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Assetto Corsa Competizione Blog Post 2: Tyres

Paul Jeffrey

RaceDepartment Editor-in-Chief
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Kunos Simulazioni physics expert Aris has been putting pen to paper once again, this time talking about how tyres behave in ACC...

If you missed it yesterday, Aristotelis Vasilakos of Kunos Simulazioni has started a new blog post series providing a very enlightening and entertaining look at the physics work behind the scenes in Assetto Corsa Competizione, promising to release regular insights into the work undertaken so far on this new and still under development racing simulation from the people behind the hugely popular Assetto Corsa.

In his latest wall of text, Aris takes us on a journey of discovery around the world of tyres, and what they do and how they react in the upcoming Assetto Corsa Competizione... all very exciting stuff, and continue to do wonders to build anticipation for the new title ahead of Early Access release on September 12th.

You can read the full blog post below:

Tyres! And rain… But first…

The Rules!
The Blancpain gt series uses Pirelli tyres. Different sizes for different groups of cars, mainly 3 sizes depending the weight bias of the car. So front engine cars usually get 325/705/18 all around, mid engined cars get 325/680/18-325/705/18 front-rear and rear engined cars 325/660/18-325/705/18 front-rear.

Compound is one and unique for all races, all circuits, all cars. This means that this single compound must work in all cars, all weather conditions and all kind of circuits.

The above information is crucial in order to understand that these kind of tyres have an extremely difficult job to do. They have to work on cars that go from 40%, down to 55% of their weight to the front. Heavy cars that go up to 1500kg at full race trim while at the same time support well over 500kg of downforce, have to withstand at least 1 hour of hard racing before changing tyres and are driven in various circuits on various ambient temperatures by professionals and gentlemen drivers. At first I also thought “hey that’s easy, I only have 1 compound to do”, then as development went forward and Pirelli and teams started sharing data (thank you so much!) I knew I was into big big trouble… Luckily I have master Stefano always willing to accept a challenge!

Let’s start with the slick tyres.

Slicks have a wide operating range. They give decent grip from 40°C and up to 130°C. Obviously they have a narrower optimum range around 70°C to 90°C. Pirelli defines optimum pressure at 29psi (almost 2bar) but most teams will run a little bit lower. Pirelli though, advises against very low pressures as it is easy to deflate a low pressure tyre on a kerb or similar conditions… For safety reasons it is prohibited to go lower than 20psi (1.4bar) as minimum inflation pressure.

The tyres are always in preheated in tyre heater racks up to 70°C but realistically around 65°C, so expect to start any session (except maybe hotlap) with tyres at around 65°C core.

Pressure in AC now influences the stiffness rate in a non linear way and differently for vertical, lateral and longitudinal. The whole footprint flexes in all 3 axis and I believe you will definitely feel this when attacking kerbs. Damping of the tyre is also affected by heat.

The heating in ACC now has 3 interacting layers. Surface, core and inside air.

The surface heat is quite active, going fast up and down while influenced from slip, flex, rolling speed, ambient temperature, road temperature, air speed and rotational speed. Obviously it exchanges heat with the inner core too.

The inner core, is influenced mainly by rolling speed, flex and surface and inner air temperature.
The inside air is exchanging heat from the core and… brake heat.

For the first time though, we are not going to show you everything, just what the real teams get to look at, which means pressure and core IMO temperatures.

Tyre wear
This is now calculated in 3 separated IMO layers in a way that camber and toe can affect different parts of the tyre wear. If you use excessive amounts of camber and toe on a circuit with very long straights, then you will experience much more wear (and heat, more about it in a minute) on the Inner side of the tyre, making braking and traction worse but not affecting a lot lateral grip… and vice versa of course. Tyre wear is also implemented in a different way. We actually simulate the tread depth and we lower the depth as the tyre wears out. So you start with 3mm of depth at fresh tyres and you wear this out. Normally the teams and Pirelli consider a tyre as a very consumed one at under 1.5mm. The tyre wear is influenced by the distance covered, but most importantly by the slip. The more you slip the tyre, the more it wears and by “slip” we consider not only actual dragging the tyre on turns, braking and acceleration but also toe and camber, so again, watch out on how you setup your car.

