View Single Post
#369: 12-04-2012, 03:56 PM
outahere's Avatar
Title: HighIntensityDenial
Location: Idaho
Car: '05 Legacy GT Wagon 5MT
Posts: 2,436
iTrader: (3)
Send a private message to outahere Find all posts by outahere Reply With Quote
Originally Posted by SBT View Post
Subaru prescribes this bias for all of it's models, some are only 1 psi difference, but all of the models have it.

It has little, if anything, to do with inducing understeer, or sharpening response. It's about weight differences front to rear, and the rotational stress that having two constant velocity axles rotating at the same speed, combined with the inherent F/R speed differences in the powertrain, causes through center differential binding.

Remove the F/R bias, and you take-out the engineered compensation differences, and the two axles battle it out in the center diff. Techs regularly check for pressure plate/clutch judder issues by increasing/decreasing tire pressures in one axle, to increase loading of the drivetrain. There's a mechanical reason for it.

Subaru used to publish this information, including a pretty detailed explanation of why you should never run tires with "more than 1/4" circumferential difference front-to-rear", now they just make that the statement that it's critically important to maintain correct tire pressures - as indicated on the tire pressure plate on the door sill. And always replace the tires as a set, never one axle only.
Or to put it more simply, the 2 psi difference ensures that the rolling radius of the front tires is equal to the rolling radius of the rear tires.