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Brad,
Because of road slope and gravity most any vehicle has a tendency to fall down the slope. There are design parameters that can reduce the sensitivity to road slope (crown) but they also tend to reduce steering feel and feedback.
To compensate for road slope many manufacturers implement cross car caster and some use cross car camber. (caster is arguably the better solution) I do not know for sure what Saab is doing on the current 9-3 and 9-5, but I assume they have some cross car asymetry. The asymetry creates a natural tendency for the vehicle to pull a little in one direction. The amount of cross car asymetry is set to combat the road slope and other vehicle asymetry. Here in the states we like to use an average road slope of about 1 degree to the right.
If cross car asymetry is implemented correctly it can cancel out the tendency of the vehicle to fall down the slope. The problem is that this works only on roads sloped in one direction. On a two way street the road is usaully sloped to the right, so the cars are usually dialed in to pull slightly to the left cancelling the pull to the right due to road slope. this means your vehicle goes straight.
Now if you drive on the higway a lot and spend a lot of time in the left lane you will be on a road that is sloped to the left. that combined with a car that is designed to pull slightly to the left will result in an annoying pull to the left.
Again I don't know what Saab did on the current models but from the experience gained by owning two I would guess that there is some cross car asymetry dialed into the vehicle.
The reason dealers have a lot of trouble getting rid of this is that the vehicles use the net build alignment strategy where there are no adjustment provisions for caster or camber. The only thing that is easily adjustable is the toe and that will not fix this issue. You can have the mechanic change the above parameters by enlongating the upper strut mounts and re-aligning the car. Apparently there are also smaller diameter bolts for the knuckle strut interface that will allow a camber change.
I have had a hard time swallowing a designed in asymetry, but have been convinced of its merits. Believe it or not it results in more customer satisfaction than complaints...
Again there are lots of reasons why it is done (not just road crown) and I don't think I should go into them all as they may be considered trade secrets. But on a very coarse level think of the vehicle as a whole and not much about even the weight distribution is symetric... The goal is to keep the majority of people happy as often as possible.
-Joe
, Thu, 3 May 2001 14:44:00
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