wait exactly what are u looking for? maximum horizontal acceleration or vertical acceleration or the net acceleration? because the answer would vary depending upon which 1 ur looking for. max horizontal acceleration occurs at the end points or the extremities where the bob is not moving as the only acceleration horizontally under normal circumstances is the tension of string (due to vector division of tension to 1 horizontal and 1 vertical component) and air resistance, which opposes the motion of the bob depending upon the velocity of the bob [a=-k(v^n)/m]. for vertical acceleration it's always the vertical component from tension- the gravitational acceleration+or-air resistance and it's max when the bob is at the vertical position. the net acceleration is the combination of horizontal and vertical acceleration and it should be a constant when only considering magnitude due to centripetal force (assuming it follows a perfect arc) and i don't really know how u can assign + or - for this one.