Ground Clearance and Center of Gravity





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  1. #1
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    Default Ground Clearance and Center of Gravity

    Hi All,

    Just been wondering about claims by manufacturers about their vehicles ground clearance. Sometimes the undercarriage looks so low but the specs say the car has 235mm ground clearance.

    So here is my question, at what point under the vehicle is the ground clearance measured? Basically, is it taken at Front left/Front Right/Front Center or Middle Left/Middle Right/Middle Center or Rear Left/Rear Right/Rear Center or is it the average of all of these above measurement. Or is it taken immediately below the Center of Gravity of the vehicle?
    Does anyone here have info on what the Society for Automotive Engineers' (SAE) standards recommend for measuring ground clearance?

    Another question, has anyone of you ever wished they new what the location of their car's Center of Gravity is (in x,y,z coordinates)? I am certain this would go a long way to help deciding on how much load to take on your roof especially after installing aftermarket suspension kits and tyres. Would manufacturers keep this CG as propritory information?

    Regards ,
    Lone Rider

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    CoG info can be gathered from manufacturer specs, even though they don't specifically state it.
    Mmmmm... How to label the axis for explanation.
    X=Lenght of vehicle
    Y=Width of vehicle
    Z=Height of vehicle

    To get the X position of the CoG look at the recommended tyre pressures. If front and rear are specced to be the same pressure, then the X of the CoG will be pretty close to the center between the front and rear wheels. If the front pressure is specced higher than the rear, it means there is more weight in the front and thus the CoG will be a bit more forward.

    The Y of the CoG will always be pretty close to the center between the two front or 2 rear wheels.

    To get the Z value you will need a bit of info. Most importantly the roll over angle on a side slope. If you view the vehicle from the front, at it's roll over angle. The CoG will be where the Y-axis center line crosses a vertical line drawn from the outside edge of the low tyre.

    Hope that helps!
    Last edited by RedLineR; 2009/10/14 at 09:52 AM.
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    Re: Ground clearance,

    As far as I know its measured from the lowest point on the vehicle, so something like the diff.
    2004 Nissan Patrol 4.8
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    RedLinerR

    Very good. Why didn't I figure that out before.
    In fact I can further your comment by saying (Highest Pressure divided by lowest pressure)xHalf the base of the vehicle = Distance to CG from Low Pressure end.

    I'll check what my roll over angle is.

    Graham. What about car with Independent suspensions, the diff could be higher than the wheel center line?

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    Good explanation Red Liner, just one thing though... for the Z axis in your example, would one not also have to look at the weight distrobution of the vehicle's vertical plain? suppose the defender is the best example, by looking at it, it appears to have a high centre of gravity in the vertical plane, yet, becasue above say 400mm off the grounds, it's mostly an empty aluminium shell - and the heave protion is below half way up - the steel chassis engine etc. Where a for example Disco has a lot of weight higer up, so the centre of gravities in the vertical (z axis) could well be similar?

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    Exactly Lone Rider!

    The only problem, which I didn't point out is that after all this you'll only have the CoG of a standard vehicle. Because the weight distribution is not uniform through Z, you can't, unfortunately, make any meaningful prediction of the movement of the CoG in Z when a RTT is added

    Edit to add:
    And SilverPuma beat me to it
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  7. #7
    4ePajero Guest

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    Ground clearance:

    Imagine the vehicle parked on the bottom of a flat swimming pool.
    Start filling the pool with water.
    The moment anything except the tyres & wheels (and most include swing arms) touch the water, the depth of the water indicates the Ground Clearance.

    (It is usually the diff pumpkin or the front cross member).

    GOC (the imaginary mass at a point, which will act similarly to the whole vehicle) is not so easy to determine.
    It is determined by integrating all the bits that make up the mass of the vehicle in 3 dimensions.

    The "reverse" way of determining the COG point (on the width of a vehicle) is to determine the roll over point (angle).
    The geometry of this situation will show where the COG has to be on the width of the vehicle.
    The position along it's long axis cannot be determined that easily.

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    Though, on second thought...
    You have the total weight of the vehicle, and you have the position of the CoG in Z

    This means that you can calculate 2 different mean densities for the roof portion, and bottom body portion. From this you can actually calculate the movement of the CoG in Z (If say a RTT is added). Again not 100%, but a pretty damn good estimation.
    SWAMBO
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  9. #9
    4ePikanini Guest

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    COG




    This COG runs from front to back. The 3D COG can be found by intersecting the COG line with the front to back balance point.

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    Quote Originally Posted by RedLineR View Post
    CoG info can be gathered from manufacturer specs, even though they don't specifically state it.
    Mmmmm... How to label the axis for explanation.
    X=Lenght of vehicle
    Y=Width of vehicle
    Z=Height of vehicle

    To get the X position of the CoG look at the recommended tyre pressures. If front and rear are specced to be the same pressure, then the X of the CoG will be pretty close to the center between the front and rear wheels. If the front pressure is specced higher than the rear, it means there is more weight in the front and thus the CoG will be a bit more forward.

    The Y of the CoG will always be pretty close to the center between the two front or 2 rear wheels.

    To get the Z value you will need a bit of info. Most importantly the roll over angle on a side slope. If you view the vehicle from the front, at it's roll over angle. The CoG will be where the Y-axis center line crosses a vertical line drawn from the outside edge of the low tyre.

