The present invention relates to the adjustment of the relative driven speed of the front and rear wheels of a front wheel steer vehicle when turning a corner. The invention has particular application in four wheel drive agricultural tractors.
It is well known that, when turning a corner, particularly a sharp corner, a vehicle's front wheels need on average to accelerate relative to the rear wheels. This is governed simply by the geometry of the paths which each of the wheels follow when a vehicle turns a corner. For example, for a 40 to 50 degree steering angle, the front wheel speed needs to increase by around 30 to 50%.
In a four wheel drive vehicle, therefore, there will be considerable wheel slip at the front wheels when turning a tight corner unless the four wheel drive is disengaged. In a four wheel drive agricultural tractor it is standard practice to disengage four wheel drive when turning a sharp corner, such as when changing direction on field headlands.
Rather than disengaging four wheel drive, it is known to provide change speed gearing for the front axle drive whereby the speed of the front axle is increased relative to the rear axle when a corner is turned. A linkage to the steering mechanism controls the ratio of this gearing automatically, so that the front axle speed is increased when the steering angle passes a certain point. Such an arrangement is known as "bi speed" gearing or a "bi speed" facility.
A problem with such arrangements is that the front wheel speed increase required to eliminate wheel slip is dependent on the exact steering angle. It is possible of course to provide multiple ratios engaging at different angles, but this obviously does not completely solve the problem. This is also a relatively expensive solution and is not generally used.
In general, therefore, bi speed facilities will have only one "bi speed ratio" and this will determine how aggressive the bi speed facility is. A high ratio will cause the vehicle to jerk on turning a sharp corner, involving considerable wheelslip and an uncomfortable ride but assisting the making of a tight turn. This type of bi speed tends to be used in agricultural tractors for use in paddy fields. A smaller ratio will make for a smoother ride and less wheel spin and this might be used for example in a tractor for use in more conventional field work. Different operators will also have preferences for different degrees of aggressiveness.