Introduction: How to Estimate the Axle Weights of a Standard 2-Axle 4x2 Class 6 Truck

About: Industrial Arts, Appalachian State University. Recession has dried up my field (commercial printing & packaging), but have found new work in staging, lighting, sound systems, sets, productions and events.

Want to add some excitement and adventure to your life?
Get a part-time job, rigging and driving a flatbed truck!

So many Class 6 trucks (26,000-pound maximum legal weight) on the road are driven by those who have a general Class C driving license, and good driving record, but little or no formal Commercial Class A, or Commercial Class B truck driving training.

But the Class C licensed truck drivers' responsibilities are still there, just the same.

As the driver of a 4x2 Class 6 Truck, you are legally responsible for axle weights and condition of the vehicle. This fact of life becomes sharply apparent at the state highway weigh stations. Any violations, and you, as the driver, may be written up and issued a ticket, with the violation(s) on your driving record, and your personal car insurance rates may increase. Your employer may also be fined for any and all violations. If the axle weight(s) of your vehicle is over the legal limit, your vehicle will be temporarily impounded: You will be ordered to park your truck behind the weigh station, and wait there until you, or your employer, sends out another truck to transfer some of the excess load off your truck. In some cases, a third party may need to get involved to send out a forklift to transfer, or rearrange the load. Your truck will then be required to run through the weigh scales again before being allowed to return to the highway. And this clumsy, embarrassing, and irritating process will usually take several hours.

But all of this can easily be avoided. Read on.

Many standard 2-axle 4x2 class 6 trucks have four heavy-duty leaf springs: Two on each axle, one for each front wheel (also known as "steer axle"), and one for each pair of rear wheels (also known as "drive axle"). And you may have already observed that the undercarriage (or frame) of the truck noticeably sags under heavy loads, as the tire-to-undercarriage (or frame) distance becomes less and less.

And these leaf springs, whose function is to cushion sharp impacts on the highway, can also be viewed as reasonably accurate, but uncalibrated, axle scales.

But to make a long story short, start by taking your empty standard 2-axle 4x2 class 6 truck, but with a full tank of fuel, to a public scale, and get a printout of the total weight of the empty truck, as well as the individual axle weights.

Public scales are sometimes quite busy, with much heavier trucks and tractor-trailer rigs waiting in line for their turn. But if there is no one waiting behind you, this is the perfect place to quickly measure the leaf spring-to-undercarriage (or frame) distance, in millimeters, before you drive away, as these scales are all on a perfectly flat surface. The best distance to measure is from the top of the leaf spring, directly over the axle, to the closest part of the undercarriage directly above it.

I've found the best way to accurately measure these distances, to within half a millimeter, is to use a standard extendable-retractable chrome antenna from an old portable radio or TV. But the antenna segments must be snug and tight for an accurate measurement. I have scrounged up two of these: One is 38mm long, extendable to 99mm. And the other is 95mm long, extendable to 377mm -enough range to work with any truck. I also cut off the screw bases of the antennas and filed them down until they were perfectly flat.

Each distance taken with the extendable antenna is then carefully measured with a metric tape measure and noted. Both measurements on each axle are then averaged. Note that this handy method for determining axle weights is not suitable for individual wheel weights and lateral (left-to-right) load balance, as the leaf springs are, simply, not accurate enough for this purpose. Legally, both wheels on a given axle should be within 3% of each other, for lateral load balance, but it's not uncommon for my unorthodox method of leaf spring measurements to erroneously indicate a lateral load balance to be off by much more -on an empty truck! But averaging both ends of a given axle completely solved the problem, as long as this method is only used for axle weights. For lateral load balance, I simply make judgments by visually examining the truck from the front, or the rear, and then making lateral load shift corrections, if needed.

When the truck is fully loaded, take it back to the public scale, and get a new printout of the total weight of the fully loaded truck, as well as the individual axle weights. Also, if there is no one waiting behind you, this is, again, the perfect place to quickly measure the leaf spring-to-undercarriage distance, in millimeters, before you drive away, as noted, these scales are all on a perfectly flat surface.

