Breakthrough: The MacPherson strut

In this series, we usually can’t be 100 per cent sure where the Promethean spark of genius came from for this or that breakthrough, but in this case we have a pretty clear idea, thanks to MacPherson generously citing previous work by Fiat in the 1920s in the original US patent, filed by GM in 1947. He may also have been thinking of various other pre-WW2 cars which used sliding strut type arrangements, possibly even of the long-travel ‘oleo’ legs of aircraft landing gear.

In any case, MacPherson’s baby was not only Chapman-esque in its simplicity and elegance. It was cheap to manufacture. It allowed the use of more sophisticated springs and dampers than the basic leaf spring or torsion bar designs that dominated up to WW2. And crucially, it was space-efficient. Although it needed a fair bit of vertical space (in the z dimension, as automotive engineers would have it) to accommodate the stroke of the strut, it was particularly compact in the ‘wide’ and ‘long’ dimensions (x and y if one insists on being geeky, and you know that I will), allowing a sophisticated suspension system to be crammed right into the very ‘corner’ of the car. No more yard-long leaf springs or spidery links and bars running hither and yon.

So the Chevy Cadet was the first car to use the MacPherson strut? Sadly, no; it all got a bit messy. GM actually cancelled the Cadet project in 1947, making it one of those great might-have-beens of automotive history – what if the 1950s had been an era of compact, light and cheap American cars rather than the decade that gave rise to 20-foot long bechromed and befinned land yachts? The disgruntled MacPherson crossed the floor to work for the arch-enemy at the Blue Oval. So the honour of being the first car with MacPherson strut suspension went to Ford, with its 1950 Zephyr and Consul models – big hits for Ford of Europe right through the ’50s and ’60s.

But the car that really lit the fuse on the MacPherson strut was, appropriately given the Latin roots of the idea, the 1971 Fiat 127. Post-WW2, European car design went a very different route to the more-is-better ethos of the US industry. Europe was devastated by high explosives, money and raw materials were scarce, and the Old World’s ancient, crowded cities, narrow streets and relatively short inter-city distances required small, cheap cars rather than V8 continent-crossers.

The challenge of getting the most people and luggage into the smallest possible metal box gradually led designers and engineers to converge on the incredibly efficient ‘supermini’ layout – a transverse engine bolted directly to a gearbox, uneven-length driveshafts turning the front wheels, the whole job lot suspended by a pair of MacPherson struts.

Previous designs like the Mini and the Fiat 128 had come up with some of these elements, but the 127 was the first car to put one small box up front for all of the oily bits, pulling along a larger box containing the humans and their chattels. To the present day, this remains probably the most efficient way of fitting five people with all their stuff into a box that uses the absolute minimum of raw materials, and move the whole lot down the road in reasonable comfort and safety.

The compactness of the MacPherson strut was an important enabler for this. Because it takes up so little space ‘across’ the car and can be packaged just inboard of the front wheels, leaving enough space for a simple end-on motor-and-gearbox unit, it is one of the keys that allowed the supermini layout to unlock the doors to the automotive kingdom.

So, is it any good? Well, if we judge success by what people are prepared to buy, then it’s a resounding ‘yes’. I can’t come up with solid figures, but given that there are about 1.5 billion cars on the planet right now, I would fairly confidently guess that there are hundreds of millions of MacPherson struts out there – so by sheer quantity, Captain MacP’s suspension was, and is, a smash hit.

But his strut has its limits. Because the top mount of the strut is bolted to the body, and is in effect a simple rigid bar, it can make it difficult to control the suspension geometry. Specifically, a strut makes it tough for chassis engineers to control camber angle changes throughout the wheel travel. This is the reason why so many sports cars choose other layouts – typically double wishbone or ‘multilink’ systems – to allow them more precise camber control, in particular. Witness the hullabaloo when Porsche moved from a strut to a double wishbone front suspension on the 911 on the 2021 GT3…oceans of journalistic ink have been shed on how this has seemingly transformed the old warhorse’s dynamics.

Personally, I would push back on this, just a little bit. I’d put the MacPherson strut in the same bucket as the twist beam rear-axle – the bucket marked ‘much maligned’. Purists might claim that any driver’s car worth its salt needs to have ‘proper’ wishbone or multilink systems front and rear. On paper, maybe so. In reality, a well set-up MacPherson strut – like a well-set up twist beam – can do wonders. If you doubt this, please try to blag a drive in any pre-2017 Renault Mégane RS – and tell me if you wish for anything other than its humble struts up front and twisty beam out back. But you won’t be able to – you’ll be grinning too much.

So, is the MacPherson strut destined to continue to dominate the mass-market, and hence carry the good Captain’s name into automotive eternity? Maybe not. It has one other weakness, which might well sound its death knell. We mentioned that it’s compact in x and y. But we also hinted that it’s not particularly so in the up and down z axis. Almost by definition, the strut comprises a long tube, that in turn needs to sit in a strut tower or ‘turret’, often sitting proud of the top of the wheel.

This presents a hard problem for one specific and increasingly important issue – pedestrian impact. If you are unluckily enough to be hit by a car with a MacPherson strut front end (Gods forfend), your body will likely crumple in such a way that your hip, torso or head may strike the bonnet roughly where the solid steel lump of the strut tower top sits. Think about getting smacked in the skull by a lump hammer – no fun at all.

It’s inevitable that VRU or Vulnerable Road User (pedestrians and cyclists to you and me) protection requirements are likely to become more stringent. Even the US, last bastion market with few standards to protect VRUs, is now starting to consider crash-test standards aimed at reducing the almost 8000 road deaths last year involving folks outside the car – a number which is, shockingly, increasing rather than decreasing. So I will climb out on a thin and shaky prediction limb here, to suggest that the MacPherson strut may well start to be replaced by other, more vertically-compact front suspension designs. Ironically, I’m even seeing some of the companies I work with looking at solutions involving leaf springs and torsion bars: systems much older than the MacPherson strut.

Progress? Regress? Just the circle of automotive life? Time will tell…