Do real cars pivot?

[Tyger]

Mods feel free to move this to another section as you see fit, but didn't want to put this in a GT or Forza thread for objectivity.

I'm a long-time fan of Gran Turismo (that's why I'm here hey) though perhaps have favoured the Forza franchise in the last few years. But one thing that kinda preys on my mind is the physics that comes into play in both games/franchises. It seems to me, and I might be alone here, that GT, for a long time, has based its physics on a central point in each car, almost like a central wheel/ball bearing, and whilst the car might still react to environmental factors, there's a habit of "gliding" as if not wholly connected with the surface.

Forza, by contrast, seems to work on the four contact points with the surface of the road, and react accordingly. I kinda hate to say it but I think PD have been somehow working on this central point vs environment model for a decade or more.

This Digital Foundry video is a little laboured, as it arguably unnecessarily focuses on very specific graphical differences, but it also shows how the cars in Forza steer into corners, rather than either darting forward in a straight line, or rotating and/or kinda sliding.

 

[BKGlover]

I've felt the opposite before, GT felt more in line with physics while Forza felt like it was moving the car around a point.

Now days, I honestly think it's a visual trick, even if unintended. GT1, GT5, Forza 1, and Forza Horizon all initially felt this way, but when I found a camera setup that didn't make my head hurt or eyes bleed that thought went away. I'll even apply this to GTS: Offline may be the best straight driving the series has had, open lobbies are a different matter for me, especially when the chase cam is placed more for a Raptor than a race car...however all that stops when I use either hood or head cams, and my times improve.

This may just be me and my overarching idiocy, but it's something I've noticed lately.

 

[Wolfe]

When things get loose a real car can pivot more or less around its center of gravity, but otherwise the front steers and the rear follows.

The biggest issue I have with a "pivot-steering" handling model is in oversteer modelling and control, which rarely turns out even well enough to be adequately controllable, much less realistic or intuitive. The concept of countersteer doesn't really fit. Another issue is how the rear outside wheel gets pivoted to the outside of the corner from any and all steering input, which is a problem when you're trying to slip by obstacles or avoid dropping that outside wheel off the edge of the road, like in Codemasters rally games.

I'm sure Forza does work off of contact points at each tire without pivot steering, though it's possible for a game to track variables at each tire while still ultimately working off of pivot steering, which is what Gran Turismo appears to do. I'm not sure about GT Sport because I did not spend much time with the public beta.

Anyway, this graphic from the long-dead official website for Enthusia is relevant:

 

[PzR Slim]

Calling @Scaff ! I'm pretty sure he did a video shortly after the release of GTS that discussed this very thing.

 

[Scaff]

The upshot is that yes cars do rotate, technically its called yaw (and is one of the three axis that make up vehicle dynamics - the other two being roll and heave/squat). Put a car on a dolly on a flatbed train car and when the train turns the car would rotate/yaw.

However what makes it complicated is that the point of yaw is always around the centre of rotation (which is the point at which the centre of gravity and the centre of the two roll centers intersect), and is dynamic because a car (unless stationary) is always subject to acceleration forces (which create squat and heave, and unless the car is perfectly balanced left/right a degree of roll), as well as rolling forces when cornering. Add in that almost no cars have a 50:50 weight balance across both axis and you have a point of yaw that is almost constantly shifting.

Now you have to add in that the tyres themselves are also subject to yaw rates (and its the difference between the front and rear tyre yaw rates that determine car balance), now the tyre yaw rates are normally dominant, up to the point at which grip is exceeded, when the cars yaw may become dominant.

As you can see its all hideously complicated in terms of the physics calculations, and for that reason many devs in the past went for a simple fixed single point of rotation, Codemasters were notorious for this, as was the early GT titles (in GT and GT2 you could actually get 4WD cars to donut around the fixed yaw axis point).

GTS appears to be built around a model of this type, but with some complexity added to allow the main cars yaw point to move slightly as the car rolls and heaves/squats.

