Lesson 6: The limited-slip differential

What a differential does in a corner

In a corner your outside wheels travel a longer arc than your inside wheels, so they have to spin faster. The differential is the gearbox in the middle of the driven axle that allows that speed difference. Without it, one tire would have to scrub across the road on every turn.

The catch is how an open diff shares torque: it always gives both wheels the same amount. That sounds fair until one tire has less grip than the other, which is exactly what happens on corner exit.

The open-diff problem

As you turn, weight moves onto the outside tires, so the inside driven tire goes light. Squeeze the throttle and that light tire is the first to break traction. Once it spins, it can only take a little torque, and because an open diff splits torque equally, the loaded outside tire gets that same little amount. The engine revs, the inside wheel flares, and the car barely accelerates. You lose drive exactly when you want it most: out of slow corners.

The locking spectrum

A limited-slip differential (LSD) sits between two extremes:

• Open: the wheels turn freely at different speeds. The car turns easily, but drive is capped by the weaker tire.
• Fully locked: both wheels are forced to the same speed, like a solid axle. Drive is superb, but the axle fights every corner, because the inside wheel has to scrub to keep up with the outside one. That pushes the car straight on.

An LSD lets the wheels differ in speed until the difference, or the torque going through it, gets large. Then it starts holding them together and sends more torque to the slower, gripping wheel. How hard it holds is the locking: think of it as a percentage between 0 (open) and 100 (locked).

More locking is not always better. It buys traction, but it also resists rotation and makes slides arrive together and more suddenly. Tuning the diff is about putting the right amount of lock into each corner phase, not the most.

The four choices in Differential Gear Selection

GT7 gives you one menu, Differential Gear Selection, with four options. Each one decides when the diff is allowed to lock.

• Normal: the car's standard differential. GT7 doesn't describe it further, but on many road cars it seems to behave close to open, so inside-wheel spin out of slow corners is common.
• One-Way LSD: locks on throttle only. Off throttle and under braking the diff stays free, so the car turns in smoothly. GT7's own magazine notes that FF cars often use one-way LSDs.
• Two-Way LSD: locks on throttle and off throttle. Very stable under braking and easy to steer with the throttle, at the cost of entry understeer.
• Fully Customized: you set the three dials yourself: Initial Torque, Acceleration Sensitivity and Braking Sensitivity. This is the one to learn.

The 1.5-way idea

Racers talk about a "1.5-way" diff: full locking on throttle, some locking off throttle. It keeps the rear calm as you brake and turn in, without the heavy entry push of a two-way. GT7 has no menu item called 1.5-way, but you build one in Fully Customized by setting Braking Sensitivity lower than Acceleration Sensitivity. The course starting point for rear-drive cars is exactly that shape.

The three dials, phase by phase

Picture the LSD as a lock that responds to the pedal. Initial Torque sets how much lock is there all the time. Acceleration Sensitivity sets how fast the lock rises as you add throttle. Braking Sensitivity sets how fast it rises when you lift or brake and the engine holds the car back.

Initial Torque: how soon locking starts

This is the preload: a baseline lock that is there before you touch either pedal. Raise Initial Torque and the diff reacts sooner and more firmly. GT7's manual puts it simply: higher makes the LSD more responsive, but it may hinder the car's ability to turn. The official magazine adds that preload accentuates the car's natural tendency.

• Rear-drive car (diff at the Rear): mid-corner, the car feels more reluctant to rotate on a steady throttle. Then, as you power out, the rear steps into oversteer earlier and more abruptly. The Front isn't touched directly, but it feels like it has to work harder to get the car turned.
• FF car (diff at the Front): the driven front tires are tied together while you're also asking them to steer, so you get more understeer mid-corner and on exit. The Rear is undriven; it only feels looser by comparison when you lift.

Lower it and the car rotates more freely at part throttle, with a gentler, later transition into the lock. The modern trend the official magazine describes is low preload with stronger locking under load, which is why the course starting points keep it small.

