BMW Drivers Club Melbourne

Tech Talk | MacPherson Struts

14 Oct 2020 2:17 PM | Anonymous

From the Neue Klasse cars in 1961 up to the 2010 F10 5 Series most BMWs used the industry dominating MacPherson strut front suspension. The F10 and subsequent models changed to the technically superior but bulkier and costlier double wishbone design.

This article explores why the MacPherson strut was so popular.

The MacPherson strut was invented by Earl MacPherson in the 1940s when independent front suspension was still something of a wonder and too expensive for all but the upper luxury or exotic sports car market.

I should point out that what we call “suspension” is actually “superimposion”; the body of the car is not suspended but supported.

The origins of the term come from the days of horse and carriage. Initially the carriage body was bolted directly to a frame underneath which two axles were bolted, also directly. Add in wooden wheels with metal tyres and a very bumpy ride results.

To improve this a flexible vertical post was fitted at each corner of the frame with two leather straps slung across the width of the frame (ie transverse) from the top of each post, one at the front and one at the back.  The body of the carriage was then attached to these straps.  That is, the body was suspended by the straps. As the posts could bend the body could now move independently of the frame, up to a point.

It didn’t take long for some genius to realise that putting the springy bit between the frame and the axles worked much better with the leather strap replaced by a metal “leaf” spring of similar shape. So the transverse leaf spring “suspension” came into being.

The axles were still rigid and the wheels still wooden.

With the advent of amazing horseless carriage and it’s death defying speeds of over 30 mph (typical horse speed beyond which one should not travel for fear of being unable to breath – or so they thought) the limitations of rigid axles and leaf springs became apparent.

Although leaf springs provide some natural damping this decreases as the springs get softer and more comfortable.

The consequence of having insufficient damping is that when the wheel goes over a bump or a hole it continues to bounce up and down for quite some time afterwards. Which is not good, especially if the wheel is bouncing around just at a time when you wanted it to grip the road.

Additional springing was introduced when pneumatic tyres were adopted but this had next to no damping making that issue worse.

So, dampers were introduced to control the wheel movements dynamically.  By about 1927 dampers were used which could provide a spring like function on the initial movement from rest, thus allowing a softer spring without allowing the wheel to crash up against the bump stops upon hitting a bump at speed. Which is why dampers are often called “shock absorbers”, or shocks for short.

In the modern designs the dampers do a lot to control wheel movements and to prevent body roll, making them a very important part of the handling role of the suspension as well as the ride.

So, we need a spring and a damper. And something to join the wheel to the body, which we do not want to be a rigid axle because with a rigid axle if one wheel goes over a bump that wheel changes its angle to the road and the other wheel has to move around too. Plus the body has to be above the axle (generally) making the floor high.

To a lot of people’s amazement, we do not want the wheels upright, and we do not want them at the same angle to ground all the time.

The wheel (front wheels in particular) needs to lean in slightly toward the centre of the car at the top (negative camber) when travelling in a straight line and sightly more so on the outside wheel when cornering. But, as the wheel goes up and down, such as over a bump, the angle of the wheel with respect to the road should not change much, if possible.

So, if the links between the body and the wheel are correctly designed the wheel angles can be optimised for cornering or going over bumps and, if you are really clever, doing both at the same time and on both sides of the car.

The double wishbone suspension is generally considered the perfect choice.  Except with a narrowish front engine car in the ‘40s the engine prevents the wishbones being long enough for an ideal design. And, it is expensive.

In a double wishbone design there are two V shaped “wishbones” (as per a turkey or chicken wishbone) one above the wheel centre line and one below, attaching the wheel to the car body.  Thus there are 6 mounting points which need bracing, rubber mounts, bolts and assembly time. There must be a steering pivot (or upright) attaching the wishbones to each other and to the hub about which the wheel spins. We also need a spring (coil now not leaf - very modern) which needs 2 mounting points which need bracing, rubbers, bolts and assembly, and a damper which also needs 2 mounting points with bracing, rubbers, bolts and assembly.

So, lots of parts and cost and space. And possibly sub-optimal performance.

Earl MacPherson was given the task of coming up with a lightweight low cost car at General Motors.  In fact, the target was more or less impossible, which is just as well otherwise some brilliant solutions may have never been developed.  MacPherson put a tubular damper inside the coil spring, instantly removing 2 mounting points and freeing up space.  He then made the damper body into a strong strut capable of holding the spring, taking the suspension loads and extending the lower end of the strut so that it reached below wheel centre, thus taking on the role of the upright – more parts and connections eliminated.  This strut leans in toward the centre of the car with the top attached to the car body at the top of the inner wheel arch just below the bonnet. When viewed from behind it makes one side of a triangular structure with the car body forming the second, mostly vertical side, and a vaguely horizontal member running from below the wheel centre toward the centre of the car making the third.

The wheels rotate about the strut for steering and the geometry is fixed by the 2 mounting points.  The bottom member (the track control arm) is a relatively long arm going under the side of the engine which reduces wheel angle changes over bumps etc.  To stop the whole thing moving forwards/backwards the anti-roll bar (which the double wishbone design also has) is attached to the track control arm and used as a brace.

Thus we have two body mounting points with attendant bracing and bolts and rubbers and one hub attachment point plus a unitary upright/spring/damper strut unit.

Brilliant.  Hats off to Earl.

Except the Chevrolet Cadet that all this was meant to appear on never made it as, amazingly, GM top brass said is was too expensive.  A more likely reason is that, at that time, only top of the range cars had independent suspension and it was unthinkable for a budget priced car to have a better suspension.

MacPherson left GM, walked across town to Ford and his invention surfaced on the UK built Ford Consul and Zephyr, quickly spreading across the entire Ford range.

Sadly, because of the Ford connection some snobs initially refused to recognise the brilliance of this design and refused to use a MacPherson strut design despite reasonable licensing deals.

Although the MacPherson strut has some geometric shortcomings a half decent MacPherson strut installation beats an 80% good double wishbone setup. By the 1970s just about everyone was using a variation on the theme at the front with many different designs for the rear suspension, from “live” axles through double wishbones to the dreaded swing axles and compact low cost torsion bar beams. 

Lotus’ Colin Chapman (of F1 fame) re-invented MacPherson’s rear strut design – which had not been a success – becoming the Chapman strut for obvious reason. This was probably the best rear axle strut design there has been, but it too was not a success with a nasty habit of breaking welded joints. The semi-trailing arm design rear suspension, such as used on BMWs since the 1957 600 model (whose suspension design, with mild evolution continued right up until it was replaced by the similar-but-different Z axle in the 1990’s), has proved to be superior overall.

The gradual switch to double wishbone front suspension has occurred for two main reasons. The first is that cars are a lot wider and engines narrower, making room for wishbones. The second is that automation and other production efficiencies have reduced the cost significantly.

On a final note, BMW used the equally innovative and even more compact motor scooter inspired Dubonnet type front suspension on the Isetta, 600 and 700 models. This design has more technical flaws than the MacPherson strut and is less suited to heavier wheel loads but was an ideal low cost solution on those vehicles.

Lawrence Glynn | Member #3
BMW Drivers Club Melbourne

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