Corvette Z06 GT3 Part 3
Balance
Corvette with slight "fast" oversteer in Rivage
When a car understeers, the driver gets to see what he is about to run into. When a car oversteers, the drivers cannot see what the car is about to run into because they are going backwards. In either case, excess understeer or oversteer is BAD.
In essence, understeering is caused when the side force provided by the front tire's grip is less than the side force provided by the rear tires's grip. But the term understeering is even easier to explain from a driver's standpoint with an understanding that a car that has understeer is requiring more steering than should be needed--it is "rotating" less than it should. Oversteering is when the car is "rotating" more than it should. A more technical term is "slip angle" which is the angle the tire's travel differs from where it is pointed. When the slip angle is higher on the front tires than the slip angle on the rear, you have understeer. In order for a tire to produce a sideward force, there must be a slip angle.
Every race driver needs to train diligently to become hyper sensitive to both of these conditions of understeering and oversteering. And the car will have different levels of understeering or oversteering in different corners and will depend a lot on what input the driver applies and when the input/s are applied.
The goal is to achieve a car that is "balanced" with not too much understeer or oversteer-especially in the corners leading to long straights.
Drivers should learn that they can partially overcome or correct oversteer or understeer by changing their inputs and the timing of those inputs. This is not ideal, but is better than being seriously slow because of a slow set up or than running off track or spinning out of control. The most common way to compensate for understeer is to trail brake further into the corner. The most common way to compensate for oversteer is to brake earlier and end trail braking earlier and avoid early downshifts—and to delay throttle application slightly on exit.
They can also make adjustments to Brake Bias, Rear Wing. ARB,s, T/C and Anti Lock Brake settings in the cockpit. (see Part 2 pictures)
Tire Pressure
Here you are dealing with iRacing’s infamous “Tire Model” that attempts to replicate the real world. In this respect, sometimes reducing tire pressure will increase grip, sometime it does not. Sometimes increasing it will increase grip. Start with the pressure in the “canned” iRacing setups and experiment.
Toe out/Toe In
Most race cars including the Corvette perform best with Toe Out on the front tires and Toe In on the rear tires. The Toe Out in the front is where the tires point outward toward the sides. This helps the car "point" on corner entry by providing an immediate higher slip angle on the inside tire at the beginning of turn in--recognizing that the inside tire has to turn thru a smaller radius.
Most race cars including the Corvette perform best with Toe In on the rear tires. This tends to increase the slip angle on the outside rear tire on corner exit as it increases sideward thrust.
If the car is understeering on corner entry--try increasing Front Toe Out. If the car is oversteering on corner exit-try increasing Rear Toe In.
Both Toe In and Toe Out increase drag and slow down the car and wear out tires--too much is bad.
Camber
Negative camber is where the tires lean in at the top. Negative camber generally makes the tires produce more side thrust--a good thing. But there is a limit and excess camber is counter productive. Too much camber leads to the inside edges of the tires overheating--especially during braking. Camber reduces the size of the contact patch between the tire and the track--making that part of the tire work harder, causing overheating. On formula cars there is air flowing over the entire face of the tire, but in GT cars the inside edge of the tire gets less air flow--overheating is worse. Camber needs to be only enough to allow the largest possible tire patch size during cornering and no more.
During qualifying, tire overheating is less of an issue. But during a race it can be serious. Most iRacing 'canned' setups have too much camber.
Testing has determined that traditional concerns regarding tire overheating is not an issue at track temps below 100F, so maximum Negative camber is fastest.
Caster
Positive Caster pushes the outside tire down when turning. This action transfers weight to the inside rear tire reducing understeer. More positive caster reduces understeer and increases oversteer. Unfortunately, iRacing does not provide for adjustment of caster on the Corvette. I am sure it has a lot.
Rear Differential Setting
The more clutch plates and the higher the preload, the more force is applied to make both tires rotate together with less "differential" action. This is a good thing in that the power is better applied to both tires and there is more force pushing the car out of the corner-especially as the steering is almost straight. It can be a bad thing in that high diff settings make the car have understeer on corner entry and corner exit.
Conversely, if the driver becomes accustomed to this differential caused understeer and has developed a habit of overcoming the understeer with early downshifting, late heavy trail braking, low Brake Bias and excess early throttle application--then getting into a car without the excess differential action will result in annoying spins,
The ideal differential setting would be enough force to reduce wheel spin on corner exit without any significant understeer through the corner. One important note: with low differential force, it is important to keep the inside tire firmly on the ground as the differential will not transmit much power thru the outside tire.
Anti Roll Bars
The Corvette has a cockpit adjustable anti roll bar in the front and rear.
The numbers are not intuitive-the higher the number, the lower the stiffness.
An anti roll bar causes the inside tire to be lifted by the outside tire during turns when the outside tire is pushed up relative to the body. Another way of thinking is the bar allows the inside spring to assist the outboard one. On the front this transfers weight to the inside rear tire increasing understeer or reducing oversteer.
So a stiffer front bar (lower number) makes the car understeer. A stiffer (lower number) rear bar makes the car oversteer.
To make sense of the numbering, think the higher the number the more grip. So a high number in the front increases front grip reducing understeer. A higher number in the rear increases grip reducing oversteer.
Rake and Ride Height
Generally speaking race cars are fastest when they sit as close to the ground as possible without hitting the track or curbs. Ride height depends on springs selected--the weaker the spring the higher the ride height at rest and the further it drops when subjected to aero loads and weight transfer. So be sure the car does not hit the ground.
Rake is the front to rear incline. Changing rake by lowering the front will tend to increase front downforce and the total downforce at the same time. Increasing aero downforce always comes at a cost producing more drag.
Finding the Optimum Settings
Run the car 4 laps (1 out plus three complete) at speed and check tire temps and wear. 20C, Mostly Cloudy, October 1 in Northern Hemisphere at 10AM. The inside of the tires should not have worn in excess of 1-2% more than the outside on the front tires and 1% on the rear tires.
The average temps of the front tires should be only slightly higher than the average of the rear tires. If the front tires have an average temperature more than 10 degrees hotter than the average of the rears, then the setup and driver's actions are causing too much understeer.
Average Front Tires 171.5 F Average Rear 160 F 11.5 F Difference=Excess Understeer
27 F difference inside to outside on Fronts 98/95=3 % delta (too much camber)
Average Front Tires 168.3 F Average Rear 160 F 8.1 F Difference=Safe But Understeer
23 F difference inside to outside on Fronts 98/96=2 % delta Good for GT car.
Keep in mind that this 4 lap "Test run" is for illustration purposes and a guide. Running more than 4 laps on a track that is hotter will yield different figures. The goal is to avoid camber settings with an excess difference in temperatures-inside to outside (but still alllowing some difference) and to "measure" the relative understeering tendency.
The iRacing "high downforce sprint" has considerably more understeer than the 55PLUS V3 setup and the front tires are more likely to overheat making handling less consistent. V3 was faster than the iRacing setup. Understeer is usually slower.
The V3 setup was designed to be "easy" to drive. Other setups with less understeer would be faster.
The next Part will deal with Dampers and Springs. Both of which are important. They tend to affect the way the car transitions from straight to corner entry to apex to corner exit more specifically to individual corners than the overall performance of the car over the entire track. Springs and Dampers affect how fast a car transfers weight in corners and when changing from accelerating and decelerating during braking and throttle application.