Another important factor for wear is surface temperature. The harder you drive the more surface heat you generate, the faster the tyres wear out. Heck you could completely destroy a tyre by doing donuts for some minutes… Obviously you are not going to monitor tyre’s surface temps as it is rapidly changing and hardly measurable in real time (in the real life), but if your core temperatures are on target, then the smoother you drive the less wear you’ll have. Also graining, blistering and flatspotting are still there, with all vibrations now acting also on the suspension movement. Pirelli points out that those tyres do not suffer much of graining and blistering, but if you keep using wet tyres on the dry, well don’t expect miracles. TC and ABS levels can also play a role here. Since flex also influences tyre heat, a stiff suspension and dampers as well as high downforce, can also influence the tyre wear…. so many things, so little time, I know.

The Overall Feelings
All of this works together, obviously in real time and affects many aspects of the tyre behaviour. This is one of the biggest improvements of ACC. Heat, wear, grip, do not just influence tyre grip but actually change the tyre behaviour. Slipangles and slipratios, stiffness rate and damping, lateral and longitudinal flex that is now also simulated, all of them change in real time, depending all of the above factors. You can expect a cold slick tyre to not only have less grip, but to be way more nasty and on the edge. So if it starts raining and you’re on slicks, before aquaplaning issues, you might have to deal with a much more nervous car behaviour because the tyres lost heat and pressure. A consumed tyre has less flex too, generates less core heat and has different peak splipangles. You might find the grip acceptable but the behaviour changed for the worse. All is extremely dynamic and lots of placebo is going to occur…Be brave and endure the difficulties ;)

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Still, there’s more. ACC now simulates variable dynamic weather and so we have…

Rain in ACC is not simulated by simply lowering the grip. We simulate mathematically an actual water film depth. Tyres go over it and depending on tread design, load, speed and more, they manage to drain the water out and have a contact with the ground… or not. If the tyre can’t drain enough water, then it starts losing contact, up to complete aquaplaning, which means total loss of grip, zero, null, nada. So in ACC the feeling you get from a wet circuit is a good grip but a constant feeling of “something is about to happen”. You might do a turn in a specific way and feel there’s more than enough grip, you might even think “hey that was easy after all, arcade™!”, only to push a tiny bit more the lap after, or have the rain fall harder 3 laps later and go completely aquaplaning sliding out of the corner. The wetness also lowers drastically the heat generated by the surface layer of the tyre, so temperature of the tyres will go down inevitably.

Slicks can go into aquaplaning very VERY easy. I strongly suggest that you watch the first laps of the Hungaroring race1 of the Blancpain GT Series to understand how cars on slick struggle on damp conditions, but also how the BMW M6 that had wet tyres could work his way from 11th position to 2nd and struggle right afterwards when the dry line started to form. You can also see him searching for wet spots to cool down the wet tyres. Here's the video. Race starts in 33:00

Also in ACC wet tyres will overheat dramatically in dry conditions and you can cool them down going outside the dry line, searching for wet spots. Beware that in such conditions it’s easy to place one side of a car in the wet spot or puddle, resulting in high rolling resistance force from the water depth (and sudden aquaplaning) that can easily destabilze your car.

I will also mention the obvious…there is no way you can stay on the track under heavy rain on slick tyres. We’re not talking being slow or having difficulties to control the car… we’re talking complete and utter loss of control and sliding around on “ice”. Fear not though, for people that want to experience the graphical majesty of rain conditions but in a less hardcore grip situation, we have a nice option slider that will lower the amount of physics water… just for fun.

