Does an 18 kg case really need two shoulders?

Yes. A one-shoulder case harness stops at 12 kg and puts everything on one collarbone, numb within 10 minutes. Two straps split it and the belt removes 30 %.

Can a sternum strap replace the belt?

No. Its force is 20 N to 40 N and it acts sideways, so it moves no vertical load, while the belt takes 30 % of it.

Are wider straps always more comfortable?

Up to 50 mm, yes. Past 60 mm the webbing bunches over the shoulder, so contact area stops growing and the edge rubs instead.

Can a single strap be swapped side to side?

Yes, and it should be. After 30 minutes on one side, move the strap across so both trapezius muscles take turns.

Is a 6 mm back panel worth having?

Above 15 kg it is. The moulded edge presses beside the spine, and 6 mm of EVA turns it into a 40 mm face.

Can a case harness be retrofitted?

Only where the shell has mounting points. Four reinforced lugs take a harness rated at 8 kN per point; without them you are drilling the shell.

Case Harness: Shoulder, Back, Sternum

Quick answer: A case harness is chosen on three numbers: weight, strap width and belt share. Above 18 kg use twin shoulder and belt; one shoulder stops at 12 kg.

A harness spreads 18 kg across support points
One shoulder: the load lands on one collarbone
Two shoulders and a belt: 30 % off here
The sternum strap carries nothing
Three numbers that set the sizes
FAQ

1. A harness spreads 18 kg across support points

Carry 18 kg on a strap and the shoulder does not feel 18 kg. It feels 18 kg divided by contact area, which is why widening a case harness strap from 25 mm to 50 mm halves the pressure at the same load.

Pressure is the easy half. The centre of gravity is the hard half. A case is a rigid box, and once it hangs on the back its mass sits 120 mm to 180 mm from the spine, generating a moment the body answers by leaning forward.

So the first question about a case harness is how close the load can sit to the back; the second is how wide the straps are.

One strap gives one support point. Two points spread the moment, and a third hands part of the load to the hips.

The moment can be estimated with two numbers. At 18 kg with the centre 150 mm from the back, the waist sees 26 N·m; a panel that pulls the centre 6 mm closer drops it to 25 N·m, and that single newton-metre decides whether the ache arrives at 20 minutes or 40.

2. One shoulder: the load lands on one collarbone

A single-strap case harness runs from the right shoulder to the left hip. The whole load passes through one point above the right shoulder, and at 18 kg that point sits on skin with bone underneath; after 10 minutes the fingers tingle.

Case harness test case fully open, three routed pockets in the lid, six empty EVA compartments in the base, both latches closed

The practical ceiling for one shoulder is 12 kg. Past that the shoulder rises on its own, the trapezius takes over from the collarbone, and the neck aches within 20 minutes.

What one shoulder does well is speed: on the body in under 2 seconds, which still suits inspection work with frequent opening. The condition is a case under 12 kg and a strap at least 38 mm wide.

3. Two shoulders and a belt: 30 % off here

Two straps split the load in half, spaced 380 mm apart so each lands on the middle of a collarbone. Below 340 mm the straps crowd the base of the neck; past 420 mm they slide off the shoulder and the wearer keeps hitching them up.

The belt is the third support point and the one most often skipped. Cinched on the iliac crest it pulls the weight down, so the straps carry only the remainder. Belt webbing runs 40 mm wide: under 30 mm it cuts into the crest, over 60 mm it jams against the hip when the wearer bends. Let the belt slip below the hip and its share falls from 30 % to under 10 %, which makes a twin-shoulder case harness pointless.

Configuration	Strap width mm	Case weight band	Load path
Single shoulder	38	Under 12 kg	One collarbone and trapezius
Twin shoulder	50	12–18 kg	Both collarbones
Twin shoulder with belt	50	18–25 kg	70 % shoulder, 30 % hip
Twin, belt and sternum	50	Above 25 kg	70 % shoulder, 30 % hip, closed across

4. The sternum strap carries nothing

The sternum strap does one job: it pulls the two shoulder straps together at the breastbone so they cannot slide outward while walking. It carries a lateral force of 20 N to 40 N, an order of magnitude below the 176 N that 18 kg produces.

Case harness sample half open, egg-crate foam up, eight EVA compartments, folding handle and metal side clips in frame

Its height is 180 mm below the shoulder line, which puts it across the middle of the sternum, where the closing lever is shortest and the strap does not cut into the armpit. Adjustment travel is 120 mm, enough for wearers from 155 cm to 190 cm; the right position is level with the elbow when the arm hangs relaxed.

Treating it as a load-bearing part is the usual mistake. Tighten it and both straps pull into a V, the inner edge digs into the trapezius, and pressure rises instead of falling. Inside a case harness the belt takes the weight and the sternum strap takes the stability.

5. Three numbers that set the sizes

Strap padding is 50 mm wide and 8 mm thick in EVA, with a 2 mm edge radius so it does not cut. Under 6 mm thick, pressure on the collarbone rises sharply; over 12 mm the padding stands off the shoulder and the contact area shrinks again.

The back panel of a case harness is 6 mm EVA, as wide as the 380 mm strap spacing. It separates the hard edge of the case from the spine and brings the centre of gravity 6 mm closer.

Set-up follows a fixed order: loosen everything, put the case on, tighten the belt, then the shoulder straps, then close the sternum buckle. Reverse it and the belt never gets tight, so the 30 % share is never collected.

Strap width mm	Thickness mm	Contact pressure kPa	Case weight
25	6	26	Under 8 kg
38	8	17	Under 12 kg
50	8	11	Above 18 kg

Case weight band	Configuration	Strap spacing mm	Belt share
Under 12 kg	Single shoulder	Not applicable	0
12–18 kg	Twin shoulder	380	0
18–25 kg	Twin shoulder with belt	380	30 %
Above 25 kg	Twin, belt, sternum	380	30 %

Conclusion

The difference between harness types is the number of support points, not the finish of the webbing. One point puts 18 kg on a single collarbone and caps the load at 12 kg; two points spread it across 380 mm; a belt adds the third and removes 30 %, so the shoulders carry 12.6 kg. The sternum strap stays a closure part and never joins the load path.