The Art of Setup Building, Part 10: Need for Diff. Speed

Differential, objective and adjustments

We arguably have the car setup up perfectly fine for us. Despite us having the car setup to our likening, maybe there's some behaviors that we want to reel in, but if we adjust the suspension we will screw up the rest of the car. That's what adjusting the Differential can do for us. Now, I could talk about why we need differentials in the first place, but that's for later. I will explain the technical details (That I know) later.

First, here's something I want you to see.

A differential set incorrectly allowing power to disappear when exiting the pit stall ...

... exiting a slow speed corners ...

... and a high speed corner. This needs to be controlled.

See what happens when the Differential isn't set up properly? This is only one of the side effects. We'll cover the other ones. First, let's see the settings.

We have 4 settings to adjust. Sounds like a lot, but one of them assists the others. That said, we need to talk about terminology, because lots of guides get confused.

Differential Effects

This will be explained in more detail in the correct posts, but is is good to have a rough idea of what the Diff does under locked and unlocked conditions. Keep in mind, this only applies on a rear wheel drive car like an F1 car.

If the Diff is Unlocked ...

• Off throttle, the car tends to oversteer.
• On throttle, in a grip limited situation (AKA you have more power than the wheels can put down), the car tends to understeer. However, if one of the tires is excessively grip limited some of the power, as seen on the initial images, will vanish in a cloud of smoke and wheelspin. This will generate oversteer.
• On throttle, in a traction limited situation (AKA you have so much grip that you can floor the throttle with no issue), the car tends to oversteer.

If the Diff is Locked ...

• Off throttle, the car tends to understeer.
• On throttle, in a grip limited situation, the car tends to oversteer.
• On throttle, in a traction limited situation, the car tends to understeer.
  

Is it clear? No? Good. Details to be here later.

Differential Preload: The Entry Preload setting

In simple terms, the Preload is a set amount of lock that the diff must beat to trigger the Accel and Decel settings. By default, the differential is locked and will unlock when a certain torque figure is hit. That torque figure is what the Preload represents. At low settings, literally any kind of torque, be it driving or braking, will allow the differential to open and slip as asked by all its other setting. At high settings, it will take more and more torque to unlock.

Now, if you remember the Torque-Power/RPM chart, you will remember that there's generally more torque on the power than off throttle. For this reason, the Preload is more important when decelerating, as opposed to accelerating. So, the main reason to adjust the Preload setting is to control turn in, as well as mid corner behavior to a certain extent, though there are better settings for that.

In terms of setting it up,  you usually want it as low as possible. If the car is snapping into the corner a bit too aggressively, dial in a bit more Preload. If it resists turn in initially, dial it back. You can get away with both a high and low setting. Preload is not that important in the grand scheme of things, but it can certainly help reel in a snappy car.

Summary

• Preload is the first step for differential work. It will control when the Diff will go from locked to unlocked. It mostly affects corner turn in, and has a slight effect on mid corner behavior.
• A low settings allows an earlier unlocking of the Differential. This induces extra rotation during turn in, and slightly more rotation mid corner. On throttle behavior is slightly affected, but should be ignored when dealing with Preload.
• A high settings will make the Differential unlock later. This induces some understeer during turn in, as well as slightly less rotation mid corner.
• If in doubt, settings 1 to 4 are good enough to work.

Differential Accel: The High Speed Setting

The first of the Diff setting we will mess with the the High Speed Setting, better know as Accel or Power setting. This setting controls how much Torque the differential will transfer from the faster spinning wheel, to the slower spinning wheel under driving (acceleration) torque. That is an important distinction. Something I'll detail later on, but I'll simplify here: Differentials are dumb, they don't know which wheel is the inside one and which is the outside one. They just see a wheel that spins faster than the other and takes action.

Tweaking the differential in high power, high downforce cars is ... interesting. A theory says that you want the highest Differential Accel setting that you can get away with without generating brutal oversteer out of slow speed corners. The other theory says you use the lowest possible Accel setting, to control inside tire wheelspin and that's it. I stick to this second theory, but the choice is yours.

Adding Differential Accel will increase Differential lock exclusively under driving torque, basically whenever the throttle is on. As explained in the Differential Effects, a locked Differential will oversteer when the car is grip limited and understeer when traction limited. In simpler terms, increasing the Accel will increasing Power On Oversteer on slow speed corners, and will increase understeer on throttle everywhere else. Decreasing the Accel will decrease Power On Oversteer on slow speed corners, but will decrease understeer on throttle everywhere else.

