A changed value in a data file loads but changes nothing in the game

What to know

A mod changes a value that the game's loader reads, such as a car's grip, and the game behaves as before. The file did load: other changes in the same file show.

The value the loader reads is a default. Later, before the race, the game replaces it with one computed from saved or menu settings (measured in POD: the garage setup).

In POD the car file's side_grip and traction_grip (+0x94, +0x98 of each wheel record) are read by load_car_file and copied into the race car by FUN_00476aa0. Then setup_defaults (FUN_004436c8) computes one grip for all four wheels from the car's third garage setting ("adhérence", 0 to 100), and car_from_setup (FUN_00442f04) writes it over the copied values: grip = 1.2 + 0.026 × setting (measured at 95, 50 and 5). The five garage settings at the end of the car file are the car screen's five figures.

Evidence

Read the running game's memory (/proc/PID/mem of the Wine process). The changed values were in the car's slot (0x61757c, +0x94), so the file had loaded. The race car (0x61aad0, +0x94) held 3.67 on every wheel with the mod and with the disc's file. A memory search for that value found the setup at 0x50769c + car × 0x1a8 + 0x13c; the decompiled writers of race car +0x94 led to car_from_setup.

What to do

Before judging a handling or rules mod by play, read the value back from the running game where the rules use it, not only where the loader puts it:

1. Find the structure the game logic reads (a memory diff while the value matters, or the reader named in the format notes).
2. Compare it with and without the mod. Equal values mean something later replaces it.
3. Search memory for the value the game uses, then find the code that writes it: often a settings or setup step (garage, difficulty, options).
4. Change the source of that value instead. In POD: the grip setting (make_mod setting FILE OUT 2 N), which shows on the car screen and changes the race car's grip.

What was tried

• Grip parameters of all four wheels set to 1.2: race totals 8:05.50 against 8:06.05 with the disc's file, within the driver's spread. The race car's grip was unchanged.