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Posted: 4/20/2017 4:56:35 PM EDT
I am building a hot rod from scratch and would like to calculate the weight over the front and rear axles before I finish building the
car, primarily to determine the required spring rates.

I have found lots of information online regarding finding the center of balance for vehicles but to use the on-line calculators or formulae I need to know
the actual weigh over axle as determined by placing the vehicle on scales.  I want to do this backwards, I think. 

I know the wheelbase and where each axle lies from a datum line, I will be able to actually weigh or estimate each sprung part as I begin
the design. 

I will know:
-         Total Weight (as it adds up while I am planning/building; this will be an iterative process)
-         Distance from the datum line to the front axle
-         Distance from the datum line to the rear axle

I would like to build a spreadsheet to perform the calculations as I enter a part’s weight and its distance from the datum line, giving me a running total of the:
-         Gross Vehicle Weight
-         Weight over the front axle (Front Axle Weight)
-         Weight over the rear axle (Rear Axle Weight)
-         Center of Balance

My problem is I have two variables (FAW and RAW) and no way to determine what they are.  My algebra is rusty, does anyone know how to do this?  
Link Posted: 4/21/2017 8:04:47 AM EDT
You will be able to approximate this, but likely will not be perfect.  You'd need to weigh each part, and know where the center of gravity of that part is.  If you do this, then your table can easily calculate the center of gravity and weight on each axle.

First, define a master datum line for calculating locations.  For simplicity, make it forward of the front bumper of the vehicle, so that everything will be on the same side of the line (no negative moments).

For your spreadsheet, you will need 2 columns for data entry.  First entry is the weight of the part, and the second is the distance from the master datum line.  Third column will be the moment, which is the product of the first two. (moment = force x distance)

The sum of the weight and moment columns will give you the total weight and moment of your vehicle.  The weight over the axles is then easily found:

D1 = distance to front axle
D2 = distance to rear axle
Wt = total weight of vehicle from the spreadsheet
Mt = total moment of vehicle from the spreadsheet
W1 = weight on front axle
W2 = weight on rear axle


W1 + W2 = Wt

(D1 * W1) + (D2 * W2) = Mt : of these, W1 and W2 are your unknowns.

So, let's make W1 = Wt - W2

we now have

(D1 * (Wt - W2)) + (D2 * W2) = Mt with only 1 unknown.  We can rearrange and simplify to get:

    (D1 * Wt) - (D1 * W2) + (D2 * W2) = Mt

    (D2 * W2) - (D1 * W2) = Mt - (D1 * Wt)

    W2 (D2 - D1) = Mt - (D1 * Wt)


and finally, we are left with

    W2 = (Mt - (D1 * Wt))/(D2 - D1)

And from there, W1 = Wt - W2

The issue will likely be determining how far from the datum each part is.  In this case, the distance is to the center of gravity of that part, which can get tricky.

Mike
Link Posted: 4/21/2017 7:57:33 PM EDT
Thanks very much for that.  I completely forgot how to reduce it to one unknown; I was thinking of simultaneous equations and other nonsense!

Again, thanks
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