You now have everything you need to estimate any load with this particular truck, especially if the fully loaded weights of the second public scale visit were at, or slightly over the legal limits. Use simple straight-line calculations to quickly determine the individual axle weights for all future loads.

For our 20-foot flatbed truck, the leaf spring-to-undercarriage differential is only 16.5mm (0.656"), between a empty truck and maximum legal load, on the front axle. But each half-millimeter represents about 38 pounds of payload. This may, at first glance, seem to be an unnervingly small distance to work with to calculate the front axle weight, but the front axle on this truck is already at 82% of its maximum capacity, when empty. On the rear axle, the leaf spring-to-undercarriage differential is 60.5mm (2.375"), between a empty and maximum legal load. Each half-millimeter represents about 105 pounds of payload.

Examples of these calculations:
(these numbers only applicable for this particular truck)

Front Axle (7,000# maximum weight, with this particular truck):
Left-Front: 96.5mm
Right-Front: 87.5mm
= Average: 92.0mm
-(77.9 x 92.0)
= Front Axle Weight: 6,936 pounds

Rear Axle (19,000# maximum weight, with this particular truck):
Left-Rear: 287.5mm
Right-Rear: 317.5mm
= Average: 302.5mm
-(209.5 x 302.5)
= Rear Axle Weight: 18,988 pounds

Note: The above calculations are only examples. You will have to devise your own straight-line formulas, for your truck.

Note that the payload capacity on the front axle, on this particular truck, is only 1/10th the payload capacity of the rear axle. In other words, the load on the front axle, of this particular truck, even when empty, is already at 82% of maximum capacity. As such, I routinely advise others, for simplicity, to think of this truck as a "giant Segway," where the payload should be balanced over the rear axle. This shortcut thinking is sound, as long as the total vehicle weight is comfortably below capacity.

But I've also noticed another driver's "26,000-pound" truck, equipped with a combined pair leaf springs / shock absorbers on each axle, with a front axle capacity of 10,640 pounds, and a rear axle capacity of 18,500 pounds, although the total weight of the vehicle is still limited to 25,995 pounds. So expect diversity in this truck classification.

Now that you have determined the weights of the front and rear axles, you may need to accurately determine the weight of the payload, as well as its center of gravity.

Center of gravity of payload (assumed forward of rear axle):
(numbers only applicable for this particular truck)
(Front Axle 6,936# loaded weight - 5,740# empty weight) + (Rear Axle 18,988# loaded weight - 6,320# empty weight)
= Total Payload 13,864#
(Front Axle Payload 1,196# ÷ Total Payload 13,864#) × 237.25" Wheelbase
= Payload Center of Gravity, measured in inches, forward of rear axle: 20.5"

If one of the axles exceeds maximum capacity, the payload will have to be carefully shifted away from the overloaded axle, and towards the other axle.

To shift payload:
(Desired Front Axle Payload# ÷ Total Payload#) × Wheelbase of truck, in inches
= Revised Payload Center of Gravity, measured in inches, forward of rear axle
Carefully measure and draw a chalked line on the flatbed for the loading crew to accurately shift the payload.

This is especially useful when it comes to shifting a maximum capacity payload forwards, or backwards, to change the center of gravity of the payload to meet the legal axle limits -or even to tell your loading crew that you can still take on, for example, another 3,000 pounds or so, and exactly where to place it. As the nearest public scale is, for example, a 12-mile round trip from our truck yard, quick on-the-spot truck weight calculations and payload shifts, etc., have proven to be a breeze, in comparison.

Please note that public scales will always trump this quick-and-not-so-dirty method of axle weights estimation. On all maximum loads, when in doubt, it's a good idea to still make a visit to the nearest public scale before hitting the open highway. After all, Commercial Class A and Commercial Class B licensed truck drivers don't hesitate to pull their trucks onto a public scale before a long-haul.

