The story behind MaDE’s RightWeight Book on mass density in design


The practice of lightweighting combines materials and design in an intimate way: you use a lighter material, or less of the same material through clever design, or maybe combine both strategies.

But on closer examination it’s just not that simple. Designing in lightness can save energy and employ less material – which is good – but what about safety and robustness? Will the superlight new café tables blow away in the wind and create traffic mayhem? We can use fabulously light materials for a product – carbon composites are an obvious example – but can our consumer customers afford it? How big a part of the pleasure of drinking Champagne comes from the weight of that impressive bottle?

If the task is to design an electric car for the city, we must make it as light as possible consistent with the kinds of uncomfortably conflicting criteria the above examples illustrate: it needs to go a decent distance on a battery charge, be safe in a collision and people have to like it enough to buy one. The vehicle weight we end up with is probably not the lightest it could be, but the lightest it should be – the right weight.

Which is how the project came to be called RightWeight. On 15 November 2010 a group of designers and engineers from influential international brands, along with academics and others – about 60 participants in total – met at the Institute of Materials, Minerals and Mining (IOM3) in London to learn more about weight and design, do hands-on evaluations and experiments with real objects, and discuss their own experiences and thoughts. The group’s consensus view was that weight is a poorly understood dimension in product design and work needs to be done to correct this. All agreed that many products in future should be lighter, more efficient and use less material, but that consumers continue to associate mass with quality, strength and safety. It’s a challenge to designers to find new ways to make lightness practical and desirable – and not just in obvious places like bicycles and luggage.

More questions than answers, so RightWeight2 was held on 28 February 2011 to find out more about the ways people relate to the weight of things, about the culture and psychology of weight. This time our speakers included two experimental psychologists, an anthropologist and a design historian – all leaders in their fields – and our delegate audience, numbering many of the same influential designers and engineers as before, plus some new faces, took part in the re-staging of classic experiments in weight perception.

Dr Bernie Rickinson launching the MaDE RightWeight Book at the Institute of Materials, Minerals & Mining in 2011


The RightWeight Book

The RightWeight book, with its intriguing ‘Z’ structure, contains narrative reports on the two events plus additional articles on the design and science that relate to products and their weight. Here is a short extract :

Apparent density

If I wanted to measure the density of a silver teaspoon, it would be quite easy. I would find out its volume by seeing how much water it displaced in a measuring jug, then I would weigh it and divide the weight in grammes by the volume in cubic centimetres to get the density in g/cm3. Of course I could just look up the density of silver – 10.5 g/cm3. The spoon is made from just one material and it is in one solid shape with no internal cavities, so the density of silver is also the density of my spoon.

Suppose now that I wanted to know the density of a chef’s knife with a steel blade riveted to a plastic handle (don’t worry for now about why I should want to do this)? I could find its volume by displacement of water just as before, weigh it and calculate the density; let’s say the figure is 4.5 g/cm3. This of course is the knife’s average density, which corresponds to that of none of its constituent materials; in fact it turns out to be the density of titanium. So a knife of identical shape and size but made from one solid piece of titanium would weigh the same.

It occurs to me now to find the density of my iPhone, but wait – what does that mean? The iPhone is certainly made from many different materials – like the knife only much more so – but unlike the knife it’s not solid, it has spaces and gaps inside and holes in the case for plugs, the speaker and so on. Yet it would still be interesting to know the object’s density, as if it were a solid thing. I can weigh it easily, and then cover it in cling film and do the volume by displacement, or measure it and do the arithmetic.

The result is a density of 2.0 g/cm3 and I’m going to call this the phone’s ‘apparent density’ – a term used to describe the ‘bulk’ densities of granular materials such as metal powders, where the density is less than the equivalent volume of solid material due to the air gaps between grains. But apparent density works well when used to describe the density of an object based on its weight divided by its perceived or apparent volume.

The iPhone’s apparent density is the same as clay brick. You could call this material, the brick, a ‘density benchmark’ for the phone. When the apparent density of the super-light carbon fibre bicycle was calculated by one of the groups in the Rightweight event, it turned out to be equivalent to that of a bar of soap: if the Specialized S-Works Tarmac bike was made from solid soap, with no internal, invisible cavities of course, it would weigh the same.