The rear dropout spacing on the six speed road bike is probably 126mm. The seven speed mountain bike wheel probably has the axle spaced for either 130mm or 135mm dropout spacing, depending on how old it is. This can be dealt with by either flexing the dropouts apart by hand a few millimeters (if there's only 4mm difference between axle spacing and dropout spacing) or cold setting the frame. But what about the size of the rims? Is the Puch frame meant for larger diameter rims, either 27" or 700c? If so, the brakes won't come close to reaching the 26" rims-