There are more considerations, however. An excessively low Accel setting will result on the faster tire, usually the inside tire in cornering, being out of controlling and spinning up at will. You need enough Diff to control that. However, an excessively high Accel setting will transfer too much torque from the faster tire to the slower tire. On one hand, this means the engine can't produce excessive wheelspin. On the other hand, the diff might get pissed off. It will transfer too much torque from the faster tire to the slower tire, which will make the roles switch. This is not good for traction or rotation. 

During cornering, we want the inside tire to be the faster tire all the time, so that the Diff works properly. The true, optimal Diff Accel setting is the one high enough to keep the inside tire spinning slightly faster than the outside. This maximizes the traction of both tires on acceleration, for maximum drive off, while ensuring rotation is sufficient and the Diff works correctly. Going below is the safe option, you lose a bit of acceleration but gain stability and rotation at high speeds.

A properly tuned up Differential, allowing solid power to be put on the road without handling issues.

Summary

• The Differential Accel setting affects the car's behavior when on the throttle.
• High Accel settings will improve acceleration exiting corners, however it will also increase Power On Oversteer on slow speed corners, and Understeer on high speed corners.
• Low Accel settings will reduce high speed Understeer, and will reduce Power On Oversteer. However, it can compromise acceleration out of corners. Terminally low values will allow excessive wheelspin on the inside tire, however.
• If in doubt, 5-10% is good enough on these cars. We have another setting that can control excessive wheelspin as well.

Differential Decel: The Entry Setting

Now we mess with the Entry setting, also known as Coast or Decel. Similarly to the Accel setting, Decel controls how much torque is transferred from the faster spinning tire to the slower spinning tire under braking torque. Now, in 99% of all decelerations, the outside tire will spin faster than the inside tire. So we can simplify this to say that Decel will transfer torque from the outside tire to the inside tire during deceleration. Also, an important note, the Decel setting referes to Differential Effect under braking torque FROM THE ENGINE. It kicks in when OFF throttle. The differential does not care about the brake pedal, it only cares about what the engine does.

As opposed to the Accel setting, tuning up the Decel setting is considerably easier. Increasing Decel will increase Differential Lock under braking torque, which mostly occurs during deceleration, turn in and a bit of mid corner. This results in added stability/understeer. Decreasing Decel reduces Diff Lock under braking torque, which allows extra rotation under braking.

Decel works a bit with Preload to determine Turn in balance. A high Preload means it will take longer for the Decel setting to kick in, while a lower Preload means Decel will become a factor earlier. You can combine both to get slightly different behaviors during low and high speed corners.

In case you are getting real wheel lockups under braking, usually caused by too aggressive downshift, you can use both Decel to try and control those.

Summary

• The Differential Decel setting affects the car's behavior when off throttle.
• High Decel settings will increase understeer. It can control rear lockups under braking.
• Low Decel settings will reduce understeer. It could increase rear lockups under braking.
• There's a big useful range of settings for the Decel setting, I'd recommend a setting over 20% and below 50%, but this is a driver comfort setting. If you like 100%, use 100%.
• If in doubt, set it at 30%.

Differential Lock: The Mid setting

The final setting on our disposal is the Mid setting, better known as Lock or Pump. This is the only setting available in the base game, and it does a solid job all in all. The Accel and Decel setting will transfer some of the torque from one of the wheels to the other. The Lock setting will restrict the difference of speed between both wheels. While Accel and Decel are like on/off switches that vary with torque, the Lock setting is more progressive (softer in effect) and varies with how much rotation the wheels have.

Increasing the Lock setting will reduce the allowed speed difference between the wheels, which makes it act as a Locked Differential. Reducing the Lock allows more speed difference between the wheels, making it behave like an Open Differential. It should be noted that the Lock setting is active at ALL times, both during driving and braking torque.

Since the Lock setting tries to restrict the difference between the rotation of the wheels, the Lock setting is useful to control excessive wheelspin under acceleration. So, my suggested usage is to use the Lock setting to control wheelspin, adjust mid corner behavior, then use the Accel and Decel settings to adjust behavior exiting corners and entering corners.

Summary

• The Differential Lock setting affects the car's behavior at all phases.
• High Lock settings will make the Diff behave as a Locked Diff, which makes it respond as I explained in the Differential Effects section.
• Low Lock settings will make the Diff behave as an Open Diff.
• From what I could find on some setup sheets, real life teams of the time used quite High Lock settings. In regards to us, we have a WIDE range of settings, anything from 10-20% to 100% is perfectly usable.
• If in doubt, it'd set this to 30%, enough to control wheelspin and allowing car behavior to be free enough.

Why all of this works?

I guess the eyes of some of you are filled with curiosity about why the F we need Diffs to start with. Allow me (try to) explain.

