Des détails sur les évolutions du moteur physique, et un billet concernant plus spécialement la SF-25 ont été publiés sur le forum de Kunos

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"If I had more time, I would have written a shorter letter." - Blaise Pascal

This post will focus on the physics (read: fiziks) and car side of development and expand a little on the changelog. Plenty has been done for 0.4 that I'll go through in this post.

The Ferrari SF-25 needs and will receive its own sticky thread.

Tire Damping
In 0.3 we introduced a higher frequency for the Suspension of 1000hz. On top of that, we have developed a new method for inherent tire damping, the lack of which seems to have been a root cause for ACC's bouncing issues and hopefully you should not see this again. The tire will now rarely really lose contact with the ground. This took quite some R&D and is part of the physical foundation of this update, making bumpy tracks like the Nordschleife, Oulton Park or Road Atlanta a pleasant experience even in very stiff cars.

Tire Inertia
We adjusted the tire inertia to better match each car's individual tire dimensions. For example, you will notice this when braking with the non-ABS Porsche Cup. The tire now has more inertia and thus is harder to lockup. At the same time a tire with more inertia is harder to spin up on power, so when you are going into a slide, the engine won't as easily rev up to the limiter - ultimately allowing better control in post peak grip situations.

Cars "stuck" in Understeer
We went down the rabbit hole of finding the root cause of RWD cars refusing to rotate on power without deliberately and forcefully breaking traction. For that we worked on two things: Combined grip and the slip ratios.
Essentially slick tires now have more baseline grip both laterally and longitudinally, but less additional grip in combined scenarios (e.g. braking + turning, or turning + accelerating). You will still have high corner speeds, better traction and deceleration, but the car will be more sensitive especially at initial throttle pickup until you are able to open the steering and then plenty of grip is available. It's quite nice to work with that. The rear will be playful, but not deadly, while also not allowing you to abuse sliding as you'll lose time and overheat the tires. Aggressive driving might pay off on a Q run, but not in a race (until someone proves the opposite...).

However, without also looking into slip ratios, the above change would not have been enough to address the inherent understeery nature. Upon open heart examination we came to the conclusion that the rear tires produced a bit too different slip ratios due to too optimistic assumptions of load related tire diameter changes. Something simple on the surface, but with large consequences throughout the entire grip calculation. This led to the inside driven tire producing more longitudinal force than it should in critical scenarios like corner exits. This force worked directly against the request of the driver to rotate the car.

Actually, there is a third component here that we want to address in the future around the overall morphing of the curve defining slip ratio related grip. Though, this has to wait for now and the expected impact is not as major as the above changes we made.

Tire Pressures, Wear and Temperatures
While further revisions and adjustments are likely, here's where we currently are on that front.
Tire wear has generally been increased. For all street legal compounds this is hardly a factor unless you drive really long. They generally produce the same grip over a long period of time. In future updates we'll make them wear faster when overdriving severely.
Road and slick tires are so far also not very sensitive to the theoretical optimal pressure, or rather not having optimal pressure. This will also be addressed in a future update.

The same is true for tire temperatures, with all road compounds being rather insensitive and having a wide operating window.

The reason for both is: Our model has become quite complex with a ton of parameters and really dialing in their relationships takes time (that means long periods of undistracted testing in night sessions). This has been done extensively on the SF-25 where you can see much more alive tires and all other compounds will benefit from these learnings in the future. There is/will be a separate post on SF-25 tires in the car specific thread.

Ideal Tire Pressures:
Road & ECO: 31.0 PSI
Supercar: 30 PSI
Hypercar: 29 PSI
Slicks: 27 PSI
SF-25: 25 PSI

Car Electronics: TC & ABS
We have reworked the TC and ABS logics on every car.

Our system allows to separately define the response of the TC intervention among a variety of Parameters:

Slip Ratio
Slip Angle
Angular Speed of the rear end
Oversteer

First of all, most TC levels will now allow more before stepping in. They will also act less rude. Some levels on race cars in particular will mostly engage due to slip ratio and slip angle, but almost ignore angular speed and oversteer (i.e. the driver counter-steers). Only on higher TC levels these factors will be considered and the TC will step in at earlier.
Ultimately, this should allow you to play with the car without TC interfering, but at the same time give you the confidence to explore the limits.

