Fifth Wheel Clunk and a Frame Rail Crack
2015 day-cab tractor, 48 in fifth wheel on a slider, 250,000 miles. The driver reports a clunk on acceleration and braking and says the trailer 'walks' the tractor. With the trailer coupled, the tractor brakes released, and a helper rocking the tractor fore-aft against set trailer brakes, there is about 1/2 in of relative movement between the trailer upper coupler and the fifth wheel top plate. The kingpin measures 1.988 in OD at the wear point; nominal is 2.000 in. Inspection also finds a 1-1/2 in crack in the bottom flange of the left frame rail, starting at a bolt hole in the rear crossmember gusset. Fifth wheel mounting bolts are at spec and the slider pins are fully engaged.
1. Is the 1/2 in of movement the kingpin or the fifth wheel jaws and lock? How do you tell?
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Do the arithmetic first. Kingpin wear is 2.000 − 1.988 = 0.012 in. Even doubled at the jaw interface that cannot produce 1/2 in of movement. Record the measurement against the OEM wear limit, but the kingpin is not the cause.
Half an inch of fore-aft movement is jaw/lock wear or a loose fifth wheel. Check:
- Jaw-to-kingpin clearance with a feeler gauge while coupled.
- The lock adjustment - many fifth wheels have an adjusting nut or screw for jaw slack. If it is already at the end of its travel, the jaws and lock are worn out.
- The top plate for dishing and wear, and the bracket, slider pins, and mounting bolts under load rather than static.
Rule out the look-alike: a loose rear suspension torque rod or a worn spring bushing lets the axle steer and feels exactly like a loose fifth wheel from the cab. Check the torque rods before you sell a fifth wheel rebuild.
Decision: rebuild with a jaw/lock kit when the lock cannot be adjusted into spec. Do not shim or tighten a worn jaw to get by - a jaw that hangs up or releases is a trailer-drop risk.
2. What is your decision on the frame crack, and how is it repaired?
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The truck is out of service until the rail is repaired.
- The crack starts at a bolt hole in the bottom flange under the crossmember gusset - a stress concentration in a highly loaded area. Left alone it runs up the web and the rail fails.
- Repair per the OEM's published frame procedure: stop-drill both ends, then a reinforcement plate or bolted/welded repair exactly as specified for that rail section. Never weld across the flange or over the web without OEM approval, and never weld a crack and call it repaired - the heat-affected zone cracks again beside the weld.
- Find the cause. A rail cracks there when the crossmember works: check crossmember bolts, gusset condition, fifth wheel mounting, and rear suspension torque rods. A loose fifth wheel is a plausible contributor and must be corrected before the plate goes on, or the repair fails the same way.
Document the repair. Frame repairs affect resale and often must be disclosed under company policy.
3. The owner asks whether greasing the fifth wheel will cure the clunk. How do you answer?
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No. Lubrication reduces plate and upper coupler wear and improves steering feel; it does not take up slack in a worn jaw or lock. A dry plate accelerates wear, so it is a contributing factor to long-term condition, but it will not remove 1/2 in of movement.
What to tell the owner:
- The clunk is mechanical slack, confirmed by the rocking test; grease cannot close that gap.
- A dry, worn plate plus a loose lock is a coupling-security issue, not a comfort issue.
- While you are there, check the upper coupler wear plate thickness on the trailers in the fleet - a thinned plate on one trailer wears every fifth wheel it touches.
Recommend the jaw/lock rebuild, a plate inspection, and a lubrication interval as part of the PM, and note that lubrication is prevention, not repair.