Centre of Gravity. Help!

[Eiskalter Mick]

I was reading gt5mania.com and thought I would try out the Weight transfer formula in GT5 on the BMW M5, but I would like help confirming where the M5 centre of gravity is.

Length: 4855mm
Height: 1469mm
Width: 1846mm


Centre of Gravity=(Length1*Weight1)+(Length2*Weigh t2)/Total Weight

As I only have the overall Weight & Length, the CoG would be the same as the length, so with that in mind, I have set CoG at half length of the entire car, and if the weight is evenly dispersed then I think it will be a fair value.

mass=1855kg
Weight=massxgravitational pull=1855*9.81=18197.55N
Force=massxacceleration=1855kg*62.5mph(1 00.5625kmh)=186.543MN

accleration=finalvelocity-initialvelocity/time=60mph(96.54kmh)-0mph/5sec=11.758mph/sec (19308m/s/s OR 19.308kmh)

deceleration=60mph (96.54kmh)/4sec=14.698mph/sec (24135m/s/s OR 24.135kmh) (.668G)

1G=32f/s (22mph)

(Front Weight Transfer) Force Down = Braking 'G' x mass x height/a

It appears that "a" is pitch centre, of front part of vehicle. In the example on GTMania pitch centre appears to be determinable by Braking G Force*Vehicle Centre of Gravity. However, I expect the relativity to change according to vehicle, so a way of determining BMW M5 pitch centre will also be helpful.

http://gt5mania.com/driving-guide/st...ight-transfer/

Front Pitch "a" = Braking G Force * Vehicle CoG = .668*2427.5 = 1621.57N
Rear Pitch "b" = Braking G Force * Vehicle CoG = .668*-2427.5 = -1621.57N

With the statements of CoG and pitch in mind, lets now try the formula:

(Front Weight Transfer) Force Down = Braking 'G' x mass x height/a
= 0.668G*1855*4855/1621.57= 3710N= 378.42kg/f (GAINED)

(Rear Weight Transfer) Force Up = Braking 'G' x mass x height/b
=.668G*1855*4855/1621.57=3710N=378.42kg/f (LOST)


Remember, I am assuming the Weight, Mass are evenly dispersed. Therefore;
Initial Front, Rear Mass = 927.5kg
Initial+Transfer = 927.5+378.42=1305.92kg (Front Mass After Weight Transfer)
Initial -Transfer=927.5-378.42=549.08kg (Rear Mass After Weight Transfer)

Total Mass = 1305.92 + 549.08 = 1855kg


/Thanks/

 

[dr_slump]

Damn, this M5 must be a monster on the brake.

 

[Eiskalter Mick]

I'm using the default values, so the car is heavy and requires heavy braking and gentle throttle, low steering input to minimise oversteer.

But looking back, the deceleration time does look odd (it was late, when I did this), I will do another 60-0 brake test.

EDIT: It takes 5s for car to come to full halt. Sorry for misleading you guys and thanks for spotting the inconsistency. I've edited Deceleration formula to reflect that.

 

[killerjimbag]

I'm using the default values, so the car is heavy and requires heavy braking and gentle throttle, low steering input to minimise oversteer.

But looking back, the deceleration time does look odd (it was late, when I did this), I will do another 60-0 brake test.

The car is a beast.I'm assuming you are talking about the M5 05 premium.

 

[Eiskalter Mick]

I'm using the M5'08 premium.

 

[Eiskalter Mick]

EDIT: I realise the table appears with lots of | vertical lines, those are just there to act as spacing.

Friends, Gamers, Humans! While I wait for the answer to my initial question, I have some data for you, from 20 laps of testing on BMW M5'08, Sports Hard Tyres on Trial Mountain cct. ABS 1, other electronic aids off. Tyre/Fuel depletion off for best chance of getting best lap time. The BMW M5 had no Aero parts attached and the car was running on purely default settings at all times. Repairs were done as soon as applicable, (oil and chassis after every session, 1 engine change applied).


There will be 2 Maximum Upgrades sessions; these are sessions where all weight, power upgrades are applied, the first will be without Aerodynamic parts, the second, with Aero parts (default values) > this opens up the door to noting down effects of individual parts in another test. The MAX setups should not be viewed in the same light as Engine/Weight setups, for reasons of additional components added, however, default aero, suspension, transmission, drivetrain will be used.

PP||||Engine/Weight||||||||||BHP/Kg|||||||||||||Time
[color=#dd2423517[/color|||Default/Default|||||||||514/1855|||||||||||1:40.126

525||Default/Stage1||||||||||514/1669|||||||||||1:39.457
534|||Default/Stage2|||||||||514/1521 |||||||||||1:37.485 [To be RETESTED for Consistency]
542|||Default/Stage3||||||||514/1409||||||||||||1:37.232

525|||Stage1/Default||||||||537/1855||||||||||||1:39.266
537|||Stage2/Default||||||||586/1855||||||||||||1:37.785 [RETESTED for Consistency, Previous test - 1:39.470]
542|||Stage3/Default||||||||607/1855||||||||||||1:38.043 [To be RETESTED for Consistency]

