Assetto Corsa Competizione: The 5 Point Tyre Model Blog

[Paul Jeffrey]

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°.

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.

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.

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).

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.

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.

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.

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.

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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[giga1tr]

Thanks a lot to Kunos for this great development.
We all love simracing and haters have no room here. I am grateful for every development in every sim and for every progress. It's great when developers keep working and listening to the community. Since realese ACC has made great progress and every developer is actively involved in the forum and blog. I've never experienced that in another game like this. The developers of ACC are Simracers like us. But they have the talent to develop a great game for us. Thanks to all developers of ACC and every simracing game.

 

[boomn]

Agree, this was very informative. I'd like to hear more about tire models, but most sims keep these things secretive and only use buzzwords to describe how good their current model is. It will be interesting to see how close this different approach can get you to sims like iRacing and rF2, which simulate a whole tire with hundreds of contact points.

The biggest thing I have learned from this is that what we normally hear about contact patch (and flex, heat, surface grip, etc) in any sim's tire model talk isn't necessarily the same thing as this issue of interaction with the road mesh

rF2 may be calculating a lot of tire data within their tyre model (as do most current-gen physical tire models, even PCars) but that doesn't mean it calculates hundreds of interactions with the road mesh per physics cycle. That's just not possible with our PCs! rF2's brush model does seem to naturally lend itself to more precise interaction, but I wonder how many brushes it is calculating and how they interact with the road mesh? Is it a few more, a lot more...?

 

[Lucio Distilleri]

I'm always amazed by haters in such a niche market.

now it's a habit
do you praise the game? fanboy
do you criticize the game? haters ...
developers should start wondering why criticism, labeling haters is the easiest thing you can do ...
behind a criticism there is always a fundamental interest because those who criticize would like the product to be better
I would never go to the pcars forum to complain, because it is not a product that interests me

 

[Troy Barman]

We live in the generation where you have to be for or against something, middle ground doesn’t exist, either the game is great or it totally blows.

 

[BrunoBæ]

We live in the generation where you have to be for or against something, middle ground doesn’t exist, either the game is great or it totally blows.

Quite honestly if you are able to read then about half of the posts in this thread is exactly like what you are looking for.
They are "the middle ground" and both positive and openminded to ACC but also to sims/games from the competition.
Just read with a bit more unbiased mind.
mkey?

 

[Deleted member 113561]

I'll bite. Current Steam rating of sims based on all reviews:

Assetto Corsa: 86%
Assetto Corsa Competizione: 73%
Automobilista: 84%
iRacing: 77%
Project Cars 2: 74%
Raceroom: 60%
rFactor 2: 81%

So it's pretty much in line with sims like iRacing or Project Cars 2 and this is from a game that has just come out of early access, so I reckon the score will improve. Assetto Corsa is the anomaly with such a high score, but IMO it's obvious a game focusing on a single series will never garner as high a score from the average consumer on Steam.

In line? You kidding me
It is far worse than most sims. PC2 was also a buggy mess at release (score reflected and still reflects that) and still has issues - yet it has 88% on recent reviews - see your numbers are misleading because they include early access reviews. That's why I pointed towards reading the (negative) user reviews, which are recent; TLDR they all say the same, it is buggy and unfinished and as someone who tested it myself, I only can confirm that assessment.

iRacing you can't take seriously because it is only part of the user base that has it linked to Steam and most core users have gotten it before it was on Steam and haven't linked it to Steam.

I disagree with AC being an anomaly and wanna point to AMS (and ie. PC2 recent reviews) - they are both just good games. Or in AC case: They had also bad reviews, especially at launch it was not a good game, but they improved it dramatically over the years.

 

[Austin Ogonoski]

I do not see anything innovative, rfactor2, 7 years ago it implemented the "brush" patch contact model much more complex and detailed than this

Correct.

 

[StefanoCasillo]

I wonder where you guys read that this was in any way revolutionary, ground breaking, history making, bettah than evah though? We deliberately tried to avoid that kind of language and approach specifically to avoid these kind of responses and yet.. there's no winning with some of you.

It's a step forward compared to what we had in ACC... interestingly enough it's actually a "step back" because AC used to have multiple contact points in the first iterations, that's all there is to it.

we're happy about the improvement and the result, hopefully you'll also be once you try it... nobody here is claiming the second coming of jeezooz... really, calm down and enjoy the weekend.

 

[tlsmikey]

I'm really appreciate the detailed update and what is being done to produce more accurate curb behavior. That's something that has plagued a lot of sims, so it's cool to better understand the problems and what was causing it. I think ACC has done an amazing job with the engineering and simulation portion of this title.

Unfortunately, what's keeping me out of it at the moment is the graphics in VR and triple screens, so this update won't get me playing more other than a few discovery sessions.

 

[callumjtc]

I played it at 1.0.5, still buggy. As said, look at Steam user reviews and you will understand.

I still get the rare bug now and again, but I got more bugs from Minecraft and how old is that game? I don't need to read the reviews, as people are much more likely to complain than to gush affection. I have played this game since the very beginning and can form my own opinion, and on my system it is going from a slightly meh start to what has developed into a strong title with potentional.

 

[callumjtc]

I wonder where you guys read that this was in any way revolutionary, ground breaking, history making, bettah than evah though? We deliberately tried to avoid that kind of language and approach specifically to avoid these kind of responses and yet.. there's no winning with some of you.

