Device for changing the speeds of the front and rear wheels in a four-wheel drive vehicle

A device for changing the speeds of the front (10) and rear wheels (14) in a four-wheel-drive vehicle (1), which is equipped with a continuously variable speed-changing mechanism (6a) for the front wheels (10) which changes the input rotational speed and outputs it to the front wheel drive shafts (9) and a continuously variable speed-changing mechanism (6b) for the rear wheels (14) which changes the input rotational speed and outputs it to the rear wheel drive shafts (13). The device for changing the speeds of the front (10) and rear wheels (14) comprises a detector means for detecting the operation conditions of the vehicle (1) and a controller (30) for variably controlling the speeds of said continuously variable speed-changing mechanism for the front wheels (10) and of said continuously variable speed-changing mechanism (6b) for the rear wheels (14) based on the operation conditions of the vehicle (1) detected by said detector means. The controller (30) calculates a tire diameter ratio alpha of the front wheels (10) to the rear wheels (14) when the vehicle (1) travels straight, sets a target speed-changing ratio obtained based on the vehicle operation conditions detected by the detector means to be a target speed-changing ratio for the rear wheels (14) in the continuously variable speed-changing mechanism for the rear wheels (14) or to be a target speed-changing ratio for the front wheels (10) in the continuously variable speed-changing mechanism (6a) for the front wheels (10), and sets a speed-changing ratio obtained by correcting the target speed-changing ratio based on the tire diameter ratio to be a target speed-changing ratio (ecf) for the front wheels or to be a target speed-changing (ecr) ratio for the rear wheels.