Assetto Corsa Competizione: The 5 Point Tyre Model Blog

Paul Jeffrey

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ACC c.jpg

In another very interesting post from Kunos Simulazioni, physics developer @Aristotelis talks about the upcoming new 5 point tyre model for the simulation.


Developed by the makers of the popular Assetto Corsa racing simulation, and benefitting from the official licence to represent the 2018 and 2019 seasons of the Blancpain GT Series, Assetto Corsa Competizione continues to be expanded and improved upon following V1 release earlier in the year - the next development set to be a new 5-point tyre model...

Five point tyre model I hear you say... pray tell, what might this be? Frankly I've no idea, so I'll leave it to Aris to explain...

In modern computing, collision detection is still one of the most resource consuming operations a physics engine has to do. It needs to be very fast, very accurate and low resource demanding, but you can only choose two of them at once… Add to the equation that in AC and ACC the cars drive on an invisible physics 3D mesh with millions of polygons, derived from the laser scan of the real circuit, and it’s clear that a compromise has to be made. On top of that, AC and ACC uses the same player physics for the AI, requiring even more resources.

The collision detection of a simulation has to be very fast and very accurate, otherwise strange things might happen to the simulation. So in order to make it less resource demanding the tyre model of AC and ACC uses one single point to determine contact of the tyre with the terrain, being this last one a flat asphalt, bumps on the asphalt, various types of curbs, grass and so on.
This solution is a quite good compromise in order to have decent performance and high simulation accuracy. It permitted us to push hard and evolve the tyre model, adding more and more features on it. As a matter of fact the ACC tyre model is one of the most evolved ones, completely dynamic with a wide range of causes that affect and influence the grip and response of the tyre. Various heating levels, different tyre wear features, various influences in tyre rigidity and damping, completely dynamic slip ratio and angles, dynamic rolling resistance in different situations, full water draining simulation etc. etc. the list is very long, very complex, innovative and often involving completely original and breakthrough solutions, derived from meticulous studying and hard work of Stefano Casillo that you won’t find in any scientific paper, as he had to build new equations by himself.

Unfortunately, while still acceptable in AC, the more advanced physics engine of ACC put in evidence the limitations of the above solution. Our tyre model started to have issues and downright buggy behaviour under certain conditions over curbs. The use of laser scan circuits, gives no doubts on specific features of the circuits. If a curb is high, has a specific angle, has dangerous angled steps in it or any other strange feature, then the laser scan will show it in millimetric accuracy. On the other hand, our company motto is that we take no shortcuts in things we are certain. If a curb is made in a specific way, that’s how that curb is going to be implemented in the sim. If that means that our tyre model is going to suffer on it, then so be it, we will have to work hard to make it better and for sure we ain’t going to make the curb smoother just to “workaround” the issue. So let’s analyse what exactly happened with our tyre model in such conditions.

First of all, let’s see how the tyre model would deal with a high but smooth curb, like the many of the curbs at the circuit of Spa Francorchamps. As you can see in the screenshot a curb like this, although it has a smooth surface, it has a quite steep angle, often exceeding 30°.

ACC a.jpg

The single contact point is approaching the curb but still stays in the completely horizontal surface of the asphalt. Even if graphically the tyre starts to be “on the curb”, the actual point still remains down on the asphalt. This also tricks the mind of the driver, because in real life, if the edge of the tyre touches the curb surface, the driver will hear and feel the tyre touching the edge and take appropriate action or at least he will know he is gradually going over the curb. In ACC this won’t happen. As an example, many people see in real life the left inner curb of Eau Rouge being dirty from tyres and think real drivers abuse the curb. They try to do so in ACC and get an instant spin. In reality, you just need to touch the curb a bit and you can feel it and make it dirty. In ACC the driver will probably think he still has space and on the next lap he will go even close. This creates also a false impression to the driver of where the limits of the car are and the driver might caught himself trying to place his wheels in places he shouldn’t do. At some point the single point will go over the curb. Instantly it will understand a 30° surface inclination. This is an extremely big change on the contact point and a huge spike in load, vector, rolling resistance and so on, resulting also in big spikes in forces and grip. Consider the following situation:

You are on the limit of grip in a turn. The outside wheels are right on the edge. You climb with your front internal tyre on a high curb. This means that you raise the front inner end of the car and obviously you load the rear tyre with even more load. The tyre, already at the limit of its adhesion, cannot afford any more load so it starts to slide. In AC/ACC the front inner tyre will also take a big spike in load and rolling resistance, so it actually brakes for a moment and throws to the suspension more forces than it should. Those forces end up to the rear suspension and tyre in big spikes and the tyre loses even more grip. Usually in very stiff racecars, the inner rear tyre might even go airborne losing all grip and forcing the differential (if locked) to move even more torque to the outside rear tyre.

