Why are these two tread (track) width numbers going the opposite direction?

[idosubaru]

XT's are EA82 and XT6 rear uses EA82 rear suspension, the XT FSM shows the following tread ("track") width: 

 

4WD XT6 front = 56.5"

4WD XT6 rear = 56.7"

 

FWD XT6 front = 56.9"

FWD XT6 rear = 56.1"  (same as all EA82 XT's in 1988)

 

I don't suppose anyone knows where they're measuring this from?  

 

But when going from 4WD to FWD: 

The front width increases

The rear width decreases

 

I would expect them to go the same direction? 

 

Is that simply due to geometry - different vehicle height and tire size and based no where on the tire the measurement is taken?

 

 

[wagonist]

I'm a bit surprised the fronts changed, but did they use different wheels from 4wd to FWD? Different offset wheels will cause the track to change.

 

The FWD rear end is quite different to the 4wd with a solid hub and bearing like a trailer.

Subaru could've changed it for a handling benefit (most FWD have a narrower rear track than front)

[Numbchux]

I don't think the parts yield any change. Control arms, knuckles, wheels. Yes the rear inner trailing arm is different, but the outer is the same. Rotor offset is the same. The tire size changed, but that shouldn't make much difference. Depending on how it's measured...but it would change both numbers equally.

 

 

I suspect it has to do with the suspension geometry at different ride heights. The front is a pretty typical arc, with the control arms at a decent downward angle at 4WD height, so as it lowers, the arc pushes the hub outwards.

 

The rear trailing arm actually has the opposite geometry. Because it's an asymmetrical triangle, and the mounts are not in-line, the arc travels outwards towards the extremes, and inwards when the arm is level. And since those arms angle downwards at ride height, as the car is lowered, the hubs shift inwards.

[idosubaru]

So if it's geometry and we're doing at home alignments on vehicles that typically now sit at lower or higher heights from factory - we can't blindly follow the track width measurements?

 

FWD to AWD tire size difference is .4" per tire, not enough to explain it all I don't think. 

 

XT6 wheels are all identical. 

[Gloyale]

Chux nailed it.

 

Front gets wider as it lowers

 

Rear gets narrower as lowers

[idosubaru]

Chux nailed it.

 

Front gets wider as it lowers

 

Rear gets narrower as lowers

 

Thanks, that is exactly what the measurements showed last night. I couldn't pull hardly a millimeter difference between the front and rear.  But it's sitting well below stock (lift kit is already on my garage floor waiting) so what you said would explain how the FSM shows .2" wider (.1" per side) but I'm almost dead even.

 

Thanks again - I'm glad I didn't follow those FSM numbers. 

 

It's been out of alignment since I got it - toe was way off - got it back last night.