[svxistentialist]

Originally Posted by ABSVX

I must be missing something here. I understood AWD to mean that all four wheels can be powered but they can also rotate independently at different rotation speeds (vs. locked in 4x4 in which the diffs are locked). If (in simple terms!) AWD works as I have understood, why would different tire diameter make any difference?? How would the transmissions/differentials know the difference between different tire diameters (resulting in different rotations from one side of the car to the other), and going round a corner which requires one wheel to rotate more than the other?? I knew SVX's were great, but are they really that SMART -- WOW I'm impressed!!

Mike

'94 Barcelona LSi 127K Kms stock and lovely.

"Smart" would be a relative term in this context Mike, but I'll make a couple of points on the questions you pose.

You are quite correct in that the front and rear differentials are there to cope with the different rotations the right and left wheels need to make when turning a corner, so this is normal enough. When this happens it puts a certain load or stress on the differentials, but if you think about it unless you are running your car on an oval circuit, the numbers of right turns equals or cancels the numbers of left turns over continuous mileage.

However if wheels on one side are a smaller size than the other, then this puts a continuous load on the differentials all the time, leading to possible damage [depending on the difference of rotation diameter] The same thing happens front to back if one axle has different diameter wheels than the other. The strain is constant in that scenario, so the default correct position for AWD cars is all four wheels having the exact same rolling diameter.

The 4EAT in the [AWD] SVX has two speed sensors measuring the rate of rotation of the front axle versus the rear axle and based on these readings controlling the required distribution of torque to the axles via the TCU, aka the transmission computer.

The control software is actually pretty clever, and Subarus can find traction when most other vehicles have given up. For that reason it's pretty important to have all 4 wheels exactly the same to establish the accurate control baseline. [and to avoid burning up the gearbox clutches when traveling in a straight line ]

Joe