ACC b.jpg

If during the whole process, you also remain on the accelerator, you will have a situation where the rear outside tyre pushes forward with less lateral grip while the front inner tyre pushes backwards. Practically you car is transformed to a tank with treads that move in different speeds.

ACC c.jpg

This is why in AC/ACC if you modulate your accelerator the issue is practically non existent but also why if you stay on the accelerator the behaviour is exaggerated.

Another example are stepped curbs. As you can see in the following screenshot of a Paul Ricard curb, the steps of the curb are not equal from side to side, but gradually become more and more deeper (or in other curbs they might go upwards).

ACC d.jpg

The tyres of a GT3 car are quite wide, around 30cm wide. This means that often the tyre can be over the curb with the outside part going over the more shallow part of the step, and the inside part going over the completely flat part of the curb, leaving only the center of the tyre hanging through the most deep part of the stepped curb. The result in real life is that while you will feel vibrations from the curb, they will not be as important as the most profound part of the curb. Again, unfortunately in AC/ACC the single contact point at the center of the tyre, will get the worse possible condition of the stepped curb, something that in reality would never occur. So again in AC/ACC the behaviour is much more harsh and critical than in real life.

Finally, some kerbs have an almost vertical step at their outside edge.

ACC e.jpg

Often the driver will ride and go over the edge of said kerb and then slowly return to the main road going almost parallel with the curb. ACC in particular is extremely critical in such situations. Adding full 3D flex of the contact point (only vertical in AC), created a critical condition in the above scenario. The single contact point would go to the vertical parallel side step and being as high and vertical, instead of climbing over it, it would start to flex outwards, practically getting trapped in a rail. The driver would see that the car wouldn’t follow his commands to reenter and at some point he would move some more the steering wheel, creating more lateral force than actually needed. The front tyre contact point would climb over the step and then obviously would have excessive slipangle that would steer the front end very fast. At the same time, the rear tyre would be in the same condition and still trapped, so it won’t be able to follow the rotation of the car and will continue straight ahead in the rail, practically inducing the car into a very fast spin.

ACC f.jpg

This is one of the most well known and widely reported “curb of death” situations in ACC and in great need of a solution.

Paradoxically, sometimes trying to make the tyre model even more accurate, detailed and realistic to drive, you get into extreme situations that are so critical and have so bad side results that can practically destroy all the good intentions and effort to offer an even better simulation.
Fernando has been hard at work on our tyre model. With the help of Stefano so that he can understand the underlying code and make sure we get no big impact in performance. Some extra code performance optimisation from Fabio was more than welcome too. So from version 1.0.7, ACC now features a 5 point contact model! We implement 2 contact points at the edge of the front of the tyre footprint, 1 in the middle of the footprint and 2 more contact points at the edge of the rear of the footprint. Each single point moves and flexes independently reacting on forces and surface contact, but also, predictably forces to move the other points together, averaging the resulting forces and vectors, giving a much better representation of what an actual tyre would do.
Examining the above 3 examples again, we can observe massive improvements of how the new tyre model is reacting.

On the first smooth high curb situation, the advantages are multiple. First of all, when the edge of the tyre touches the curb it activates the sound and properly moves the FFB steering wheel, thus communicating at the driver the correct width and position of the tyre. Furthermore the contact points at the edge of the tyre, get the spike of the steep angle of the curb, but their forces are averaged to the rest of the contact points that are still on a flat surface. The tyre actually “climbs” over the curb, instead of instantly finding itself on top of it. There are no more load and angle spikes except the realistic load changes.

ACC g.jpg

So obviously if you are too aggressive the rear tyre will lose grip and can still provoke a spin, but the result is no more exaggerated so there are no more excuses for bad driving ;)

On the second example, the contact points now include the whole width of the tyre and if that’s the case successfully keeping the middle of the tyre in the air while also being spread longitudinally in the footprint length. There is always a contact point touching the surface at the front or the rear of the footprint even if the tyre is rolling on the steps. On top of that, the extra points are controlling for load spikes and avoid situations of excessive rolling resistance or vectors that point backwards to the car motion.