It's a step forward compared to what we had in ACC... interestingly enough it's actually a "step back" because AC used to have multiple contact points in the first iterations, that's all there is to it.

we're happy about the improvement and the result, hopefully you'll also be once you try it... nobody here is claiming the second coming of jeezooz... really, calm down and enjoy the weekend.

Good work and we really appreciate the proper solutions to a real issue

 

[mms]

I wonder where you guys read that this was in any way revolutionary, ground breaking, history making, bettah than evah though?

I might have missed some posts, but I don't see anyone implying they expected this to be revolutionary (or the other things you mentioned).

I don't think some people comparing to whatever exists in other sims, or just stating that this isn't something 'revolutionary' or 'innovative' should bother anyone, the most important is that you guys are improving ACC one step at a time, and hopefully it will follow the steps of AC to become a fantastic sim.

And I wish for everyone at Kunos to enjoy the weekend too!

 

[Deleted member 113561]

I still get the rare bug now and again, but I got more bugs from Minecraft and how old is that game? I don't need to read the reviews, as people are much more likely to complain than to gush affection. I have played this game since the very beginning and can form my own opinion, and on my system it is going from a slightly meh start to what has developed into a strong title with potentional.

Minecraft has less bugs than ACC, fact.

Make it more obvious you are a fanboy and biased.

 

[Cote Dazur]

I might have missed some posts, but I don't see anyone implying they expected this to be revolutionary (or the other things you mentioned).

You might have missed a few, they probably are on your ignored list, with good reason. They are just here to stir trouble and demonstrate one more time how frustrated they are with anything that is done by Kunos. Best to ignore them, as they truly are insignificant. Always the same too, almost predictable, especially the famous one.
You would be wondering if something happened to him if you do not see his derogatory comment. Thanks fully he is still around, we do not wish anything ell happened to anyone even to the sh**tdisturber.

 

[Oldzeb]

when will the new IU?

 

[NDG]

Very good read, from OP to every single argument. What I find odd is they even thought of using a single point in the first place. This 5 point model definitely makes sense, but what doesn't is why even start with one? Some kinda math problem all your classmates solved in high school, and finally you solved it at 40.

 

[ermo]

@kunos / @Aristotelis

Cheers for the excellent writeup!

Am I to understand that the point (pun intended) here is that there are now 5 brush elements in contact with the ground at all times (or, well, most of the time), simulating the edges and the middle of the contact patch in an (elongated?) quincunx pattern?

Something like this when viewed from above (or below) where each point represents the end of a bristle on the brush, which is connected to ... the rim (for purposes of calculating height under 3d deflection)?

front (direction of travel)
^^^^^^^
o     o |
   o    | contact patch length
o     o |
-------
contact patch width

With PC2 (and I guess PC Pro and AMS2), I believe SMS settled on a grid of 3x11 points per contact patch with a physics calculation rate of 600 steps per second, which obviously has the drawback that the engine simply can't run the same model on the AI cars (I saw some tests being done with a CPU core per wheel, which helped mitigate the performance cost at least a little). I also saw tests with more points per contact patch and more simulation steps per second, but they weren't workable on console CPUs AIUI.

Your brush model also appears to be quite advanced (aquaplaning etc.); do you also simulate heat propagation from surface through the various elements of the tyre/rim/air/brake setup in any way, shape or form?

In terms of AI cars, do they also run full player suspension/aero models in AC and ACC or do you have to simplify them a little due to physics calculation budgets?

Finally, how many times a second do you do physics mesh collision calculations for the contact patch?

Inquiring minds would like to know.

All the best with ACC!

 

[M D Gourley]

One more thing…
In our never-ending research for more realism, we knew that we had to improve our Traction Control systems.

Absolutely enjoying ACC 'Medium Rain' racing on every track...outstanding...especially on 'Spa' where I came across a Traction Control necessity at around 'Kemmel' just before the Kemmel Straight when driving the Bentley. I heard the water splash hitting the underside of the Bentley when driving through a water pool and the Bentley gave a small speed wobble with having the TC on full, but I was able to control it and carry on. I thought I would reduce the Traction Control by half and try driving over that same spot again at 'Kemmel...result, speed wobble and lost control....AWESOME...great to see the Traction Control working...keep up the great work.
Screen shot SPA at 'Kemmel' with full Traction Control...had a small speed wobble but controlled

Screen Shot SPA at 'Kemmel' with Traction Control at Half strength...and losing control...awesome

 

[callumjtc]

Minecraft has less bugs than ACC, fact.

Make it more obvious you are a fanboy and biased.

I am only a little bit of a fanboy but I'm not sure where the bias comes from? To be biased I'd have to have some reason to support Kunos and their sims. That reason is that I like and enjoy the sims they have produced. So the "bias" is in fact, in my opinion, deserved support.

 

[Deleted member 113561]

I am only a little bit of a fanboy but I'm not sure where the bias comes from? To be biased I'd have to have some reason to support Kunos and their sims. That reason is that I like and enjoy the sims they have produced. So the "bias" is in fact, in my opinion, deserved support.

Wait, so now you admit that you are a fanboy ... and then ask where the bias is coming from? Then try to turn it into "deserved support". No further questions your honor.