Gas Spring Force and Mounting Geometry

Everything is a moment about the hinge: the lid's weight acts at its center of gravity, which for a uniform panel is halfway along it, and the strut pushes along its own line at whatever perpendicular distance the mounting points give it. That perpendicular distance is the MOMENT ARM, and it is almost always much shorter than the lid's, which is why struts are rated in the tens or hundreds of pounds for lids that weigh far less. The moment arm is also the design variable and the cheapest one to change. A 40 lb hatch with its center of gravity 18 in from the hinge makes a 720 in-lb moment, and two struts on a 4 in arm each need 90 lb. Move the body-side mount out to a 6 in arm and the requirement falls to 60 lb each -- a third less force for a change in one mounting hole location. Move it in to 2.5 in and each strut needs 144 lb, and the hatch becomes genuinely hard to pull closed against them. Two struts halve the requirement, which is why almost everything uses a pair. The catch a first-time designer meets is that both moments change through the swing and they do not change at the same rate. The lid's moment FALLS as it opens, because the horizontal distance to the center of gravity shortens toward zero at vertical; the strut's arm typically grows and then shrinks. A strut sized only at the closed position may hold the lid there and then fling it open, or hold it open and refuse to close, which is why the required force is worth checking at more than one position. Static moment balance at the positions entered. It does not model the gas spring's own force curve, which rises as the rod compresses and falls with cold weather -- a strut is noticeably weaker on a winter morning -- and it does not account for damping, the end-of-stroke behavior, the strut's free length and stroke against the geometry it has to fit, friction at the ball ends, or the fatigue life of the mounting brackets and the panel they bolt to. The strut manufacturer's force curve, stroke, and mounting recommendations govern.

horizontal reach = cg distance x cos(opening angle); lid moment = weight x that reach; force per strut = lid moment / (struts x perpendicular moment arm); the same at a second angle and arm for the comparison.

Static moment balance about the hinge -- lid moment against strut moment through its perpendicular moment arm -- by name. Checked at the positions entered; it does not model the gas spring's own force curve, its rise as the rod compresses, or its loss of force in cold weather. The strut manufacturer's force curve, stroke, and mounting recommendations govern.

A statics calculation on the user's own geometry; no manufacturer force table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: lid_weight_lb, cg_distance_in, opening_angle_deg, moment_arm_in, struts, second_angle_deg, second_moment_arm_in, horizontal_cg_in, lid_moment_in_lb, force_per_strut_lb, total_force_lb, second_force_per_strut_lb

• The arm is the design variable moving the body-side mount changes the required force far more cheaply than changing the strutstatics
• Both moments vary through the swing the lid's falls toward vertical while the strut's arm grows then shrinks; check more than one positionstatics
• Force curve not modeled a gas spring stiffens as the rod compresses and is noticeably weaker coldgas spring manufacturer data