Physics right or wrong?

Frederic Schornstein · Jan 4, 2014

Disclaimer:
First of all please don't make that a fanboy discussion. I always try to be objective and leave my personal preferences out of the way. I am always willing to accept, if something I don't like on emotional basis, is better than what I like on emotional basis.

Ok I hope I covered that part and this won't get into a "You are a fanboy of this and that thread".

Prequal:
Like most people I eagerly awaited the release of Assetto Corsa. I read about all the licensed content, loved the screenshots and everything. I didn't drive the tech preview, because I didn't want to get a false impression, because it was all still WIP. The day AC was released on Steam I bought it.
I started really testing it after the first update were the FFB was fixed.

I do really like the game and wanted it to be good and so far my only issue are the physics. Most people praise them and they get a lot of love and I do confess, that in external videos they look great.
The car dives in on the braking, you see all the weight transfer and everything. No other sim offers that!

Example:
But as soon as I drive it, it feels just so wrong to me. Just one example: I tested the BMW Z4 GT3 at Monza. I know the track very well and usually drive GT cars, so a perfect combination for me.
Despite all the massive understeer you get everywhere in AC even with setup tweaks, I wanted to point out something you can judge on a much more objective level. So I will talk about the braking distance.

Every racer knows how hard the first braking zone in Monza is. You have a low downforce car and arrive at 260+ kph and have to break down to 60-80kph. So if you look at a lot of GT3 onboards on youtube even with cars, that do have ABS and are a bit restricted due to BOP. They all break before the 200meter board.
That said, I can brake at the 300meter (270kph) board and have the car at a total stand still at 150meters. If I brake for the corner I can brake at or after the 150 meter board.
So we already have a 25% shorter braking zone compared to a real life car with equal or less power and ABS, which I didn't use.

Test conditions:
So now people would start arguing, that I maybe run more downforce and stuff and that is the weirdest thing. I tried to figure out the "worst" braking performance. I used hard tyres and removed all the downforce from the car for this test.

I don't want to get to much into the details of the cornering behaviour and the understeer and that you can just pull on tons of lock and don't get any turn-in oversteer.

Is my opinion qualified?!:
A lot of simracers don't have a lot of reallife experience and I didn't race a GT3 car in my life, but just a quick background:

I drove Race07, rFactor1 and now rFactor2 and I am a pretty good driver in rF2 especially in rear wheel drive touring or GT cars. Even in the new Civic I was racing for wins after 30mins on an unkonw track, so I can quickly adapt to new cars and tracks etc.

In my free time I did some kart races with friends and even on an unkown track with for me unkonw more powerful karts than the average rental karts on a bit cold track I got within 1,9 seconds of the track record within 15min and reduced it to 1,5 seconds in a further 15min.
Keeping in mind that real professional Kart drivers practice their and I had maybe 2 or 3 hours track time in karts ever, I would say I am not a bad driver.

So when I jump out of rF2 into a kart it just feels like home. I apply nearly the same technique and everything. When I jump into AC it all feels wrong. It is so hard to get wheelspin. You can turn the wheel so much, that you would end up in a wall in real life.

I also spoke to some guys with actual racing experience and they got the same feeling.

The end:
I really wanted AC to be a very good sim and I do love everything about it, but these physics keep me from driving it.
In a sim I don't want it to be easy, I want it to be as realistic as possible, but in AC you can apply some really bad driving technique to get quicker laptimes.

Frederic Schornstein · Jan 7, 2014

Michael Hornbuckle said:
I don't care if you don't like my attitude and you don't get to tell me where, how, or when I can post. Sorry that since I don't share your views I am a troll. FO

You didnt share any view there and I only said if you dont mind, so I never told you what to do. So keep it calm as I do

Michael Hornbuckle · Jan 7, 2014

EDIT: Nothing offensive now

Frederic Schornstein · Jan 7, 2014

Michael Hornbuckle said:
Whatever, You called me a troll and yet you are the one calm.......again, FO

I didnt call you a troll at all. You only made a trolling statement in that last post. I dont think your other comments here were the comments of a "troll", but your last question was only there to provoce others and was never an argument in this discussion. So it was not useful and the only thing what it could have done, would make it something I never intended to do (see the first part of the OP).

If you havent understood it now, I am sorry

David O'Reilly · Jan 7, 2014

Great tech discussion men. I consider myself informed by it.

Matt Sentell · Jan 7, 2014

Joco Gorenc said:
Sure, but no need for a billion calculations to do that right? It seems to be working fine at this stage in AC, but one thing I find strange is that the tires heat up so quickly when going straight, and once at 100C they dont cool off as well...

Not likely a billion, no.

But a few more than are done on single layer models.

