A pound of flesh

The ‘why’ is easy. Chief vehicle engineers are constantly called upon to decide between various technical solutions – in fact, I’d say this is their main role. It’s up to them whether they will make that front wing in good old steel, or go for a lighter injected plastic to save a kilo or so. Will they choose a steel bonnet, or cough up for the aluminium one and save maybe 2.5kg? Simple cast-iron engine mounts, or pony up for more sophisticated structural-composite ones? Even tyres – go for the Michelin offer that is 700g lighter per corner? Or buy from their competition, even though their product is heavier? (You’d be amazed how much variation there is between different tyre makers’ offerings in the same nominal sizes…).

How to decide which of these competing options to go for? Or put it another way – let’s say you have a mass-reduction budget of £500 per car. Where to put your money? Where will you get the most bang-per-buck, or rather kilo-per-quid?

To find the answer you have to have a guideline, a £-per-kilo go/no-go figure on which you will base your decisions. This is the guideline that will help filter the almost-infinite range of options available. Without that, your chief engineer is flying blind, making decisions by gut feeling alone. Not a great idea, when folks have trusted you with large amounts of money and with thousands of people’s livelihoods.

Now things get tricky. How do you put a monetary value on losing a kilo? Here’s the short version – car makers try to figure out the value to the end customer of lopping mass off the car, and then convert that into how much they can spend in terms of cost. To be clear, cost and value are two very different things. Let’s say you tick an option that costs you, the end customer, £1000. Some fancier seats, perhaps. This does not mean that the car maker simply passes that £1000 on to the seat supplier to go buy some nicer fabrics.  As a very (very) rough rule of thumb, that supplier may see £500 – the rest needs to be reserved for engineering costs, logistics, assembly costs, insurance etc – and, yes, a profit margin if the car manufacturer is interested in staying in business So, very roughly, the value to cost ratio is 2:1.

So, what is the value of reducing the mass of a car? First and most obviously, there is less to pay for raw material. Reduce 10kg of steel and, well, you just saved about £1.50. Put it in the bank. Next, acceleration. Isaac Newton was right: F really does equal ma: lighter cars will accelerate faster, and car companies can put a value on that – they know customers will pay a little more for cars that have more impressive 0-60mph and in-gear acceleration figures.

Fuel consumption comes next. There is a fairly direct and predictable relation between mass and mpg, and again, clear data on how much extra customers will pay for more fuel-efficient cars. Finally, CO2 figures. Thanks to the various governmental CO2 bonus/malus schemes, there is a very mechanical, quantifiable relation between reduced mass, reduced grams of CO2/mile figures, and hence savings for the end customer in reduced taxation. This final contribution is mathematically significant these days – and we’ll come back to it later.

Sadly, however, it’s notoriously difficult to put a monetary value on what many of us at Ti value most – ‘feel’. As noted above, it’s pretty easy to figure out how much extra a customer will pay for a 0.1 second improvement to a 0-60mph sprint time. It’s extremely hard to figure out what they will pay for ‘agility’ or ‘fun’ or ‘playfulness’, or any other of those inadequate subjective words with which we struggle to describe cars like Caterhams, Elises or…dare I do a Prosser and mention the Alpine A110?

In any case, car makers still do their level best to quantify these things, feed all these data into sophisticated mathematical models and spit out mass reduction guideline numbers. And these numbers might surprise you.

I’m going to stick my neck out here by quoting some figures, or rather some ranges of figures, but first, some disclaimers. We are discussing mass-produced vehicles here – not products of low-volume marques like Morgan or Koenigsegg. I’m also deliberately keeping the ranges pretty wide because I don’t want to betray any trade secrets. But most car makers will estimate the permissible cost of a 1kg reduction at somewhere between £5 and £10. Only.

Yes, the numbers really are as low as that. And the greatest contribution to that is in saved CO2 penalties. In other words, if saving a kilo costs less than £5, a chief engineer will just do it – automatically. Above £10/kg is a no-go zone: the proposal is rejected. Between £5 and £10, the proposal will be seriously studied and assessed against a myriad of other factors to consider. Things improve a little as we look at more ‘sporting’ cars – the equivalent numbers might go as high as £15 to £30/kg.

Now, let me end – uncharacteristically – on a slightly depressing note: EVs. I already mentioned that the biggest mathematical value-driver is in reducing CO2 penalties. Of course, with electric cars, tailpipe CO2 emissions are already zero. So, you can’t reduce them anymore – the ‘value per kilo’ calculation becomes ‘divide by zero’, i.e. infinite.

Things get worse. The ‘fuel consumption’ of an EV is equivalent to its energy efficiency, measured in kWh/miles. Surely reducing mass will help here? Nope. Not as much as you think. Because modern EVs all have very efficient regenerative braking systems, so much of the energy lost in accelerating the vehicle (or propelling it up a hill) is recovered in the braking phase (or as it coasts back down the hill). So, infuriatingly, it is actually less cost-effective to save mass on an EV than it is on a small ICE vehicle with no regen braking.

Hence, when we now spin our number-crunching machine, it spits out a depressingly low number for how much our well-intentioned chief engineer can spend on mass-reducing his or her EV: numbers as low as £2/kilo. Let me tell you, that eliminates a hell of a lot of mass-reduction possibilities. This partly explains why modern EVs are so portly – the economics of putting them on a diet simply don’t work out.

Have I depressed you enough? Could be worse…you could be Antonio in the ‘Merchant of Venice’.