CG/Rollover calculator

Collapse
X
Collapse
First Prev Next Last
 
  • Filter
  • Time
  • Show
Clear All
new posts

    CG/Rollover calculator

    I've been thinking about something for awhile now. (Dangerous I know) I would like to know the angle that my project truck could take on a side hill without tipping over. I'd like to make an inclination meter for the dash. I think I could devise a test to determine the angle, but it would be kind of a PITA to do. Then I got wondering if you couldn't figure it out with some scales. Let me explain.

    Lets say we have a hypothetical vehical. For this discussion, we'll say the tires are "rock hard" and the axles are bolted to the frame. (No suspension) It is 6' wide from the outside of the tire treads. It weighs 4000 lbs. it has perfect weight distribution and the CG is 3' off the ground. If we tip one side up 45 degrees, there will be 4000 lbs. on the two downside tires and no weight on the upside tires. If we were to tip it 46 degrees, it would rollover. First, does everybody agree this would be true? If not, explain.

    Assuming this is correct, I'm thinking that we could work backwards by weighing the machine flat on the ground and then again with one side tipped up a fixed amount. Let's say 15 degrees. A certain amount of weight would transfer to the low side tires. I'm thinking that we could make a calculator that we could feed in the total weight and the weight transfer and it could tell us how high the CG is and the ultimate "tipover" angle.

    I realize that angle would be less because the "real world" tires at low psi and soft suspension would increase the weight transfer, but it should be close.

    Now, I think I could come up with an equation to accomplish this, but it would be nice if one of you "computer wiz" guys could make a calculator that we just feed the data in like a link calculator or the ratio calculator.

    PS, I think the mods here should put sticky for the link calculator and the "Grim Jeeper" ratio calculator. If this works out, it also could be put in a sticky.


    Comments and flames welcome.



    Check out my project
    https://irate4x4.com/general-4x4/2779-amphibious-hemtt

    #2
    Seems like a mathematical nightmare. Bust out the forklift and a magnetic angle finder on the dash and find out exactly.

    Comment


      #3
      The proper way to determine your CG accurately will have your vehicle a foot or two from finding the rollover angle. Pretty easy at that point to find the tipping point.

      Getting the correct CG is the hard part in what your asking. Bad numbers put in a calculator = bad results
      Kevin

      Comment


        #4
        Originally posted by Byro View Post
        Seems like a mathematical nightmare. Bust out the forklift and a magnetic angle finder on the dash and find out exactly.
        If you do this^ - run a safety strap or chain from near-side frame rail to forklift's load backrest in case you pass the tipping point. Unless you go apeshit overboard, the strap or chain will only see ~100# of tension when just over-center.

        PLEASE BUY AMERICAN SO WE DON'T HAVE TO LEARN CHINESE
        The 4th little pig made his house of reinforced concrete, with wolfskin rugs in every room

        Comment


          #5
          Originally posted by Byro View Post
          Seems like a mathematical nightmare. Bust out the forklift and a magnetic angle finder on the dash and find out exactly.
          Doesn't seem that hard to me. I did forget something. You would need to account for the width of the vehical. So I'm thinking you need three things.

          Width from outside tire tread edge to outside tire tread edge.

          Total Weight when flat on ground.

          Weight transfer to low side tires. (At some specific angle)


          There would be some error due to one side being heavier than the other. (Drivers or pax side diff) But I wouldn't think that would be dramatic.
          Check out my project
          https://irate4x4.com/general-4x4/2779-amphibious-hemtt

          Comment


            #6
            In NJ we have 'modified height vehicle' inspections...part of it was weighing the vehicle on flat ground, then you drove one side up onto a concrete berm, and they measured the amount of weight transfer to the downhill tires. If it exceeded a certain amount you'd fail inspection, this was known as the 'tilt test' or stability test.

            I forget what the threshold was or how it was calculated, but yes what you're asking for can be done in theory.

