Air density from temp is calculated in GT6

  • Thread starter KinLM
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Just going to post what I put in the Motec thread here... it might be relevant, I was trying to puzzle out the variables:-



"Okay, so technically speaking, the original Veyron set a record at 253.81mph (113.632 metres per second). According to standard equations for Drag, at that speed a Veyron, at sea level (Ehra-leissen as a town is ~63m above sea level), with an air pressure of 100kPa, and at 15°C is experiencing 5802.176 N of resistant force thanks to drag. In order to overcome this drag, it requires 895.08 (metric Horsepower). SSRX at 12 noon is 27° from memory... so if we simply change the air temperature to 27°, the hp required to hit that speed drops to 859.28... or flipped over it would reach 257.36mph assuming everything else stayed the same.

Assuming PD use the same relatively simple equation for determining drag... they would include...

Air Temperature - We see that PD display the air temperature of the circuit at the start of the race, so let's assume they do factor this in to air pressure and therefore air resistance, in which case they need the following two things as well...
Air Pressure - I've assumed 100kPa - PD could use a constant, or a local average? this is required for Drag force.
Specific gas constant - This is a constant, I'm assuming PD base this on 'Dry' Air, and therefore the value would always be the same... it would vary if we were talking about humid air for instance....

from these three things we get Air Density, so
Air Density - as above
Relative Velocity - i.e. the cars speed, although it's relative to the air the car moves through, so a head wind, or tail wind would have an effect. (5 mph tailwind takes the 859.28 PS above down further to 825.75 PS).. however the game doesn't appear to factor in windspeed... so this is just the car speed.
Drag Coefficient - 0.36 according to Bugatti when the car is hunkered down.
Frontal Area - 2.07m²

I would say this is not difficult, and I'm pretty sure that PD factor all this stuff in. The question of environmental conditions does mean that SSRX can give a different top speed to other tracks -- for instance if all fake tracks are considered to be at Sea Level for instance. They may use a constant Air pressure across all tracks, they might give each it's own air pressure, hell.. it could even be dynamic!"
 
Interesting. I will test at Le Mans and see what happens. In real life, the cars are faster when the air is cooler. The "happy hour" is during the morning. Also, even the trees can influence. The section of the Le Mans track between Mulsanne and Indianapolis becomes the fastest because of the trees( more oxygen?).
 
LeGeNd-1
Alright peeps, I finally retested the X with the hottest temperature. I was expecting a few km/hs improvements, but ended up with 30 km/h so my top speed is now 767 km/h (480 mph) :eek: :bowdown: :D

Settings
SSRX arcade mode race
Professional difficulty
Track time 15:00 (should give you 27C / 80.6F)
Grip reduction LOW
All driving aids OFF
Racing Soft tyres
Don't forget to oil change and wash (every little bit helps)

Suspension
Ride height max min
Spring rate min min
Damper (C&E) 1 1
ARB 7 7
...
Click to expand...

I have a question about your tune. Isn't it unrealistic that you have Front Ride Height to the max? :\ It doesn't make sense in real life, does it?
 
In real life, raising the front would most likely be a major flip or stability hazard as it's prone to create a lot of lift under the front wheels. It could be that the max front height of the car is lower than the minimum rear though. Or it could be a typo.
 
Out of interest, do we know that in real life raising the front and dropping the back wouldn't be the fastest Aero set-up? Assuming that certain consequences may not be modelled by the physics engine?
 
That's a car specific question, and without knowing the actual heights and what they mean for body orientation it's hard to give a solid answer. In general it exposes more of the tires and wheel wells to high speed flow which leads to large amounts on unnecessary drag. It will also change the amount of air going over rather than under the car, which can impact diffuser efficiency. Angling the car up also changes the force vectors on local areas of suction on top of the car. They will start to point in a more reward direction and local areas of thrust might become neutral or draggy.
 
Exorcet
That's a car specific question, and without knowing the actual heights and what they mean for body orientation it's hard to give a solid answer. In general it exposes more of the tires and wheel wells to high speed flow which leads to large amounts on unnecessary drag. It will also change the amount of air going over rather than under the car, which can impact diffuser efficiency. Angling the car up also changes the force vectors on local areas of suction on top of the car. They will start to point in a more reward direction and local areas of thrust might become neutral or draggy.
Click to expand...

