Setting up 36GP cars

Here I describe how I make revised 36GP setups for v2.0. The steps might or might not be used for other non-aero cars, your mileage might vary, but worked well for me to do the Silber and Motor Union.

Reference data

It starts by finding references. I have started to do a Google search with the original cars name and the word 'bore' or 'stroke' - one of those is needed to calculate the new engine brake parameters, and if a page contains it, usually it contains all other necessary data. For example: http://et.mercedes-benz-clubs.com/mediawiki/index.php/Kategorie:MB_750-kg-Formelrennwagen/en#Mercedes-Benz_W_25_racing_car.2C_1936 is very detailed about the Mercedes, and http://www.supercars.net/cars/121.html is good for the Auto Union.

Data needed:

• mass
• wheel and tire sizes
• engine capacity
• number of cylinders
• stroke (or bore)
• compression ratio

Data might be useful:

• engine power and torque
• mass distribution
• gear ratios
• fuel tank capacity
• front and rear track

Generally, I try to modify as few parameters as possible, for example I did not change wheel tracks or other geometric parameters, I left them as they are. They have relatively low effect on handling, but might develop graphic bugs, like a wheel inside the car body.

Next I will go through modifications section by section. Note: the handling is mainly characterized by the weight distribution and the grip distribution (that is the front/rear tire width ratio).

Section Car

• mass: usually the empty mass of the car is given, I add 70 kg to it for an average driver.
• front-rear weight repartition: this is mass distribution, if real life data is too "drastic" it might be a bit off (for example the Auto Union is said to have 60% at the rear, I made only 58% for increased driveability).
• fuel tank: if any data is known, modify accordingly. For example The Mercedes is said to have 215 liters, I guess cars with smaller engines have smaller tanks, too.

Section Engine

• brake linear coefficient and
• brake coefficient: these can be estimated from thermodynamic considerations, the octave file enginebrake.m does this. Engine data between lines 4-8 needs to be filled in. Then LinBrakeCoeffAir? is used for the "brake linear coefficient", and "brake coefficient" = - ConTorque/{maximal torque of the engine}.
• torque curve - adjust only if it is seriously different than real life numbers.

Section Gearbox

• Use gear and rear differential ratios from reference, if found.
• Efficiencies might either remain, or a thumb rule might be used:
  • for foreward gears efficiency = 0.98 - (gear ratio)/100
  • for reverse and differential efficiency = 0.97 - ratio/100. In both case if ratio is smaller than one, use instead 1/ratio.

Section Steer

• steer lock: start with 25 degrees, and increase/ decrease it after some test run, the car should be able to navigate around the tightest hairpin with maximally turned steering wheel.

Section Brake System

• front-rear brake repartition: set after testing. Brondehach is a good track to test, it is perfect if the car is not too oversteered while braking into T1, and is not too understeered while braking for Dingle Dell (the fast right-left-right esses at the top of the hill).
• max pressure: again set after testing. 100% pressed brake pedal should lock the wheels, while 80-90% pressed should not lock them in a straight.

Sections Front Axle and Rear Axle

Remove "roll center height" lines, as they are unused in the code.

Section Rear Differential

• For ratio and efficiency see the Section Gearbox.

Various Wheel sections

• rim diameter and
• tire width: use the references. Note: 1 inch = 25.4 mm.
• inertia and
• mass: the wheel rim and the tire is modelled by homogeneous cylinders and discs, the wheelmass.m octave file does the calculation. The rim diameter (line 4), tire width (line 5) and the tires outer diameter (line 11) should be updated, the other parameters can be kept fixed. Then I add 5 kg to mtotal to get the mass ot the wheel, this is for the brakes, as their mass is not handled separately in the code. And Thetatotal is the inertia of the wheel.
• ride height: should be around 15 cm, usually they are already in this range. I set the rear ride height 10 mm lower, it helps to stabilize the rear.
• toe: -0.1 deg for the FR, +0.1 deg for the FL, 0 for the rear wheels.
• camber: set to 0, as this is unused in the code.
• operating load: the sum of the four operating loads (in N) is 12*mass of the car, and the front rear ratio is equal with the ratio of the front/rear tire width (denoted by wf and wr, respectively). This means the front operating load = mass*6*wf/(wf+wr), while the rear operating load = mass*6*wr/(wf+wr). I round them to the nearest whole number.

Anti-Roll Bar sections

• spring: start with 10 kN/m at front and rear, fine tune after testing.
• suspension course: should be the same than for the suspensions
• bellcrank: set to 1.0 for easy calculations.

Suspension sections

• spring: I use 2 things to calculate it: the spring go in average 5 cm down under the mass of the car; and the front springs are 1.5 times as much as the rear ones. This gives for the front spring rate Df = 60*m, and for the rear spring Dr = 40*m/100 (in N/kg), where m is the mass of the car in kg. Then they are rounded to the next whole kN.
• suspension course: I set it to 10 cm, that is 2 wheels can keep the whole weight of the car when completely pressed.
• bellcrank: set to 1.0 for easy calculations.
• slow bump,
• slow rebound,
• fast bump,
• fast rebound: the car handles well if it is not oscillating, that is it is overdamped. I calculate the necessary damper strength with the damper.m octave file, it looks after various oscillations and rotations, and finds the needed strength. Car and suspension data should be filled in the first 17 lines. The last 4 returned values are the slow rebound values for the front left, front right, rear right and rear left wheel in N/(ms). I round it up to the next whole kN/(ms), or when the difference is less than 100 N/(ms) to it then I add 0.5 kN/(ms) more (for example 8652 N/(ms) will be 7 kN/(ms), 8923 N/(ms) will be 9.5 kN/(ms)). Then slow bump is approximately half of the slow rebound, fast bump is approximately 1/3 of the slow bump and fast rebound is approximately 1/3 of the slow rebound - approximately means they are rounded to, say, 0.5 kn/(ms).
• bump limit velocity and
• rebound limit velocity: set them to 0.1 m/s.

Brake sections

• mu: set to 0.2, it is realistic for iron or steel brakes.
• piston area: use 50 cm2 by default, but it can be used to tune brake balance. Smaller area means less braking power on that wheel.