The differential: your entry-stability & exit-traction lever

What a limited-slip diff actually does

A differential sits between the two driven wheels and lets them turn at different speeds — which they must, because the outside wheel travels further than the inside one through a corner. The catch with a plain open diff is that it always sends torque to the wheel with the least grip. Lift the inside wheel under power and it just spins uselessly while the gripping wheel gets nothing.

A limited-slip differential (LSD) fixes that. It locks the two driven wheels together by a variable amount, capping how much faster one can turn than the other. Instead of one wheel spinning free, both are forced to share the drive torque and put power down. How hard it locks — and in which phase of the corner — is what you tune. That is the whole game.

The three numbers: preload, power & coast

An LSD is not one setting but three, and they each act in a different part of the corner. The trick is that they answer different questions: how much lock at rest, how much under throttle, how much off throttle.

• Preload — the always-on locking torque, present before either ramp engages. Because it is a fixed baseline, it dominates at low torque: the light-throttle, low-speed and turn-in moments where neither ramp is doing much. It is what holds the car together across the throttle transition.
• Power ramp — the lock that builds under throttle (on drive torque). This is your exit lever: it governs exit traction and exit stability. More power lock puts the power down and steadies the rear out of the corner.
• Coast ramp — the lock that builds off throttle (under engine braking, or on a trailing throttle). This is your entry lever: it governs entry stability and how the car rotates into the corner.

On-throttle vs off-throttle: two halves of the corner

Off throttle, into the corner, the coast ramp decides how tightly the rear axle is tied together. More coast lock binds the two rear wheels, so the back stays planted and predictable on entry — at the price of less rotation, because a locked axle resists taking the tighter inside line. Less coast lock frees the wheels to turn at different speeds, so the car rotates more into the apex, but the rear gets livelier and can step out under trail braking.

On throttle, out of the corner, the power ramp takes over. More power lock shares torque across both driven wheels so they hook up — strong exit traction and a steady rear — but it drags the car straight, which reads as power understeer. Less power lock lets the inside wheel spin up, freeing rotation but bleeding traction and making the rear nervous as load shifts around.

Sitting under both is preload. At corner entry and low speed, when you are barely on the throttle or coasting in, neither ramp is loaded much — so the fixed preload is most of the lock the car feels. Raise it and the whole car feels more tied-down and consistent through the transition; lower it and entry rotation comes back, but so does a twitchier platform.

The trade-off: rotation vs traction & stability

Every diff change is the same bargain, pulled in two directions. More lock — anywhere — buys stability and traction but costs rotation: the axle resists the speed difference the wheels need to follow different arcs, so the car is dragged straighter and pushes wide, especially on power. Less lock buys rotation back but costs traction and calm: the inside wheel is free to spin up, so you get wheelspin on exit and a more nervous, less predictable rear.

This is why the diff is such a powerful balance tool — and so easy to over-use. It is not the only lever: the anti-roll bars set steady mid-corner balance and the springs set the platform. The diff is the one that specifically owns the transient moments — the rotation as you turn in and the traction as you fire out — that the other levers leave alone. It is part of the same picture as the rest of your mechanical balance, just acting where torque, not just cornering load, is in play.

Read it first, then move it — one number at a time

A diff change is only the right answer once you know which phase is hurting. SimRace.app pulls your full setup and a stand of specialist race engineers logs the telemetry, corner by corner. They watch the three tells that map straight onto the three settings: entry rotation (is the car darty or lazy turning in?), exit wheelspin (is the inside wheel lighting up under power?) and mid-corner diff lock (how tied-together is the axle through the low-torque part?).

From there the move is obvious and specific: a nervous, over-rotating entry off-throttle → add coast lock or preload; wheelspin and a snappy rear on exit → add power lock; inconsistency right around the throttle transition at low speed → tune preload. And like any good engineer, they hand it to you one number at a time, judged over a few laps, so you actually feel what each change did before stacking the next.

FAQ

The setup glossary

The differential is one lever among several. The rest of the toolkit, one page at a time — and it all comes together back on the SimRace.app home:

• Brake bias
• Mechanical balance
• Anti-roll bars
• Camber, toe & caster
• The full setup guide →