An infinitely variable friction wheel transmission of this type usually has input and output discs arranged coaxially on a common shaft, arranged together in pairs and whose inside surfaces are designed in a toroidal shape, with friction wheels arranged between the pairs of input and output discs.  These friction wheels are in frictional contact with both the input discs and the output discs, and transfer the torque transmitted to them from the input disc to the output disc by virtue of friction-force contact, the rotation speed of the friction wheels being greater, the larger is the distance between their point of contact with the input disc and the rotation axis.  In contrast, the rotation speed of the output discs is the higher, the closer the point of contact between the friction wheel and the output disc is to the rotation axis.  Accordingly, by swivelling the friction wheels, the rotation speed of the output discs can be adjusted infinitely variably and as desired.  For this purpose, the rotation axles of the friction wheels are, in each case, mounted on a support which can be controlled by means of a swivelling device. 
An infinitely variable friction wheel transmission of the said type is described in DE A 197 54 725 by the present applicant.  This transmission comprises an input shaft connected to a torque converter or a fluid-operated clutch of a vehicle engine, with two transmission units arranged coaxially with respect to the input shaft.  The input discs and the output discs are mounted on a torque shaft which can move slightly in the axial direction relative to the input shaft.  The two input discs are mounted on the torque shaft in rotationally fixed positions with respect to it, but are able to slide axially on it.  A roller-shaped pressure device is mounted so that it can move axially on the input shaft but is rotationally fixed with respect to it, and this acts upon one of the input discs in the axial direction, pushing it towards the associated output disc.  The upper ends of the supports carrying the friction wheels engage in two recesses in the housing and are fitted and able to rotate therein, wheel, brings it closer to one of the two discs, while if one of the discs is axially fixed, the control force can act upon the friction wheel. 
Since the transmission ratio is adjusted by displacing the friction wheel tangentially with respect to the transmission axis in these infinitely variable friction wheel transmissions, swivelling forces from the drive input and output discs can act upon the friction wheel since they have to be pressed against the friction wheel to transfer the torque.  In the known infinitely variable transmissions, the friction wheel in each transmission unit is arranged such that its swivel axis is positioned at the mid-point of the torus formed by the associated drive input and output discs.  At the contact points of the friction wheel with the associated drive input and output disc, when the transmission ratio is adjusted so-termed normal forces are produced.  To prevent the resultant torque imposed on the friction wheel from producing any undesired modification of the drive transmission ratio, it has already been proposed by the present applicant in DE A 198 26 057 that to control the transmission ratio, the difference between the normal forces acting on each wheel by virtue of the contact pressure forces exerted by the associated input disc and output disc should be compensated by producing a control force, such that with the friction wheel held axially, this control force, which leads to a tilting of the friction wheel, brings it closer to one of the two discs, while if one of the discs is axially fixed, the control force can act upon the friction wheel. 
The purpose of the present invention is to reduce the structural complexity of an infinitely variable friction wheel transmission and, at the same time, avoid undesired transmission ratio deviations caused by elastic deformations, in particular of the input and output discs when they press against the friction wheel.