How full is a "full" truck? Not sure? That's a shame, because when you contract for truckload freight, you pay for the whole vehicle, whether you fill it or not. As I'll show you, the regulations around what constitutes "full" for weight are very complex. In addition, the 3D jigsaw puzzle to pack product into the trailer space, distributing weight correctly and minimizing damage is exceptionally challenging. Get it wrong and you are paying to ship air.
It is much easier to plan to approximate rules or guidelines than to figure out what's really going on and it is common in the CPG industry to plan transportation loads based on these approximate rules. Unfortunately, these approximate rules are very dependent on what you are trying to load (as well as who made up the "rule") so you will rarely encounter the same "rule" twice. Typically what you will see is based on weight, pallet positions or cube. Rules that restrict what's loaded to maximum limits like:
- 40,000 lbs of product
- 2,500 cubic ft of product
- 48 pallets of product.
None of these are right and they routinely result in shipping air.
Depending on the product and the vehicle, I can safely, and legally, load much more than 40,000 lbs of product, (much) more than 2,500 cubic feet many more than 48 pallets.
One particularly bad example I have encountered said "a truck is full when there is 38,000 lbs of product in it". If I can find a way to, legally and safely load 45,000 lbs in the trailer it’s as though 7,000 lbs of product just shipped free, effectively saving 15% ((7,000/45,000 = 15.5%) in freight cost.
So, why is this so difficult to get right? Let's look at the regulations around weight.
Weight Limits
If your product is "heavy", you will probably hit a weight limit in loading. "Heavy" in this instance is roughly 20 lbs per cubic foot or more. Much less than this and you will probably hit a space limit first (see below).The government sensibly places restrictions on how heavy a loaded vehicle may be and how that weight must be distributed to be carried safely. To get a feel for the complexity involved, here is a link to the relevant page from the US Department of Transportation. Stay just long enough to get confused then head back here :-) Bridge Formula Weights
To summarize (and simplify):
- The total weight of the vehicle (including product, packaging, pallets, fuel, driver, etc...) must not exceed 80,000 lbs.
- There are limits for the weight on individual axles (shown below in this graphic from the US Department of Transport)
Note for the mathematically inclined:
If anyone is really interested in being able to calculate what weight should be on each axle given a particular layout of product in the trailer you need a little physics/engineering math. Here is a great example of how a beam transfers load with an interactive calculator.
With the right math, you can calculate the center of gravity for the product and how that weight would be distributed to the axles. Move the center of gravity forwards (e.g. by moving heavier product to the front) and you take weight of the rear wheels and transfer it to the tractor unit. Quite how that weight then gets distributed to to axles 1 through 3 depends on where the "kingpin" connection (between the trailer and the tractor unit) is relative to the tractor units axles. You can build such a model in Excel.
So, can you plan to a total rig weight of 80,000 lbs? No, sorry, there are only so many options for how to layout product in the vehicle and there may be no layout that balances weight well enough across all axles to max out the 80,000 lb overall limit.You need to find the layout that gets closest to that limit and live with the loss.
Alternatively, you may run out of space in the vehicle before you get close to a weight limit.
Space Limits
If product is reasonably light (low density) we will probably be constrained by space before weight becomes a problem..
A reasonably standard trailer's internal dimensions are approximately
- 52' long
- 8' wide
- 8' (usable) height
That's a little over 3,300 cubic ft of space available to you. However, you are typically loading with palletized product and you will not get to use most of this space. Let's look at how well you can use the floor space first.
A standard pallet for US grocery is 40" wide, 48" long. The ability to fill the trailer floor-space depends a lot on how well these palletized units fit.
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In each case there is floor space you cannot use. Load "narrow:narrow" and you have already lost almost 20% of the available space.
Now let's look at vertical space. This is much more variation in the height of trailers and in the height of doors to those trailers. If the door height is restrictive , perhaps because the door rolls up inside the trailer, that will limit the product you can get into that trailer. Let's assume for now that we can safely get to 8' high.
How much of the vertical space we can use depends on what we are loading. Some product cannot be stacked, pallet on pallet, without causing damage. Some pallets are too tall to allow anything (except an unusually short pallet) to fit stacked on top of it. Very short pallets may be able to stack 3 or even 4 high.
If I assume 40" high palletized product, double stacked, we can use 80" of the 96" vertical space (83%).
