Haruna Say wrote ...

Hi all,

Decided to start up this thread to get some discussion on "realistic" car physics. Even though GR isn't a sim at all, I wanted to bring car physics that were very different from the high-power and sliding physics typical on most user's cars.

Power
According to a carmaking tutorial on the F1Cup site, power is actually measured in kilowatts (kW) rather than horsepower (BHP/HP), the commonly agreed belief on the vast majority of carmakers. However, to me, using either BHP and kW would make cars accelerate very slowly, moreso for heavy American V8-powered muscle cars (which were known for at least respectable 0-60 times). Therefore, I used the SI unit measurement of torque (nM) for the power value on all of my cars.

Weight/Mass
Everyone thinks it's kg. I agree... except that with commonly used slowdown values, this makes cars again very slow. My common solution is to half the mass for the GR model's weight (this is the setup used for all of the FMGR cars).

Slowdown
Slowdown as its name implies controls how much the car's top speed is limited (some kind of magic value). I used to use the common slowdown value of 0.1 on tarmac, but 0.01 allows cars to go much faster. On dirt and ice, I use 0.02, and on sand and snow, I used 0.05.
Grip helps to counter the small slowdown values.

Grip
Used to be wild guessing.

Then I developed a formula (still in refinement) for tyre's tarmac grip, so that I would not have to guess the amount of grip.
It factors in three things:
1) Front tyre's width (A)
2) Rear tyre's width (B)
3) Compound from 1 to 9... (C)
- 1 being crappy gripping "Economy" tyres
- 2 being whateverish "Comfort" tyres
- 3 being mediocre "Road" tyres
- 4 being OK "Sport" tyres
- 5 being fair "Super" tyres
- 6 being good "Semi-Slick" tyres
- 7 being "Racing Slick Hard" tyres
- 8 being "Racing Slick Medium" tyres
- 9 being "Racing Slick Soft" tyres

The formula comes out like this:
(0.7+(C*0.1))+((A+B)*0.001)

...so if we use...

235 mm tyres front
275 mm tyres rear
Super road tyres

(0.7+(5*0.1))+((235+275)*0.001)

Our tarmac2 grip is 1.71.

For tarmac1, use this guideline if you are using my formula:
If the compound/tread type is less than 6 (road tyres): Tarmac2-0.1
If it's a racing tyre (compound value more than 5): Tarmac2-0.2 (it's because of "marbles" effect)
Historic tyres (pre-1980) have a -15% grip penalty.

Off-road values
- For racing tyres, the grip on any non-kerb or non-tarmac surface should not exceed 0.6, on kerbs, it should be the respective tarmac grip -1
- For road tyres, the values vary on the compound:
Gravels and Sand: 0.6 (Economy), 0.725 (Comfort), 0.85 (Road), 0.725 (Sport), 0.65 (Super)
Mud and Snow: 0.4 (Economy), 0.55 (Comfort), 0.7 (Road), 0.6 (Sport), 0.5 (Super)
Ice: 0.35 (Economy), 0.5 (Comfort), 0.65 (Road), 0.55 (Sport), 0.45 (Super)
RWD cars: -0.05 less grip on all off-road surfaces.
FWD cars: No grip penalties/bonuses.
4WD cars: +0.05 grip on all off-road surfaces.

Air Resistance
As a general guideline, if the real drag coeffiency is not availible, use a value that is somewhere between 0.28 and 0.5. Excepted are very large and normally non-aerodynamic vehicles such as 18-wheelers. Some futuristic automobiles also have drag coefficients that are less than 0.27. "Blockier" cars should have more air resistance, more round and "streamlined" shapes should have lower values.

Downforce
Downforce influences the cornering at medium and up speeds mostly, but mildly influences low end as well. Higher downforce increases the grip at speed and will not change if you, for instance, slide the car. This means you will always have a steady 'flow' of downforce, which can allow you to tune out bad sliding behaviors.

As a general guideline, refer to the table below:

No downforce - Most production cars. Examples: Your grandpa's station wagon.

5-15 downforce - Common sports cars with spoilers. Examples: Subaru Impreza WRX STi (all production models), Ford Mustang Cobra R, Toyota Supra, etc...

