Ronin of the Spirit

Because reality is beautiful.

Suspension Primer II

So people seem to believe that the chronology of mass produced suspension designs relates directly to suspension quality.  The usual theory goes, beam axle, swing axle, McPherson, wishbone.   First let’s look at the proviso “mass produced”.  Define mass produced.  Is it 100 units?  100,000 units?  Beam axles were the first mass produced in the sense that they were produced en mass since around 1000 BC.   But you can’t just say they were the first mass produced in cars just because the Model T had them.  The Model T began production in 1908.  By that time, Decauville’s indpendent front suspension patents had already expired, and they had been making a car with independent front suspension for a decade, beginning in 1898.

Further, equal length wishbones are usually seen as a post McPherson strut development.  Actually, they were proposed in a 1934 technical paper by Maurice Olley.  The McPherson strut was not invented until ’49, but mid 1920’s technical papers by FIAT would show that they invented the concept then. So here to set you straight is the Israel Walker “real story” of suspension history.

Remember, there are four parts to suspension: axle, linkage, springs, and shocks, and you need all four. A brilliant axle with poor linkages, crappy springs, and lousy shocks will be crappy suspension.  Let’s look at Model T as a baseline.

First off, there is nothing wrong with a Model T’s suspension.  The roads the T was made to run on were crap.  So the T had to have tall wheels.  The tires available were skiny so it had to have narrow wheels.  If you have tall skinny wheels clawing over rough terrain you need a huge amount of articulation.  The motors available are heavy and weak, so the car (and suspension) has to be light.  A modern engineer, given those requirements would give you the exact same suspension Henry did.

Beam axles get a bad rap.  This site has a pretty standard Pros/Cons list for beam axles.  They’re simple, and strong, with good camber control, but have high unsprung weight, gyro stability issues (the fact they have two rotating masses on a stick causes weird vibrations) bump steer (up and down make the wheels turn left and right), take up to much space, and poor road holding.

This is where critical thinking comes in.  Simple compared to what? Strong in reference to what? High unsprung weight compared to what? Gyro stabilizing issues compared to what? Large compared to what? And some research about bump steer and road holding.

The problem here is that they there is more difference between Model T’s beam axle suspension and a modern high end beam suspension than there is between a kitten and tiger.  Strong in reference to what?  Weight.  They carry more load with less component weight than any other system.  This is why semis have them. And if they are the strongest referenced to weight, that means that they are one of the lightest systems, not the heaviest.  Yes, they do have gyrostability issues if they have weak or poorly designed linkages.  ALL suspensions have stability problems if they have weak or poorly designed linkages.  The bump steer is fixable.  Road holding is fantastic, if properly designs.  As is the case with all suspension designs, road holding is poor if the overall execution is poor.  Finally, the “too big” one is a plain lie.  Most minivans use beam axle suspension in the rear precisely because of how little space it takes up, allowing more cargo space in the back of the vehicle.

I think what they meant, was it takes up the wrong kind of space.  Beam axles must run in a line from wheel to wheel, meaning that you can’t put, say ,an engine, in the space between them.  The original reason that GM went to independent front suspension in the 30’s was to mount the engine between the wheels instead of behind them.  It was a stylistic and not an engineering decision.  This was reflected in the fact that early GM IFS ate the heck out of tires.

We must compare apples to apples.  It’s not fair to say that the beam axle suspension of a 1908 model T designed to conquer roads that would have appalled the Romans and do so for as cheaply as humanly possible, compares unfavorably 21st century dual wishbone designed for glass smooth roads and with cost no object.  We never learn anything from comparisons of maximized systems to un-maximized systems.  If we want to set high performance as the baseline, than lets look to racing.

The first Indianapolis 500 was raced in 1909.  The last time a car with beam axle front suspension would win? 1962, at 150 mph.  Sprint cars, racing 1200lb vehicles with 800HP engines on dirt tracks still use them, again at around 150 mph.   Further, they are the preferred axle of choice for many extreme motor sports, like rock climbing.  The Humvee has has been troubled by it’s lack of beam axle suspension.  It’s wide articulation, fully independent suspension is far more weight sensitive than beam suspension.  As such, the Humvee becomes dangerous to drive when overloaded by say, improvised amour.

Beam axle is far more simple.  As such, it costs less to maximize, and more importantly, has less fail points.  In the crushing loads incountered in racing, rock climbing, and warfare, the beam axle wins.   Tune in next time, for a bit less detail about the swing axle, the wishbone, the McPherson strut and double wishbone, and more.

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January 1, 2009 - Posted by | Engines, skepticism, Small Car, Transportation, Uncategorized | , , , , , ,

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