Combine that with "narrow:narrow" loading and we max out a truck at 83% * 83% = 69% space utilization. From a "cube" standpoint the trailer is only 70% full and it's at capacity. Hence rules like "the trailer is full at 2,300 cubic feet of product" when the trailer has over 3,300 cubic feet of air.
Depending on product dimensions and the ability to stack product we may be able to load much more than 2,300 cubic feet or much less.
Reducing Damage
We could spend a lot of time on this, but for now suffice to say, we would like to load the trailer so that product is less likely to get crushed, to move as the vehicle corners or to land in a heap at the front when the driver must, necessarily, brake. This puts additional constraints on what product can go where in the vehicle, reducing, again, the weight you can carry and/or the volume you can load into the available space.
Equipment limitations
If the pallet handling equipment (for either shipper or receiver) can't stack pallets or can't handle them turned, you will necessarily lose a lot of payload capacity. Unless this is for very short trips where you can pay for the freight-cost with handling labor savings, you need to invest in warehouse equipment.
Not all trucks/trailers are the same. By design they can be very different, tractor units weighing anywhere between 11,000 and 20,000 lbs. Trailers can be vary by a few thousand lbs too. Even equipment of the same make, model and year can be different depending on its setup (kingpin position) and the addition of aftermarket parts. (Mount a new fuel tank too near the front and see it use up the limited capacity you have on the steering axle). If weight is an issue for you, you will need to work with your carriers to understand what equipment they are bringing in. Lightweight equipment is worth more to you, heavyweight equipment should be avoided or contracted at a low enough rate to offset the loss in carrying capacity..
Pulling it all together
Each of these sets of restrictions, weight, space, damage and equipment are complex: accurately modeling any of them is a challenge. Depending on what you load: heavy or light product, slightly oversize, stack-able or not, the factors that constrain you continually move. And, beyond modeling it, you need to optimize: to find the selection and layout of product that maximizes your vehicle loading.
My Take
If you're lucky enough to have consistently-sized, palletized product with consistent low or high density that is consistently stacked and loaded on consistent carrier equipment you may be able to build a simple rule that really does max out your truck loading. Good for you! For the rest of us, such "rules" are poor guesses at best and can leave a lot of money on the table.
You are NOT going to build this in Excel unless you have a lot of functional knowledge, very advanced skills in mathematical-optimization and the ability to program your own optimization code. I have built a load optimization tool (It's a weakness, I like to know how things work). I have also built my own tools for inventory-optimization, neural-network modeling, genetic-optimization, forecasting and many other needs.. Some of these tools are still in production use today, others were essentially learning opportunities. In this instance, I chose to buy the software because it has richer functionality than what I chose to build myself.
(On a technical note, application-specific,heuristic optimization routines solve these problems well and quickly. Mixed-Integer-Programming can get you most of the way, but personally I can't see how to embed some of the damage-reduction ideas into a linear objective function and as the heuristic works so well, you don't need the overhead or complexity of integrating a math programming tool)
Any load building “optimizer” that asks you to specify a maximum weight or cube or number of pallets is really just automating these approximate rules rather than helping you truly max out the load. An optimizer that is implemented as a stand-alone package is interesting but not really useful: you need this capability integrated into order-processing, deployment and transportation planning to be effective. You need the right tool for the job. These tools do exist, they work and it's not worth your time or effort to write them again.
There are a number of tools in the market that work in this space. Google "load building optimizer" to see some. I have not reviewed them all, far from it, but I can tell you that not all "optimization" is the same. Having "optimize" in the sales literature does not mean it will do a great job for you or that any actual optimization is really taking place.
If you want a quick recommendation I suggest you talk to Transportation | Warehouse Optimization. they have a great load building tool and can extend this further into optimizing case-pick routing, pallet builds and shipping locations.
Let's assume you are doing a reasonably good job today without a load optimizer. What would an extra 5% off your freight spend be worth to you? Enough to invest in the right tool for the job?
A final thought
These rules-of-thumb can be very persistent. Having implemented a system to drive increased payload coming out of one manufacturing site, I was perplexed some months later to see that the load factor had shrunk back to where it started. It turns out that the warehouse supervisor at the plant was adjusting each and every load plan manually to fit with his interpretation of the “rules”.
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