Understanding tire (tyre) pressures

Hi all,
Can anyone tell me about tyre pressures in AMS2. The reason I ask is that in ACC there is a recommended tyre pressure range for both GT3 and GT4 cars and you can drive laps and tune your tyre pressures accordingly. Even in the original AC there are little apps that can help to see if your tyres are in their optimal pressure range. I understand there are a lot of car classes in AMS2 so I'm just asking about GT3 mainly. If there is a generic explanation too for how to work out what tyre pressure should be for any car class, that would also be good to know.
Thank you,
JoshOz.

 

Pressures and temps in AMS2 don't seem to work as I would expect so I just forget pressures and work on temps. Annoying buy just my impressions.
I hope I'm proven wrong and I finally get to understand AMS2 better.

 

I recall reading here a while ago that the target pressures were in the same ballpark as PCARS2? This slide came from one of the threads… apologies for not finding the original post, and double apologies if the information is not accurate!

EDIT: removed attachment -- see following posts for much better answers!

 

Inner-Medium-Outer (I-M-O) tyre temps are the basis for tyre setup in AMS2.

I minus O must be targeted between 6 to 8 Cº at fronts for a close to ideal camber in AMS2.

M minus (I+O)/2 tell us how close to linear they are. Obviously the only manner to linearize these temps are changing tyre pressure. If M > (I+O)/2 it means an overinflated tyre, negative values and you get an underinflated tyre. Ideal is M equal to (I+O)/2. Linearizing those temps result in the optimum contact patch. So, the ideal pressure may vary according to the ideal temps. In this type of physical modelling you will not see a table of ideal pressures. Very different from ACC.

Problems: AMS2 v9 API passes wrong values of I-M-O temps via Shared Memory and more than this, tyre temps are integer numbers (no decimals). Those temps are ok via UDP but they continue to be integers and the result is diminished precision (and leaps in resulting calculus). Since v1.3.3.0 a new API was introduced and these 2 fails were corrected but no telemetry app I know was upgraded to use v10 till now.

I'm excited to this new era.

 

The simple answer is that tire pressure is not the most important parameter even though ACC has thought an entire generation of simracers that there is a magic pressure value that is valid for all settings and all track's conditions and all cars on all axles. That is not true to real life and it's one of the unrealistic sides of ACC physics: tire core's temperature is the driver of grip in real life as that is what activates the rubber chemical properties that allow them to glue to the tarmac, not pressure.
Pressures definitely help optimizing the tire's contact patch and lateral stiffness as well as having an impact on temperatures but it's not the key parameter to look at.
Also it is not true at all that tire pressures and temperatures do nothing in AMS2: you may not notice much of an issue if your race is 10 minutes or less, but on a full stint the impact on temperatures and tire wear is visible.
With that said, on GT3s in AMS2 you should try and keep tires within 85-95C. Above that range or below your tires will grip less than optimal.
Most importantly you want your most loaded tires front an rear to have a max of 10-12C but better 7-10C differential, to prevent the car balance from swinging too wildly across a stint. A very large differential between F/R will make your car very squirrely after a few laps.
Finally you should try and have 3-5C differential between internal and outside values on a single tire.
Typical settings that affect these values are not only pressure but also (if not predominantly) camber, toe, downforce and suspensions settings.
Also note that there will be tracks were one tire does not heat up as it is barely used: typical example is front right at Brands Hatch. In that case there is little to be done there apart from using low pressures.

 

Extremely informative posts @Ettore and @newtonpg !

 

Thanks for that Ettore. Also thanks to others who posted above. I have been thinking that you get the pressures right then worry about temps. I think the reason for my question is that I’ve always found car handling in AMS 2 to be a little too 'slippery' for my liking and I have tried to work out if it’s something to do with car setup or just inherent in the physics of the game. JoshOz.

 

I have a question regarding this. It's an honest question, since I never understood this line of reasoning.