    Hope that helps!
    I'm not sure if I agree with this. My Defender is specced at 3.3bar back and 1.9 front, but is front heavy (when empty). It will be back heavy if fully loaded (1 ton capacity). Or am I just being dof and not understanding you?
    Erin Bosch (082 342 1909)
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    how hipnotic is that picture of 4e's? it's great!

    My Defender is specced at 3.3bar back and 1.9 front, but is front heavy (when empty). It will be back heavy if fully loaded (1 ton capacity). Or am I just being dof and not understanding you
    ?

    the puma accodrding to ve boekie, is 2.2 front and 2.8 rear (empty) and 3.3 (rear) if you're loaded in the back and or towing. SO, the manufacturer recons that the more you load one area the more tyre pressure you need. logically then because they say the back needs more tyre pressue, the back is heavier pushing the CofG aft. that being said, I have not yet felt the defender popping a wheely when you drop the clutch because of it.

    I agree with Imvubu - the defender is nose heavy when empty
    I agree with redliner that "front pressure is specced higher than the rear, it means there is more weight in the front and thus the CoG will be a bit more forward."

    Thus i don't agree with land rover. Sho but now i'm confused.

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  12. #12
    4ePajero Guest

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    If you want to get a quick indication of the position of the COG along the (long) axis of the vehicle, take the actual axle weights and apply RedLiner's principle.

    The front and rear overhang will however have an effect on the accuracy of this calc (I think )

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    Quote Originally Posted by 4ePajero View Post
    If you want to get a quick indication of the position of the COG along the (long) axis of the vehicle, take the actual axle weights and apply RedLiner's principle.
    100% agreed.
    Manufacturers spec tyre pressure to get the tyre to wear down correctly. In order to achieve this the tyre pressure must be increased as the weight on that axle increases.
    So there is a direct correlation between axle load and tyre pressure. So you can use either.

    I'm very puzzled by Imvubu's figures
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  14. #14
    4ePikanini Guest

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    If your overhang extends further you move the COG towards that extension and that will re-proportion the weight on the wheels due to the leverage of the overhang over the closest axle.

    If I see it right in my minds eye you can use the different weights on the axles to calculate ( I don't think it will be linear though ) the COG ( front to back )

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    Recommended tyre pressures are only partially a function of anticipated load on the tyres . In other words recommended tyre pressures (front vs rear) are a poor indication of the COG position.

    The reason for different pressures , front and rear , is to ensure balanced handling. (ie not too much under steer or oversteer) This handling balance is a function of , amongst other factors, weight distribution (ie COG) , suspension and steering characteristics and power application split between front and rear.

    A typical , lightly laden 4x4 vehicle will have a 60 : 40 front to rear bias. But its recommended tyre pressures will typically be 2.0 bar front and 2.4 bar rear ie not what we would expect if weight loading on the tyres was the only factor governing tyre pressures.

    Dave

  16. #16
    4ePikanini Guest

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    Pajero Owner's Manual

    1.8 Front < 2.0 Rear
    Jetta Owner's Manual

    2.2 Front > 2.0 Rear


    I suppose it is with regards to balance/COG/weight distribution

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    Silverpuma,
    Load distribution along the Z (Vertical) axis doesn't matter. At the roll over angle the car is about to Roll over, meaning for that instant the mass of the vehicle is virtualy concentrated/centered along the Z axis.

    RedLiner,
    Once again, I think you are spot on with your approach to estimation of how the CoG moves as you load the roof rack.

    4epikanini's diagrams are a perfect depiction.

    Imvubu, I dont think the specs on your landy are confusing. Your car is optimized for heavy loading. I am thinking along the lines of, when you have a ton of load which due to the orientation of empty cavities in your car will invariably be toward the back, most of the load will be supported by your rear axle hence 3.3bar pressure. This is pretty much like Silverpuma explained. So RedLiner's method of estimation still stands I think. And it should be applied with the thought of laiden and unlaiden vehicle load scenario.

    By the way, 4ePikanini, the Jetta is a front wheel drive. I would expect most of the load to be toward the front wheels, laiden/unlaiden, to maximize tractive effort.

    Thanks, for the excellent replies.
    What about ground clearance do you believe what the manufacturers claim?

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    Quote Originally Posted by 4ePikanini View Post
    Pajero Owner's Manual

    1.8 Front < 2.0 Rear
    Jetta Owner's Manual

    2.2 Front > 2.0 Rear


    I suppose it is with regards to balance/COG/weight distribution
    I think it's because the Jetta has the engine diff and gearbox all at the front axle. On the Jero those weights are distributed to the rear.
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    Quote Originally Posted by Lone Rider View Post
    What about ground clearance do you believe what the manufacturers claim?
    With all the regulations around advertising and automotive testing I recon the figures will be pretty accurate. Might be a bit of bending of the slide rule here and there, but still pretty accurate.

    Where big faults might creep in is where wheel sizes of the local model might differ from the world-model. If the local specification guys tried to keep the costs down by going for the smaller wheel, and they copy ground clearance from the world model brochure you might get a big difference.
    SWAMBO
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  20. #20
    4ePikanini Guest

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    Quote Originally Posted by RedLineR View Post
    I think it's because the Jetta has the engine diff and gearbox all at the front axle. On the Jero those weights are distributed to the rear.
    Makes sense - Occam's Razor!

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