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Finding Car's Center of Gravity;
IN part of this section, I shall try to teach you how to determine what your actual front to rear weight distribution is by using stuff you already have, without having to go pay a chassis shop to scale your car if you are on a tight budget or there are no scales near you. Of course you are better off to scale the car but we have not always had individual wheel scales in drag racing so here is an old fashioned method that has worked for years in some of the baddest cars there was.
With 4 wheel scales it is easier to find the center of gravity and center of balance. You take the instructions I gave and use them in reverse to find where it is from the front to the rear. So if you know the total weight and the weight on each end then you will be able to calculate the percentages on each end with that info. Then you just use your wheelbase and figure for example 45 % of weight on the rear translates to 45 % of your wheel base from the rear would be the center of the car so that should be the balance point.
Write all of this info down so you will keep it in a log. Any adjustment changes you will write down and the results of them always.
where is your actual center of gravity of the car.
What is your actual wheelbase
what is your rear tire diameter
Place a weight in the driver's seat equal to the weight of driver. Load fuel tank, water reservoir and make the car balance the same, as it would be weighted while at the starting line.
Get two jack stands. (I actually even place short angles on them so there is an actual point instead of the larger top of jack stand).
Now you will think I am nuts.
You are going to balance the entire car on only two jack stands, one on each side. Do not give up. The car will perfectly balance. Mark that place with a piece of thin "3M FinishLine Tape" (Buy at Paint and body supply) attached to each side of your car in a level vertical line. That is the point of front to rear balance of your car. Good to write this down by measuring from balance point to the center of both front and center of rear wheel hub.
Make a mark where a level vertical line is at the center of balance, to the ground or floor. Mark a level vertical line where the rear-housing center is. Mark a level vertical line where the front hub center is. All of these three vertical lines will be used to determine your actual front/rear weight distribution.
Measure your exact wheelbase from center of rear housing marked vertical line to center of front hub marked vertical line. Write it down.
Divide the distance measured horizontally level on the ground/floor from the rear housing center to the vertical level line that is at the car's point of balance by the measured wheelbase. That will = your rear weight percentage or percentage of weight on the rear wheels. Write it down.
Divide the distance measured horizontally level on the ground/floor from the center of front hub to the marked vertical line at car balance point by the measured wheelbase. That will = your front weight percentage or percent of weight on front wheels. Write it down.
Lower car to the ground and bounce it a few times to get it to be at normal ride height.
measure from ground level to center of camshaft. That should be close to the height of center of gravity. Write this measurement down.
At the exact place previously established as the car's balance point, at the height of the center of camshaft, will be your
theoretical point of center of gravity of your car. Mark that spot on the door or fender by crossing two thin pieces of tape in an
X. Take a picture of side of car with tape on it.
Measure all the discovered dimensions, including the measurements from front axle and rear axle to that place where the car
perfectly balanced. Write all this down in a notebook to keep.
With someone helping stretch the thin tape from the place you determined was the center of gravity of your car, in a downward angle to the bottom and center of where your rear tire is contacting the pavement. This will be the Theoretical Instant Center line (Neutral Line). You will keep this imaginary line in your mind forever once you begin using it. It is only a point of reference for you. Take a pic of tape positions on car from the side.
Take a picture of the side of your car with the tape showing the angular line from the bottom of rear tire to the place marked to show cam height at the balance point that we are calling the car's center of gravity.
That imaginary line is to be used as a reference as theoretical "Line of Instant Center". Some call it "Neutral Line". The actual (or active) point of instant center is the actual point where a straight line extended from your top bar and your bottom bar will intersect in an imaginary line looking like a triangle shaped like a Ladder Bar.
Use the narrow 3M Finishline tape, which you will find at auto paint and body supply houses or paint stores. If you use a magic marker, or pen or pencil to mark on your car you will have a nightmare removing the marks. Finishline tape will stick on and will easily peel off.
Place the 3M Finishline tape in a vertical level line on the car at that point of balance. Which is easier when you have scales to weigh the ends with. At the camshaft ht place a piece of the 3M Finishline tape in a horizontal line at camshaft ht. That X where the two pieces of tape cross should be your Center of Gravity of the car. That is Center of Gravity. That is not where you want your Instant Center. I use Center of Gravity as my reference point for pretty much everything related to suspension adjustments.
Adjust Instant Center & WHY;
As you move your actual instant center further forward or below that Theoretical "Line of Instant Center" (Neutral Line) (It is an imaginary
angled line, where you had your tape) The rear suspension hit will decrease, but the front will have a tendency to rise on it's suspension. Likewise, If you move your actual "Instant Center" back or up toward the Theoretical "Line of Instant Center", the rear suspension hit will increase.
As you move your actual or active Instant Center point forward of the Imaginary line of Instant Center (Neutral Line) you will promote more front suspension lift and less rear tire hit.
As you move the actual or active Instant Center to the rearward you will give it more tire hit and greater rear suspension reaction and less front suspension lift.
As you adjust for increased rear tire hit, you will be at the same time be adjusting from less effect of actual bar lift in the front.
When you adjust for decreased rear tire hit, you will at the same time be adjusting for more effect of actual bar lift in the front.
If you reach the Ideal, as the tires hook the car will rotate on the rear axle and will also try to lift the front, as the car is moving forward.
Hopefully you will know that a low torque engine will need more adjusted Hook or Tire plant than will a high torque BB car. As the power level at launch increase you will need less and less adjusted hook in your suspensions. Sounds strange but that is the way it is.
Thanks for all that info it's added to the knowledge overload I'm just not getting it I'm thinking there's just a simple method my car is a driver not a race car or anything like that
No problem. This is what I have been using to try to convert to 4 link on my car. The ideas and bar adjustments will all be the same. It is nice that he explains what changing the bars will do to the chassis. There was much more to it; jumping into shocks, coil springs, locations, etc
Okay guys I took a stab at it this is what I came up with i just put it where it looked right let me know if you think it will work if not how to fix please I really appreciate it
If it were me I'd have pivot points parallel with the axle. From my screen it kinda looks like it could bind up when the axle and frame travel up and down.
So I should bring my top link up
They way it looks to me and what I think 23dodges10 is saying is the front lower link pivot doesn't seem like it will allow the suspension to cycle without binding. Typically the pivot points all should be parallel to each other and the rear axle (triangulated upper arms just need to be parallel to the other respected end) unless heims joints are being used which allow more misalignment.
Last edited by Sproket; 01-20-2017 at 04:44 AM.
I think I may have it bear with me guys true rookie here so the the parts of the link that move need to be the horizontal that way they will move up and down with ease like this or am I wrong
Yes, they have to pivot on the through bolt. A heim joint can go either direction as long as the movement is within the range of motion. If you cycle your suspension - it will likely not move at all because of the vertical mounting of the front pivot on the lower link.
So do you think the way is mocked up will work