The point is that if you want to be able simulate the rapidly changing surface temps of a tire, which is the part that determines CoF since it's what's in contact with the road, you need to do a bit more work. But more to your previous comment about it not being that meaningful, here's an interesting graph from Michelin that I believe was in relation to F1 tires:

The relevant piece is the box labeled friction coefficient, which is a three-dimensional visualization of CoF vs tread surface temp vs sliding speed. Reading 3D charts on a 2D slide like that, as opposed to being able to interact with it and place a pointer for exact readings, is a bit of an acquired talent. But if you focus on the front side in terms of sliding velocity, at 100C you're around the peak CoF of 1.8. At 150C your CoF has dropped to approx. 1.5. That's roughly a 16% reduction in CoF(!).

(CoF is not the only factor determining available grip, especially in the dry, but it is by far the largest and I'd rather avoid going into that detail.)

And the point is that in the physical world this surface temperature can change very rapidly indeed. In the FLIR video up above you can see changes of 20C in less than a second, and that's with a driver staying nicely within limits. He's only sliding the back of the contact patch, in other words, not the whole tire. A tire in a full slide, at high speed, might increase by 100C or more in a second. And between those two extremes there's a great deal of variability depending on driver talent, style, and how hard he's pushing. Again looking at the graph, the peak of 1.8 is across a fairly narrow range. Perhaps 20C, if we're being generous? I think it's fairly obvious that a driver can significantly influence available grip by how well he or she can stay near that optimal range.

In a sport where fractions of a percent are considered meaningful, I'm not sure how one can dismiss this kind of data as unimportant. Consider that in a typical 1m30s lap, a mere 2% of that would be 1.8 seconds. At the sharp end of the grid people consider 1.8 seconds to be completely off pace. And in these debates it's often said that if you aren't within a few tenths of the top times, or let's say half a second, then you shouldn't even comment on things like handling balance or other physics issues. That would mean a difference from the top times of just 0.5%.

I think a reasonable person, looking at the available data (which is admittedly limited in the public domain), would have to conclude that what might at first seem to be minor variations in tire temperature may in fact have a very significant influence on performance. Why else would real race teams, increasingly so at the higher echelons, place so much value on monitoring them?

Matt Sentell · Jan 7, 2014

It seems as if this discussion will soon derail again so I may just leave you guys to it. For the record, I'm not trying to influence anything, let alone anyone to stop playing AC. I was just driving it yesterday. It's a very good sim with a lot of promise.

The reason I usually stay away from discussions about sims in public forums is that it seems there's a psychology at work with many people where no criticism or even questions can be leveled at their favored sim without it being interpreted as an attack on their personal choices and beliefs.

All of these sim developers - the serious ones at least - are on a journey of discovery and invention themselves. They aren't nearly as defensive about well-reasoned critique and questions of their work as the community is.

Joco Gorenc · Jan 7, 2014

Cool write-up thanks. I dont defend it if I dont believe it, as I said I do find some things strange but it seems ok in general. For sure there can be improvements made. Long way to v1.0.

Paul Lock · Jan 7, 2014

If my house wasnt even halfway built, and the builder gave me 'early access' to it, I (personally) wouldnt be too concerned that it was not yet perfect.

Stereo · Jan 8, 2014

My impression of the surface temps. is that they're closely linked to how much load you're putting on them - the front tires go +20C or more right at turn-in (mostly on the inboard side of the profile, presumably due to camber), and then slowly heat through the corner, as the rest of the tire picks up temperature too.

The thing about that is that the accuracy of the coefficient of friction is most important when you're trying to wring the maximum forces out of the tires. Aside from being necessary to doing accurate heat simulation, it doesn't really matter what temperature the front tires are in a straight line - they're just producing minor aligning forces to keep the car straight.

So if you have a single layer model that heats/cools appropriately (one step removed from the surface's current conditions can be compensated of course) that then returns coefficients of friction appropriate to that current state, the actual temperature at the skin becomes academic - if you're putting 1G lateral force on the tire, it'll have some temperature higher than the core, but you can compensate in your lookups, and heat the core at the expected rate until the system reaches equilibrium, or you stop turning.

It's interesting that the ideal temperature changes as sliding friction goes up. That could even argue in favor of the single-core-layer model - the faster surface heat generation means a steeper gradient of temperatures through the carcass, so the 'equivalent' core temperature might actually be a straighter line across slip speed than the surface temperature. On that plot it looks like peak is at 50C for low slip, 100C for high slip...

In any case more data is always welcome

It's fine to say the tires are too grippy in some instances, but better if you can point to data showing how they should perform. And as long as they're dynamic in the right ways, it leads to fair realistic racing even if the entire field is 0.1 second faster than in the real car.

Deleted member 151827 · Jan 8, 2014

I hate these threads! Isn't this supposed to be about the fun?

BanjoMaster · Jan 8, 2014

Doug T said:
I hate these threads! Isn't this supposed to be about the fun?

No, 'dis are serious business!!!!!1!1!

