# Thread: Vertical Centre of Gravity

1. ## Vertical Centre of Gravity

Hi guys/gals,
My First post here!
Just wanting to know how to get the Vertical Centre of Gravity of trailer mounted pump.
I've found this link, but it is very sensitive to accuracy.

http://www.longacreracing.com/articles/art.asp?ARTID=22

Pump weights 5871kg
Wheelbase 3554mm
Front axle goes from 2935kg to 3329kg
But this puts the VCG very high (~3300mm)
It should be around 1540mm

Any other methods?
I have Autocad, could I do this based on geometry and these figures?

Cheers

2. So do you have a real trailer to take those measurements or are you just plugging in guesses?

You don't provide any dimensions of the pump but my guess is the COG for the system is just below the COG of the pump. You also need to take into account the mass of the trailer. If you draw a line from the COG of the trailer up to the COG of the pump, the COG of the system will be somewhere along that line. And if for example the trailer weight 10% of the pump weight, then the COG will be 1/10th of the distance down from the pump COG. Does that make sense?

3. Thats the way to do it.

Another way is to lift the entire trailer with cable. The CG will be located in straight line below the mounting point of the cable.

Move your rigging slightly to attach in a different location. Lift again. CG lies on the new vertical line.

The intersection of those two lines is the CG.

A photo of each condition could be overlaid to find the intersection or you can make some kind of marks on the pump while it is hanging to mark that line.

4. I have a load cell on an overhead gantry and performed the above method on this pump.
I have also performed Erich's method, but the lifting point was fixed, so could not shift the rigging to get the trailer/pump level.
Can I still work out the VCG if I can measure the distance from the ground to the 4 corners of the trailer when suspended by the single lift point? 1 end was 720mm, the other 775mm above the ground

5. Originally Posted by Dinga04
Hi guys/gals,
My First post here!
Just wanting to know how to get the Vertical Centre of Gravity of trailer mounted pump.
I've found this link, but it is very sensitive to accuracy.

http://www.longacreracing.com/articles/art.asp?ARTID=22

Pump weights 5871kg
Wheelbase 3554mm
Front axle goes from 2935kg to 3329kg
But this puts the VCG very high (~3300mm)
It should be around 1540mm

Any other methods?
I have Autocad, could I do this based on geometry and these figures?

Cheers
HI Dinga

Just a couple of questions before I work it out mathematically.

Can you supply the total weight of installation, i.e trailer with pump.
What type of trailer is it?

Is it a drawbar trailer or a powered trailer?

Does the trailer have any tanks fitted, i.e. fluid if so they must be full when weighed.

When you say the front axle goes from 2935 to 3329kg, I take that to mean the difference in weight before and after jacking to a given height?

Note, any height can be used but not greater than 45 degrees.

If I think of anything I have forgotten I will ask again.

Auto Engineer

6. I used once to calculate the center of gravity: In your calculus book find the "Papus Theorem". Good luck: Benoit Ventimiglia.

7. If you can hoist up the assembly, do this and let it swing simply (no jiggling). Neglecting the mass of the lifting chain or cable etc, the system is susceptible to a pendulum calculation where, for small amplitudes, the centre of gravity will be approximately
g (2 PI T^2)
below the top of the chain. T , the period of oscillation should obviously be determined over several cycles. Keep the amplitude very small in order to minimise the errors implicate in the approximation to a simple pendulum. Don't forget you will also have errors as a result of the trailer suspension being fully extended when the assembly is lifted.

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