Alignment

[lee]

Once upon a time I read a tractor manual (boring life huh), and they specifically said they used a difference in front/back ratio to "pre-load" the front wheels. (The pre-load term is mine because I don't remember/know what to call it)

I realize a tractor doesn't see the miles a car does, but given what mbtoloczko documented, is it possible that some pre-load on the front was designed in? Is it better than letting the rear "push" given the differential system on the Subaru (and I have no idea what this means for the different styles between NA and rest of the world).

Maybe I'm still beating a dead horse here, but it seems the designers would assume the system could not be maintained perfectly, and therefore prefer one type of bias to another.

[Beav]

Pre-loading, as it applies to four wheel drive, is accomplished by placing a higher numerical ratio gearset in the rear differential. This arrangement provides the front wheels the ability to 'pull' the vehicle slightly when steering over a poor traction surface. Prior to push-button AWD, etc. most four wheel drive vehicles had this arrangement. Typically a difference of .02 - .03 was the norm, i.e. 3.54 & 3.57, 4.09 & 4.11.

Back to the inflation theories - no matter what you do to ensure an equal rolling radius at each tire there will be factors that can't be allowed for at all times. The growth of the radius due to centrifugal force is compounded by tire construction which not only varies by manufacturer but also by age, density variations in the rubber, etc. Tire temperature will cause growth also, the heavier nose of the car plus the heat generated by steering will make the fronts heat up quicker and to higher temps than the rear. No matter how you measure it if you're not operating in a steady state there will be variables that are impossible to factor. The rolling measurement is about as close as you can get, if done after the tires are warmed to their average temps and measured over a normal route at normally driven speed. It may provide a way to kill an afternoon though...

[Ron Mummert]

I wonder if drivers of 18 wheelers have these discussions.

Ron.

If you pump up the front tires to the 44-46 range you will get a far lower differential in front/rear rolling radius when using a 3-4psi pressure delta.

Obviously only do this on a 44psi max or higher tire.

What I'm saying is that much of the problem you're encountering is a result of excessive flex in the sidewalls of the front tire... i.e. you're riding on cushy pillows.