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Measuring Vehicle Luggage Space – A Process a Little Like Tetris

The volume of a car trunk (boot) is an important purchase criterion. According to common DIN, ISO and SAE industrial standards, it is determined by filling up the trunk with special boxes. In addition to using the trunk of a real car, this process is more and more performed with virtual models in a CAD system. Both practices require time-consuming manual work.

Luggage compartment space is still measured manually with the help of standardised blocks according to Skoda. Technicians arrange in the luggage compartment standardised 1-litre blocks that measure 20 × 10 × 5 centimetres. Arranging the blocks is governed by a technical standard (ISO3832). Although this may sound pretty simple, the process has its pitfalls. “What’s important is that the volume we arrive at with the blocks and then state in the vehicle’s technical specifications must be repeatable at any time,” says Ladislav Kraus, who is responsible for luggage compartment development, explaining the fundamental principle. Theoretically, then, you should be able to perform the process yourself and come up with the same figure as the technicians.

Standards For Determining Luggage Compartment Volume

In real-life measurement, the technicians’ experience usually enables them to arrive at a better result than the computer, with the average improvement being around 5 litres.

The declaration of the volume of a vehicle trunk in a catalogue is done in litres, according to standards ISO 3832 and DIN 70020. The sole measuring equipment permitted is a one-litre box (cuboid) with spatial dimensions 200×100×50 mm. The maximum number of named boxes that can be placed inside the luggage compartment without deformation specifies its volume in litres. This defines a comprehensible
and reproducible method to determine the usable cargo space of a passenger car and facilitates comparing different automakers. This measurement method was originally published by the German Association of the Automotive Industry (VDA). It was later incorporated into the international ISO 3832 standard and is part of the German standard DIN 70020, which generally specifies the dimensions and weights of a vehicle.

The US-American standard SAE J1100 also specifies packing with boxes. However, the boxes used shall represent differently sized suitcases, They range from suitcases with 67 l of volume to small shoeboxes with just under 6 l. Additionally, there is a unit module, styled in the form of a golf bag, which can also be packed. The SAE standard also defines exactly how many boxes of which type may be used and in which order they are to be packed.

“But you’d have to comply with the given rules, just like ŠKODA technicians do. For example, you’re not allowed to force the blocks in, though there is some leeway: after all, carpets and upholstery allow for a little give. The rules also dictate how far the blocks can go, depending on the version of the technical standard – you can measure the volume up to the edge of the rear seat backrests or up to the parcel shelf, which may result in large differences in different models. But the blocks must on no account raise the height of the parcel shelf” says Kraus.

Luggage compartment volume is part of the brief for car development: obviously, the developers don’t simply leave the resulting boot space to chance. The luggage compartment has to satisfy more than just practicality requirements: structural resilience is also crucial. The compartment is therefore meticulously prepared, including the strength of its components. “The initial design work is done on a computer, including a simulation of the volume measurement. The programme can do this automatically using standardised blocks – virtual ones, this time – or the computer operator can do it,” Kraus explains.

Of course, the technicians are hugely experienced in this measurement process and know how to make the most of every suitable space. As a result, they have so far always managed to arrive at a slightly better volume result than computer simulations. This ‘fight’ for extra litres in the boot begins before they start arranging the blocks in the boot, though.

Paradoxically, though, this is a tough job for a computer. “It can easily take a whole weekend to calculate. That’s because it looks for the best possible arrangement. If a block doesn’t fit, the programme tries to work out how far it has to backtrack to make optimal use of the space,” adds Peter Hancko, one of the luggage compartment constructors at ŠKODA. Arranging blocks in a 3D modelling programme will take someone about one working day. Arranging physical blocks is a question of a few hours, depending on the size of the compartment. “In real-life measurement, the technicians’ experience usually enables them to arrive at a better result than the computer, with the average improvement being around 5 litres,” says Hancko.

Fair Measurement Value

The technicians are genuinely capitalising on their experience, not using ‘tricks’, even though the technical standard does permit some finessing. “It is permitted to remove everything from the luggage compartment that isn’t bolted down, for example. But we don’t go that far – we want the volume to be realistic and consistent with the actual usable space,” Kraus points out. Even so, the space beneath the luggage compartment floor is included in the measurement, and when the measurement is being done with the rear seats folded down the space beneath the folded rests is counted. The technical standard allows this provided that this space can be reached after the doors are opened – it is defined as an additional luggage space.

Measuring with the seats folded down is a relatively complicated task. The front seats need to be put in the right position as well, for example. “Dummies are used to put the seat in a position defined for an average person by the technical standard. We then place a polycarbonate partition behind the seat to firmly delimit the space we can arrange the blocks in. This stops the blocks from slipping between the seats,” Hancko explains. Various other storage spaces located behind the car’s front seats are also included in the total volume with folded down seats: these include compartments in the doors, by the mudguards and so on.

Box stack according to ISO 3832 computed with RMAGS TrunkPacker software. (© Daimler)

That’s because the technical standard dictates that the biggest measured luggage compartment volume is stated, and various extra features can naturally reduce this volume.

With TrunkPacker software from RMAGS the trunk volume of a car can be computed fully automatically. The average number of man hours necessary to determine the trunk volumes of all model series at Daimler has been reduced by 70% with the TrunkPacker software. This enormous time saving is possible because the software integrates seamlessly into the workflow of volume determination: One can directly transfer the trunk geometry from the CAD system to the TrunkPacker, do the calculations and export the packings produced back to the CAD system. Also volume determinations can now be carried out much more frequently, which enables a better influence on the product development and finally an optimal trunk design.