ACC h.jpg

This greatly improves acceleration over stepped curbs, as in example at the exit of turns, which in the past, drivers would avoid in order to not harm their acceleration.

Finally, on the most important third example, as clearly explained before, the multiple points now permit the tyre to “climb” over obstacles. So when the edge of the tyre hits the vertical step of the edge of the curb, those contact points start to flex and go parallel “entering the rail”, but the rest of the contact points, still push through the direction and push also the edge points to climb the edge. The driver doesn’t have to do anything with the steering wheel, and the tyre simply goes over the edge of the curb without any dramatic situations.

As an extra bonus, we have also added a new dynamic feature to the tyre flex behaviour. As you know the footprint of our tyre model now flexes in three dimensions. Going even further in tyre simulation, the lateral flex provokes the tyre to lower its profile. This means that the more the tyre flexes laterally the more the ride height lowers. Obviously the change in ride height is minimal, but in a car with proper simulated aerodynamics we know that even one millimetre is important to the handling and balance and so this new tyre model feature plays an important role to the car’s handling. You might notice a bit less understeer on power exit with some rear and mid engined cars, since the lateral flex of the tyre will bring the nose very slightly lower. Gives a bit more control to the front end of the car.

The new tyre model feels even more accurate to drive, permitting placing the car with more precision and absorbing bumps and undulations better with more predictability. All of this is now possible without any hits in terms of performance, which is practically a miracle and win win situation for all of us. As I’m writing this article, we are working very hard on balancing the various values and testing performances and handling, so that laptimes will remain more or less equal and general balance of the cars and your setups won’t change, except maybe for better precision, stability and predictability of the tyres both on and off the curbs.

One more thing…
In our never-ending research for more realism, we knew that we had to improve our Traction Control systems. With the strict rules of the Blancpain series and the BoP in place, there’s not much performance to be found in terms of aerodynamics, chassis, engine and suspension. But the electronics, although regulated, are still a big open field and automakers spend a big amount of their budget to improve such systems.
In street cars, the Traction control engages on the aperture of the accelerator (drive by wire), on the engine ignition timing, on the engine ignition cut and on the brakes. With a combination of all of those controls, the engage of the TC can be smooth and highly efficient.
In GT3 race cars, the control of the throttle and the brakes activation is prohibited by the rules. The TC can “only” modify the ignition timing which lowers up to a point the engine power delivery and if this is not enough, then it will start to totally cut the ignition at a high frequency, resulting in the characteristic engine rattling and vibration.
Been able to lower the ignition timing before cutting totally the ignition, is very important because it permits a more gradual power delivery and a more accurate control at the initiation of a sliding, giving the driver the possibility to modulate throttle application or work around the slide with steering inputs.
To better control the TC engagement, the ECU not only tries to estimate the sliding and slipping of the tyres, but now also uses gyroscopes that measure the yaw rotation of the car, sensors on the steering wheel, and many other parameters so that it can accurately calculate the acceleration of the yaw rotation of the car and understand and predict if it is controllable by driver or if it has to intervene to slower such rotation and give the driver the time to react.
All of the above is now simulated in ACC, delivering a more advanced TC intervention that not only can make you faster and safer but also, incredibly enough, make the car handling more enjoyable even over the limit! Honestly I never thought I would say this for a electronic system…

All of the above, was not a small feat by any means. It took hard problem solving, analysis, non stop testing and balancing. We are very proud of the results as we think they push the simulation realism even further and we are confident that all of you will enjoy ACC even more and appreciate the efforts to not deliver you just another racing game, but really push the boundaries of simulation. As with such complex systems, we hope everything will work flawlessly but if you find any issues, please report everything to our support forums and we will do our best to resolve them.

Original post HERE.

Assetto Corsa Competizione is available now on PC.

For more from the world of ACC, why not head over to our Assetto Corsa Competizione sub forum and get yourself into the thick of the action? We have a great and knowledgeable community, plus some pretty epic League and Club Racing events, if I do say so myself. Go on, treat yourself!



