Vibration at 100 km/h After New Tyres
A 2019 four-wheel-drive pickup with 55,000 km has a vibration felt through the floor and steering wheel between 95 and 105 km/h; it fades below 90 km/h and above 110 km/h. Four new tyres were fitted 2,000 km ago and have been balanced twice on a static, non-road-force balancer, which removed none of the complaint. There is also a slight pull to the right and the steering wheel sits about 8 degrees off-centre. Workshop checks give: right-front tyre radial runout 1.7 mm against a 1.0 mm limit, right-front wheel radial runout 0.4 mm, right-front road force 21 kg against an 18 kg limit, left-front road force 9 kg. Wheel bearing play and propshaft universal joints are within specification. Alignment readings are total front toe of -0.20 degrees against a specification of +0.10 degrees with a tolerance of plus or minus 0.15 degrees; camber and caster are within specification and thrust angle is 0.05 degrees.
1. Explain why two balancing attempts did not help, and interpret the runout and road-force figures.
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Balancing corrects mass distribution around the wheel; it does not correct a tyre that is out of round, has a stiff spot, or is mounted off-centre. The road-force figure of 21 kg on the right front against an 18 kg limit measures variation in force as the loaded tyre rolls, which is what the driver feels as a shake. The runout split matters: wheel runout of 0.4 mm is acceptable, but tyre radial runout of 1.7 mm is not, so the tyre or its seating on the rim is the main contributor. At 100 km/h a tyre of about 2.35 m circumference turns roughly twelve times a second, and a first-order once-per-revolution disturbance at about 12 Hz matches the 95-105 km/h band. That is why two static balances did nothing: the balancer told the fitter where to put weights, not that the assembly was eccentric. Match-mounting, rotating the tyre on the rim so its high spot opposes the wheel's low spot, is the next step.
2. Set out the order of repair operations and justify why the alignment should not be done first.
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Order matters because each step changes the next. First, re-seat and match-mount the tyre on the rim and re-check road force; if it remains above the limit, replace the tyre and re-measure. Second, verify the wheel and hub face for cleanliness and runout. Third, road test at the complaint speed to confirm the vibration has gone. Only then perform the alignment, because a vibrating vehicle cannot be assessed for pull and because adjusting toe will not remove a shake. The alignment figures do explain the pull and the off-centre wheel: total toe of -0.20 degrees against a specification of +0.10 degrees with plus or minus 0.15 degrees is outside tolerance, and should be set with the steering wheel centred and the vehicle at kerb weight. The thrust angle of 0.05 degrees is fine, so the rear axle is not causing the pull.
3. The customer says: just do the alignment, it is cheaper. How do you respond, and what goes on the record?
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Answer with the physics rather than with authority: the alignment corrects the pull and the steering wheel position, but the shake is a once-per-revolution force variation from the tyre, and toe adjustment cannot change it. Offer both, but sequence them, tyre correction first and alignment last, and explain that if the tyre is replaced or rotated afterwards the alignment may need repeating. Quote the tyre work separately and show the road-force and runout readings on the job card so the decision is the customer's and is documented. If the tyre work is still declined, record the refusal and the advice given. This protects the workshop if the vehicle returns with the same complaint, and it gives the customer information to choose with rather than a sales pitch.