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A method of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a motor vehicle with a four-wheel drive is provided. One axle is driven directly and the other axle, as a hang-on system, is coupled by way of the clutch. In order to create a method of controlling th

A method of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a motor vehicle with a four-wheel drive is provided. One axle is driven directly and the other axle, as a hang-on system, is coupled by way of the clutch. In order to create a method of controlling the shiftable clutch in a drive train between the front axle and the rear axle, and a corresponding system with an improved availability of the full four-wheel drive characteristic with a corresponding protection of the driving dynamics of the overall vehicle, the clutch is continuously acted upon by torque in an adjustable manner by way of a pilot control measure.

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What is claimed is: 1. A method of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a four-wheel drive motor vehicle, the method comprising the acts of: driving one axle directly; driving the other axle coupled as a hang-on system by way of the shiftable clutc

What is claimed is: 1. A method of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a four-wheel drive motor vehicle, the method comprising the acts of: driving one axle directly; driving the other axle coupled as a hang-on system by way of the shiftable clutch; continuously acting upon the shiftable clutch with torque in an adjustable manner by way of a pilot control measure; determining values for an inequality of wheel tires between the front and the rear axles using a tire tolerance logic; determining a distortion of the drive train based on the determined values, wherein values of the wheel tire inequality between the front axle and the rear axle is measured in a form of a tire diameter deviation and, wherein determining the tire diameter deviation, comprises the acts of opening, in a neutral driving condition, the shiftable clutch; and determining a rotational speed difference between the front axle and the rear axle when the clutch is opened. 2. The method according to claim 1, further comprising the act of determining tire diameter deviations, without opening the shiftable clutch, by determining a rotational speed deviation between the front axle and the rear axle while the clutch is partially tensioned. 3. The method according to claim 1, wherein for determining the tire diameter deviation, measurements are carried out at two points. 4. The method according to claim 3, wherein by using the measurements carried out at two points, values at limits of Δn=0 and M=0 are determined via linear interpolation, wherein Δn is a speed difference between the front axle and the rear axle, and M is clutch torque. 5. The method according to claim 3, wherein the measurements carried out at two points utilized torque values of the shiftable clutch which are as far apart as possible. 6. The method according to according to claim 1, wherein by using the measurements carried out at two points, values at limits of Δn=0 and M=0 are determined via linear interpolation, wherein Δn is a speed difference between the front axle and the rear axle, and M is clutch torque. 7. The method according to claim 1, wherein the pilot control measure, via the tire tolerance logic, actively intervenes only when the distortion between the front axle and the rear axle leaves a dead or intervention-free zone. 8. The method according to claim 7, wherein the intervention-free zone is selected as a function of a respective speed of the vehicle. 9. The method according to claim 1, wherein a range about a characteristic Cardan torque is determined above which, when the front axle is too small, the pilot control measure no longer limits the clutch in order to utilize driving torque at the front axle for accelerating the vehicle. 10. The method according to claim 1, wherein the act of continuously acting upon the clutch occurs on a basis of a defined torque distribution between the front axle and the rear axle. 11. The method according to claim 10, wherein the defined torque distribution between the front axle and the rear axle within the drive train is implemented in certain ranges of driving dynamics, including when the front axle is too small in a trailing case and/or when the rear axle is too small in a drive case. 12. A method of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a four-wheel drive motor vehicle, the method comprising the acts of: driving one axle directly; driving the other axle coupled as a hang-on system by way of the shiftable clutch; continuously acting upon the shiftable clutch with torque in an adjustable manner by way of a pilot control measure; determining values for an inequality of wheel tires between the front and the rear axles using a tire tolerance logic; determining a distortion of the drive train based on the determined values, wherein values of the wheel tire inequality between the front axle and the rear axle is measured in a form of a tire diameter deviation and, determining tire diameter deviations, without opening the shiftable clutch, by determining a rotational speed deviation between the front axle and the rear axle while the clutch is partially tensioned. 13. The method according to claim 12, wherein for determining the tire diameter deviation, measurements are carried out at two points. 14. The method according to claim 12, wherein by using the measurements carried out at two points, values at limits of Δn=0 and M=0 are determined via linear interpolation, wherein Δn is a speed difference between the front axle and the rear axle, and M is clutch torque. 15. A method of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a four-wheel drive motor vehicle, the method comprising the acts of: driving one axle directly; driving the other axle coupled as a hang-on system by way of the shiftable clutch; continuously acting upon the shiftable clutch with torque in an adjustable manner by way of a pilot control measure; determining values for an inequality of wheel tires between the front and the rear axles using a tire tolerance logic; determining a distortion of the drive train based on the determined values, wherein the pilot control measure, via the tire tolerance logic, actively intervenes only when the distortion between the front axle and the rear axle leaves a dead or intervention-free zone, and, wherein, during cornering by the vehicle, when leaving the dead zone, a respectively tolerated distortion torque is defined by a suitable steering-angle-dependent characteristic curve for the vehicle. 16. A vehicle system, the system comprising: a front axle; a rear axle; a shiftable clutch arranged in a drive train between the front axle and the rear axle of the four-wheel drive vehicle; wherein one axle is driven directly and the other axle is coupled by way of the shiftable clutch as a hang-on system; and a control unit operatively coupled to continuously and adjustably act upon the clutch with torque based upon a tire diameter deviation determined by a rotational speed difference between the front axle and the rear axle when the clutch is opened in a neutral drivina condition. 17. The system according to claim 16, further comprising: rotational speed sensors operatively coupled with wheels of the front axle and the rear axle; and wherein the rotational speed sensors provide inputs to the control unit. 18. The system according to claim 17, further comprising: a transfer case, wherein the clutch is part of the transfer case. 19. The system according to claim 16, wherein the adjustment of the clutch is based upon a tire tolerance logic.