The trailing arms we normally refer to are actually semi-trailing arms i.e. non-perpendicular to the chassis centreline and are quite dynamic apparatuses. Arms mounted perpendicularly to the chassis are true trailing arms and are limited to the basic function of swinging the wheel up and down in an arc.  

Pivot Angle
The pivot angle for a semi-trailing arm is usually around 8-10° from perpendicular, depending on the arm's length, travel and the amount of body roll it has to contend with. This pivot angle creates camber gain in bump, which improves cornering in the same manner as it does in the front suspension. It also causes camber loss in droop and some minor toe changes too.  

Ideally (negative) camber should gain slightly more than the maximum degrees of body roll (to account for flex in the tyres' sidewalls). A good starting point would be to match the camber gain of the front end. Of course, the shorter the arms, the more pronounced the effects of camber and toe will be and conversely, the longer the arms, the effects will be less pronounced. 

Camber in droop isn't that important in the rear during cornering (from purely a suspension point of view) because the inside tyre doesn't contribute much to the handling. However, because semi-trailing arms have the advantage of some positive camber in droop this results in the outer CVs being more favourably angled towards the inner CVs, thereby allowing maximum wheel travel at full extension. 

However, and again, from purely a suspension geometry point of view, there's no advantage in the outer CVs having less angle than the inner CVs, so positive camber in droop should be monitored as too much will affect scrub.

Semi-trailing Arm Pivots
Semi-trailing arm pivots are normally parallel with the bottom of the chassis (horizontal) and can be set up in one of two ways: The tube across the rear of the cabin (which the arms pivot off) can be a straight tube with uneven length pivot mounts that locate the arms' pivot axes at the desired angle.

The pivot tube can be bent in the centre (at the required pivot angle) and identical pivot mounts then welded to it with their axes parallel to each side of the tube.

The semi-trailing arm configuration provides camber gain in bump and camber loss in droop (which also helps to minimize variances in axle length through the arm's travel). Some toe-out will occur in bump, but as body roll increases, this actually aids in over-steering through turns.

A small amount of static camber should be included in the arms (usually by angling the hub carriers) and needs to be balanced somewhere between the CVs' maximum angle and the maximum roll angle.

True Trailing Arm Pivots
Since the pivots are perpendicular to the chassis centreline, the wheels will see zero camber change/gain.

Toe will not change through the arms' travel, but the absence of any change in camber will restrict the amount of travel due to the limitations of axle length and the ability of the CVs to plunge. This is because the outer CV on a true trailing arm moves away from the inner CV exponentially quicker in bump and droop compared to a semi-trailing arm set-up. 

Axle plunge can be minimised by placing the outer CVs behind the inner CVs (in plan view) and angling the axles back and out from the trans so that at full droop, the outer CVs become aligned with the inner CVs (in plan view and side view). 

True trailing arms can see camber if the pivot brackets are at an angle to the horizontal, but this will simply result in static camber and with zero camber gain.

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