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Old 1 Jan 2007, 22:21   #1
rabieng
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Centre of Gravity of Race Car

Hi Everyone,

Out of interest, has anyone ever come across any equations (that include unsprung mass, unequal track width, different wheel diameter) used to calculate centre of gravity of a race car. There must be something out there on this. So far all the equations I have found are based on simple models for a "rigid" car.

I would like to put together a little excel sheet where I can punch in a few things like vehicle weight measured at individual corners, (=total weight), tilt angle of car, wheelbase, track width, mass of unsprung weight per corner, wheel dimensions etc and have it calculate the Centre of Gravity.

Any help on this topic?
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Old 2 Jan 2007, 02:55   #2
johnny yuma
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are we talking about if superman got under,and was to lift the car with one finger where would he place that digit,or something more complex ?

IF its that simple a practical rather than theoretical test seems in order. You
know -- level concrete,2 jacks,a plank...
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Old 2 Jan 2007, 08:51   #3
rabieng
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Yeah thats right johnny! Talking about that point on vehicle (or in vehicle space) where all the weight of the vehicle is said to act downwards (balancing point as you described it).
The simple jack, level concrete and wheel scales system is excellent but usually most of the equations out there are used to calculate CoG of unsprung mass (i.e including weight of wheels and part of the suspension). There must be a way to calculate CoG for sprung mass only or taking into consideration tire deflection due to weight shift on jacking and stuff like that... so a more complex model of simple test.
Anybody have any ideas? Links to websites, equations you know of etc would be helpful. Thanks
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Old 2 Jan 2007, 10:23   #4
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Why not replace the springs with solid bars and measure the normal way? Then you can ignore the effects of suspension movement when jacking the car. Setup/solid wheels will eliminate the tyre deflection too.
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Old 2 Jan 2007, 13:46   #5
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would not cornerbalancing the race car do this indrectly??

Put the race car, with drivers wt, and fuel, on four scales.

(Longtech makes scales. )

read the four scales measurements. Then make adjustments to suppension and or balast to get the four corners wt as close as possible to each other. This putting the CoG in the center (or almost centered) of the car.

Rember Fuel loads change during the race, so the CoG also changes slightly.
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Old 2 Jan 2007, 14:29   #6
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Agree, corner weight scales are the easiest way to find the plan position of the CoG.

If you want to do something more interesting... find the CoG as accurately as you can with the car horizontal, then jack one end as high in the air as the body allows and corner weight again. You will find that the CoG has moved proportionally closer to the lower end of the car than if it was a completely flat object. The two measurements can be used with a bit of scale drawing or trigonometry to find the height of the CoG.
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Old 2 Jan 2007, 17:06   #7
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How about using the crank as a starting point??
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Old 3 Jan 2007, 00:05   #8
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I haven't tried this as my race experience is in a cumbersome old sedan where niceties like C of G are the least of worries,but how about-

1.find the 'rocking point' of the car front/back at 90 DEG centreline

2.find rocking point for side/side-a bit trickier but yes line of crankshaft might be a good guess.

3.intersect two lines and bingo

4.build up your data by repeating 1 and 2 with various ballasts,unsprung weight removed etc.Soon you may have enough to make your own spreadsheet.

5.If your racecar can generate .8g,is it a simple arithmetic proportioning
inside to outside wheels of the front and back wheelsets scaled weights ?? eg if front wheels carry 300 kg total static,at 8g is outside wheel carrying 240 kg ?
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Old 3 Jan 2007, 13:47   #9
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Further to my insights above, here's a good article on how to find the CoG height. Front/rear and left/right is pretty simple really.
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Old 7 Jan 2007, 22:41   #10
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Thanks Guys

Thanks for your replies, found them helpful.

Cheers
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