This is how I understand it having had a long talk with wire wheel friendly technician. The problem with using standard cones is that there is no guarantee that the inner face of the splined hub (where it bells outwards from the splines) is accurately flat compared to the rims of the wheel - it doesn't need to be because this face does not locate the wheel on the hub, it is the inside of the splined portion that does that. The accuracy is better in modern wheels (eg from factory or MWS), very poor in older/classic wheels. Thus the inner cone on a balance machine may (if lucky), or may not, locate the wheel accurately flat - resulting in run out which may be axial (rim wobbles in and out laterally) or circumferential (rim wobbles up and down i.e. eccentric). If this amount of wobble is large it will be evident visually, but it can be quite small and within the range that can be balanced out on the machine. If balanced on the machine and put back on the car, the wheel will now be located accurately on the hub by the splines, and thus be out of balance when driven, indeed all four wheels could well be out of balance. Sorting this out is difficult, because when the wheel is put back on the machine, the location may be slightly different, resulting in a new balance point, which is still off when put back on the car. Compare this to a standard steel or alloy car wheel, where a (usually accurate) circular hole is present in the flat face of the wheel centre, and a cone will locate this accurately, so a wheel balanced off the car will usually be in balance when put back on the car. Balancing on the car is possible, but is only correct for one location, so if you take the wheel off and move it by one stud or a few splines, or put it in another location, it cannot be guaranteed to be in balance.
Accurate, correct balance makes a big difference - mine was recently done in house by Beamish Morgan, who are co-located with the Aston Martin Workshop and between them know a bit about this.
Please note I am not an engineer.
Nigel
