>>Anyone any idea why?
A wheels-on balance, with only one sensor placed under the suspension can not give the same resolution when considering dynamic balance.
A wheel-off balancer will typically have two sensors spaced along the machine's axle, and the output from these two sensors can be combined mathematically* to first obtain the values of static and dynamic imbalance, and then to take these imbalance values and the geometry of the wheel to calculqate the two balance masses.
* Static imbalance is a rotating force, while dynamic imbalance is a rotating moment, and in general terms, these produce sensor outputs which (in broad terms) are in phase with each other for static imbalance, and mutually out of phase for dynamic imbalance.
A wheels-off balance using a single sensor can't perform this seperation between static and dynamic imbalance with the same degree of precision (if at all!). As each vibration happens once per revolution, they have the same forcing frequency, and so, it's not possible to filter out the two parts of the response in the frequency domain.
Dynamic imbalance tends to try to alternately steer and camber the wheel as it rotates, and this is a motion which is more stiffly resisted by the suspension components, where static imbalance tends to make the wheel hop, in the direction where the suspension is designed to be compliant. This means that the resonance caused by static imbalance is at lower frequency, and hence at lower speed that that caused by dynamic imbalance. Static imbalance is typically at about 30 - 40 mph, while dynamic imbalance usually begins nearer 70.
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