Differential

The differential controls how much the left and right wheels are free to rotate independently.
In the words of Dallara, differential and weight distribution are the most important setting controlling the mechanical balance of the car.

In simplified terms, imagine a car going through a corner. The outside wheel needs to cover more distance (larger radius) in the same time of the inside wheel, and thus needs to rotate faster. Forcing both wheels to rotate at the same speed (locked differential) is not ideal for cornering.
Conversely, imagine a car braking hard into a corner. The wheel with less weight load will have a tendency to lock earlier; connecting it to the other wheel (forcing it to rotate at the same speed) helps prevent locking.

There are two independent settings: coast and drive.

Drive

• Controls the behaviour of the differential on-throttle (under acceleration).
• Higher values (eg 80) cause more understeer when cornering, but also less wheelspin and thus more control on corner exit.
• Lower values allow higher cornering speed but may lead to spins under acceleration (corner exit).
• Wheelspin on exit can also increase tyre temperature throughout a race.

Coast

• Controls the differential off-throttle.
• Higher values (80) increase cornering ability off-throttle (corner entry) but decrease stability under braking.
• Lower values (30) increase braking stability and efficacy (think ABS, in a way) at the price of reduced cornering on entry.
• Very low values may lead to snap oversteer under braking; if both rear wheels lock at the same time the rear of the car will step out in a similar way to applying the handbrake.

Lower “drive” and higher “coast” values make the car more responsive to throttle inputs.

Clutches (discs) influence the maximum lock (wheels rotating at the same speed) achieved. They affect both coast and power differential in the same way.

In the image below, compare the lock (bottom row) achieved with the same ramp values (top row) at different clutches (discs, middle row). You will notice that the discs act as a lock multiplier (4 discs have 2x the lock of 2 discs; 6 discs have 3x the lock).

Values in the image above are taken from the real life F317 manual. They seemed to be in line with our simulation early on, but now the iRacing car allows a much larger range of clutches. Values will be updated if and when available. Regardless of the exact lock values, the concept remains valid.

You can use the image to tune the differential in a very simple way:

• if you choose settings to the left you have more on-throttle understeer and off-throttle oversteer
• If you choose settings to the right you have more on-throttle oversteer and off-throttle understeer

Regardless of the mechanics (we have very little info), the preload can be used to tune the F3 behaviour.

First of all, preload has effect on both slow and fast corners in entry, mid-corner and exit. By contrast, the ramps and discs/clutches have more effect in slow corners and affect entry and exit, but less so mid-corner.

Higher values (eg 100) produce more oversteer on-throttle and more understeer off-throttle.
Lower values (eg 20) produce understeer on-throttle and oversteer off-throttle.

You can increase the car reactivity on corner entry by decreasing the preload.
If you want the car to turn better on exit, instead, choosing higher preload values should help you.

More info on the differential at pages 27-28-29 of the F317 user manual, including a comparison of the lock in all the available settings.