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This article/blog is good info.
Because now its easier to understand why most racing sims has huge problems going over curbs.
Hehe yes they have often problems even when the tires are just going near a curb :laugh:
At the moment Im mainly using Raceroom and can honestly say that a 5 point input to its curb behaviour would have the possibility to make a great sim even better. ;)

Raceroom possibly has the least advanced tire model. It feels like they took rF1, and then used less than half of it's tire simulation. But there are things to like about it too. DiY FFB certainly is not one of them. :D
 
Yes and 999/1000 people has no idea what they are looking at when they watch that ttool video. Including me.
Basically you can get a real life tire, cut it in half and replicate the structures you see there in that tTOOL, it's mind blowing if you are curious.
Best thing is that you can just do it yourself and no need to just take the word of the developer in a propaganda video, bad thing is... well... you need a real life tire and some can be pretty expensive for you to buy to just cut it :roflmao: ofc you can try to build your own or find some pics

But idk why rF2 fanboys need to post it here. Cant we just feel happy for Kunos improvements? ffs
 
Excellent writing. One step further to get simulation closer to the reality. I'm curious to know how other simulators deal with this aspect.

One thing that more always seem to be arcade in any race sims is the harsh way that tires react to the surface. They're are in kerb or in tarmac in a on/off way which is far away from the real experience. In karts I can get the feeling of tyre being on two surfaces and predict the reactions from it. Let's see.
 
I’m sure there’s a few different approaches to tyre simulation and I’m sure that all of them have their benefits and drawbacks. The cool thing is that there are developers out there (and I’m not going to name them) that are actively pursuing and constantly trying to improve and evolve this rather then throwing something together and quickly moving on to the next project, those are the true pioneers in sim racing.
 
Basically you can get a real life tire, cut it in half and replicate the structures you see there in that tTOOL, it's mind blowing if you are curious.
Best thing is that you can just do it yourself and no need to just take the word of the developer in a propaganda video, bad thing is... well... you need a real life tire and some can be pretty expensive for you to buy to just cut it :roflmao: ofc you can try to build your own or find some pics

Do you honestly think that if you had a tire cut it in half, then you somehow could replicate it ? :)
 
I do not see anything innovative, rfactor2, 7 years ago it implemented the "brush" patch contact model much more complex and detailed than this
All that flex complexity and the tyres don't even react to the track temperature. I can't imagine nothing more unreal for a racing simulator than the tyres being the same at 15 or 70 Celsius tarmac.

So S397 has their homework to do too.
 
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Do you honestly think that if you had a tire cut it in half, then you somehow could replicate it ? :)
With a look up table, and obviously its limitations, yes, it's possible. Some pro applications use solutions slimilar to these but in real time (I dont think a home PC processor can run a proper tire model in real time anyway), see Cosin FTire and I think rF Pro is the same
Personally I dont mind much, it's cool to look at and build your own if you are curious but I just need a sim to be convincing, I dont mind if they get a proper tire running or if they need something like an A380 landing gear data in order to replicate a GT3 tire, if it works it works :D
 
I'm always amazed by haters in such a niche market. We have (talking as a simracer too), like 3 or 4 dev studios that make highly advanced and innovative vehicle physics engines, completely custom made.
Those devs, push each other and deliver every year or even sooner, often updating existing and on the market titles for free, instead of waiting for the new one and charging for it.
As a dev I was impressed and amazed by the dirt model of iRacing and the dynamic rubbering/drying line of rF2 so to give you an example. It pushed us to do other things better, we implemented tons of active stuff, 4wd, 4ws, aero, hybrid systems and so on. They did so too in some of their cars.

If you see the history of those titles, each advancement of one, being in physics, graphics, licensing, sound, has pushed the other one to get better. Do you remember the initial tyre model of iR? well try it now. Do you remember the licensed cars and laser scan circuits of rF2? Well check again now. Do you remember the rain and night and sound of AC? Look again... It is a very healthy competition.

The end user, being hardcore alien, passionate simracer, or simply a videogamer that likes cars, are the main winners from this. They get software of amazing quality and realism at thousand times less from what they pay for their HW. Yes of course we can go on forever and analyse how the gameplay is not all you want it to be, how one thing or another doesn't work as you think it should and so on. Still, I'm pretty sure the vast majority of you have enjoyed their favorite sim for at least 10 hours... usually for years. With a similar price you usually get just a night at the cinema and pizza.
So even if you don't like a sim or dislike a developer for whatever psychological issue that I can't know or understand, you still have only to gain if a sim makes progress... sooner or later it will make your sim of choice to become even better.

Make setups, not war.
 
Thanks for the hard work @Aristotelis and everybody at Kunos. I haven't tried the new tyre model yet but I am definitely curious to see the difference in behaviour, particularly with the Lamborghini.