Forza I can't comment on as I haven't played it this gen, however the likes of PC and AC both take into account the yaw rates for the car and the tyres, as well as both being truly dynamic. Enthusia also gets a nod for being one of the first (if not the first) console titles to head down this road (along with RBR).

This book...

https://www.amazon.co.uk/dp/0837602262/?tag=gtplanetuk-20

...is a good start in understanding how this all affects a car in motion, and covers in in a easily digested manner.

 

[SimTourist]

Do a donut in a car to test a game's physics. For example in Project Cars 1 you literally can't do a donut in a RWD car, which to me says a lot, the car sorta jerks around.

Forza 5 for comparison

 

[RYAN]

I always found forza to be understeery and the break in traction always felt way too snappy and not gradual enough. I actually prefer the physics in Forza Horizon 4 than FM7. FM games always seem really rigid and stiff.

 

[Moglet]

As you can see its all hideously complicated in terms of the physics calculations, and for that reason many devs in the past went for a simple fixed single point of rotation, Codemasters were notorious for this

Anyone who ever played any of the early Colin McRae Rally games can attest to this, you could often see the cars rotating around that central point as they left the ground and landed again. It was very jarring.

As for Forza, one of the newest vehicles in Forza Horizon 4 (the DeBerti Silverado) has an issue with the steering that really reveals the rotation system in action as it's possible to make the vehicle turn left while the wheels are still turned the opposite way. The front end of the car basically ends up sliding across the surface its on as if the tyres have no grip. I'll try and get a video of it.

 

[Wolfe]

Do a donut in a car to test a game's physics. For example in Project Cars 1 you literally can't do a donut in a RWD car, which to me says a lot, the car sorta jerks around.

Yep -- donuts, powersliding, weight shifting, abruptly lifting the throttle, a wheelspinning start from an uneven spot, those are the sort of things I do.

Project CARS is weird, because both games behave oddly with collisions or when a car becomes airborne. At those times, the car doesn't behave like a freely rotating body like in Live for Speed (or GTA for a non-simulator example). It tends to behave in an erratic or rigid manner that suggests there's a pivot-steering sliding brick model under all that sophistication. Yet PCARS2 is close enough to the real deal that I find that hard to believe.

On the other hand, I suspect the old ISImotor engine was the foundation for the Madness engine, since it's what Ian Bell and co. had worked with before, and there are similarities in the UI/menus.

PCARS2 made big improvements to oversteer modelling and behaves close to right most of the time, but it still doesn't quite nail it like LFS or even EPR.

 

[VXR]

I never really got on with the way cars pivot in PCars 1 and 2. FM and GTS feel more natural to me, but then I play chase cam with a pad and only in-car when I use the wheel.

 

[LeGeNd-1]

None of the modern day sims have the central pivot feel to me (GTS, FM, AC, PCARS). The last game I played which I could definitely say had center pivot was probably Codemaster's GRID 2/Autosport. It's really obvious. The newer games might be "based" on center pivot but if so they've covered it up well it probably doesn't matter for most driving situations. Also you need to remember that camera movements, especially in chase cam can have a huge impact on how your brain perceives movement. Assetto Corsa's chase cam is a joke and it looks like the physics are wack, but we all know it's one of the best sims out there.

 

[Tyger]

Some really interesting comments. I think the aerial diagram from Enthusia illustrates what I was getting at pretty well. I also play GT and Forza with a controller/chase cam and it often looks to me like the cars in GT, when steering around a corner, rotate on this central pivot so the front of the car say rotates right, as one would expect with steering and all, but the back also rotates left, without necessarily losing traction and oversteering (which is programmed separately for both models). Then depending on how fast you went into the corner, the whole car tacks left towards the outside of the corner, or right, towards the apex.

I've put this in slightly clumsy terms as it's hard to explain what I mean, but in reality whilst the rear wheels rotate on the spot to some extent when steering, they shouldn't actually move laterally unless they break grip. The front wheels do move left or right because they are being pointed in that direction.

