Gas Spring Sizing for Hoods and Covers
Size gas struts for a hood or cover: from hood mass, centre-of-gravity distance, the attachment points on the hood and frame, and the opening angle, the moment equilibrium about the hinge gives the required spring force over the opening angle, a recommended extension force with reserve, the spring lengths in the closed and open position with the stroke, and a hand-force check for opening and closing, live with every input.
Calculation
Results
Notes
- Residual force in the closed position 16 N is positive: the spring pushes the closed hood open, a latch or catch is recommended.
Design rating as an approximation: constant spring force assumed; the real gas-spring progression and the final type selection remain manufacturer-specific (catalogue).
- Recommended extension force F1
- 380 N
- Spring force used F_sel
- 380 N
- Maximum required force F_req,max
- 341 N
- Stroke
- 131.4 mm
Key values
- Spring length closed L_F(0)
- 208.1 mm
- Spring length open L_F(α_max)
- 339.5 mm
- Closing force F_hand(α_max)
- 33.3 N
- Residual force F_hand(0)
- 16.4 N
Sketch: side view with hinge, hood and spring
Calculation in your browser, inputs go to our server only when you export or save.
Formulas and fundamentals
Moment equilibrium about the hinge
The hood is a rigid body rotating about the hinge. In the closed position (horizontal) the load moment from its own weight is at its maximum, because the centre-of-gravity lever L_S then acts fully horizontally. As the hood opens through angle α, the centre of gravity rotates with it and the load moment decreases with the cosine:
For the hood to stay in equilibrium (or to be held in a controlled way) at every opening angle, the moment of the gas spring(s) must counteract this load moment across the whole opening range.
Geometry: hood point, spring length and effective lever arm
The attachment point on the hood rotates with the hood about the hinge and, at any opening angle, is located at:
The frame point R = (x_R, y_R) is fixed. The spring acts along the line R-D, its length is L_F = |D-R|. What matters for the moment about the hinge is not the spring length itself but the perpendicular distance from the hinge to this line of action - the effective lever arm h. It follows from the cross product of R and the direction vector D-R:
Required spring force and extension-force recommendation
At each opening angle, the force one spring must supply follows from the moment equilibrium, shared across n_F springs:
The calculator evaluates this formula on a 1° raster across the full opening range [0, α_max] - the maximum is not necessarily at α = 0, since the load moment and the lever arm can move in opposite directions. The recommended extension force F1 is this maximum with a 10 % reserve, rounded up to the next 10 N step (a typical catalogue increment). Model limitation: real gas springs do not have a constant force but a characteristic curve with progression - on compression (retraction), the force rises by roughly a factor of 1.2 to 1.4 relative to the extension force F1. The calculator deliberately simplifies to a constant force; the final type selection (e.g. Suspa, Stabilus) is based on the manufacturer's characteristic curve.
Hand force when closing and residual force when closed
With the chosen spring force F_sel per spring (default: the recommendation F1), the holding force required at the handle, at distance L_G from the hinge, when closing the hood from the open position is:
A positive value means the user must pull, because the spring wants to hold the hood open more strongly than its own weight would require. This value is evaluated at the fully open angle α_max (hand force when closing) and at α = 0 (residual force in the closed position): if F_hand(0) is negative, the hood's own weight dominates, the hood stays shut on its own and must be lifted to open it - the normal case. If F_hand(0) is positive, the spring pushes the closed hood open; a latch or catch is required.
Spring lengths, stroke and dead-length plausibility
The geometry gives the installed (closed) and extended (open) spring lengths and their difference, the stroke:
As a rough buildability guideline for gas springs: the extended length should be at least twice the stroke plus a dead length of around 60 mm for the cylinder and rod overhang (L_F(α_max) >= 2·stroke + 60 mm). If this is not met, the chosen attachment points are unfavourable for a gas spring that is actually available.
