These are old articles
for previous models of iRacing cars that may have changes significantly. This
was all designed for a TNT Racing app in 2015, but is now free to the public as
a general guide. Some links may be broken and can no longer be updated.
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Setting up the Car
There are very few options in the SB compared to other high-performance vehicles. Options available include the Anti-Roll Bar (ARB), tire pressures, Ride-Height (RH), and fuel. Beginning with the tire pressures, an understanding of how pressures change while on-track and how they are manipulated by weather and track conditions will help determine the appropriate changes to be made. Colder weather reduces the temperature in a tire. As the weather becomes cooler, a static tire’s air will condense and reduce in overall pressure. Hotter weather has the opposite effect. In a real-world racing tire, it is common for crews to increase tire pressures 1PSI per 10degF of temperature reduction and vice versa. This is a good baseline to apply to iRacing scenarios. Should track temperatures increase or decrease, the racer must be aware that the tires are affected. Track temperature is truly the concern since brake heating is currently not modeled in iRacing and has no effect on increasing tire temperatures and pressures at all. Track temperatures on the iRacing service are a function of ambient air temperature and sky condition. Overcast conditions give the coolest track temperatures and clear skies give the hottest temperatures. The difference between the two is almost 30degF! With default weather, little consideration is given to the differing weather settings, but outside of default weather, changes should most definitely be made. Lower tire pressure settings provide slightly more grip, though the sidewall of the tire is less capable of handling high-force requirements. This premise is important due to the nature of each track differing in its requirements. Tracks that have predominantly high-speed corners (such as LRP) need more pressure so the tire sidewall can withstand the higher loading. Tracks with low-speed sections as the key features (such as Summit Point) should be attacked with lower pressures to account for the extra grip available at slow speeds.
When determining how much to change, one to two PSI in either direction is acceptable starting at a 21PSI baseline. Over-inflation causes the middle to be convex and under-inflation causes a concave condition. If you have lower pressure, as high speed cornering occurs, the tire in incapable of maintaining its upright optimal structure and will roll over slightly on its outer sidewall and won’t provide a good a solid slip angle range. Higher pressures during slow-speed corners are ok, but they won’t be using the full surface of the tire if too high and will also cause a reduction of optimal slip angle ranges. Wearing out the tires in the SB, with the extremely limited number of laps per race, is not an issue. Simply drive the car hard and focus on treating each tire on the car with similar wear as you drive balanced and under control. Off-track situations, lockups, and sliding the rear will increase tire temperature and pressure, resulting in slight handling changes. To cool off a tire, simply use less slip angle or brake/accelerate more gently, but only use this if you have the option to relax your lap-times. After 15-20 seconds of easy driving, the tire temperatures and pressures generally will return to normal.
A lower ride height (RH) keeps the car out of a larger amount of air passing over the vehicle. If the rear is higher than the front, the air will pass over the front and be in contact with the rear pushing on both ends, though raising the rear of the car will provide a higher percentage of downforce in an over-steering fashion. Raising the rear RH is not important just for the air over, in fact, it is often more important for the air going under the car. Some vehicles achieve around 50% of the downforce from the under-tray area and around 30% from the rear alone. The deviation of the floor plane from the horizontal is called the “rake.” Positive rake means that the rear is higher than the front and the more you have, the more turning ability the car has, sometime to the detriment of aerodynamic performance. The SB in iRacing only has the ability to raise or lower the front, but the same principal applies. If you want more positive rake, lower the front which increases the angle of the floor plane from the horizontal. Use as low a front end as possible, but if you are scraping the ground on a bumpy circuit (such as Sebring), raise the front RH enough to stop this unwanted contact. Scraping the ground results in time lost as friction forces are applied to the car and deviations in weight transfer occur as well. The significant bumps in the track must be learned by the driver so they are avoided unless required for an optimal line and the bump is manageable. Examples of bumps would include curbing if the curbing use would be detrimental to the handling of the car (weight transfer, inability to brake over the curb, or inability to use throttle over the curb and there is time lost). RH choice should be a function of scraping and rake, though there are very few tracks that require anything other than the lowest height setting.
The Anti-Roll Bar (ARB) is exactly that. It reduces body roll which adds to spring roll resistance. At its most basic, the ARB is a bar that connects the left and right sides of the car (position of the ARB can be on various parts of the suspension, but usually the lower control arm on cars with independent suspension systems). As a car turns, there is a certain amount of body roll induced. This body roll can be reduced by an ARB which adds a selectable level of resistance to the amount of body roll. The purpose of the suspension is to maintain maximum tire contact with the road. Body roll, which reduces the suspensions ability to keep the tire parallel to the racing surface, actually tilts the wheel away from the corner direction. The ARB keeps the tire in a more parallel direction though it has its consequences. The ARB limits the tire tilt, keeping a larger contact patch on the racing surface which gives more grip to that individual tire (the outside tire). The trade-off is the inside tire is now reduced in its ability to push down on the racetrack as it is slightly lifted up (less load/weight on the inside tire). The optimal scenario is an ARB which improves overall front or rear-end traction by harnessing more available load on the outside tire while allowing the inside tire to remain connected sufficiently to the road and handle surface imperfections. Too high an ARB and the inside tire is incapable of independent response to road imperfections causing a front-end push or rear-end loose condition depending on which end the ARB is installed or manipulated. To get the grip level just right, track time and a little telemetry analysis helps tremendously. It is easier to find the grip level on-track as the telemetry output can be confusing or misleading at times. Looking at telemetry (Figure XYZ), a driver may be able to spot a situation where the ARB is too high resulting in the lifting of a tire off the ground. In this instance, this might actually be preferred in some high-performance cars as the loading on the outside tire compensates for this inside wheel lift, but this is a generally undesirable situation. Look instead for a loose or tight feeling to the car and work to improve traction in the front or rear as desired.
The SB only has a rear-end ARB. Tuning this to your liking is a matter of understanding the ARB and feeling its effect on-track. A lower rear ARB works great for slow-speed turns requiring a greater independent suspension effect while a higher rear ARB works well for high-speed turns as the downforce created on the rear helps to push the car down and wheel lift is less likely. In practical terms, generally a higher rear ARB causes a tight-in, loose-off sensation and vice-versa in the SB.
Simple weight of the fuel is enough to reduce times. Programs like ispeed
perform fantastic fuel burn per lap calculations to help the driver realize required fuel load per race length. Simply multiply fuel burn by lap number to calculate total fuel requirements. As the fuel burns off, depending on the location of the fuel tank, a car will develop slight handling changes. Since the fuel tank in the SB is in the rear, the driver can expect a slightly lighter rear-end sensation the longer the run goes. Compensating for this change in feel is important as the driver will notice a loose condition developing.