Car suspensions have a simple task to describe but hard to actually do: control
wheel movement and keep the tire in proper contact with the road despite things like bumps/depressions in the road, physics trying to make the car tip over during hard cornering, etc.
In a nutshell, suspensions often can't perfectly satisfy these goals... the
wheel+tire can tip in/out (not be vertical or perpendicular to the ground) and can steer slightly as the suspension compresses or extends over bumps. Good suspensions keep the
wheel straight & vertical; bad suspensions allow a lot of this type of motion.
The issue experienced/described by Soledad sounds like toe angle is changing so one of the rear wheels is pointed slightly differently from the "straight ahead" and the rear end of the car tries to steer a bit. I'd have the dealer examine the bushings for problems.
Long-winded explanation if you really want to know, or have insomnia:
Suspensions are basically arrangements of pivoting parts that ideally make the
wheel go straight up-and-down as it passes bumps/drops in the pavement. Unfortunately straight up-and-down is rather difficult to obtain. Try this test: hold your right arm straight out from your side, pretending it is a suspension piece. Wrap your right hand around a pen or pencil and hold your hand+wrist so the pen is perfectly vertical. Now rotate your shoulder to move your hand up and down while holding your wrist constant. The pen moves in an arc, right - it doesn't stay perfectly vertical. That's a very dirt simple/basic suspension design - a "swing arm" as used on VW Beetles decades ago.
Now move your hand up and down again but this time let your wrist flex so the pen stays vertical. If you imagine you had two right arms - one at your shoulder and one attached to the base of your rib cage - holding the pen, your arms, the pen, and your body would basically form a rectangle - that's like a dual "A" arm suspension or part of a modern multi-link suspension. As the
wheel (i.e. the pen) moves up/down over bumps in the road both wrists flex so the arms stay parallel and the pen stays vertical too --> the tire stays "flat" on the road for better grip. But... notice the pen is moving in-and-out from your body as it goes up-and-down... a tire doing that on the road ends up pushing your car left/right as it goes up-and-down. So we still don't have the "ideal"
wheel movement of pure vertical motion over road bumps.
Now imagine holding "both of your right arms" straight out again... then lean your body to the left as though the body of the car were leaning in a high performance turn. Your hands, and the pen (
wheel+tire don't forget), try to rise as you lean... so let your arms and wrist flex to keep the pen at a constant height (
wheel + tire staying on the pavement). Again, you'll see/feel the pen needs to move inwards, towards you, as you lean... a
wheel+tire has to move sideways on the pavement as the car body rolls. Also, because your two right arms are a constant length, when your body leans that forces the pen to lean - our rectangle has become a parallelogram. A tire that is leaning doesn't have its tread flat on the pavement and thus can't operate at maximum grip.
A multi-link suspension has arms that simulate that rectangle but the rectangle is distorted a bit - more of a trapezoid shape - with unquall length arms. When done correctly, it reduces that
wheel tilt motion as your body leans.
In my example, your arms are "locating" the
wheel laterally from you, the car body. But what keeps the pen (
wheel) from moving fore/aft --> imagine somebody else facing you and pushing the pen straight backwards. Your arms would not be able to resist that much... the same is true on a car. So cars have a fore-aft link to resist this type of force. That link is much like our first example: a single arm with two pivots. So now when the pen (
wheel) hits a bump in the road and moves up or down, your arms make it want to move in/out too... and this fore/aft link makes it want to move fore and aft. With all those links trying to hold the pen (
wheel & tire) it's almost impossible to keep the "geometry" perfect - i.e. the radii of the various arcs the pen is moving through. Plus these links attach to slightly different points of the car's
wheel assembly so that screws up the radii a little too. The result of all these links pushing/pulling in different directions (roughly 90 degrees apart) is that the pen moves in/out, fore/aft, and doesn't quite stay vertical. And it can also twist along a vertical axis. What is a
wheel+tire doing when it twists about a vertical axis? It steers - that's what your front wheels do to steer the car!
Loading the car with weight on just one side compresses that side of the suspension a little more than it's counterpart on the other side of the car... so the wheels on that side of the car get slight steering changes - called "toe" (think pigeon toed feet) changes. Remember the first pen example - the pen moving in an arc and not remaining vertical? The angle the
wheel makes to the ground, when viewed from the front or rear of a car (i.e. how the
wheel seems to tilt in/out) is called camber. That pen moving away from vertical has a lot of camber change. Good suspensions have little to no toe and camber change for most suspension movements; crappy suspensions change either or both quite a bit. The whole point of complex "multi-link" suspensions is to allow as much
wheel up-and-down travel over road imperfections while minimizing toe and minimizing the camber change of the
wheel
Car suspension parts end in rubber parts called bushings. Think of bushings as stiff springs: they have a little "give" to them, compared to the strong metal parts of the suspension, to absorb any imperfections in those radii. They also filter out a lot of the high frequency road noise. Whenever the
wheel moves up/down due to a road imperfection, or you make a turn, etc. these bushings "give" a little (they're springs, right?). This "give" alters the relationship between the suspension arms and the
wheel assembly... screwing up the suspension's ability to control that
wheel. A bad bushing, worn out one, or improperly installed bushing is basically a really soft one, introducing a lot of slop to the
wheel assembly... so the suspension can't keep the
wheel toe & camber under control.
mike c.
