Differential settings: preload, power, and coast

A limited-slip differential (LSD) controls how hard your two driven wheels are forced to turn at the same speed. Lock them together and the car drives straight and puts power down; let them spin independently and the car rotates. Most race cars run a clutch-type LSD, so you get three knobs: preload (the always-on baseline lock), the power ramp (lock on throttle, which is corner exit), and the coast ramp (lock off throttle, which is corner entry and braking). Every diff change is the same trade: more lock means more traction and stability, less lock means more rotation.

Preload: the always-on lock

Preload is the baseline locking torque that holds both driven wheels turning together until the torque difference between them exceeds it. It’s measured in Nm. An ACC GT3 set to 170 Nm preload stays fully locked until the torque difference between the rear wheels passes 170 Nm, then the diff starts to open.

Typical ACC GT3 preload runs ~20–300 Nm, with most cars working in a ~50–120 Nm band. Lower preload (say 80 Nm) gives more turn-in, more rotation, and a more agile car that can feel loose or snappy on entry and may spin an inside tire on power. Higher preload (200 Nm) makes the car stable and planted, costs you rotation, adds understeer, and helps traction off the line. In ACC, GT3 cars only let you change preload — the power and coast lock are fixed by homologation, so the whole diff conversation collapses to one slider.

Power ramp: on throttle, corner exit

The power ramp sets how much the diff locks under acceleration, which is corner exit. More power lock means more traction off the corner — the inside wheel can’t spin up freely — but the locked rear pushes the front wide, so you understeer on exit. Less power lock frees the rear, adds rotation and oversteer on exit, and risks wheelspin.

iRacing and AMS2 express this as a ramp angle in degrees, and the direction is the gotcha everyone gets backwards: a smaller angle number means more locking. Some sims show it as a percentage lock instead, where bigger means more lock.

Coast ramp: off throttle, corner entry and braking

The coast ramp sets how much the diff locks when you lift or brake, which is corner entry. This is the setting behind the most common complaint — the AMG GT3 that pushes wide while coasting through a corner. More coast lock keeps the car straight and understeering on the brakes and on lift. Less coast lock opens the diff, frees the rear, and adds rotation on entry, at the cost of snap-oversteer risk when you trail off the brake. To chase more entry rotation, open the coast ramp and pair it with a softer front anti-roll bar and a stiffer rear spring.

Symptom to setting cheat sheet

Symptom	Change
Understeer on entry / coasting in	Less coast lock (open the coast ramp / lower preload)
Understeer on exit, won’t take throttle	Less power lock
Oversteer / snap on entry, off the brakes	More coast lock
Oversteer on exit, rear steps out on power	More power lock
Inside wheel spins up mid-corner	More preload or more power lock
Car too lazy, won’t rotate anywhere	Lower preload

How each sim exposes it

• iRacing: preload (Nm) + power ramp angle + coast ramp angle, plus clutch/friction-face count on some cars. Remember smaller angle = more lock.
• ACC: GT3 cars give you preload only; GT4, GT2, and other classes vary.
• AMS2 / Assetto Corsa: clutch plates (friction faces) multiply the locking effect — more plates = more locking, so more resistance to wheelspin on exit but more lift understeer on entry. AMS2 also gives power/coast as ramp angles.
• F1 23/24/25: on-throttle differential as a percentage. F1 24/25 communities run ~100% on-throttle almost always; on older F1 23 you’d back it off ~5% for rear grip on exit.

Do I adjust it mid-race?

No — not in iRacing, ACC, or any sim with a real clutch-type LSD. The diff is set once in the garage and lives with you for the stint. The live, per-corner knob is brake bias, not the differential. The F1 games are the exception: they let you sweep on-throttle diff and brake bias on the fly, which is where the confusion comes from.

Check your inputs before the setup

A lot of “diff understeer” isn’t the diff. When you release the brake and immediately get on the throttle, weight transfers to the rear, the front goes light, and the throttle locks the diff while the front has no load — so the car pushes straight. The fix is in the right foot: finish your braking, let the front load and the car rotate, then feed the throttle in. Sort the inputs before you touch the setup. For the braking technique that loads the front, see trail braking and setup theory, and the pedals that let you trail off consistently.