Why we need Differentials?: The Spool

The first logical point we could want both wheels to rotate at the same speed. That way when we put the power down, both wheels get as much power as possible. This maximizes acceleration, assuming both wheels have enough traction to drive that power to the ground. This is what a Spool does, and it is perfect when accelerating in a straight line.

However, problems appear when we try to take corners. You see, when we take a turn, the outside tire takes the long way around the corner, this means it must rotate fast. The inside tire, however, takes a shorter path through the corner, so it should rotate slower. Do you see the problem? Corners mean the tires must rotate at different speeds, but the wheel rotation is locked due to the Spool. The result? The car will go straight, terrifying understeer will ensue. This isn't the only issue, but I'll talk about that later when we discuss setting the differential itself.

We need the wheels to rotate at different speeds through the corners. That's why the Differentials exist. To be exact, the Open Differential. However, there are issues as per usual.

Why we need to control them?: The Open Differential

So, if the Spool is bad for cornering because the wheel rotation are locked between them, what about we unlocked them? That's why the Differential exists. The objective of the Differential is to allow the wheels to rotate as quick or as slow as they need to. That means, when we enter a corner, the outside tire will rotate fast. The inside tire will rotate slow. Everything perfect, right? ... Of course not.

While cornering is much better overall, accelerating now becomes a mixed bag. While the Spool makes sure both tires get equal amounts of power (remember, Torque x Speed = Power. Same Torque, same Speed, same Power for both wheels), the Open Differential makes sure the tires get equal amounts of torque. This is a big difference. Why? Well, if both tires have enough grip, this is no problem. If one of the tires lacks enough grip, that tire will start spinning, unable to put the power down. This is common for the inside tire exiting a corner, or if one of the tires is over a kerb or into the grass. This kills the acceleration, as the other tire could probably put that torque to good use.

So, we know the Spool is horrible while cornering, and that the Open Differential is horrible while accelerating. What do we do, then?

We combine them, of course.

The solution: The Limited Slip Differential

The LSD, and yes I will call it that, basically combines the best aspects of the Open Differential and the Spool, by adjusting how much Torque each wheel gets. Not only that, but that effect is adjustable. Off throttle, we can make the LSD behave like an Open Differential, allowing the car to rotate and turn into the corner without too much resistance. Under throttle, we can make the LSD behave like a Spool, allowing the wheels to put down as much power as they can, to maximize acceleration when exiting corners.

Now, we don't really want to completely make the LSD completely Open off the throttle, as that will cause oversteer. We don't want to completely make the LSD completely Locked on throttle, as that's scary to handle. The positive about the LSD is that we can adjust it to our likening, making it one of the best tools to adjust car behavior. We can slightly lock the Diff under throttle, or we can lock it a lot. We can slightly lock the Diff on throttle, or we can lock it a lot.

There's a slight problem. When the LSD kicks in, it kicks instantly and fairly aggressively, regardless of how much effect is needed. It usually isn't a problem, but we might want a softer effect from the Diff ... but if we do that, car behavior might become unstable. We need to think about something else ...

The alternative solution: The Hydraulic Differential

So, instead of the Differential biasing Torque from one side to the other, we could have it react to how much the wheels are rotating relative to each other. If the difference of speed between them increases, the locking effect increase. This is what a Hydraulic Diff does, using a pump and viscous fluid to reduce the speed difference. The higher the difference, the higher the effect. And since it depends on the wheel difference, the effect will slowly scale up depending on how much effect is needed.

The only issues are that the Hydraulic Diff works at all times, so can't have different settings for On Throttle and Off Throttle, and that one of the tires MUST start slipping before the Hydraulics start working. Both of these disadvantages are not present in a typical LSD. The problem of these two differentials is the adjustability, as you'd have to remove them from the car to then adjust it and that takes about 20 days to do. F1 teams can't be bothered with that.

So, the LSD is quite aggressive but versatile, the Hydraulic Diff is a bit too passive. Both take too long to adjust. The solution? Combine them!

The ultimate solution: The Electronic Differential

The Differential F1 teams use, and the one we have at our disposal, is an Electronic Differential. It combines the effects of a typical LSD, a Hydraulic Diff and is quickly adjustable since the driver can adjust it with a few switches on the steering wheel ... well, at least in real life, in F1C we can only use those switches in the setup screen. 

Real Electronic Diffs also are computer controlled, so they change their settings on the fly. They are absurdly expensive for a reason. F1 teams spend a lot of time during practice sessions tweaking the differential since it affects the car so much.

You know, if it's good enough for real teams, it is DEFINITELY good enough for you.

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