532|||Stage1/Stage1||||||||537/1669|||||||||||||1:37.803
546|||Stage2/Stage1||||||||588/1669||||||||||||1:38.318 [To be RETESTED for Consistency]
552|||Stage3/Stage1||||||||607/1669||||||||||||1:37.200

540 ||Stage1/Stage2||||||||537/1521||||||||||||||1:37.657
548 ||Stage1/Stage3||||||||537/1409||||||||||||||1:37.117

556|||Stage2/Stage2||||||||596/1521||||||||||||1:37.734
562|||Stage2/Stage3||||||||596/1409||||||||||||1:36.612

560|||Stage3/Stage2|||||||| 607/1521|||||||||||1:37.527
567|||Stage3/Stage3||||||||607/1409||||||||||||1:37.475

585|||MAX/MAX UPGRADE||||708/1394|||||||||||||1:36.363
597|||MAX2/MAX2|||||||||||708/1394|||||||||||||1:35.826

I found Stage2/Stage3 to be a very surprising result, it really was a last lap wonder, in a car, I didn't like; as it was too light (with plenty of invalid laps) but the potential is clearly there for greater lap times once tuned. Up until that point, Stage3/Stage1 was the favourite for best set up.

No matter which set up you choose, you're going to have to deal with Oversteer. From what I've read on gt5mania, loosen your rear Spring Rate & Dampers to stop weight transfer from being applied too quickly to rear outside tyre, but be sure to increase ARB, as soon as you notice body pitch & roll (causing constant change in tyre grip and turbulent aerodynamics). In the case of understeer, the weight transfer is shifting to front outside tyre, similar to oversteer, loosen Front Spring Rate, Dampers but be aware of body roll.

Again, Stage2/Stage3 setup was very light, so be sure to add some weight to front to keep some vertical load on front tyres when accelerating, so that they do not lose grip.

Default/Stage1 was joyous driving pleasure in comparison to the higher lap times, the car felt well balanced, oversteer was minimal, the car felt comfortable.

Default/Stage2 was less comfortable, more competitive, being lighter it sure felt quicker, but the rear tyres were punished on many corners.

Default/Stage3 was more of the same from Stage 2, but the Outside tyres is what took a beating.

Here is a hypothesis I will test later during the week, I will run the same test with tyre wear on, I feel that Weight Stage3 will be the first to pit, then Stage2 and it's between Default and Stage1. If this is true, the question then is: Do the Lower Grade Weight Stages make up lost time and pass the Higher Grades when the Higher Grades need to pit? Meaning that though they would run slower lap times, the overall race time of the Lower Weights would be better. This would give tuning at Higher Grade levels a model to work towards, the ability to have quick laps and a stable chassis to preserve tyres. > You may think, well, obviously. But you've haven't driven the model you are tuning towards have you? So you don't know how many laps you expect the tyres to last, if this hypothesis is true, you will know how many laps you want to get out of those tyres when you have tuned the car.

As only the Engine/Weight upgrades were changed, I cannot account for how other upgrades (like exhaust pipes, ECU) will affect lap times and handling on these particular setups.



Max1: like asking an evil mother-in-law a cash loan, one must be very delicate. Weight upgrades make front end too light, and the car is prone to oversteer. The advantage over high scoring Engine/Weight setups is not, as great as expected, hindered even by some of the upgrades.

Max2: a much greater relief, despite only having an Aerodynamic advantage and the time reflects this, less oversteer to contend with, but obviously the lack of front end weight is still noticeable.

 

[Eiskalter Mick]

Trial Mountain BMW M5'08 Lap Time Calculation Formulae

Power=Work Done/Time

Work Done = Force*Distance

Force = Mass * Acceleration

Acceleration = Final Velocity - Initial Velocity/Time




Mass = 1409kg

Actual Power = 596Bhp

Acceleration = 19308m/s (Acceleration = Final Velocity - Initial Velocity/Time = 60-1/5 = 11.758mph/sec (19308m/s/s OR 19.308kmh))

Force = 186.543MN (Force = Mass*Acceleration)

Work Done = 281057.995638MJ

1W = 0.001341Bhp

Calculated Power = 205734.486W = 276Bhp

Distance = 2.474miles. = 1.506666Km (Trial Mountain cct. info - mygranturismo.net)


Best Lap Time 1.36612 seconds

Calculated Time
1.3612 seconds

NOTE: The calculated Power suggests that I had 320Bhp Not in Use.

 

[Ealirendur]

Trial Mountain BMW M5'08 Lap Time Calculation Formulae

...

NOTE: The calculated Power suggests that I had 320Bhp Not in Use.

Interesting approach. I assume this is for a spherical M5 driving in a vacuum?



I had a very quick look to see if I could find the CoG data inside an rFactor M5 model, but no dice. I know some of the tuners have sources for this sort of data...

One approach you could try would be to start with 2-3 cars you do have the CoG data for, and check how ballast (at +50/-50) affects the weight distribution. Then do the same to the M5, and you may be able to make a reasonable approximation.

Also, the [CODE][/CODE] tags are very handy for table formatting with text (fixed width font).

EDIT: Found some data you might use as a base, or to find simlar resources: http://www.eng.auburn.edu/~dmbevly/mech4420/vehicle_params.pdf (but check PDFs, kids!)