Worked example
Reference example with the default values: hood mass m = 25 kg, centre-of-gravity distance L_S = 400 mm, handle distance L_G = 700 mm, two gas springs (n_F = 2), opening angle α_max = 60°, hood attachment r_D = 250 mm, frame point x_R = 80 mm, y_R = −120 mm. In the closed position, D(0) = (250, 0) mm, the spring length L_F(0) = √(170² + 120²) = 208.09 mm, and via the cross product (80·120 − (−120)·170 = 30000) the lever arm h(0) = 30000/208.09 = 144.17 mm. The load moment is M_L(0) = 25·9.81·400·1·1e-3 = 98.1 Nm, giving F_req(0) = 98100/(2·144.17) = 340.2 N.
Evaluated over the full 1° raster, the maximum required spring force is not exactly at 0°, but at around 9°, with F_req,max = 341.0 N - the load moment and lever-arm curves work slightly against each other here. With a 10 % reserve and rounding up to 10 N, the recommendation is F1 = 380 N per spring.
In the fully open position (α_max = 60°), D(60°) = (125.0, 216.5) mm, the spring length L_F(60°) = 339.50 mm and the lever arm h(60°) = 95.20 mm - smaller than in the closed position. The stroke is L_F(60°) − L_F(0) = 131.4 mm; the dead-length plausibility check passes (339.5 mm >= 2·131.4 + 60 = 322.8 mm).
With F_sel = 380 N, the hand-force check gives: closing from the open position requires F_hand(60°) = (2·380·95.20 − 49.05·1000)/700 = 33.3 N at the handle (well below the 80 N guideline, so pass). In the closed position, F_hand(0) = (2·380·144.17 − 98.1·1000)/700 = +16.4 N - positive, meaning the spring pushes the closed hood open slightly; a latch or catch is recommended (warning).
Frequently asked questions
Which extension force F1 should I choose?
The calculator determines the maximum spring force required across the whole opening range and, with a 10 % reserve rounded up to the next 10 N step, suggests an extension force F1. Since real gas springs are only available in catalogued force steps (usually 50 N or 100 N increments depending on the manufacturer), choose the next larger available step from the relevant manufacturer (e.g. Suspa, Stabilus) and re-check the hand force with that actual value.
One or two gas springs?
With a single spring (n_F = 1), the force that one spring must supply doubles compared with two springs for the same geometry. Two springs (mounted symmetrically on both sides) are standard for wider or heavier hoods because they share the load, introduce the forces symmetrically and avoid the hood twisting as it opens. A single spring is enough for narrow, light covers with a central mount.
Why doesn't the hood stay open or stay shut?
If the hood does not stay open on its own, the chosen spring force is too low for the geometry at that angle - the hand-force values then go negative and the rating flags it. If the closed hood instead pushes itself open on its own (F_hand(0) positive), the spring force there exceeds the weight moment; a simple latch or catch helps - reducing the spring force usually isn't sensible, since it is needed in the open position.
Where should the gas spring be mounted?
It is essential that the spring's line of action (R-D) never passes through the hinge - there the effective lever arm h would be 0 (dead point) and the required force would go to infinity. A frame point below the hinge usually increases the lever arm over part of the opening range and allows a more compact installed length. Check the full force curve F_req(α) in the chart, not just the end positions, to spot proximity to a dead point.
What is gas spring progression, and why does practice deviate from the calculation model?
The calculator simplifies by assuming a constant spring force. Real gas springs, however, have a progressive characteristic: on compression (retraction), the internal pressure and hence the force rises by roughly a factor of 1.2 to 1.4 relative to the extension force F1 (as stated by the manufacturer at full extension). The actual final spring selection is therefore always based on the manufacturer's characteristic curve and approval, not just on the guideline value computed here.
Does the piston rod need to point down or up?
The usual manufacturer recommendation is: piston rod pointing down (towards the frame point R, if that is the lower attachment point). The lubricating oil inside the cylinder then continuously wets the seal at the piston rod, reducing wear and break-away friction and extending service life. In a horizontal or reversed installation, the seal can run dry, leading to leakage and loss of force over time.
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