"Lately, every time I take off from a stop or gear down, I feel a distinct 'thud' coming from behind the cab. It feels like the trailer is sliding back and forth a fraction of an inch. My jaw is locked and the handle is in, but something feels loose. Is there a way to tighten this up, or is the fifth wheel worn out?"
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Mechanic's Expert Answer"A fifth wheel isn't a 'set it and forget it' part. As the metal wears down, that gap between the lock and the kingpin grows, turning a small vibration into a heavy hammer."
Most modern fifth wheels (like Fontaine or Jost) have an adjustment bolt specifically designed to take up this slack. If you ignore that "clunk," you aren't just annoying your ears—you’re putting massive stress on your kingpin and your tractor’s drivetrain.
The 3 Signs You Need an Adjustment:
The "Slack-Back" Thud: If you feel a jerk when you accelerate or a push when you brake, your locking jaw has too much "free play." Most manufacturers recommend no more than 1/16th of an inch of play. Any more than that, and you're hammered by the trailer every mile. Difficulty Uncoupling: If the handle is extremely hard to pull, even when you've backed into the pin to take the pressure off, your adjustment might actually be too tight or the wedge is sticking. A properly adjusted fifth wheel should snap open with firm but smooth pressure. Visual High-Hitching Risk: Look at the jaw when it's locked around the pin. If you see a massive gap or if the locking bar isn't fully seated across the throat, the "over-travel" is gone. This means the metal has worn down so much that the lock can no longer safely wrap the pin.
💰 The Bottom Line: Adjusting a fifth wheel usually takes one wrench and 10 minutes of your time. Replacing a ruined kingpin on a trailer or a cracked fifth wheel plate can cost you $2,500 plus downtime.
Pro Tip: Before you adjust the bolt, clean the grease off. You can't see the true wear if it's buried under 5 pounds of old, gritty grease. Steam clean the plate, adjust it until the play is gone (but the jaw still moves freely), and then hit it with fresh, high-quality extreme-pressure grease.