click for larger image!! This trace shows the right-rear height trace from my Class B Car at Iowa. This value can be used to set a starting point for the right-side track bar height when building a new setup.

The amount to have the bar rake initially set to can be found through a quick telemetry check by checking the right-rear ride height in the corners, which I’ve shown in the picture above.  Here I’ve isolated turn 1 at Iowa and can see that my lowest right-rear ride height was 5.85”.  I started the right-rear corner at around 7.75”, so the rear is dropping just under 2” in the corners, meaning I should start the track bar with a 2” rake and work from there.  This is rarely going to be the setting with which you race, but it’s a great starting point since having the bar flat in the corners not only aids in aerodynamic performance, but also reduces the jacking force amount from the bar itself.

It should be noted that this does often have some exceptions where this isn’t the ideal way to set the track bar.  Typically, you’ll find the most performance with this configuration at a very fast track, especially large speedways.  As the track gets shorter, speeds reduce, and grip level decreases, the jacking forces from the track bar can be used to both keep the rear end up and keep weight off the right-rear tire and on the left-rear tire by simply running the right-side of the bar lower than the left-side.  This does hurt aerodynamics, but the added grip can be very beneficial at slower tracks.  The other major exception would be Daytona and Talladega, where running the most rake that is legally allowed will help to pull the back of the car down out of the air.  If you ever get a good look at a NASCAR Cup or Xfinity car upside-down at either of these tracks, take a good look at the track bar and you’ll see it way up in the back of the car with the right-side as high as possible.

The relationship between the right-rear ride height and right-side track bar setting ultimately links the track bar height directly to the right rear spring rate, so these two settings need to be considered together when making spring adjustments on the right-rear corner.  If you soften the right-rear spring, it can result in extra travel at that corner, requiring a higher static right-side track bar height.  Conversely, if you run a stiffer right-rear spring, the track bar needs to start lower due to less rear travel.  Of course, these are simply starting points, so adjusting the track bar is necessary while tuning.  This is also the reason why a lot of people choose to run the same (or very similar) rear spring packages at every track in the Class B and Class C oval series.

Other Linked Settings

There are a lot of garage settings that will rely on another specific adjustment, but most are far less strict than the two I’ve gone into detail with.  Here’s a quick list of a few things that need to be considered together when setting the car up to make sure you don’t work yourself into an unsolvable problem down the road:

• Bumpstop rate and Packers: Changing the bump stop/spring rate will always require a change in the amount of packer shims you have installed.  Stiffer springs will keep the front end up in the air, while softer springs will allow more travel.  Keep that in mind when changing bump stop rates.
• Rear Springs and Ride Heights: Rear spring rates will change how far the rear end will travel so the heights need to be changed accordingly.  Since you’re typically looking for the same ride height at full travel, the starting height will need to be raised or lowered when you soften or stiffen the rear springs.
• Rear Spring Split and Crossweight: Higher rear spring splits will result in less force on the left-rear tire, especially on corner exit.  With the higher split, crossweight will also need to be increased to keep the car stable on exit and entry.  The opposite is true for lower spring splits.

Final Thoughts

The truth that nobody wants to hear is that everything in the garage will, somehow, affect something else on the car.  It’s for this reason why you need to be aware of what you’re changing in the car and how it will affect the rest of the car to keep from unintentionally adjusting multiple things at once.  We’re all told to “adjust only one thing at a time” and this often means being aware that one adjustment changes multiple things.  It’s very easy to make a few small adjustments and completely ruin the car, and in many cases it can be traced back to simply not keeping up with how the car is changing before it ever hits the race track.
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To keep up to date with The Commodore’s Garage, return to Sim Racing News every other Friday afternoon and “Like” our page at https://www.facebook.com/CommodoresGarage

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