Engineering Notes

How Many Newtons Does Your Lid Need? — F1 Force Calculation

12 July 2026 · COMSTAN Engineering

"How many Newtons does this lid need?" — it is the question we get asked most, and the answer comes from moments, not from a scale. What holds a lid open is not the gas spring's force itself but the moment that force generates about the hinge axis. The same 400 N gas spring can work perfectly on one lid and fail to lift another at all, depending entirely on where it is mounted.

The formula

The practical calculation is set up as follows:

F1 = (m × 9.81 × L_G) / (n × L₁) × R

m — lid mass (kg). The real mass, including glazing, cladding and lock hardware.
L_G — horizontal distance from the hinge axis to the lid's centre of gravity (mm).
L₁ — perpendicular distance from the hinge axis to the gas spring's line of action, i.e. the moment arm (mm).
n — number of gas springs (2 on most lids).
R — safety/reserve factor: 1.2 – 1.3. This margin is what makes the lid "sit" firmly in the open position.

Worked example

Take a 15 kg engine bonnet: centre of gravity 400 mm from the hinge, gas spring moment arm 120 mm, two gas springs to be fitted:

F1 = (15 × 9.81 × 400) / (2 × 120) × 1.25 ≈ 306 N

So you would select ~300 N per gas spring. With this force and the stroke value, our configurator suggests the size group itself — in this example the typical result is the 8/18 group.

The four most common mistakes in the field

"When I pushed it by hand, it felt about this much." A hand pushes at the end of the moment arm; the gas spring works on a much shorter arm. The two are not the same thing.
Assuming the centre of gravity is the middle of the lid. On lids with glazing, locks or spoilers, the centre of gravity is not at the geometric centre.
Replacing only one gas spring. In a twin system the load is shared equally; with one worn spring and one new one, the lid twists and the seals take side load. Always replace them as a pair.
Picking the mounting point by "whichever hole lines up". Shortening the moment arm by 20 mm increases the required force by 15–20%. If the mounting point changes, the calculation must be redone.

Shortcut: Our configurator does not do this calculation for you — but if you don't know the force, include the lid weight, the hinge-to-centre-of-gravity distance and the mounting points in your quote request; we will calculate F1 for you and send it together with a technical approval drawing.

Once you know your force, the rest is easy: enter the stroke and length, and let the configurator generate the size group and part code.

Open the Configurator →