Acceleration Sensitivity: lock on throttle

This sets how much the diff locks as you add power, so it rules the exit. Raise Acceleration Sensitivity and the inside wheel stops spinning away the drive, so you get better traction out of slow corners.

• Rear-drive car: both Rear tires push together, so you accelerate harder, but when they let go they let go together. Too much gives sudden power oversteer and makes the car harder to place on exit. A very high value can also push wide on part throttle, before the rear lets go, because the locked axle resists turning. Too little, and the inside rear flares and you crawl out of hairpins.
• FF car: a bit more helps the Front put power down. Too much makes the locked front axle pull the car straight, so the nose runs wide on exit: exit understeer.

One more warning, straight from GT7's manual: a high Acceleration Sensitivity can make the car unstable when you release the throttle. You feel it as a twitch at the moment you lift mid-corner, because the lock that was steadying the axle drops away at once. If you feel that, come down on Acceleration Sensitivity, or bring Braking Sensitivity a little closer to it.

Braking Sensitivity: lock off throttle

This sets how much the diff locks when you lift or brake, so it rules the entry. Raise Braking Sensitivity and the driven wheels are held together under engine braking. The car feels planted on the brakes and you can brake later, but it resists turning in. The manual's wording: more effective when decelerating, with more risk of understeer.

• Rear-drive car: the Rear stays calm and straight as you trail-brake. Raise it too far and the Front can't pull the car into the corner, giving entry understeer. Too low and the rear can feel loose or step out as you lift into the turn.
• FF car: locking the Front axle while you steer and brake makes the nose push on turn-in. That's why FF setups keep this low, which is one-way style.

Drivetrain by drivetrain: where to start

The physics is the same everywhere; what changes is which axle carries the diff and where the engine's weight sits. Start from the table below, drive, then move one dial at a time.

These are the course's coaching numbers, not game defaults. They sit near the low end of the typical 5 to 60 range on purpose: it's easier to add lock until a problem goes away than to find which of three high numbers is causing trouble. If your car's sliders run on a different range, place the values at a similar point in that range.

FR: engine in front, rear drive

The classic case for a 1.5-way shape. A light rear loves to spin the inside wheel out of hairpins, so Acceleration Sensitivity does most of the work. Keep Braking Sensitivity below it so turn-in stays sharp, but not so low that the Rear wanders when you lift into the corner.

MR: engine in the middle

More weight sits over the driven Rear tires, so there is more natural traction and the car rotates eagerly. That also means slides arrive faster. Start at the same numbers; if the car snaps on exit, lower Acceleration Sensitivity before anything else. MR cars react sharply to a lift, so be wary of running Braking Sensitivity very low.

RR: engine behind the rear axle

Huge traction from the weight over the Rear, and a light Front that tends to push on entry and mid-corner. Start at the same numbers. If the nose won't bite as you turn in off the brakes, try a step lower on Braking Sensitivity before reaching for the suspension, and go easy on Initial Torque, which adds to the push.

FF: front-wheel drive

Here the diff sits on the axle that also steers, so every bit of lock fights the steering wheel. Keep Braking Sensitivity and Initial Torque low so the Front turns in freely, and use Acceleration Sensitivity to stop the inside front tire spinning. The official advice for FF exit understeer is that more LSD can help, but too much makes it worse. Add it in small steps and stop the moment the nose starts running wide on throttle. The Rear isn't driven, so its balance comes from the suspension, not the diff.

4WD

4WD cars have a diff on each axle, and the official approach is milder locking at the front and stronger at the rear, tuned for front response on entry. Lesson 7 covers that along with Front/Rear Torque Distribution.

Symptom to fix

First decide whether the diff is the culprit at all. The diff only changes things where torque flows through it: on throttle, on lift and under engine braking. If the car pushes or slides while your throttle is steady and nothing changes when you vary it, the springs, bars and alignment are better places to look.