I know technically there is the yaw factor and some complex physics at work in real life, and always a little "drift" laterally, but not to a very visible extent unless you are, well, drifting or oversteering into a potential spin. To me this gives a sense of detachment of the car in GT from the surface it's on, and it occasionally feels like it is almost constantly sliding around, just in a controllable way. The over-use of tyre squeal sound effects only adds to this. And I do agree that in Forza cars feel like they break traction more suddenly, but this feels more realistic to me. For the most part, the rear wheels shouldn't be moving laterally, and regardless of weight distribution (which has its own set of physical effects) a car for the most part points with the front wheels, and pivots on the back wheels. When the rear tyres give up under lateral load/force/loss of friction, it should snap on you.

 

[Dave A]

I find the cars in Forza behave looser than in GT from a lateral point, however I have no experience of GT Sport, GT6 was the last GT game I played extensively and FM7 is the most recent Forza Motorsport game I've played extensively. I must admit that there was a notable period of time playing neither inbetween GT6 and FM7 so maybe that view is skewed by fond memories of ye olde times.

The video @SimTourist posted of drfiting in FM7 shows how the car pivots from the centre once the lateral forces in the game exceeds a certain amount demonstrating a pivoting point that moves as the force increases. I beleive it's pretty simialr in most AAA racing sims these days including Gran Turismo. It's how well it's disguised and built around that tends to make most difference. A lot also has to do with camera work and control setup too IMO.

The problem games have is they have to meet expected requirements for graphics, sounds etc. as well as in the case of a sim like GT or FM, feeling realistic. Having more cars on track, more sounds, higher resolutions, better effects etc. all takes it's toll and concession have to made and often are in places the players can't see, physics.

This is also why in a lot of games if you make contact with an AI vehicle, the AI vehicle behaves differently to your vehicle. It's not unheard of for the AI to be running on a considerably more basic physics model than the players vehicle is because the game can only process so many physical calcualtions per second, and how the players car feels and behaves is ultimatley the most important.

One day gaming computers and consoles will hopefully become powerful enough to provide the graphics and gameplay features like larger grids etc. that consumers generally want while also handling more advanced and complex physical calcualtions so less or no fudging is required (I think no fudging what-so-ever is a tad too optomistic though). As it stands, physics in games have to get fudged somewhere and all games fudge them, it just comes down to how well they hide the fugde.

 

[LeGeNd-1]

One day gaming computers and consoles will hopefully become powerful enough to provide the graphics and gameplay features like larger grids etc. that consumers generally want while also handling more advanced and complex physical calcualtions so less or no fudging is required (I think no fudging what-so-ever is a tad too optomistic though). As it stands, physics in games have to get fudged somewhere and all games fudge them, it just comes down to how well they hide the fugde.

This pretty much. I mean there are still some things in the universe that physicists can't even crack with all their particle accelerators and supercomputers, so what hope does our plebian consoles or gaming rigs have? Unless you simulate every atom's properties and movements in the game, technically nothing will ever be a true or realistic simulation. It's just a matter of using mathematical equations to get as close as possible for the end user.

I always think that there are 2 ways to approach the physics issue. Bottoms up or top down. Bottoms up you try to get all the physics equation as close to reality, you input parameters for the car and the end result will come by itself. It might be accurate for 80% situations, but there will be times when it doesn't match because we don't have all the equations of the universe (or the computing power). With top down approach you set out with clear performance figures that you want to achieve for each car (acceleration, top speed, lateral g, braking distance, etc), and you fudge the equations until you get those numbers right, even if "under the hood" the power/weight figures are out of whack. Which one is right? Well I think ultimately that can only be judged by each our individual perceptions of reality and how our brain compares that to the simulation/FFB. I mean we all agree that GT is more real than NFS, but when it comes modern day sims the differences are just splitting hairs really.

 

[sesselpupser]

All-wheel steering (like Nissan's Super HICAS) and torque vectoring (like Mitsubishi's Super AYC) can be used to make a real car yaw, otherwise they wouldn't under normal conditions. Provided the road surface is flat, the car's alignment isn't completely broken and none of the tyres have lost their lateral traction enough to slip, the rear wheels will just follow the path the front wheels take, more or less.