The unit used by Tim Payne Overdrive Designs Ltd is the "Laycock J" or "2XP" type as was manufactured by Laycocks of Coventry, and which is still in production today. This is a well proven design of many years standing and has been fitted to numerous types of vehicle. It is an epicyclic gearing arrangement, (see Fig 1 below), with a cone clutch either locking the input shaft to the output shaft for direct drive, or locking the centre gear to the case, thereby dictating that the gears on the input shaft rotate not only around their own axis, but also around the inner gear, so forcing the output shaft to rotate faster than the input.   This is pleasant from an engineering aspect as the loads are distributed within the gear structure, and not via main support bearings as is the case with a conventional fixed centre gear arrangement. A cam is attached to the input shaft and this cam operates a hydraulic pump, the lubricating oil doubling up as the hydraulic power medium. When the operating switch makes contact, a solenoid is actuated which moves a valve allowing the hydraulic pressure to engage the cone clutch via a pair of pistons. By the addition of a flanged input shaft and front closing plate the unit is made up into a self contained gearbox, complete with breather, filler, level and drain plugs. The closed unit is then mounted in a carrying cradle and this is suspended on two rubber mounts on a cross bar which clamps to the vehicle chassis. A third supporting point is the drive shaft connecting the overdrive to the gearbox. This shaft includes either a stainless steel flexible coupling, or a single universal or Hooks joint. Both these couplings give excellent axial alignment whilst allowing an element of angular offset, as may be experienced by chassis flex. more.............