Vibration at 88 to 95 km/h After a Driveline Overhaul
A 2017 tandem-axle tractor with 1.6 million km on the chassis and a reman transmission fitted 300 000 km ago. The driver reports a vibration in the cab floor and steering wheel that appears at 88 km/h and disappears below 80 km/h. It is still there with the clutch depressed, coasting in neutral at the same road speed, and it is slightly worse loaded. Tires are new and road-force balanced, wheels are torque-checked, and all U-joints and the centre bearing were replaced last month. Ride height measured at the rear bags is 6.5 in against a 7.5 in spec, one bag was recently replaced, and the fifth-wheel plate was lowered 2 in during a rebuild. Drive tire size works out to about 500 rev/mile; axle ratio is 3.55.
1. What does coasting in neutral at the same road speed eliminate, and how does frequency help you place the fault?
Read the worked answer
Eliminated: with the clutch down and the transmission in neutral, engine and clutch speed fall away while the vibration stays tied to road speed. That removes the engine, the clutch, and anything upstream that spins at engine speed. What remains is anything turning at wheel speed or driveshaft speed: wheel ends, tires, hubs, brake drums, driveshaft, U-joints, centre bearing, pinion, and axle gears.
Frequency estimate. At 90 km/h with a 500 rev/mile tire (about 311 rev/km), wheel speed is roughly 90 × 311 ÷ 60 ≈ 465 rpm ≈ 7.8 Hz. Driveshaft speed is that times the 3.55 ratio ≈ 1 650 rpm ≈ 27.5 Hz.
So the question to answer on the road test is: is the shake a slow, once-per-wheel thump (around 8 Hz, wheel end) or a fast buzz (around 27 Hz or a multiple, driveline)? A vibration analyser or a phone-based frequency app settles it in minutes; seat-of-the-pants 'it feels like a tire' does not.
Note 'worse when loaded' already tilts toward a driveline angle or U-joint problem, because torque load changes the working angles and the joint load, not the tire.
2. Given the ride height and fifth-wheel changes, what do you measure and what are you looking for?
Read the worked answer
Check driveline geometry before anything else, because the chassis history is a red flag:
- Ride height. Bags at 6.5 in against a 7.5 in spec is a full inch low, which tilts the pinion and changes every working angle. Fix the suspension and ride height first - measure again after. A recently replaced bag on one side also warrants checking for an uneven or leaking height control valve.
- Working angles. With a digital inclinometer, measure at the transmission output, both ends of each shaft, the centre bearing, and the pinion flange. For a two-piece shaft with a centre bearing, the transmission-to-first-shaft angle and the second-shaft-to-pinion angle should be roughly equal and opposite, typically within about 1°, and each joint should be running somewhere around 1–3° rather than near zero or beyond about 4°.
- Phasing. Confirm the yokes are in phase and the slip joint is splined on the correct spline - one spline off gives exactly this speed-specific buzz.
- Centre bearing height. Shimming the centre bearing is the normal correction; lowering the fifth wheel and dropping ride height may have pushed the pinion angle out of range.
- Not the driveline? Then go back to the wheel ends: hub runout, drum balance, and a loaded road-force check on the drive tires.
Correct the geometry, then road-test loaded at 88–95 km/h - that is the only proof.
3. Your angle readings show the pinion joint running at 5.5° while the transmission joint is at 1°. What is the correction, and what do you tell the customer?
Read the worked answer
The joints are far apart in working angle and that mismatch is the vibration source. Options, cheapest and least invasive first:
- Restore ride height. Get the rear suspension back to 7.5 in at the bags; that alone changes pinion angle and may bring the joints into line.
- Shim the centre bearing up or down to split the difference between the two joints, then re-measure - the goal is equal, opposite angles within about 1°.
- Axle wedges or a pinion-angle shim at the axle if the centre bearing cannot take up the difference.
- Re-check phasing and U-joint condition while it is apart; a joint that has been running at 5.5° may already have brinelled caps.
Tell the customer: the fault is geometry, not a worn part, so replacing U-joints again will not fix it. The 2 in fifth-wheel drop and the low ride height changed the angles - that is the root cause. If the fifth wheel must stay low, the correction is shimming, and it should be road-tested loaded, because loaded is when he feels it. Document the before and after angle readings on the repair order.