The weighting game

| December 15, 2005

Onboard scales show your gross, and axle totals as you load.

When it comes to what you weigh, there will be times you have no doubt your rig is legal. You have an experienced eye, and you know what you have on board and where it is. It’s way under and all in the right places.

But weight and weight distribution are not things to gamble with. So what happens when your certainty begins to weaken as a load gets heavier and you know you must be 100 percent certain? You know you’re going to have to make a run to the scales and back. Unless, of course, you have onboard scales.

A typical 80,000-pound rig with a single front axle and drive and trailer tandems will have 12,000 pounds on the front axle and 34,000 on each tandem.

Onboard scales give the driver an instant indication of weight distribution so he or she won’t have to drive back and forth to a scale to check and redistribute it.

How onboard scales work
The pressure inside air springs is controlled by a leveling valve. The valve’s purpose is to adjust the firmness of the spring to the load so the trailer will always ride smoothly.

As you load a trailer, the leveling valve will add enough air to restore the trailer’s ride height. So there will be a consistent relationship between the pressure in the air bags and the weight on each axle.

Onboard scales measure the air pressure in the air springs on each axle group with an electronic sensor. The sensor converts the pressure into an electronic signal. It then sends the signal to a ComLink (Air-Weigh) or a hand-held reader (TruckWeight).

The digital processor in the ComLink or reader has calculated the relationship between air pressure and weight for each axle. Instead of sending a signal representing the air pressure, it mathematically converts the pressure to a digital reading of the axle’s weight.

Air-Weigh tractor system installation
Air-Weigh, of Eugene, Ore., has been making an onboard scale system that can give a driver accurate axle weights on the dashboard for some time. We visited Dennis Trucking in Philadelphia, where Steve Spriggs, customer support and training manager for Air-Weigh, installed a system on a Mack CH600 tractor.

The first component in the system is the sensor. The sensor’s signal is transmitted by wire to a ComLink, which is a type of processor called a “reader board.” Another wire then conducts a different signal, representing the axle weights, to an Air-Weigh gauge installed in the dash.

Spriggs only took about an hour to install the system, even though he was unfamiliar with the tractor model. He says that where you need to run an air line all the way from the suspension up to the dash, you should add another half hour.

  1. Find a place for mounting the system’s digital gauge. In this case, Spriggs found a round orifice in the Mack’s dash that was perfect and removed the rubber insert. The gauge needs a hole that is about 2 1/16-inch in diameter.

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