In the future: More noticable TC & ABS related sounds across all cars.

ABS has also been revised. Generally our system was too perfect, operating at too high frequencies, having it too easy controlling the tire in too tight margins. The operating frequency has been lowered, while the window of its engagement has been widened. Additionally, there have been adjustments to how independent each tire can be controlled individually by the system. Especially on older cars more imperfection should be noticable.

Since this is a vast update, please report any bugs you might come across.

Citation :

SF-25 Electronics

There's lots to explain, so let's get straight to the point:

Many things on this car are adjustable. I'll introduce them one by one. The main setting are the performance modes in which all of the other settings are preselected for you. However, you are always able to change individual settings. All these modes are only necessary because F1 has a very tight rulebook.

Regulations

The maximum fuel flow rate can never exceed 100kg/h - the cars are INCREDIBLY efficient for what they do.
The ERS consists of 3 devices: The battery, the MGU-K (deploy and brake recovery) and the MGU-H (waste heat recovery, both in terms of turbo pressure and effectively zero turbo lag, but also as a second charging or deploy device).
MGU-K: This may only use 4000kj per lap from the battery and it may only feed 2000kj back into the battery - kind of a designed energy shortage.
MGU-H: The MGU-H can charge infinitely towards the battery, and it can also deploy energy directly through the MGU-K - this energy does not count towards the 4000kj or 2000kj budget. This means MGU-H energy is basically "free" and gets harvested and deployed or forwarded to battery just as you drive (since it’s unlimited, the development of the MGU-H has become expensive, and thus it will be removed in 2026)
The MGU-K may never charge or deploay with more than 120 kw
the MGU-H is not regulated in that regard and information is non-existent, assumptions range from 30 to 150 kw. After some testing with the energy budget, we went for the lower end of the spectrum, with the MGU-H providing a maximum of 30 kw - which are basically available at all times

The key challenge now is to make the most of this while driving. Use enough fuel to be fast, use little enough to make the race distance. The ERS can help you both with saving fuel, or passing and going for lap time when it matters. The car has a maximum of around 1000hp, ~160 of that coming from the ERS. To manage these systems there are a variety of settings:

EBB: Electronic Brake Bias.
This shifts the brake bias forward the more you press the brake, but it also accounts for the current recharging intensity, shifting the BB even further forward. There are 5 EBB settings:

Map 1 is +0% BB shift forward when pressing the brake 100%.
Map 2 is 2%
Map 3 is 4%
Map 4 is 6%
Map 5 is 8%

Engine Maps
There are 12 maps, always 2 have the same power and fuel consumption, but have different throttle shapes.

Map 1 is full power and consumption, a linear throttle shape, while Map 2 has a more aggressive throttle shape.
Map 3 and 4 are Race maps, with 4 having the aggressive shape.
Map 5 and 6 are even more lean maps that should last a race distance, 6 aggressive shape.
Map 7 and 8 are Lift and Coast maps, very low power and consumption, with 8 having a progressive throttle shape,
Map 9 and 10 is for battery charging with full ICE power and consumption (pretty much equal to 1 and 2 - maybe i'll make more use of it putting it between Quali and Race),
Map 11 and 12 are Wet maps, they have full power and consumption but both are progressive throttle shapes, with 12 being very slow to pick up power for easier wet driving

ERS Maps
These alter the logic as to how energy is deployed and harvested.

Map 1 will drain the battery to 0 at maximum output from around 160kph, through all gears.
Map 2 will generally deploy less and will target to keep 20% of charge in the battery for potential "OVERTAKE" usage.
Map 3 is the map designed for the race where you dont have to worry about anything. It will deploy a little less than 2 and target to always have 40% in battery - in 8th gear you will also start charging.
Map 4 deploys all battery and is for the lift and coast scenario, here 8th gear will deploy maximum ERS even at lower throttle positions to save fuel at the end of straight.
Map 5 is for charging, very low deploy, aggressive coast charging (so advised to use with EBB 4 or 5).
Map 6 is for wet and deploys as 2, but at higher speed when you have enough grip

MGU-H Charge/Deploy
This toggle defines if the energy goes to battery (charge), or if you rather feed it into mgu-k, reducing the kj budget draw, e.g. in Q.

