Front Tire Cupping and Pull After Alignment
A bus returns two weeks after a four-wheel alignment with a complaint of a pull to the right and a shimmy at 55 mph. The alignment printout from the previous visit shows front toe within spec, camber within spec, and caster within spec, but cross-caster was 0.8 degrees (right more positive). The front tires show diagonal cupping on the outer shoulders of both front tires, worse on the right. Tire pressures are correct. The bus has 95,000 miles on the front tires, which are due for replacement. The steering damper was replaced at the last visit. Wheel bearings show no play. The customer rotated the front tires side-to-side after the alignment.
1. What is the most likely cause of the shimmy and pull, and how do the tire condition and rotation factor in?
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Diagonal cupping on the outer shoulders points to a tire/wheel condition (balance, runout, or worn dampers) rather than alignment alone. The customer rotated the front tires side-to-side after alignment, which can move a cupped tire to the other side and change the pull direction - so the current right pull may be tire-induced, not alignment-induced. The 55 mph shimmy is classic wheel balance/runout or tire irregularity. Cross-caster of 0.8 degrees is within many specs but can contribute a mild lead. First: inspect tires for cupping/runout, rebalance or replace the cupped tires, check wheel runout, and road test before re-aligning. Do not chase alignment until tire/wheel causes are eliminated.
2. The customer insists the alignment is wrong. How do you prove or disprove it?
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Road test on a level, crown-free surface and note pull direction. Swap the front tires left-to-right (or install known-good tires) and re-test: if the pull reverses or disappears, the cause is tire/wheel, not alignment. Measure wheel runout and balance. Re-check alignment readings with the vehicle at proper ride height and fuel/load condition, and confirm the previous printout's cross-caster. If readings are in spec and the pull follows the tire swap, the alignment is not the cause. Document the test sequence and results so the customer sees the evidence.