            Comment


              #7
              I had friends that did something similar in their senior project in college. They used scales to determine fore and aft, as well as side to side weight balance, but they also needed the height. The solution was to use scales on the rear tires and a forklift to lift the front up. With the front tires a known height off the ground and a now known weight transfer to the back tires, the CG height could be calculated. As I recall they had to get the front 4-5ft off the ground. I don't see why that wouldn't work side to side also. Also, if you know the cg height you can figure out the tipping angle and vice versa. I could probably dig up the math on the above project if needed.

              Kevin

              Comment


                #8
                Originally posted by Ghetto Fab. View Post
                I had friends that did something similar in their senior project in college. They used scales to determine fore and aft, as well as side to side weight balance, but they also needed the height. The solution was to use scales on the rear tires and a forklift to lift the front up. With the front tires a known height off the ground and a now known weight transfer to the back tires, the CG height could be calculated. As I recall they had to get the front 4-5ft off the ground. I don't see why that wouldn't work side to side also. Also, if you know the cg height you can figure out the tipping angle and vice versa. I could probably dig up the math on the above project if needed.

                Kevin
                I might be thinking wrong, but it would seem like it wouldn't work fore and aft because of overhang front and back. There really isn't any over hang side to side. The reason it wouldn't seem to work with overhang is because that weight would never shift to the low side tires until you were near vertical. Again, Maybe I'm thinking wrong here.

                in any case, my friend just got some racing car scales in an estate sale. I maybe borrowing them if they work.
                Check out my project
                https://irate4x4.com/general-4x4/2779-amphibious-hemtt

                Comment


                  #9


                  Click image for larger version
Name:	fetch?id=6415&d=1590124130.gif
Views:	179
Size:	2.54 MB
ID:	8120

                  μολὼν λαβέ
                  '94 Ford Bronco XL / Sky's ORD 6"lift / Sterling 10.25 dually / HMMWV tires / Bilstein shocks

                  Comment


                    #10
                    How do you think that these people are going to obtain the wight on each tire to start plugging those numbers into a calculator?

                    Comment


                      #11
                      Originally posted by Crimsen View Post
                      How do you think that these people are going to obtain the wight on each tire to start plugging those numbers into a calculator?
                      I hate to be sarcastic, but maybe they would use some kind of scale.


                      So, my first thought is take the transferred weight and make it a percentage of the total. In the case of my hypothetical truck from the first post, the weight transfered was 50 percent of the total. (At 45 degrees) If I take 50 percent of the width (72"), I come up with 36" for a height to the CG. Could it be that simple?

                      Of course that only works at 45 degrees, but I think I could make an equation account for a lesser angle. Let me ask a question. If 2000 lbs. transfers to the lower tires at 45 degrees, would I assume 1000 lbs. to transfer at 22.5 degrees? Or do you think the weight transfer would not be linear?
                      Check out my project
                      https://irate4x4.com/general-4x4/2779-amphibious-hemtt

                      Comment


                        #12
                        This might help. It can be done side to side also.

                        This comes from
                        www.longacreracing.com





                        CENTER OF GRAVITY HEIGHT



                        Finding the center of gravity height can be done in several ways, none of which are accomplished very easily and without some work. Presented here is the easiest method. The center of gravity height is calculated by weighing the car when level and then raising the car at least 10 inches at the rear and weighing the front again. Enter the data into the program below to calculate your center of gravity height.