For your convenience, one of our Pre-Trip Truck Inspection Check Lists has been uploaded, as a Microsoft Word file, so you can download and modify it to suit your unique vehicle specifications and requirements.

Feel free to leave comments, as information specific to other vehicles will be especially helpful and interesting.

Stay tuned for additional truck inspection tips and pointers.

Step 1: Kicking the Tires: Checking Truck Tire Pressures

The best time to do a quick check of your truck tires, as well as the air pressure in the tires, is before loading up your vehicle. All Class A truck drivers carefully check each and every tire before heading out with their tractor-trailer rigs to the open highway.

But as the driver of a 26,000-pound Class 6 truck, where it's not uncommon for one of our trucks to log no more than 40 miles over a 72-hour period, quick, no-nonsense vehicle checks are our main focus.

Check the recommended tire pressure, posted on a label inside the driver's door, specific to your vehicle (the same label that has the long VIN number). And ignore the tire psi noted on the tires, which is often higher than that posted on the vehicle. Inflating the tires to pressures above the recommended tire pressures stated by the vehicle manufacturer will only slightly increase the mileage of the vehicle, but will seriously reduce the footprint of the tires, reducing the ability of the truck to make sudden, controlled stops. The recommended psi pressures, for front and rear tires, are to be taken when the tires are cold. That is, ideally taken after vehicle has not been driven for 12 hours. Cold tires, if driven less than 3 miles, are also in a "cold psi" state.

You may have already noticed that our attached file: "Abbreviated Pre-Trip Truck Inspection Check List" still appears quite lengthy, especially for a quick local round trip. But, in actual practice, everything is checked out very quickly. The driver does a fast, but complete walk-around outside of the vehicle. The Check List is designed to follow the natural and logical sequence of checks around and under the vehicle, as the driver completely circles the truck and returns back to the cab.

An effective and quick method to check the tire pressures: I give each of the 6 tires a good smack against the sidewall with a short length of 1" electrical conduit pipe, striking in-line with the center of the hub, slightly off-perpendicular to the axle, so as not to strike the metal wheel rim. Any tire that is under-inflated, or over-inflated, will be clearly "out of tune." My "tool" for this is a 22.5-inch length of 1" EMT (26.64mm I.D./29.54 O.D.) electrical conduit pipe. Weighing 1.21 pounds, it has just enough heft to strike the sidewall of a 10R 22.5F truck tire for a quick-and-sure tire pressure check, without causing damage to the tire. Any rogue tires are then given a closer examination with a standard stick psi tire gauge.

This quick tire-banger check is also effective in detecting an overloaded axle, as the tires will appear somewhat flat, yet produce a higher pitched sound when struck, indicating higher than normal tire pressures.

See attached photos of a basic truck driver's tire psi check kit. Note the heavy-duty 2-way truck tire stick gauge, required for the four tires on the rear axle. Also, be sure to pack a heavy-duty dual-wheel 2-way truck tire inflator chuck with standard 1/4-inch NPT male quick-connect air hose fitting. Note that I also pack a high quality analog dial psi tire gauge, as stick psi tire gauges can go bad very quickly.

This is also the perfect time to do a quick-check of the condition of the tires: Sidewalls and treads should be free of deep cuts, embedded metal objects, and damage. It's not uncommon for a small chunk to be missing from the tread, exposing the steel belts of the body of the tire, requiring a replacement of the tire. If driving on retreads, be sure to verify that the tread-to-tire bond is solid.

