In the June’s Sensor Sensibility article, the negative effects inconsistent tire size can have on transmission shift scheduling had been mentioned, particularly with BMW, Land Rover, Dodge and Nissan Maxima vehicles.
For completely different reasons, AWD transfer cases that use viscous clutch, pump couplers or clutch control, all suffer from inconsistent tire size. These are active transfer cases which can provide on-demand torque biasing based on wheel speed inputs. If the rear wheel
speed signals indicate they are slipping, torque is ramped up to the front wheels. Full torque is returned to the rear wheels once a no slip condition is detected. This on-demand torque biasing is done so smoothly the driver has no idea which wheels are driving the vehicle.
Inconsistent tire sizes are misunderstood by the operating system as a slipping condition causing the transfer case to become overly active needlessly. This results in premature damage to the transfer cases internal components. In some instances the damage causes the transfer case to lock up in AWD which then causes wheel hop on turns and reoccurring front differential failure.
The quickest way to check for consistent tire sizes with both high end passenger cars and trucks with active transfer cases is with a stagger gauge. While the vehicle is on the ground, you can quickly measure the width of each tire. They are all to be within 1/8 inch of each other.
Sometimes repeat failure is enough to make one drink. It may then become a staggering revelation when you learn that a no up-shift condition or repeated blown transfer case was all due to an underinflated tire. Why stagger out of a bar when you can stagger the car or truck tire with a gauge?
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