TRUCK WEIGHT DISTRIBUTION
THE MATH INVOLVED IN FIGURING WEIGHT DISTRIBUTION

By Allen Collier

Let's say that we've decided to buy the truck described below. The truck's Gross Vehicle Weight Rating (GVW) is enough to carry our 19,500# anticipated body and load. But we realize that the individual axles have ratings too and that the total weight carried needs to be divided between them so that each axle gets its proper share. And, realizing that the truck length and body length that we select will determine this, before placing our order, we want to determine the best truck length/body length combination that's possible. Let's say that the truck we're considering has the following specifications.

Gross Vehicle Weight (GVW). . .28,500#
Truck Empty Weight . . . . . . . . . . 9,000#
Front Axle Rating . . . . . . . . . . . .  9,000#
  Front Axle Empty Weight . . . . . .  5,700#
Rear Axle Rating . . . . . . . . . . . . 19,500#
Rear Axle Empty Weight . . . . . . .3,300#
Front Axle to Back of Cab (ABC) 69"
Distance between body and cab (CC) 3"
Truck lengths available: 225" wheel base-156"cab-to-axle and 237" wheelbase-168" CA

How far the center of the truck body is ahead of the rear axle, of course, has to do with how much weight is transferred to the front axle and how much remains on the rear axle. More specifically, if the center of the truck body is forward of the rear axle a distance equal to 15% of the wheelbase, then 15% of the body/load weight will be transferred to the front axle and 85% will remain on the rear axle.

First we need to determine how much of the body/load each axle will safely carry by subtracting each axle's empty weight from its rated capacity, which I do in the table below.

Next we need to know the percentage of total body/load it will take to properly load the front axle. We know that what's left will stay on the rear axle.

Front Axle Body/Load = Percent of Body/Load Allowed on Front Axle
Total Body/Load

3,300 ÷ 19,500 = .169 or 16.9% (Round to 17%), the percent of body/load that should go to the front. The balance, 83%, of course should go to the rear axle.

We would like to use a 20' body, and the truck sales representative says that the customary length wheelbase for this body is 225"(156" Cab-to-Axle). Let's see how the weight distribution works out using this body length/truck length combination.

DETERMINING PERCENTAGES TO FRONT AND REAR AXLES
USING A 20' BODY ON A 225" WHEELBASE (156"CA) TRUCK

We remember the formula for this as: CA-CC-1/2 BL ÷ W/B = % to Front Axle. CA is for Cab-to Axle, CC is Cab Clearance (between body & cab), BL is Body Length and WB is wheelbase. What we want here is how far the center of the body is ahead of the rear axle, then see what percent this measurement is of the wheelbase. Half the body is 120", and the cab clearance is 3". Take this 123" from the 156" CA and we get 33" for the distance the center of the body is ahead of the rear axle, and 33" divided by 225" equals .146 or about 15% to the front. The remaining 85% will go to the rear axle. These percentages are not far off the ideal 17/83 percentages that we hoped for and probably about as close as we'll get with the available truck lengths, but, just to check, we'll work the formula on the 237" WB (l68"CA) truck, still using the 20' body.

DETERMINING PERCENTAGES TO FRONT AND REAR AXLES
USING A 20' BODY ON A 237" WHEELBASE (168"CA) TRUCK

CA-CC-1/2 BL ÷ W/B = % to Front or 168"-3"-120"÷237" = .189 (Round off to 19%). Since the 19% transfer will slightly overload the front axle, we decide that the shorter wheelbase truck is the one we'll choose, even though the weight distribution is less than ideal.

And thinking in terms of what's ideal, just for the heck of it, let's see what the ideal truck length would be to transfer 17% of the body/load to the front axle when using a 20' body. We'll not round off decimals so we can check our work.

DETERMINING IDEAL TRUCK LENGTH
USING 20'BODY WITH 17 % OF WEIGHT GOING TO FRONT

ABC + CC +1/2 BL÷ % to Front Axle = Wheelbase. ABC being Front Axle to Back of Cab.

How about this strategy? We'll work from the front axle backwards to determine how far back the center of the body will be. To get this we'll add to the ABC measurement the cab clearance and half the body length -- (69" + 3" + 120" = 192). Now won't this 192" measurement be equal to 83% of the wheelbase? So 192 ÷.83 = 231.32" Wheelbase. This ideal wheelbase is about 6" longer than one truck that's available and about 5" shorter than the other one.

We'll check our work by using the CA-CC-1/2 BL ÷WB formula to see if this body length/truck length combination will produce a 17/83 distribution. 162.32-3-120 = 32.32 which divided by the wheelbase of 231.32 equals .1699 or almost exactly 17%

While we're playing with figures, Let's figure what the ideal body length would be on a 225' wheelbase, 156" CA truck where we want 17%-83% distribution. We'll not round off the decimals so we can check or work.

DETERMINING IDEAL BODY LENGTH
USING 225" WHEELBASE TRUCK (156" CA) WITH 17% TO FRONT

By definition, the center of the body will rest at a point forward of the rear axle a distance equal to 17% of the wheelbase? This distance will be 38.25" (17% of the 225"WB) Then, wouldn't you say that the remaining distance to the cab less the cab clearance would equal half the body length? What is the remaining distance? It's 114.75" ( 156" CA less 38.25" less 3" CC). Then 114.75" X 2 = 229.5" or 19.13' for the ideal body length. Since body lengths come in even number increments, a 20' body is about as close as you will get to the ideal. The formula for this exercise might be 2(CA-CC-(% to Front X WB) =Body Length

Again let's check our work with the CA-CC-1/2 BL ÷WB = % to Front formula to see if these measurements produce a 17/83 distribution. 156-3-114.75 = 38.25 and 38.25 ÷ 225 = .17or 17%.

SOME COMMENTS

As a practical matter, there's not a lot of instances where truck buyers or truck sales people do these computations. There's a customary body length for nearly every length truck, and most people use these customary lengths, and since most truck operators have some discretion as to how they load their trucks, these customary-length bodies usually work out all right.

Also, many truck bodies are built for specific length trucks. Garbage packer bodies, wrecker bodies, roll back bodies and beverage bodies come to mind.

Truck operators ,though, who have no discretion as to positioning their loads should pay close attention to their truck length/body length combination. Those hauling lumber or pipe of a particular length, those hauling grain or other bulk or liquid commodities should pay close attention to weight distribution.

Most medium and heavy duty trucks are offered with one standard sized front axle and two other heavier optional axles. To think that one customary body length is suitable for all three sized axles is just not right. If a truck owner is paying extra for a heavier front axle, he certainly needs weight distribution that will load that axle.

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