According to federal standards, the minimum tread depth for the front tires of a 4x2 2-axel truck is 4/32nds of an inch, and the minimum tread depth for the rear tires is 2/32nds of an inch. But, in actual practice, the state highway patrol and state weigh stations usually do not lower the boom on a vehicle until the minimum tread depth of the front tires of a 4x2 2-axel truck is 2/32nds of an inch, and 1/32nds of an inch for the rear wheels. But while this may seem generous, all it takes is one “major” groove, on only one tire on your truck, to fall below this standard, and the truck be shut down and ordered off the road until the tire is replaced. A "major" groove can best be described as a groove(s) that contains treadwear indicators or bars, that are deep inside the groove of a new tire, but flush with the surface tread of a worn-out tire.

If you are in the habit of running on retreads, it's recommended that the tires be pulled when the treads are down to 4/32nds of an inch, in order to preserve the tire casings for a good, solid retread.

Step 2: What Is the Actual Miles-Per-Gallon Range of Your Truck?

Rumors abound as to how far a given truck can be driven on a full tank(s) of fuel. And one of your first responsibilities with a new, or unfamiliar truck is to find out. The issue here is that many fuel gauges in the dashboard of many trucks are intentionally set to be ultraconservative, as to the true amount of fuel. In other words, in an effort to eliminate the possibility of the truck becoming stranded on some lonely back road, out of fuel, the fuel gauge reads "empty" when there is still plenty of fuel still in the tank.

But the savvy truck driver not only knows how to load up his truck to near capacity, and properly balance and distribute his load over the axles, he also knows how to GPS navigate the truck along the quickest route, and seeks out the nearest 24-hour diesel fueling points only when truly necessary.

Your first step, and the easiest one, is to determining the miles-per-gallon your truck is capable of:  Completely fill up the fuel tank(s) to capacity (to just below the neck of the tank that the fuel cap screws on), and carefully note the odometer reading. Run the tank(s) down, preferably on open highway hauls, until near-empty before filling up again to capacity, carefully making note of the exact amount of fuel purchased, as well as the odometer reading. Dividing the miles driven, by the fuel consumed, will yield a highly useful miles-per-gallon highway rating for that truck. You'll also need to note how heavy a load was involved in these driving conditions. For example, if your truck was driven at 80% payload capacity, to a distant point, unloaded and returned back to the truck yard with no payload, then your mpg notes should indicate that the truck was driven on the highway, at 40% payload capacity. Multiplying the highway mileage by .67x will yield a good working city mpg rating for the truck.

Your next step is to determining the true, usable capacity of the fuel tank(s). It's not uncommon for about 7% of the rated fluid capacity of a fuel tank to be unusable. In other words, if the truck was run until the tank(s) ran dry and the motor stopped, and the truck coasted to a complete stop on the roadside, there would still be up to about 7% of fuel remaining, sloshing around in the bottom of the tank(s). So I advise being a cautious cat, and factoring in that 7% in your calculations. For example, on the flatbed truck featured on this webspace, the fuel line attaches to the fuel tank about one inch from the absolute bottom of the tank, in agreement with the 7% rule. The rated fluid capacity of the tank will also be noted somewhere on the tank, usually stamped in (50 gallons on my truck, for example). The true, or usable capacity of the tank will the rated capacity less 7%. I've made measurements of fuel tanks (length, width, depth, radiused corners, in-step, etc.) and did the math, and all of them are quite accurate as to their rated fluid capacities.

The final step is to fashion a simple wooden fuel dipstick. I prefer a 11/16-inch diameter wooden dowel, sanded smooth, long enough to touch the inside-bottom of the fuel tank, and also long enough to never slip into the tank and become lost. A round wooden dipstick is easier to completely wipe off any fuel so that it doesn't smell up the truck cab. I use a "Sharpie" fine point felt-tipped permanent marker, as it's fuel-proof. But make all preliminary markings in pencil, before finalizing them with the Sharpie. The first mark is "0" gallons, based on the top level of that 7% on unusable fuel. Next, dip the stick all the way into the fuel tank, vertically, until it touches the bottom of the fuel tank. Carefully mark, with a pencil, where the bottom of the fuel neck, at the top of the tank, touches the wooden dipstick. Withdraw the dipstick from the tank, wipe it dry, and carefully mark, with the Sharpie, completely around the dipstick at the true empty level and label "0" and "empty" (about an inch from the end of my dipstick, for example) and do the same for the true maximum fuel capacity ("46.5" gallons on my dipstick, for example).

