How to Choose a Gas Strut: Force, Length and End Fittings

Choosing a gas strut is a matching exercise, not a catalogue pick. Four things have to line up — the force it makes, how long it is extended, how short it is compressed, and how it attaches at each end — and all four are set by what you’re lifting and where you can mount it. Get any one of them wrong and the lid will still fight you.

If your geometry is already sorted and you just need the numbers, the gas strut measuring and sizing guide covers extended length, compressed length, stroke and end fittings. If you’re still working out how much force the lid needs, start with how to calculate the correct force in Newtons.

How a gas strut works

A gas strut is a sealed cylinder of pressurised nitrogen with a piston rod through the end. The gas pressure pushes the rod out, and that push is what balances the weight of your lid, door or hatch.

Two things follow from that. First, the force is rated in Newtons (N) and it belongs to the strut — it doesn’t change to suit your application, so the strut has to be chosen to match the load. Second, force isn’t constant: it varies with temperature, which is why a strut that feels right in a cold workshop can feel stronger in a hot car park, and it falls away over the years as the seal slowly passes gas. That ageing is normal. A strut that suddenly loses most of its force has usually failed rather than worn out.

The four things to settle before ordering

• Force, in Newtons. Set by the lid’s weight and how far the strut sits from the hinge.
• Extended length — the strut at full travel, with the lid open.
• Compressed length, which gives you the stroke and tells you whether the strut will fit with the lid shut.
• End fittings — ball socket, clevis, eyelet or threaded stud, and the size each one takes.

You can’t choose a strut from force alone, and you can’t choose one from length alone. Both fall out of the same set of measurements.

Mounting position changes the force you need

This is the part that catches people out. The further the strut’s mounting point sits from the hinge, the more leverage it has on the lid and the less force it needs to hold the same weight. Move that mounting point closer to the hinge and the same lid needs a stronger strut.

So a strut that was correct on one toolbox can be wrong on a slightly different one, even at the same lid weight. Decide the mounting positions first, then work out the force. Working backwards from a strut you already happen to have rarely lands in the right place.

Where a lid is heavy or wide, two struts are usually a better answer than one larger strut: the load arrives at two points instead of one, the lid is less likely to twist, and each mounting area carries less force.

Matching the ends

Both ends have to match the hardware already on the lid and the frame, or the brackets you plan to fit. Ball sockets are sized by the ball they clip onto; clevis ends by the pin and the width of the fork; threaded ends by the thread size and pitch. If you’re buying struts and brackets together, specify both ends at the same time.

Materials and corrosion

Struts generally come with a painted or stainless body and a plated or stainless rod, and which suits depends on where the strut lives. Sheltered workshop cabinets and inland underbody toolboxes are generally well served by a coated strut. Coastal canopies, boat trailers, marine boxes and anything that sees regular high-pressure washing are better off stainless throughout — and stainless brackets and fasteners with it, because a mixed-metal joint is where corrosion starts.

Worth checking too: dust and grit on the rod. The seal wipes the rod every time the strut moves, so abrasive dust on the shaft wears the seal. Where a strut is exposed to a lot of dust — a ute canopy on gravel roads, a camper trailer — a boot or gaiter is worth considering.

Common applications

• Toolbox and underbody lids: usually a single strut with short travel, with load carried into a framed panel or rib.
• Canopy side windows and rear doors: light panels, so force is modest and end-fitting size matters more than the rating.
• Caravan and camper trailer hatches: often two struts for a wide lid, and usually stainless given the environment.
• Camper trailer beds and fold-outs: long travel, so confirm the compressed length fits the closed position.
• Fabricated steel hatches and machine guards: weld-on brackets let you place the mounting points exactly where the geometry wants them.

Checking a strut that’s past its best

• Lid won’t stay up: force has dropped, or the strut was under-rated for the geometry from the start.
• Lid slams down: seal failure or a strut at the end of its life — replace it rather than propping the lid.
• Lid rises too fast: strut over-rated for the lid, or the mounting point sits too far from the hinge.
• Uneven lift on a twin-strut lid: one strut has lost more force than the other. Replace them in pairs so the lid stays balanced and both mounting areas take equal load.

Before you order

Have these ready and the order is straightforward:

• Lid weight, or the material and panel size it’s made from
• Distance from the hinge to each mounting point, lid end and body end
• The extended and compressed length the mounting position allows
• End fitting type and size at each end
• One strut or two
• A material suited to where the strut is mounted

Trailer Parts Direct supplies gas struts and matching brackets and end fittings for toolbox, canopy, caravan, camper trailer and fabrication applications. Send through your measurements and a photo of the mounting area and we’ll confirm the specification before you drill anything.