Just out of curiosity, have you experimented with many more contact points across the tyre? Say 30 points (one contact point for every 10mm of the tyre) or would this be too demanding on CPU?
 
Thanks for the hard work @Aristotelis and everybody at Kunos. I haven't tried the new tyre model yet but I am definitely curious to see the difference in behaviour, particularly with the Lamborghini.
It's not out yet, sorry. We're fine tuning and tweaking. Also v1.0.7 brings many other nice things too, so all has to come together.

Just out of curiosity, have you experimented with many more contact points across the tyre? Say 30 points (one contact point for every 10mm of the tyre) or would this be too demanding on CPU?

Honestly no because the hit would be too high. We tried 3 points which was already very good and we were about to settle with that... but 5 points was even better so we pushed for the later.
 
I'm always amazed by haters in such a niche market. We have (talking as a simracer too), like 3 or 4 dev studios that make highly advanced and innovative vehicle physics engines, completely custom made.
Those devs, push each other and deliver every year or even sooner, often updating existing and on the market titles for free, instead of waiting for the new one and charging for it.
As a dev I was impressed and amazed by the dirt model of iRacing and the dynamic rubbering/drying line of rF2 so to give you an example. It pushed us to do other things better, we implemented tons of active stuff, 4wd, 4ws, aero, hybrid systems and so on. They did so too in some of their cars.

If you see the history of those titles, each advancement of one, being in physics, graphics, licensing, sound, has pushed the other one to get better. Do you remember the initial tyre model of iR? well try it now. Do you remember the licensed cars and laser scan circuits of rF2? Well check again now. Do you remember the rain and night and sound of AC? Look again... It is a very healthy competition.

The end user, being hardcore alien, passionate simracer, or simply a videogamer that likes cars, are the main winners from this. They get software of amazing quality and realism at thousand times less from what they pay for their HW. Yes of course we can go on forever and analyse how the gameplay is not all you want it to be, how one thing or another doesn't work as you think it should and so on. Still, I'm pretty sure the vast majority of you have enjoyed their favorite sim for at least 10 hours... usually for years. With a similar price you usually get just a night at the cinema and pizza.
So even if you don't like a sim or dislike a developer for whatever psychological issue that I can't know or understand, you still have only to gain if a sim makes progress... sooner or later it will make your sim of choice to become even better.

Make setups, not war.

This was my point a few posts up exactly. Thanks for all the hard work you and the Kunos team put in and the thousands of hours of racing time it has provided me.

Please keep it up
 
I'm always amazed by haters in such a niche market. We have (talking as a simracer too), like 3 or 4 dev studios that make highly advanced and innovative vehicle physics engines, completely custom made.
Those devs, push each other and deliver every year or even sooner, often updating existing and on the market titles for free, instead of waiting for the new one and charging for it.
As a dev I was impressed and amazed by the dirt model of iRacing and the dynamic rubbering/drying line of rF2 so to give you an example. It pushed us to do other things better, we implemented tons of active stuff, 4wd, 4ws, aero, hybrid systems and so on. They did so too in some of their cars.

If you see the history of those titles, each advancement of one, being in physics, graphics, licensing, sound, has pushed the other one to get better. Do you remember the initial tyre model of iR? well try it now. Do you remember the licensed cars and laser scan circuits of rF2? Well check again now. Do you remember the rain and night and sound of AC? Look again... It is a very healthy competition.

The end user, being hardcore alien, passionate simracer, or simply a videogamer that likes cars, are the main winners from this. They get software of amazing quality and realism at thousand times less from what they pay for their HW. Yes of course we can go on forever and analyse how the gameplay is not all you want it to be, how one thing or another doesn't work as you think it should and so on. Still, I'm pretty sure the vast majority of you have enjoyed their favorite sim for at least 10 hours... usually for years. With a similar price you usually get just a night at the cinema and pizza.
So even if you don't like a sim or dislike a developer for whatever psychological issue that I can't know or understand, you still have only to gain if a sim makes progress... sooner or later it will make your sim of choice to become even better.

Make setups, not war.


Amen Brother!
 
Could you please update us on the current status of driver swap animations, pit animations and actually driver swapping with an AI character?
These were all advertised and are not in the official release, why?

Pit animations are returning. Driver swap with AI I don't remember about that being advertised so as it is not my sector I don't know what to tell you. I'm sure that if it's in the plans it will appear soon in the forum news
 

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