Ford DT Diff
Drive ramp is the locking under acceleration, whilst coast ramp is the locking off throttle.

Preload is the amount of torque that will be applied to the opposite wheel if the other wheel is completely unloaded.

Clutch plates is just the number of clutch plates in the differrential, the clutch plates are what ties the two wheels together, so more clutch plates equals more locking force during a lock condition.

This is just in simple terms, if you really want to know what a limited slip differential does and how it works there are a few good reads below:

Clutch-type Limited Slip Differential - How it works.

If you are having trouble getting the iRacing Ford GT to turn you might have some joy by Lowering the Coast, Drive and clutch plates on the diff.

What the differential settings on the Ford GT are used for?

• # of Clutch Plates
- More clutch discs = more locking power.
• Preload
- More preload = more stability under braking and more understeer on entry/mid corner
• Drive Ramp Angle
- Greatest impact on corner exit, Higher angle = understeer, Lower angle = oversteer
• Coast Ramp Angle
- Greatest impact on corner entry, Higher angle = oversteer, Lower angle = understeer

Remember with all setup changes there will be compromises depending on the sort of track. Personally I think the diff plays an important role to getting a good feeling car on each track.

The number in the Ford GT garage is the ramp angle not the locking force. The lower the angle the higher the friction and locking. So a higher number will provide less locking. A lower ramp angle will create more understeer not a higher angle.

Not all diff's in iRacing are the same, some use locking force where a higher number will equal more locking such as in the F1 and I think the HDP.

Personally, I'm using 60 for both ramps with 2 clutch plates. I tried 60/60/1, but kept spinning the rear wheels trying to get power down at corner exit. To fix this I added the second clutch plate.

The higher the number the more the car is "allowed" to turn by way of the inside tire not trying to resist the twisting force in the diff. That's not the same as oversteer. It's more like "actual steer" because it's just LETTING the car turn.

BUT... You'll spin up the inside tire and cook it. So you lower it until the inside stops spinning up, then keep lowering it (one tick at a time) until you get to where the car will rotate by itself in part throttle turning, rotate via throttle on when on throttle exits, and doesn't spin up the inside tire.

So... The lower you go, the less rotation (not the same as "understeer") you get under mild exit acceleration. Once you get hard on the throttle the game changes. Lower means MORE rotation in the lower gears, less in the upper gears (it's all about the torque being applied at the tire patches).

Ramp angle power side
When drive torque is applied, there’s spreading force acting which locks the diff and forces wheels to rotate at same speed. The lower the angle the closer the force vector will be to 90 deg, i.e. torque translates to more spreading/locking force.

What happens with car?
If torque is not enough to spin the inside rear wheel than more diff lock will result in power on understeer. Inside wheel will try to resist desired rotation of the car by trying to rotate at same speed as outside wheel (all this while travelling on smaller radius arc which requires less rotational speed than outside wheel path arc)

If there’s enough torque to spin the inside wheel, or one is attacking the inside curb aggressively and inside wheel spends some time in the air, than without sufficient diff lock torque will be wasted for spinning inside wheel instead of accelerating out of corner.

How do these conditions effect handling?
Well if there’s excessive inside spin than we: loose forward thrust, effect on rear axle cornering ability is two fold – inside wheel spent all its ability to generate acceleration force in longitudinal direction and its contribution to cornering force is close to nothing As all the torque is spent on spinning inside wheel, outside wheel is not doing much in longitudinal direction so has almost all its cornering power available. How it will effect handling will depend on individual wheel normal force but in general car might be stable or if outside wheel cornering force is not sufficient it will powerslide but will do it relatively gently.

If there’s enough diff lock to transfer torque to outside wheel - than: If there’s enough torque to spin BOTH tires than car will tend to more aggressive power on oversteer as both rear wheels spend their acceleration capacity in longitudinal direction and there’s nothing left for lateral acceleration.

If both wheels are not spinning than it’s a matter of how much of overall acceleration capacity is spent for longitudinal. If not much (like in higher gear where we don’t have enough torque) than high diff lock will lead to understeer. If due to springs/arb’s/aero rear axle is almost fully saturated for lateral acceleration than even relatively small demand for forward thrust may leave less than enough cornering power and car will tend to oversteer.

Ramp angle coast side
Here everything is the same but in reverse (in some sense) due to drive torque becoming negative. So lower angle will force the wheels to rotate at same speed adding to stability under braking and corner entry/mid corner understeer. Higher number will make the diff lock less leading to less rear stability under braking but car is more willing to rotate in corner entry/mid corner.

It seems to me if you're experiencing lift off snap oversteer, you need less ramp angle on the decel side or more plates. It makes sense to me that it should work this way in real life as well.

Clutch plates
Clutch plates is what gives you friction surfaces, so more clutch discs more locking power.

I'm not sure why, but it feels as though when you add clutch plates you add something like a damper to clutch actuation. It seems like with one plate, when driving a constant radius and playing with the throttle, the diff will act "quicker". With three plates any throttle modulation will have the same magnitude effect, but it will take longer to get to that magnitude after the throttle is changed.

Diff preload
This is clamping force that translates to amount of torque needed to rotate the wheels at different speed. More preload (which is achieved by shims/thickness of clutch pack) will lead to more stability under braking and more understeer on entry/mid corner. It will also somewhat smoothen diff lock under power as there’s already some locking so no sudden lock when power is applied.

Looking at diff settings with use of data acquisition I tend to do it by looking at individual rear wheel speeds, throttle trace and gyro primarily.