Just a wild guess , but when hauling with "Off Highway " trucks , we use to load them heavier on the truck than the trailer . We had more breaking on the truck than the trailer with "Retarters and Jake Brakes" plus the usual brakes going down steep hills . If we had adverse grade to pull up , we had better traction . With the extra axles on the T880 , the driver could load heavy on the truck for traveling on off highway roads to have the same effect . Then when he entered the public roads , put the extra axles down and be loaded to a legal weight .