Apparently, by law you have to pin your colours to one sim's flag for YOUR ENTIRE LIFE. Otherwise those flying ghost things from Harry Potter come round your house and wizz in your kettle, put cheese in your DVD player, etc.

Simon Roberts · Jan 8, 2014

BanjoMaster said:
No, 'dis are serious business!!!!!1!1!

Apparently, by law you have to pin your colours to one sim's flag for YOUR ENTIRE LIFE. Otherwise those flying ghost things from Harry Potter come round your house and wizz in your kettle, put cheese in your DVD player, etc.

I like cheese, Especially cheddar.

BanjoMaster · Jan 8, 2014

Simon Roberts said:
I like cheese, Especially cheddar.

I like cheese too, just not rammed into my doovd player! Brie in particular is a real pain in the arse to remove from consumer electronics systems...

Fardrum · Jan 8, 2014

I Frederic thanks for your interesting contribution.
I have experience on karts and sports cars on a real track, and I find them to be very different.
the kart has no differential and requires a completely different technique, furthermore load transfer are much more istantaneous on a kart, and if it's a kart without gearshift there's also a different technique involved in driving to keep the engine rpms pretty high to avoid loosing power when exiting turns.
So saying that you can switch from a real kart to a sim-racing game (and not a kart simulator) and feel at home leaves me with some doubts, but i cannot exclude that my understanding of the kart world is completely wrong, in fact my kart lap times are not even that good after all.
But speaking to advanced kart drivers they all have confirmed that karts are a different world although many basic concepts do apply in the same way to both worlds.

Hope I have understood correctly the meaning of your post.

Frederic Schornstein said:
Disclaimer:
First of all please don't make that a fanboy discussion. I always try to be objective and leave my personal preferences out of the way......

OMISSIS
....
In a sim I don't want it to be easy, I want it to be as realistic as possible, but in AC you can apply some really bad driving technique to get quicker laptimes.

David O'Reilly · Jan 8, 2014

Doug T said:
I hate these threads! Isn't this supposed to be about the fun?

Doug
Personally I love this tech stuff as long as we can avoid becoming defensive about our favourite game.
When I am out there getting my tyres up to temp to the knowledge that I need to get heat into all layers feels good.
After reading this thread it helped me to find 1 sec on my quali time by managing temps better.
When I look at my Motec data and see it takes 6 laps to get to full operating temperature in the Nissan Gtr GT1 it all makes sense.

So each to their own but the techy stuff increases my enjoyment.
I haven't had so much fun since I read the Hoosier race tyre manual to better understand how to manage the pure Crossplys on the Howston H4/6 in RF2.

Robert Nocoń · Jan 8, 2014

David O'Reilly said:
Doug
Personally I love this tech stuff as long as we can avoid becoming defensive about our favourite game.
When I am out there getting my tyres up to temp to the knowledge that I need to get heat into all layers feels good.
After reading this thread it helped me to find 1 sec on my quali time by managing temps better.
When I look at my Motec data and see it takes 6 laps to get to full operating temperature in the Nissan Gtr GT1 it all makes sense.

So each to their own but the techy stuff increases my enjoyment.
I haven't had so much fun since I read the Hoosier race tyre manual to better understand how to manage the pure Crossplys on the Howston H4/6 in RF2.

True, true. I'm a telemetry freak. Setup freak. I want sth that gives me challenge not only on 1 level but 2312892 levels. So when I see that rf2 tyre tech is so much more developed and AC tyres that are just 4 round, black circles with 1 layer... it is obvious which one I choose. I seek driving experience but even best driving experience won't give me enough joy if I can't analyse things in depth and see that even smallest details matter. That's what keeps my blood going around in my veins faster.

Goes To 11 · Jan 8, 2014

Matt, your argument appears to based on the conjecture that the AC tire model is a simple one layer model. This appears to be incorrect based on the very fact that you can change the sim to be driven by surface temperatures in the ini files. You do appear to be correct that the model is driven by the temperature of a single layer rather than multiple layers. However, that doesn't mean the temperatures in the surface layer are ignored by the model. Lets say that grip is a function of core temp (ct), surface temp (st), road temp (rt) and slip angle and other factors (so).

Grip = f(ct, st, rt, so)

I would think that it is probably fair to say that surface temp is a function of core temp, road temp, slip angle and other factors:

st = g(ct, rt, so)

so we can completely remove surfact temp from the grip function:

Grip = f(ct, g(ct,rt,so), rt, so)

If you just look at the final function, you might say it is bogus because it is missing the surface temp but in reality it is included in the function. Obviously that is way way oversimplified but I think it makes my point.

I think that also might suggest why Kunos found that core temp a better driver than surface temp. While surface temp is probably a funtion of the above factors, the opposite isn't true. Core temp is not only going to be driven by surface temp but also by temperature coming from the brakes. But this is all really conjecture since we don't know how Kunos modelled the tires.