CoG Height Measurement: Weight transfer is negligible on raising height from one end of the vehicle.

[ShuklaPK]

Hi

This is the 1st time I am measuring CoG of a vehicle.

Method Followed:

1. All fluids filled.
2. All 4 suspension locked.
3. Load calculated at 4 wheels before raising the height from front.
4. Height of the vehicle lifted from front axle hard points (Not tires).
5. Weight calculated on the rear wheels kept on 2 Load cells.

Problem:

Load Cell values for Weight on Rear wheels is showing same when raised at 180 mm & 760 mm from front and in between these two heights weight is varying randomly.
Value on the load cell before and after raising the vehicle is same and on increasing the height upto a limit & further the weight on rear wheels is reducing.
What can be the possible reason for this?

PS: I got the load cell checked, it is working perfectly fine.

TIA

 

[MintJulep]

How long is the vehicle?

 

[IRstuff]

What's the performance of the load cell and weight of the vehicle?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[GregLocock]

I suspect a plot of the data would be helpful.

Cheers

Greg Locock


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[ShuklaPK]

Length of the vehicle: 4.5 Metres
Wheel Base: 2750
Image attached with the data captured for different heights.

 

[Tmoose]

How sensitive Are the load cells to side loading?

What provisions did you make in your test rig to reduce/eliminate side loading when the projected wheelbsse gets shorter as one end is raised?
And similarly protect the load cells?

Since the front pair goes to zero when the front is raised, I'm guessing the load cells are left sitting on the floor.
Are you using a floor jack under the "front axle" ?

 

[cibachrome]

Forget the electronic crap. Its not working. Use mechanical scales, or maybe you should try working with a 1/20 vehicle model with some postal scales to practice your technique !

 

[3DDave]

Check

https://www.sawe.org/papers/1802

 

[MintJulep]

Pythagoras says that your CG isn't moving very far.

2750[sup]2[/sup] >>> 180[sup]2[/sup]

2750[sup]2[/sup] >> 760[sup]2[/sup]

 

[btrueblood]

The axle hardpoints move aft relative to the contact patch of the tires. Unless you put the load cell on the rear axles, you are complicating the c.g. calculation. Why would you not block up load cells and lower the front axle onto them?

 

[LionelHutz]

Why are you trying to measure the weight transfer this way?

 

[Tmoose]

It is a clever way to determine Center of Gravity height. When it works.

 

[GregLocock]

It is the usual method for cars. I can't make any sense of the measurements.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[BrianE22]

Check how sensitive your load cells are to moment loads by rolling the car an inch or two back and forth.

 

[btrueblood]

You say the suspension was "locked", which I took to meant the springs/shocks etc. But, did you mean you applied brakes on all 4 wheels to prevent wheel rotation? If you jacked from the hard point (axle) of one end, the opposite end (with locked brakes) would drag slightly across the load cell, which might compromise the reading from the cell.

 

[SwinnyGG]

btrueblood has explained what I suspect is the cause of the problem.

Jack the car up, and center each load cell under the contact patch as accurately as possible. Tightly block both rear wheels in place (in both directions) and then do your lifting with all brakes released.

 

[ShuklaPK]

Thanks everyone for your valuable inputs to this thread.

We were facing the following 2 problems:

1. The way vehicle was lifted had some issue, after a point the whole vehicle started lifting up like a cantilever beam.
Now, at every height we put down the front wheels on Wooden blocks and then measure weight. The varying weight with height seems logical now (Weight on rear wheels increase with increase in height).
2. The rear wheels were not positioned properly on the load cell as rightly pointed out by @btrueblood (Thanks a lot).

We also repeated the activity by lifting the vehicle from the rear and measuring weight on the front wheels.

We got CG values in the tolerance range of +/- 10 mm that is acceptable to our requirement.

Thanks
Pratik Shukla