dbur

Radius doesn't count for anything. You want your axles all to rotate at the same rate to keep stress off the viscous coupling. Each revolution of the tire goes down the road a distance equal to the rolling circumference, not related to the radius which is distorted by the weight of the car pressing down on it (only at the bottom though ) Measure the circumference of the tire off of the vehicle with a tape or ribbon.

While unloaded radius is exactly related to the circumference, you will get more measurement error trying to measure the radius and then calculating the circumference, or matching radius tire to tire.

Is the rolling circumference the main issue or is it the rolling radius? Even with the recommended 32 psi front and 30 psi rear, the front tire bulge just above the contact patch is a bit more than the rear. The part of the tire that controls the revs per mile is the radius from the bottom of the tire to the centre of the drive axle. I have a leak from a nail in the left rear tire that is too close to the sidewall to patch and if I get a new tire, I should be able to run slightly lower pressure to match the rolling radius of the other tires.

 

The tires are Michelin Pilot Exalto 205/55-16 which are no longer made. The tread depth seems to be close to new tire specs, so I may be able to get away with one tire of a similar design by adjusting pressure. I have measured the height of the bottom of the wheel plastic centre piece from the ground and it seems to vary from 10.25 to 10.75 inches which would equate to a much larger variation than 0.25 inches of circumference. As expected, the lower values are at the front.

The same applies and makes no difference, and here is why.

 

Your AWD system does have a VTD set up. It will split up the ratio 45/55. However, it still operates the same manner as a continuous AWD, maybe a little better though. So if a set of tires are larger in circumference than another say the rear, the system sees they are spinning faster than the other set. It will do it's job and transfer the torque to the wheels that are not spinning as fast. This will cause damage to the clutches because the vehicle is not really stuck at all and burns the clutches/solenoids out. The AWD system just thinks it is.

 

There is a thread on here that describes all the types of AWD that comes with our cars.

 

Edit- Found it

http://www.legacygt.com/forums/showthread.php?t=48112

From that other thread:

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In normal operation, power is distributed equally to the front and rear wheels. Plates are alternately attached to the front and rear output shafts inside the viscous coupling. When a rotational difference occurs between the front and back wheels, the plates inside the viscous housing shear inside the contained fluid (a type of silicone) heating it and causing the fluid to thicken. The thickened fluid causes the plates to transfer torque from those that rotate faster (the slipping wheels) to the plates that rotate slower (the wheels with the best traction).

 

This no-maintenance system is simple, compact and virtually invisible in its operation. The system can distribute torque from a 50:50 torque split for maximum traction to mostly front or rear wheel drive.

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Two questions:

1. is this silicon fluid contained inside the housing in a secondary chamber with no access externally? I know my tranny is filled with 75-90 gear oil and when you remove the drive shaft it runs right out the back, so it seems to all be one chamber inside.

2. is there a way to tap into the control system and set up an indicator light that could tell you when clutches are engaging? That way if you see it going on all the time when you are not on a slippery surface you would know something was going on.

I just replace a tranny on a 95 Legay L wagon that had been run with different wear level tires on the front and rear. Seems there is a lot of iffiness to measuring the circumference to within 1/4" (tire pressure, vehicle weight, on or off the car...) and a positive feedback that the clutches aren't over active would be comforting.

Thanks.