Wouldn't every tire have an ideal (hot) pressure to ensure the biggest possible contact patch, and wouldn't the tire manufacturer know what that is and tell you?

Aris often stated in his streams that ideal pressures given in ACC are not for optimal grip per se, but for the optimal contact patch. Of course you need heat in the tires and all that. My question is, why would the same tire need different hot pressures to achieve ideal contact patch in different conditions? The ideal pressures given are hot pressures, not cold. It's true that you need more air in the tire on a cold day vs a hot day, but the end result is the same. The cars are all homologated and weigh more or less the same as well, the tire was developed specifically for GT3 cars afaik. The ideal pressures also changed a bit with the new DHE tire Pirelli introduced in 2020.

 

Things get a lot more strange when you look at the few road cars, vintage and TSI cars in Game. RL it feels very unconfortable, unsafe and wobbly when you drive the Polo TSI at 1,6bar. You will actually lose grip and control, not gain some. I usually run 2,4 bar RL front, 2,2 bar rear and I never had it "slide" in the dry. When you apply that in AMS2 you get a sledge,

 

Not Ettore, but I can answer this one.

tl;dr, the "ideal" tire pressure doesn't really exist.

Well yes, but actually no.
Yes, the tire always has an ideal pressure, but as you mention yourself conditions can change a lot. This condition can differ so much, that a manufacturer simply cannot know the ideal pressure/set-up for every single permutation of all the variables.

You mention that all cars drive the same tire, so why do they need hot pressure, right? This is mainly because these cars are not as similar as they seem, also the setup has a big influence on this.
The tire does not have a set hot temperature, it changes during the race (sometimes even within the lap). How these temperatures change depends on your driving style and, again set up. Steelcast has discussed this at length, so I won't spend too much time on it, but a setup can easily result in tire temp differences of over 5 degrees celsius. 5 degrees is enough to require a different pressure for a tire. This even applies if the conditions on track don't change.
So taking this into account;
and considering the fact that each track has a different ideal setup,
also that each driver has a personal driving style,
and track conditions can also change
and that race setups are different from qualification setups,
We can see an ideal pressure doesn't exist, so the manufacturer will definitely not be able to tell you what's best.

There's a lot to take into account, but personally, I feel this isn't the most important setup parameter. And if you're keeping standard setup guidelines in mind, I find that the ideal pressure follows easily from that.

Hope this makes sense.

 

Tire pressures matter for the amount of rubber in contact with the tarmac.

Tire temperatures matter for the quality of grip with the tarmac.

If your optimal contact patch results in being outside the optimal operating temperature for the rubber, then you need to compromise your contact patch in favor of temperature and grippier rubber.

In the end, the objective is to maximise grip via temperatures and contact patch. You want to find the combination of both that will give you the highest amount of grip (without getting excessive wear unless it’s a sprint race).

 

To add to Koen's answer, the "optimal" hot pressure is a compromise between the friction coefficient and cornering stiffness of the tyre:

Speed Secrets: How To Dial In Tire Pressures - Winding Road

Too high and your tyres lack grip (because too small contact patch), too low and they don't have enough cornering stiffness to carry the loads put on them. The problem is that the loads are different depending on whether the tyre happens to be on the inside or on the outside while cornering. It is highly unlikely that the same pressure is optimal for every tyre for example at Silverstone, where the left-side tyres experience much more load over a lap than the right-side tyres.

Furthermore, downforce adds load to the car in a dynamic way and so must be factored into the tyre pressure calculations.

 

Thank you everybody for your inputs!

