A driver’s complaint that starts at 80 km/h

A common story: a car passes MOT, tyres have 4 mm tread, brakes are smooth, but the steering wheel shakes between 78 and 84 km/h and settles above 90. The driver suspects alignment. At a spinboss balancing check, the left front wheel needs 25 g on the inner rim and 15 g on the outer; the right rear shows a 12 g static imbalance. That is enough to produce a narrow-speed shimmy because the wheel’s imbalance force rises with the square of road speed.

The cause: rotating mass, not steering geometry

Wheel alignment affects tyre wear and straight-line tracking; it does not create a vibration that appears and disappears in a 6 km/h window. Imbalance does. A 5 g mass at a 0.18 m rim radius on a wheel rotating at 80 km/h (22.2 m/s) creates roughly 13.7 N of centrifugal force: F = m r ω². At 120 km/h the same 5 g creates about 30.9 N. The suspension bushings absorb small forces at low speed; near 80 km/h the wheel’s rotation frequency can excite a resonance in the steering and subframe. The takeaway is simple: speed-specific shimmy points to the wheel-and-tyre assembly before it points to alignment.

What the Spinboss check separates

A spin balancer measures static and dynamic imbalance and tells the technician where to place weights. It cannot see a tyre’s stiff spot, a belt shift, or a flat spot from a locked brake. A road-force test adds a loaded roller and measures force variation in newtons; a common pass limit is around 10–12 kgf (about 100–120 N) for passenger cars, though vehicle sensitivity varies. If a wheel balances to zero grams but still shakes, the problem is tyre uniformity or runout, not missing weights. The solution is to match-mount the tyre to the rim or replace the tyre, then rebalance. This separates a 20-minute weight correction from a tyre replacement.

The fix and the recheck

After balancing, the technician should torque the wheel nuts to the vehicle specification in a star pattern, not with a rattle gun. A 110 Nm spec, for example, is not “tight enough” by feel; over-torque can distort the brake disc and create a new vibration. The road test should reproduce the original 78–84 km/h band on a smooth road. If the shimmy is gone, the cause was rotating mass. If it remains at the same speed, the next step is a road-force measurement and a check for bent rims or worn control-arm bushings. The result is a documented diagnosis rather than a parts guess.