15-30 downforce - Mid-range sports cars with spoilers and/or undertray diffusers. Examples: Ferrari F430 Scuderia, Porsche 997 Turbo/GT2, Chevrolet Corvette ZR-1 2009, etc...

30-80 downforce - Moderately modified versions of the low-end sports cars, and supercars. Examples: MINE'S Skyline GT-R (R34), Mugen S2000, Lotus Sport Elise, Ferrari F40, etc... NASCAR falls under this group as well.

70-115 downforce - Ultimate track-day oriented supercars and racing-type touring cars. Examples: BMW Motorsport Team Germany 320si Touring Car, Ferrari FXX Evoluzione, McLaren F1 LM (road car), Edo Competition 997, HKS Evolution "Time Attack", etc...

100-140 downforce - FIA and ACO based GT2 and GT3 racing cars, and FIA's WRC rally cars. Examples: Flying Lizard Motorsport 997 GT3-RSR, Risi Competizione F430 GT, Spyker Squadron C8 GT2, Subaru Impreza WRC 2008, Citroen C4 World Rally Car, etc...

130-150 downforce - FIA and ACO based GT1 racing cars from after 2000. Examples: Vitaphone Racing S7-R, AF Corse MC12 GT1, Aston Martin Racing DBR9, Chevrolet Corvette Racing V6.R GT1, etc...

150-165 downforce - All JAF SuperGT/JGTC based GT300 racing cars and all DTM post-2000 and pre-1994 cars. Examples: ARTA/ASL Garaiya, Studie-Glad Asada Racing Z4M, Audi A4 DTM, etc...

155-175 downforce - All JAF SuperGT/JGTC based GT500 racing cars and all DTM cars from 1994-1999. Examples: Xanavi Pitwork Motul Z, Alfa-Romeo 155 DTM, Opel Calibra DTM, etc...

170-190 downforce - ACO based LMP (any class) cars. Examples: Audi R8 LMP, Audi R15 TDI, Peugeot 908 HDi FAP, Pescarolo C60, etc... Also, any of the GT1s from between the years 1994 to 1999 fall under this category. Most modern open-wheel cars that push beyond 400 bhp and do not compete in F1 will likely fall into this category.

180-200 downforce - All FIA based Group C (any class) cars. Examples: Mercedes-Sauber C9, Toyota Minolta 88C-V, Mazda 787B, Jaguar XJR-8, Nissan R89C, etc...

200+ downforce (Strava's editor) - Pretty much any F1 car after 1972. Modern-day Indy and Champcars will fall under this category, too. Other notable examples include the Chaparral 2J Chevrolet and the Porsche 917/30.

Sliding
I find sliding value not very useful. I almost always set it to zero since most of my cars have enough grip to slide on their own anyway.

Balance
Dependant on the weight and drive of the car - the heavier it is, the lower it should be...:
FF (front engine, front wheel drive) : 0.5 to 0.8
FR (front engine, rear wheel drive) : 0.5 to 0.9
MR and RR (mid/rear engine, rear wheel drive) : 0.6 to 1.0
4WD: 0.5 to 0.9

Other stuff
Y-value in "Car Data" influences the center of gravity.
Driver's head can be used to influence the center of gravity (thanks Krisu).
Try messing around with wheel placement to fine-tune handling (fictional cars).

Interesting stuff
Tyre width
- JGTC/SuperGT cars in the GT500 class will typically run 300 mm wide tyres on all four corners.
- FIA/ACO GT1 cars will almost always run 300 mm (front) / 330 mm (rear) wide tyres.
- FIA/ACO GT2 cars will almost always run with 290 mm (front) / 300 mm (rear) wide tyres.
- Typical production cars will run anywhere from 175 to 235 mm.
- Most sports cars will run 255 mm to 305 mm tyres, with exceptions of smaller cars such as the Lotus Elise.

Drag coefficents
- Typical examples
- Carfolio also has drag coefficients for several cars not covered by Wikipedia

Tyre grip and slowdown
- Because paint reduces friction of a surface, tyre grip worsens when driving on painted tarmac and/or trackside "kerbs". This effect becomes far worse if said painted surface becomes wet.

Hope this provokes some discussion.

Haruna.