Tractor axle loading

As the fifth wheel moves weight is transferred between the axles on the power unit, or chassis. Many people do not fully understand this, and the example above is a graphic representing a built truck that the customer was unhappy with. Our situation is represented by 150 lbs. of weight for every inch the slide moves. Since sliding 5th wheels move in two-inch increments, there is a 300 lb. change at each location.

The customer intended to use the setting of zero, or bogie, but as the arrow indicates, they were 2,200 lbs. over the Federal Bridge allowance for tandem axles of 34,000 lbs. Using our 300 lbs. rule, you can see that at 16 inches ahead of bogie (+16, and potentially even +14) the Federal Bridge weight is satisfied.

We effectively transferred weight off the drive axles by sliding the 5th wheel forward. But, that weight had to go somewhere. And since the new location is ahead of bogie, that weight was added to the steer axle (locations at bogie add no trailer weight to the steer axle). Our expected front axle rating with these settings in 13,700-14,000 lbs. This will work, ONLY if we have specified a 13,200-14,600 lb. rated front end (axle/springs/steering) AND 16-ply tires.

Solutions are quite easy in paper, once the vehicle is built the only answers typically are unwanted trade-offs or risk. Had this salesperson ordered a 14.6K rated front end, no problem, since they had a 24.5-inch slider 16 inches ahead would work. They had enough swing clearance. But they had a 12K front end with 14-ply tires. 

If you need help with Weight Distribution attend one of our classes, or utilize the calculator we have at www.zfactorconnect.com.