Measurement - Free Weight

The placement of free weight is critical to the performance of many hobby vehicles. "How much?" and "Where?" are two of the biggest questions facing the custom designer. "How much?" is easier to answer. "Where?" is the focus of this discussion.

Planning

First, find out "How much?" free weight you will need. enter this weight in the chart below as the value of FW. The "(___)" is for you to write your units, (oz), (g), etc., in.

To find out where to place the free weight, find the center of mass of your vehicle without the extra weight - this is the empty weight of your vehicle. Enter the value for the total weight in the chart below as EW. The "(___)" is for you to write your units, (oz), (g), etc., in. Also, enter the center of mass you found as EWx, EWy, and EWz. These were called CMx, CMy and CMz respectively, but those names will be given to your desired center of mass below. Fill in the values for EW x EWx, EW x EWy and EW x EWz from the totals for the center of mass of the empty weight vehicle or just do the multiplications indicated.

Where do you want your vehicle's center of mass to be? You must answer this question yourself. Once you decide, put the values in the chart below as CMx, CMy and CMz.

"Where Does The Free Weight Go" Worksheet

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Free Weight:  FW       = ____________ (___)

Empty Weight: EW       = ____________ (___)
        (___) EWx      = ____________, EWy      = ____________, EWz      = ____________
              EW x EWx = ____________, EW x EWy = ____________, EW x EWz = ____________

Desired CM:   
        (___) CMx      = ____________, CMy      = ____________, CMz      = ____________

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Total Weight: W = EW + FW = ____________ (___)
              W x CMx  = ____________, W x CMy  = ____________, W x CMz  = ____________


W x CMx - EW x EWx = ____________ 
W x CMy - EW x EWy = ____________ 
W x CMz - EW x EWz = ____________ 

FWx = ( W x CMx - EW x EWx )/FW = ____________ (___) with respect to the X ref. line
FWy = ( W x CMy - EW x EWy )/FW = ____________ (___) with respect to the Y ref. line
FWz = ( W x CMz - EW x EWz )/FW = ____________ (___) with respect to the Z ref. line

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Now that you have all the information, you need to do some calculations.

First, add the empty weight, EW, to the free weight, FW, to get the total weight. Enter it in the above chart.

Multiply the total weight, W, by each of the parts of the desired center of mass location; CMx, CMy and CMz. Enter these three total weight product values in the above chart.

Subtract the empty weight products, EW x EWi, from the total weight products, W x CMi, and enter these three product differences in the chart above.

Finally, divide the three product differences by the free weight to get the location of the free weight, FWx, FWy and FWz.

If the free weight location is outside the outline of your vehicle, you will need to design your vehicle to cover that location so you can attach the weight there. If you do change your design, you'll probably have to figure out where the weight goes again. If you can't redesign, you need to use a different desired center of mass and do the last set of calculations again.