I'm wondering because if we knew what aspect you'd have to eliminate in real life physics for it to be as it is in the game, it might point to whats causing the issue. Perhaps there is no aero model for the underside of the car, and tilting it backwards just presents less frontal area.
 
I wouldn't expect the aero model to work like that, but I can't be sure. Cars probably have a coded CDA (CD times area) that changes slightly with downforce and maybe ride height. They wouldn't need to simulate flow over the car, and probably wouldn't be able to with very much detail.
 
Stephanos82
I have a question about your tune. Isn't it unrealistic that you have Front Ride Height to the max? :\ It doesn't make sense in real life, does it?
Click to expand...

No, it's not meant to be realistic. I was exploiting the "wheelie" glitch that has been in Gran Turismo games since GT2 that helps you get a higher top speed. If it's any consolation, it doesn't really make that much difference to the Tomahawk (only gives an extra 7 km/h). My previous tune has front and rear ride height at minimum to reduce frontal area drag, which is more realistic.

If you are wondering why max front and min rear gives higher top speed, it's because of the way downforce is simulated in GT. Downforce is a vertical vector force acting perpendicularly (90 degree) on the car's body. So if you make the ride height max front and min rear, the car's body will be tilted like this \ (exaggeration lol). Hence the downforce (still acting 90 degree to the car's body) will now act to push the car slightly diagonally FORWARDS (\ <--). And since you get more downforce with higher speeds, you get pushed more, getting even more "front"force and so on.

Of course, in real life this doesn't work and you will flip like Mark Webber at Le Mans, but until PD fixes aero physics this exploit will always remain.
 
LeGeNd-1
No, it's not meant to be realistic. I was exploiting the "wheelie" glitch that has been in Gran Turismo games since GT2 that helps you get a higher top speed.
Click to expand...

So, does that mean that the physics engine is pretty much the same as PS1 age but a bit more matured or evolved? :\
 
Vegard
It's been like that for a while.

I can't physics:


Note the feedback effect as the car picks up speed.
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Yup, that's why I said it's been there since GT2.



Stephanos82
So, does that mean that the physics engine is pretty much the same as PS1 age but a bit more matured or evolved? :\
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Some aspects of it are. Tires and suspension have improved, but aero needs a LOT of work. But hey, look at the bright side, this just shows how advanced GT1 and 2 were back in the day :P
 
LeGeNd-1
767 km/h (480 mph) :eek: :bowdown: :D
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LeGeNd-1
Offline I think this is the limit, online with a group of drivers in organized drafting and getting everything perfect I reckon you could nudge 800 km/h :crazy:

So there you have it folks! Try to beat my record and enjoy chasing jumbo jets in the process
Click to expand...

I did a 3 lap race with the AI and by (herding) separating them into smaller groups I got up to 787 km/h.
Could propably hit 790 , but that is very difficult when the AI is moving at over 200 km/h slower speed..

Using wheel and controller in party play , I could do 830 km/h by taking turns in drafting.

Without draft I've got up to 726 km/h in downhill.
 
ederss7
Interesting. I will test at Le Mans and see what happens. In real life, the cars are faster when the air is cooler. The "happy hour" is during the morning. Also, even the trees can influence. The section of the Le Mans track between Mulsanne and Indianapolis becomes the fastest because of the trees( more oxygen?).
Click to expand...

nascarfan1400
Its my understanding, at least with stock cars, that the colder it is, the faster you go...
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If you look at the drag force alone, it gets weaker when air density decreases, and air density decreases when temperature gets higher. If you take the combustion process changes into account when air density or air composition changes it gets waaay more complicated... i don't think PD models this.
 
MadMax
I did a 3 lap race with the AI and by (herding) separating them into smaller groups I got up to 787 km/h.
Could propably hit 790 , but that is very difficult when the AI is moving at over 200 km/h slower speed..

Using wheel and controller in party play , I could do 830 km/h by taking turns in drafting.

Without draft I've got up to 726 km/h in downhill.
Click to expand...

That's awesome :D I didn't think of using 2 players and drafting that way.

I wonder if we could get 16 players online for organized drafting. Get them all in a line spaced just so everyone are within the draft. The last car will be the top speed car. Every car in front moves out of the way when the last car is about to catch up. I bet we might crack 900 km/h then :D
 

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