1. A TRACTOR HAVING A REAR AXLE CASING, REAR WHEELS AND A GUARD FOR EACH WHEEL AND PROVIDED WITH AN ANTI-ROLL BAR ASSEMBLY, COMPRISING AN ANTI-ROLL BAR AND SIDE SUPPORTING UPRIGHTS THEREFOR, EACH UPRIGHT HAVING A LOWER PART, SAID LOWER PART HAVING UPPER AND LOWER ENDS WITH THE LOWER END BEING MOUNTED ON THE REAR AXLE CASING AND EXTENDING BETWEEN THE ADJACENT REAR WHEEL AND THE GUARD THEREFOR, A LATERAL EXTENSION AT THE UPPER END OF EACH LOWER PART PASSING BETWEEN THE TREAD OF THE REAR WHEEL AND THE GUARD THEREFOR AND HAVING AN OUTER END, A SHOE FOR THE EXTENSION, AN UPPER PART EXTENDING UPWARDLY FROM THE OUTER END OF THE LATERAL EXTENSION AND THE ANTI-ROLL BAR BEING RIGIDLY SECURED TO SAID UPPER PART WITH ITS RESPECTIVE ENDS PROJECTING BEYOND THE REAR WHEELS SO THAT IN THE EVENT OF THE TRACTOR FALLING ON ITS SIDE THE THRUST ON THE PROJECTING END OF THE BAR CAUSED THE UPRIGHTS TO BE SLIGHTLY DISTORTED SO THAT WHAT NOW CONSTITUTES THE UPPER SHOE BEARS ON THE TREAD OF THE ADJACENT WHEEL THEREBY RESTRICTING FURTHER BENDING OF THE SUPPORTING UPRIGHTS.