It is critical to verify the following when performing a pre-trip inspection of a truck-tractor or three axle vehicle?

• A. That first drive axle tires have a tread depth of no more than 1.6 mm
• B. That the slack adjuster moves at least a few inches when you pull on it
• C. That the axle assembly has no breaks, cracks, holes, broken seals or bends
• D. That with the fifth wheel coupling there is space between the plate and apron

Answer: (C)

The most important thing to verify is the structural condition of the axle assemblies. In a truck-tractor or three-axle vehicle, the axle carries heavy loads and dynamic stresses; any signs of damage—breaks, cracks, holes, damaged seals, or bends—indicate a serious safety risk because failure could occur under normal operation. This is why confirming the axle assembly is free of such damage is the critical pre-trip check.

Tire tread depth is also important, but it should be checked against the minimum allowed depth, typically 1.6 mm, ensuring the depth is at least that much. The statement implying tread depth should be no more than 1.6 mm would permit dangerous wear, so it’s not the correct verification.

Slack adjusters should move when brakes are applied, but not excessively. Describing movement as “a few inches” isn’t a proper measure and would suggest a braking system issue, so that phrasing isn’t the correct pre-trip check.

The fifth wheel gap relates to how the trailer couples to the tractor and is important for safe coupling, but it isn’t the critical axle-condition check you must verify first.