Questions truck drivers ask

The old estimates of 250 or 380 pounds are accurate only when the axle loads are very heavy. They are much less accurate with lighter loads.

To predict the effect of moving the tandems, we recommend using a new mathematical formula that considers both the axle weights and the distance the tandems are moved.

If the distance to move the tandems isn't very large, the app will estimate the axle weights and tell you how many holes and which way to move the tandems for the optimal balance.

It is an approximately linear relationship.

Below a trailer's air suspension, there are usually two axles, eight wheels, and the brakes. In our test, these parts weighed about 3,000 pounds when suspension pressure reached zero.

For this method, take a calibration weighing with the tandem load above 31,000 pounds while staying within the limits that apply to your equipment and route. Subtract the estimated 3,000 pounds below the suspension, then divide the difference by the pressure shown on your gauge. Now you have the weight change that corresponds to one psi.

Always check the applicable axle limits, bridge limits, road rules, and company procedures.

For winter roads, experienced drivers recommend increasing the drive axle load by 6 to 6.5 percent.

For example, in the warm season, if the weather is good and the driver is experienced, he can reduce the rolling resistance of the wheels by moderately shifting the balance to the trailer tandems. Moderately, by 1,500 to 2,000 pounds, but no more than 3,000.

But remember, these numbers come from the practical experience of experienced drivers. It is not an official norm and not a rule. For now, these are practical guidelines that need scientific verification.

To calculate how much axle weight changes when you slide the fifth wheel: take the total weight being shifted with fifth wheel, divide it by the truck's wheelbase, then multiply that result by the distance you slide the fifth wheel.

With a wheelbase of approximately 230 inches and a modern fifth-wheel slide increment of 2 inches, moving the fifth wheel by one position changes axle weight by about 230 pounds, assuming you are already close to the limits of 12,000 lbs on the steer axle and 34,000 lbs on the drive axles.

For years brochures have repeated the old number: "500 pounds." That information is outdated and does not reflect how modern trucks are built.

A gauge like this often displays pressure in steps of one psi. If it shows 57, the actual pressure might be 56.6 or 57.4. The display alone cannot tell you.

Therefore, under less than ideal conditions, the pneumatic method deviated by 310 pounds for the drive axles and 105 pounds for the trailer tandems.

Don't be afraid to learn, but remember where error can come from: the air suspension, uneven ground under the truck, and a driver in a hurry.