                        Before you begin:
                        • Be sure that all fluids are full
                        • Replace each shock absorber with a solid link to eliminate suspension travel
                        • Make sure the tires are inflated to the maximum pressure as specified by the manufacturer to eliminate any sidewall flex

                        Note: If these steps are not taken, the calculations will be inaccurate
                        Center of Gravity Height Formula Definition of Variables
                        • CGH - Center of Gravity Height
                        • WB - Wheelbase (inches)
                        • TW - Total weight
                        • FW1 - Front weight LEVEL
                        • FW2 - Front weight RAISED
                        • FWc - FW2 - FW1 (change in weights)
                        • HT - Height raised (inches)
                        • Adj - Adjacent side (see below)
                        • Tan q - Tangent of angle (see below)
                        • CLF - Left Front tire circumference
                        • CRF - Right Front tire circumference
                        • C - (CLF + CRF) / 2 (average circumference)
                        • r - Axle Height
                        Related Formulas



                        Kevin

                        Comment


                          #13
                          Interesting. I notice they don't account for overhang, so I guess it doesn't matter. I don't follow the theory behind the circumference of tires. I realize a racing car could have differnt height tires. Maybe they do that if your WB is measured at the axle center. Then the staggered heights might make a slight differnce in the true WB. Wouldn't really matter side to side.

                          The idea of locking the shocks is interesting. They say at least 10". I'm not sure if you need more or less for a side to side measurement. (Because of the shorter width vs WB)
                          Check out my project
                          https://irate4x4.com/general-4x4/2779-amphibious-hemtt

                          Comment


                            #14
                            Originally posted by gt1guy View Post
                            This might help. It can be done side to side also.

                            This comes from
                            www.longacreracing.com





                            CENTER OF GRAVITY HEIGHT



                            Finding the center of gravity height can be done in several ways, none of which are accomplished very easily and without some work. Presented here is the easiest method. The center of gravity height is calculated by weighing the car when level and then raising the car at least 10 inches at the rear and weighing the front again. Enter the data into the program below to calculate your center of gravity height.

                            Before you begin:
                            • Be sure that all fluids are full
                            • Replace each shock absorber with a solid link to eliminate suspension travel
                            • Make sure the tires are inflated to the maximum pressure as specified by the manufacturer to eliminate any sidewall flex

                            Note: If these steps are not taken, the calculations will be inaccurate
                            Center of Gravity Height Formula Definition of Variables
                            • CGH - Center of Gravity Height
                            • WB - Wheelbase (inches)
                            • TW - Total weight
                            • FW1 - Front weight LEVEL
                            • FW2 - Front weight RAISED
                            • FWc - FW2 - FW1 (change in weights)
                            • HT - Height raised (inches)
                            • Adj - Adjacent side (see below)
                            • Tan q - Tangent of angle (see below)
                            • CLF - Left Front tire circumference
                            • CRF - Right Front tire circumference
                            • C - (CLF + CRF) / 2 (average circumference)
                            • r - Axle Height
                            Related Formulas


                            This is correct.

                            Here it is derived:

                            Step 1
                            Click image for larger version
Name:	2020-05-24 11-58.jpeg
Views:	143
Size:	250.9 KB
ID:	10657


                            Step 2
                            Click image for larger version
Name:	2020-05-24 12-13.jpeg
Views:	147
Size:	275.2 KB
ID:	10658
                            4606 Ultra4 Stock Class Early Bronco
                            @SharpMotorsports4x4 - Instagram/Sharp Motorsports - Facebook

                            Comment


                              #15
                              Originally posted by WaterH View Post
                              The idea of locking the shocks is interesting. They say at least 10". I'm not sure if you need more or less for a side to side measurement. (Because of the shorter width vs WB)
                              For what it's worth - in the stability test thing I mentioned here in NJ, (and for the purposes of what you're trying to do), you don't want the shocks locked out. The whole point is that the CG shifts because we have these long-travel, floppy suspensions. Knowing the static location of the CG on level ground isn't all that helpful for knowing your tipover angle as it'll shift as the body moves around the suspension.

                              Overall height is not really an issue in NJ as there are already other limitations for that; the point of the stability test is to see if your suspension is extra-floppy to the point that too much weight % gets transferred for a given incline.

                              Comment

                              Advertisement:
                              Advertisement:
                              Advertisement:
                              Advertisement:
                              Advertisement:
                              Advertisement:
                              Advertisement:
                              Working...
                              X