But you'll also need to properly calibrate your dipstick, for those fuel levels that fall between true empty, and true full. As most fuel tanks are somewhat complex in shape: Step-tanks, cylinder tanks, etc., and simple, even-interval gallon calibrations will just not do. But if the ends of the fuel tank are flat and vertical, here's a trick, for those who find doing more complex math gives them a headache:  Simply make a cardboard template, actual size, radiused corners and all, of one end of the fuel tank. At this point, be sure to make the dimensions of the cardboard template a little less, so as to approximate the inside dimensions of the tank. If the tank is a step-tank, then carefully draw out the end-view of the in-step on the template. Next, measure the length of the fuel tank and divide it into the length of the step. Go back to your cardboard template, and multiply that number (will always be less than one) times the width of the step, and redraw the in-step to the narrower width to truly reflect the effect it has on fuel tank levels. Next, take the short distance on the wooden dipstick, from the end to the "0" true empty level, and draw it out as a horizontal line across the bottom of your cardboard template. Cut the template completely across, along the "empty" line, near the bottom of the tank, and throw the thin strip away. Cut out the in-step and also throw it away. If the bottom of the fuel neck is not at the top of the tank, then draw it out as horizontal line below the top of tank, and also cut it out and throw it away. What you have now is faithful to your wooden dipstick, and the "empty" and "full" marks on the dipstick should align perfectly with the vertical dimension on the cardboard template.

Next, accurately weigh the cardboard template. This weight will serve as a reference for the true fuel capacity of the tank, in the calculations that will follow. Then mark, along the dipstick, as well as the cardboard template, 9 somewhat evenly spaced intervals (exact measurements are not critical, at this point). But the 9 points is not an arbitrary number, since it ultimately represents even 5-gallon increments on the finished dipstick, for this particular tank. These 9 points are then carefully drawn out as 9 horizontal, parallel lines across the cardboard template. The dipstick and cardboard should completely match up on this. Starting at the top of the cardboard template, completely cut off the top (9th section) and throw it away. Accurately weigh the remaining cardboard template and divide that weight by the weight of the full cardboard template. Multiply the result times the true gallon capacity of the fuel tank to yield an accurate lower fuel level, in gallons, to pencil in on your wooden dipstick. Continue the process, step by step, down to the last section. Although you will now have a calibrated dipstick, it will be marked in an unwieldy series of odd gallons and fractions of gallons. At this point, it's safe to use straight-line interpolations to plot even groups of gallons (5, 10, 15, 20, 25, etc.). When done, carefully draw those corrected gallon marks, every 5 gallons, with a Sharpie, completely around the dipstick, and labeled with the appropriate gallons for each.

Also, you should mark a dotted "Empty!" line at about 10% of the tank's actual capacity (or at the 5-gallon mark, for example, on my 46.5-gallon wooden fuel dipstick), above the "0" gallon mark on your wooden dipstick, since "0" is when the truck runs out of gas, and coasts to a stop on the roadside. And if your truck has left and right fuel tanks, be sure to base all mileage on the tank with the lowest level, as dual tanks seem to never have perfectly balanced fuel levels.

Lastly, Clearly label your dipstick, with a Sharpie, as to exactly which truck it was designed for. And be sure to keep a record of the measurements and gallon marks, in case the dipstick gets lost.

A side note:
In aviation and marine applications, fuel tanks can be so irregular and complex that the only method is to completely empty the fuel tank, then add exactly 2 gallons of fuel, at a time, until the tank is full, and the do-it-yourself dipstick is fully calibrated. But do a quick check on the internet first, as many have already posted their DIY fuel dipstick calibration notes for specific production aircraft and mass-produced boats.