A hatch that feels manageable by hand can still need a carefully specified gas strut. The reason is leverage. To calculate hatch opening force correctly, you need more than the hatch weight stamped on a drawing or measured on a scale. You also need its centre of gravity, hinge position, strut mounting points and the angle at which the hatch must start lifting.
This applies whether you are fitting struts to a ute canopy, caravan bed base, toolbox lid, boat hatch, machinery guard or heavy cabinet door. Choose a strut only by the hatch weight and it may be too weak at the start of travel, overly forceful when closed, or unable to hold the hatch safely in its fully open position.
What hatch opening force actually means
Gas strut force is rated in Newtons (N), not kilograms. The rating describes the force the strut pushes along its own shaft. That push must create enough turning force around the hinge to counter the weight of the hatch.
A hatch creates its greatest lifting demand when its centre of gravity is furthest horizontally from the hinge. For many top-hinged lids, this occurs when the lid is closed or near closed. As it opens, the centre of gravity moves closer to directly above the hinge and the torque from the hatch weight reduces.
The catch is that the gas strut’s leverage also changes through its travel. A strut mounted at a poor angle can have very little mechanical advantage when the hatch is shut, even if it is a high-force unit. That is why mounting geometry matters as much as the force rating.
The basic hatch opening force calculation
The calculation starts with torque, sometimes called moment. Torque is the turning force that acts around the hinge.
Hatch torque = hatch weight in Newtons x horizontal distance from hinge to centre of gravity
Weight must be converted from kilograms to Newtons:
Weight in Newtons = mass in kilograms x 9.81
For a practical estimate, multiplying by 10 is usually close enough at the early design stage. For final selection, use 9.81.
Once hatch torque is known, calculate the required force from each strut:
Force per strut = hatch torque ÷ number of struts ÷ effective strut moment arm
The effective strut moment arm is the shortest perpendicular distance between the hinge pivot and the strut’s line of force. It is not simply the distance from the hinge to the mounting bracket. This is where many calculations go wrong.
Worked example
Consider a 35 kg toolbox lid using two gas struts. The lid’s centre of gravity is 450 mm from the hinge when closed. At the closed position, the strut geometry provides an effective moment arm of 120 mm.
First, convert hatch mass to force:
35 kg x 9.81 = 343 N
Then calculate hatch torque:
343 N x 0.45 m = 154.35 Nm
Divide that load between two struts:
154.35 Nm ÷ 2 = 77.18 Nm per strut
Now divide by the effective moment arm:
77.18 Nm ÷ 0.12 m = 643 N per strut
This indicates a starting requirement of approximately 650 N per strut at that position. In practice, you would normally allow a suitable safety and performance margin, then verify that the selected strut does not make the lid excessively hard to close.
Add a sensible allowance, not an oversized strut
A calculation is the starting point, not the complete specification. Hatch weight can change with handles, locks, lining, spare wheels, insulation, glazing or accessories. Friction at hinges can also vary substantially, especially on equipment exposed to dust, salt or hard use.
A margin of around 10 to 30 per cent is commonly considered, depending on how accurately the hatch mass and geometry are known. A well-measured steel toolbox lid with sound hinges may need only a modest allowance. A large canopy door, marine hatch or machinery cover with variable fittings may justify more.
More force is not always better. Overpowered struts can make a hatch difficult to pull down, place excess load on hinges and brackets, or cause the lid to spring open abruptly. On a lightweight lid, that can become a safety issue rather than a convenience.
Measure the geometry at more than one position
A reliable design checks the hatch at closed, halfway and fully open positions. The critical point is often near closed, because the hatch has high gravitational torque while the strut may have limited leverage. However, some installations need close attention at full opening to make sure the strut still holds the hatch against wind, vibration or a slight slope.
For each position, record the hinge centre, the hatch-side strut mounting point and the body-side mounting point. Measure from the centre of each pivot or ball stud. A scale drawing, CAD model or full-size mock-up makes this far easier than trying to estimate angles by eye.
You need to confirm three things: the strut produces sufficient lift near the closed position, it does not bind or reach full extension before the hatch stop, and it holds the hatch at the required opening angle.
Finding the centre of gravity
For a plain, uniform lid, the centre of gravity is usually near its geometric centre. Real hatches are rarely uniform. A canopy door may have glass, a latch assembly and a framed lower edge. A caravan bed base may carry mattress weight. A machinery guard may include handles and protective panels.
The most dependable method is to support the hatch on a pivoting edge and balance it on a suitable support point while it is removed from the vehicle or equipment. The balance point indicates the centre of gravity. For larger or awkward assemblies, use a lifting arrangement and measure carefully, with appropriate safety controls.
If removal is impractical, work from a drawing and add the approximate weight and location of major components. It is less exact, so allow for that uncertainty in the final strut selection.
Check strut length and stroke before choosing force
Correct force will not compensate for the wrong strut length. Measure the distance between strut mounting centres with the hatch fully closed and fully open. The extended length of the strut must suit the open measurement, while the compressed length must fit the closed measurement with clearance for fittings and bodywork.
The required stroke is the difference between those two measurements. Avoid selecting a strut that bottoms out before the hatch reaches its closed position or fully extends before the hatch reaches its intended opening stop. Either condition can damage the strut, brackets, hinges or hatch structure.
Also check the physical space through the entire movement. A strut barrel can foul on shelves, weather seals, internal trim or framework even where the mounting centres look suitable on paper.
Account for the direction of travel and installation details
Gas struts are generally installed with the rod pointing down when the hatch is closed. This helps keep the rod seal lubricated and supports longer service life. There are exceptions where packaging or application requirements prevent this, but it should be the default arrangement where possible.
The mounting brackets must be attached to material capable of handling both the steady load and the repeated opening cycles. Thin sheet metal, fibreglass and lightweight aluminium panels may need reinforcement plates or purpose-designed brackets. A high-force strut fitted to weak mounting points can tear out long before the strut itself fails.
Temperature is another practical factor. Gas strut force rises in hot conditions and drops in cold conditions. In Australian conditions, a canopy hatch that is acceptable on a mild day may be noticeably stronger in summer heat. For applications with a wide operating temperature range, avoid designing too close to the minimum required force.
When the calculation needs specialist input
A straightforward rectangular lid can often be specified from accurate measurements. Complex applications deserve a closer review. This includes double-hinged or offset hatches, gullwing doors, very heavy lids, hatches with changing loads, power-assisted systems and installations where failure could expose people to injury.
The same applies where a hatch must open to a precise angle, clear overhead structures, remain stable in wind or meet site-specific safety requirements. In those cases, providing hatch mass, hinge-to-centre-of-gravity distance, closed and open mounting-centre measurements, opening angle and photos of the installation will give a strut specialist what they need to recommend a workable setup.
Gas Struts can assist with standard replacements and custom applications where the original force rating or mounting arrangement is unknown. Accurate measurements lead to a safer result, fewer fitting issues and a hatch that opens with controlled, dependable assistance.