Recharge level
This adjusts the general recharge intensity, but there is little point in going lower than 100% - as this scales the recharging power down, in case its too aggressive for turning (unlikely). Also recharging intensity is also already part of the ERS Maps

Performance Modes
The performance modes are where everything comes together and choosing those should be enough to not worry about the rest. Each performance mode uses one of all the above maps so that they suit one another.

Push (PSH) is there for qualifying and will drain the battery to 0 and will deploy aggressively (while staying in the 4000kj budget), it will also have maximum fuel consumption, that will not last a race distance. EBB is low to assist rotation at turn in. MGU-H is set to deploy.

Race (RAC) is for the race, but will use too much fuel to make the distance most likely, but it gives you decent power for a few push laps. ERS will slowly drain the battery. EBB is one step more towards comfort and mgu-h will recharge the battery and try to keep 20% at all times

Balanced (BAL) is a leaner Race mode, using the battery in a way it will never empty and you will always have some ERS assistance. EBB even more conservative and secure, 8th gear will charge battery, targets 40% charge state

Lift and Coast (LIC) is for when you are trying to save fuel, very lean and low power, ERS will drain to cope, coast charging is minimal to coast longer, 8th gear will alraedy charge battery

Charge (CHR) Max fuel and power, very low ERS deploy, aggressive charging during coast, also charging in 8th gear

Wet Max power and consumption but later ERS assist, progressive throttle maps, EBB even more on the save side

The modes are dialed in in a way that BAL should be the "no worry" map for a full GP distance of 305km. Some tracks might require some saving, some will not. Definitely bind the overtake button as each track has a different profile and might allow for more ERS usage than the map would deploy. You can also just switch to a different ERS map temporarily within the perf mode, but this will be overwritten when changing performance mode again.

Of course you also have DRS, which when open, loses you about 10% of downforce, making the rear quite lose. Currently you can open DRS in every DRS zone, regardless of session or gap to car in front.

For reference: On Imola in qualifying Piastri did 1:14.7 - the car already feels mindblowingly fast, but reaching that time is not easy.

SF-25 Tires

Extensive work went into the SF-25 tires and if this proves itself in your hands, we will follow this principle for all compounds.

The target was to create clearer temperature profiles between the 3 compounds. They have different operating windows, heat slower or faster, rely more on surface or core temperature, wear faster/slower and also have bigger/smaller grip drop-offs from wear and temperature, are more or less sensitive to overdriving. We still might have to separate the baseline grip levels further, but we're doing this carefully and in isolation of all other things. Also dynamic track improvements in the future will play a role here.

Currently they should be 1.5-2s apart on COTA. It's difficult to test as the car has such a wide performance window power wise, the tires are difficult to get and keep in their window and then it requires the driver to perform on the single lap where they have optimal grip.
To keep things "easy" ideal core and surface temperature window are the same, yet differ per compound - the combination of core and surface in the window matters.

We are taking the SF-25 to show and explore what we can do with our tire model now that we have added a lot of functionality. It consists of A LOT of values and dialing them in is tedious, but rewarding when it works out eventually. Since we "only" have three and not all compounds, you will most likely end up with some tracks not working with the soft, as it just overheats quickly (e.g. COTA), but also some tracks like Monza are unlikely to work for the hard as it just doesn’t build enough temperature.

Soft (C5) 95-110C where its optimal, wears fast, big grip drop off when too hot and when wearing, relies more on surface temperature than core temp (ratio 50:50), heats the surface very quickly, is sensitive to overdriving by generating a lot of temperature but it will grain when you push above the peak slip angle

Medium (C2.5) 100-125C (importance of core to surface 60:40), less sensitive to all the above

Hard (C0) 105-140C (importance core to surface 70:30), even less sensitive to all the above

They all want 25.0 psi currently as ideal. IRL F1 has very different mandatory minimum pressures per track, we can't really do that and it would be (even more) confusing.