I think my error was to concentrate on the optimal contact patch too much without taking into account all the other factors like sidewall stability. I have the book Speed Secrets lying around somewhere, I guess I'll re-read it quite soon

 

The short answer is that the first task of tire pressures is to counter the vertical load acting on them: the vertical load is mostly due to self weight and downforce. As downforce changes by track so does the optimal pressure on a given car. Furthermore, different cars have different weight distribution and also the weight total and its distribution varies across a stint as fuel is used. So really Aris is not telling you the complete truth and ACC is a bit of a cheat in that their optimal pressure (even when you disregard all other factors except the ones above) is static and does not account for any of those variables.
Secondly, if you think about that, downforce varies per corner: if you think about the Maggot's/Beckett's esses at Silverstone they will require a lot of tire pressure to achieve optimal contact patch, while an hairpin like the one at Suzuka will require far less and would probably enjoy a much more deflated pressure to have an optimal footprint as downforce will be much less.
Therefore, depending on the type of circuit, you may be better off giving priority to one or the other direction.
On the cars weight: cars have up to 100 kg weight difference (7-8% of the total weight, not totally negligible and it should reflect on optimal tire pressure for sure) but that is not really the number you should look at. You should look at the weight on each axle and differences there are even more stark: imagine the weight on the front axle and rear axle for AMG and 911 and you will see that the difference in percentage on the same axle is quite big.
What Pirelli is giving is the minimum pressure to prevent structural damage but that is not an optimal pressure for performance.
Also, when it comes to performance (meaning the amount of grip the tire can exert) the correct window of temperature is far more important than the optimal amount of contact patch in real life and the two things can't be separated as Aris seems to imply. The two matters are related and often are subject to a trade off: you might be able to optimize the tire grip in a condition where the contact patch is not optimal and still be faster because of the increased grip due to temperatures management. Vice versa on some tracks like Monza, you may not optimize the pure grip but increased pressures leading to higher top speed may more than compensate for that on long straights (within reason).
The above is the reason why I said previously that ACC has some severe physics simplifications that are sold as the bible when really they are just shortcomings/cheats.

 

I love these discussions, but it would be great if the developers would stick their nose into these conversations once and a while to confirm / deny.. or correlate what's being talked about to what the game actually does. I understand that they're busy, but it was just a thought.

 

i never understood tire pressures in games.
i get the idea and i can make them work to get a decent grip/temp mix, but when comparing the pressures to the one i have in my daily driver IRL, using similiar pressures would be way too less (assuming i am using pressure from a roadcar from ingame)
anything below 2 bar makes the car irl very floaty and behave very weirdly, if not unsafe. no idea how in games u are regularly using way less and not get the same effect

only game that has ever seemed to be close to reallife was live for speed, where even the sidewalls have a very similiar flex to what i can see IRL and the floaty/swimming effect is also very close

btw i am not comparing gt3 tires to my reallife car tires

 

i think devs replying to such threads, would lead to something like ACC, where everyone is just aiming for the perfect value.
watching any semi-pro youtuber you will see that they are constantly aiming for 27.6psi regardless of track or car

i think the whole not knowing what the exact "best" value is, keeps things interesting. i could prolly look up the calculation to get the ideal numbers, but it would ruin the enjoyment of tinkering with the setup for me

i once did a P3 (i think) race on suzuka and came up with the stupid idea to just run max pressure to see if the long straights would gain me a time advantage.
and i did end up winning that race, simply cuz i was sprinting away from the corners so much faster, even tho i lost a bit speed through the turns.
this try and learn is what really makes it fun

 

The shortcommings part is probably just as severe conserning AMS2.
Which could also explain why it is suggested to not use the i-m-o temp difs to anything important in AMS2

ByTheWay: As allways its a good strategy to take all explanations shortcircuiting from RL to virtual sim(plifications) with tons of salt. Tons of . Because socalled sims like ACC and AMS2 (and others) are lightyears away from a 1:1 relation to the complications in RL.

 

It's very simple: street tires have a way different construction from race tires IRL. Race tires can accept a far lower pressure IRL too, reason being their construction is much more resistant and rigid. Such thing would be unacceptable for road cars as noise and harshness would be unacceptably bad on top of the fact that race tires only run in a controlled environment for few miles, road tires will need to stand for 25k km even when they are racy Michelin Cups...