NHTSA Complaint #10872897 — 2006 BMW Z4

THE PROBLEM OF THE STICKY STEERING OF THE Z4 IS RELATED TO THE WORM/REDUCTION GEAR MECHANISM THAT TRANSMITS THE TORQUE FROM THE ELECTRIC MOTOR TO THE STEERING SHAFT. THE CIRCULAR REDUCTION GEAR HAS A NYLON CROWN WITH TEETH. ALL THE SURROUNDING PARTS ARE METAL (STEEL OR ALUMINIUM). THE THERMAL EXPANSION OF THE NYLON IS 3X BIGGER THAN THE SURROUNDING PARTS. TO COMPENSATE FOR THIS EXPANSION, THERE SHOULD BE A CLEARANCE BETWEEN THE WORM GEAR AND THE REDUCTION GEAR. UNFORTUNATELY BMW AND THEIR STEERING SUPPLIER ZF NACAM CHOSE THIS CLEARANCE TOO SMALL. WHEN THE NYLON GETS WARM, THE CLEARANCE WITH THE WORM GEAR BECOMES ZERO AND THE REDUCTION GEAR IS STUCK WITH THE WORM GEAR. THE FRICTION INCREASES DRAMATICALLY AND YOU WILL THEN "FEEL" THE ENGAGEMENT OF THE INDIVIDUAL TEETH OF THE REDUCTION GEAR. THIS IS A DEFECT IN THE CAR DESIGEN AND NEEDS TO BE RECALLED.