Staying on the dynamic track subject, here’s how a track surface changes through different conditions.
  • A green track will get gradually rubbered. marbles can appear at the side of the rubbered line
  • If rain starts then (depending on the force) it will wet the track and the rubbered line will start to be very slippery. You might be forced to avoid it or explore alternative lines.
  • If rain keeps on pouring heavy enough, it will clean the rubbered line and you might be able to turn back to a more traditional racing line. When this happens? I don’t know, try, experiment and find out!
  • If rain keeps on going, puddles and “rivers” might start forming. Those also might force you to try different lines again. Puddles and rivers are placed in specific realistic places on the circuits, derived from actual drivers feedback and their onboard videos.
  • If rain is lighter or stops and many cars are lapping, a dry line might form or simply a “less wet” line. You will obviously have more grip over the dry line but wet tyres will overheat.
  • Finally puddles will be the last to dry out, so watch out even if the track is slightly damp and slick tyres are faster, puddles can still catch you out.
Obviously this is a generic description of how the whole system works: in reality and when the whole thing will be finalized, your experience might vary a lot and can become more unpredictable. The whole idea behind it, is to have deal with unpredictable conditions that will force you to adapt.

Special guest…

Did you know that real drivers will go over the marbles to collect them on their slick tyres so that they gain a kind of “tread” which lowers a tiny bit the risk of aquaplaning? Of course you’ll have to deal with less grip and vibrations, but nothing is worse than aquaplaning and it might help you until you go in for your pitstop… or the rain might go away and you’re f***ed… ops!

Then we have tyre damage… but that’s something we still working on and I’ll explain it to you later.

So, what’s next? Aero I guess… back to writing. (where’s my coffee!)

Assetto Corsa Competizione will be available to purchase on Steam Early Access from September 12th 2018.

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Check out the Assetto Corsa Competizione here at RaceDepartment for the latest news and discussions regarding this exciting upcoming sim. We intend to host some quality League and Club Racing events as well as hosting some great community created mods (we hope!). Join in the discussion today.

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Like the sound of these blog posts so far? Looking forward to ACC day 1? Let us know your thoughts in the comments section below!
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I broke off in his post (in the AC forum) and watched part of the Hungary race - starting at 33 mins. Those experiences will really elevate this Kunos title. Not only is this so cool for ACC, but for future Kunos projects like AC2 etc.

This technical detail is great, it really is, but I'd love them to put minds at rest regarding UE performance too. Sure, the game lands on early access soon - so we'll determine performance (smoothness) for ourselves, but explaining what control they have in the engine, and how they've managed the quality to deliver a smooth experience - is pretty vital too. I know it's not more important than physics/tyres/susp/weather, etc, but gameplay that isn't smooth - can undermine all of those things IMHO.

AC ran extremely well for me. Kunos seem focused on quality, as they already said they won't port ACC to consoles if the current crop cannot cope with the new level of simulation. So I'm confident that they'll have tweaked the game enough for it to be a great title on PC, but I'd love to hear about this as well. It'll be nice to read the other topics they are creating in the build up to launch too.

This title is coming at exactly the right time for me, it'll really help the genre I feel.
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Alex Townsend

Loving the sound of this more and more. Seems it's going in the right direction and hopefully will be the successor to GTR2 that so many of us crave, but more so!

Great work Kunos and thanks to Aris for supplying these blogs giving us insight to the detail of work being carried out. I feel this is raising the bar for this simulation making it as close as possible to the real sport.
Me: This dog right here, it's the best puppy ever... I love it so much
Guy: YoU cAn NeVeR pRoVe oR mEaSuRe It iN aNy WaY, sO iT Is ObJeCTivElY FaLsE.

Come on now guys, let the poor guy show some admiration atleast without :poop:ing on him lol. What's the big harm in it. Are you that insecure about your *sim of choice* that him throwing a vague compliment towards its competitor is now immediately looked at as a personal attack against your inner belief system or what? What is this tribalistic behaviour LMAO. Damn, have some chill folks. I know us simracers are a special kind of "special" but wow. xD