[Magic inside!] Need Help With Differentials

[DeathMetalRacer]

Even with the help texts I'm confused about which ones I would want turned on/off and why. Could someone please explain. The diff setup in PC1 was so much easier to understand.

[Jussi Karjalainen]

EDIT: Clutch type differential calculator here: Project CARS 2 Differential Calculator v0.9 You can't directly edit this one, it's for sharing. To use it save a copy of it to your own drive and use that.



It was easier to understand in pC1 because it was vastly simplified from how they really do it. =)

OK so we have a few different differential types available to you, which you can (depending on the car) turn on and off at will. You can (and we have) also combined different differential types for more dynamic behavior and to better represent the behavior of some of the more active differentials found on modern cars:

1) Spool. This is basically equivalent to welding the driveshafts together, no differentiation possible, both driven wheels will always rotate at the same rate (within reason, there is some flex modeled in the driveshafts). If you turn Spool on, no other differentials will mean a thing, since it will solidly lock the two sides together.

2) Geared LSD. This is your torque sensitive type differential. It allows the wheels to rotate at different rates as long as both sides can handle the torque thrown at them. When one side can't handle the torque (i.e. threatens to slip) you start to get locking effect and more torque sent to the side that can handle more torque. The higher the bias ratio, the more locking you get, roughly following this curve (thanks to Torsen differentials for releasing them):



So for example power bias of 2.0:1 equal roughly ~30% accel locking. This locking percentage is roughly equal to the old style of adjusting diffs in pCARS 1, though since the principle of the diff is different and our differentials work more accurately than before when transmitting forces, you'll get somewhat different results.

3) Clutch LSD. This is your traditional LSD that you'll find in other games and in pCARS 1, though the way you adjust it is closer to how they do it in real life. You have 4 controls:

Preload: Works exactly as in pCARS 1, this is the always on minimum amount of locking in the diff. If you find that the car is unstable between the transitions from throttle to off-throttle, or the other way around, you might want to increase this.
Clutches: The number of clutch faces in contact with each other. This is effectively a multiplier, the more clutch faces you have in contact with each other the more overall locking you will get for both power and coast side.
Power ramp: This is the ramp angle for the power/acceleration side of the diff. The higher the angle, the less locking you get. Lower angle leads to more locking.
Coast ramp: This is the ramp angle for the coast/deceleration side of the diff. Works just like the power side, but affects off throttle behavior.

The insides of the clutch differential look a bit like this (thanks to intothered.dk):



You don't really need to fully understand the principles of how they work though. Basically you can use the ramp angles to select how much locking you're getting out of the clutches for each direction (20 degrees = maximum, 90 degrees = minimum = none) and the numbers of clutches will then increase or decrease the total locking force.

Summary: Low angle = high locking. High angle = low locking. More clutches = more locking. Less clutches = less locking.

A differential calculator for the clutch LSD will be included in my upcoming Project CARS 2 Suspension Calculator, which will convert the settings into numbers similar to what you saw in pCARS 1.

4) Viscous: A viscous type differential, that works on the speed difference between the two wheels. The higher the difference in wheelspeed, the more locking effect you get. Because it doesn't care about input torque at all and scales with speed, the effect is quite progressive and even subtle, but on the other hand it requires that one side slips before it offers any locking. We're using the industry standard measurements, showing how many Nm of locking torque you get for 50 rpm difference between the two axles. Higher number = more locking.

5) Ratcheting: This is a funky type of differential that was used way back in the early days of racing and in very limited capacity in more modern times, usually as a mandated differential rather than by free choice. It offers full lockup like a spool on acceleration, but is completely open on deceleration. So you get the possible clunkiness of a spool when accelerating and the instability of open diffs on deceleration. Trans-Am in the 90s was one series that mandated these differentials. Often known as the worst kind of differential for racing, apart from an open differential. =)


EDIT: On the center differential of some cars you also have a setting for rear power distribution, meaning you can adjust how much of the power is being sent to the rear wheels. Note that this might seem a bit high on some of the cars, for example the Loose and OEM setups on the Audi R8 V10+ are set to IIRC 90-95% rear torque distribution, instead of the 15/85 or 30/70 Audi has claimed at times. There's a reason for this though, and it's basically that front/rear torque distribution is not a simple thing that you can just put an easy number on, it's constantly variable and pretty much never static.

In reality most of these types of cars cars (including Audi, Lamborghini, Nissan, and others) have a normal RWD driveline, where all the power goes to the rear. Then they add a connection from the rear differential to the front differential, with either a viscous, electronic or mechanical differential in between. So what you have is a rear wheel drive car that then transfers a bit of torque to the front based on front and rear differential gearing differences and slip differences between the front and rear. The rear power balance value can be used to adjust for the different front and rear final drives (so that some torque is always sent to the front), while the locking settings will affect the dynamic behavior.

By adjusting the rear power balance setting as well as the differential properties of the center diff, you can simulate different kinds of "modes" that modern active systems have for different road conditions, or even properties of older static viscous based ones. 0% rear power balance makes the car a pure FWD, 100% makes it a pure RWD, assuming there's no other locking applied (locking will still transmit torque towards the other side). Yes, this means you can make an FWD Lambo if you really really want to... =)

EDIT2: To further explain the "front and rear differential" bit: If you make the front differential's ratio longer than the rear differential's ratio, when the tyres are rotating at the same speed the driveshaft to the front differential wants to rotate slower than the rear differential. If there's for example a viscous locking connection between the two, the faster spinning rear will cause the viscous LSD to transmit some torque to the front. The bigger the difference in ratios (or the higher the locking in the viscous LSD), the more torque will be sent to the front in all conditions.

[MXR SMILER]

Quality, thanks... Should be a sticky for this...

[DeathMetalRacer]

Jussi, can't thank you enough for your response!

I do have a couple more questions if you all don't mind.

1. If I'm using geared LSD and my car wants to spin out at corner exit do I want to increase or decrease power bias? What about spinning at corner entry? Would I increase or decrease coast bias?

2. In PC1 I learned that if my car felt unstable at corner entry or corner exit I could decrease preload to stabilize the car. Does this still apply in PC2?

[Jussi Karjalainen]

1. Generally less power bias means more stability on throttle (you're allowing the inside rear to spin more, transmitting less torque to the outside rear), generally more coast bias means more stability off throttle (you're locking up more when there's a grip difference).

2. Weird that corner entry would be more stable with less preload, usually more locking = more stability when you're not on the throttle and are turning, but preload effectively works like it did before, just more accurately physics simulation wise.

[Aldo Zampatti]

Quality, thanks... Should be a sticky for this...

Jussi's master class is too good to go unnoticed it. I concur.

Sticky

[DeathMetalRacer]

I wonder if it makes a difference that I race in game pad as to why lowering preload helped me gain stability out of the car.

[Jussi Karjalainen]

Possibly, and would of course depend on what it was to start with.

Cheers for the Sticky btw.

[OperatorWay]

Thanks for that!
This old "How Differential Gear Works" video is also a good easy-to-understand introduction to the basics:
http://www.youtube.com/watch?v=K4JhruinbWc

[Roger Prynne]

Another quality post from Jussi, I don't know how you do it mate.