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Shock Absorbers And the Boing-Oing-Oing-Oing Factor

We kind of take our suspension systems and our shock absorbers for granted.  We don’t tend to think about them too much until that time that the mechanic sucks in his cheeks, shakes his head and says “Your shocks are just about gone, mate and you’re going to replace them at a cost of $oodles a pop.” (Apologies for inadvertent sexism but where are all the female grease monkeys?)

However, if you remember back to the days of riding home-made go-karts, a basic skateboard or (I won’t tell on you) the trailer, you probably know why cars and vehicles in general are fitted with suspension systems. Without suspension, you feel all the bumps in the road. Every. Single. One. While this is great fun when you’re a kid riding in the trailer and getting bumped up and down, it’s not so much fun for longer trips and certainly doesn’t do your spine any good at all. It doesn’t do any good to anything delicate you’re transporting, such as eggs, or if you’re trying to take a blancmange or sloppy chili con carne to a potluck dinner.

The full suspension system involves the wheels (pneumatic tyres), the springs, the shock absorbers and the links. Describing all of this and all of its variations would take ages and could take up several posts, so we’re going to talk about the part that does a lot of work that you might not realise: the shock absorbers.

Contrary to what you might think from the name, a shock absorber doesn’t have the job of soaking up the jolting, bouncing and jouncing that comes from hitting a bump. That’s the job of the springs. What the shock absorbers do is control the harmonic oscillation. That’s a long and rather technical term for what we’re going to call the boing-oing-oing-oing factor.

Although some of the springs in your vehicle’s suspension don’t look like Slinky Springs, mattress springs or trampoline springs, they are still springs and behave like any other spring. (For those interested, the weird ones we see in vehicles are usually leaf springs.)  Now, when you stretch a spring then let it recoil, which is what happens when your car goes over a bump, what happens? If you can find a handy trampoline or Slinky, you can try this out for yourself. (Don’t try this on the hair of a curly-haired person unless you want to absorb the shock of a slap in the face. What’s more, hair tends to be self-damping unless its gelled like crazy.).  Subject the spring to a sudden extension then let it compress by tying something to the end of the Slinky then letting it bounce out, or by letting yourself bounce down onto your bottom on the trampoline. What happens?

What happens is that unless you act to stop it (technically known as damping), you get the boing-oing-oing-oing factor. After you’ve bounced down on that trampoline, you’ll get bounced back up again, or the weight on the end of the Slinky will bob up and down. The initial boing will result in lots of oing-oing-oings, with each oing getting smaller.

Now, in a car, you want the initial boing as you go over a bump. What you don’t want is the oing-oing-oing, as this is downright uncomfortable as well as terrible for the handling.  Vibrations aren’t good for the human body if they go on for some time (stop sniggering!). To stop the oing-oing-oing as the spring continues to vibrate after the initial shock, the vibration needs to be damped. This is the job of the shock absorbers. They’re actually dampers, which is nothing to do with that bush barbecue favourite consisting of flour and water wrapped around a stick and baked over hot coal.

Shock absorbers take the kinetic energy of the oing-oing-oing and turn it into some other form, usually heat energy, via friction. This is usually done by using the force of the oing-oing-oing to shove oil from one compartment to another through tiny holes either between an inner tube and an outer tube (the twin-tube system) or from one end of a tube to another (the monotube system).  This sounds weird but it works.  Think of a syringe or a cake icing gizmo.

Of course, there’s a price to be paid for anything and I don’t just mean what you fork over to the mechanic every once in a while. With any system of shock absorbers and suspension, you have to trade off comfort versus handling. On the one hand, a super soft and completely damped suspension irons out all the bumps beautifully but handling is compromised – too soft and the wheels start dancing all over the place and lose grip.  On the other hand, if you want the handling to be crisp and a bit of extra grip and road feel during cornering, you pay for this with extra bouncing. The stickier the liquid inside the shocks and the smaller the hole it has to go through, the stiffer it is. To use the cake icing analogy again, think of the effort it takes to push really sticky icing through a narrow nozzle for a very fine line.  This takes a lot of force on your part, and if this was your damper (shock absorber), you would have super stiff, sporty suspension.  Use a wider nozzle (for fancy star shapes) or make the icing runnier, and it splurts out really quickly.  That’s comfort suspension.

The designers of modern cars are smart enough to know that you can’t please all the people all the time, and that people are not likely to buy one car for when they want to have a smooth ride and another for when they want performance. This is why they’ve now come up with adjustable suspension systems that allow you, the driver, to pick what you want when you want it.

The older systems of adjustable suspension did this by allowing you to make the holes (which are called apertures or orifices to make them sound fancy) in the damper tube big or small.  Not a bad system as far as it went. These got fancier as time went by with sensors that adjusted the hole size depending on how bumpy the road was and how stiff you needed the handling.

The one drawback of the hole-size-based systems is that they were comparatively slow to react to the situation. After all, the signal had to get from the road to the sensor to the apertures, which then had to move from A to B. The designers decided it would be much quicker and better for handling and comfort combined if you could somehow make the liquid inside the shocks thinner or thicker depending on what you want.  Although heating would make the liquid thinner (it does this anyway), this would be even slower and cooling for a stiffer suspension would take longer still.

Enter magnetorheological fluid (called MRF by designers). This combines oil with easily magnetised particles. OK, it’s oil chock full of iron filings because iron, as we all know, is attracted by magnets.  This is fun stuff – even iron filings by themselves are a lot of fun to play with if you have a magnet, and the more powerful the magnet, the thicker and stickier the clump of iron filings gets.  This video explains how it works:

 

And that’s exactly what happens in a magnetic shock absorber. If you remember your high school physics, which is probably where you got to play with magnets and iron filings, you may recall that any suitable iron rod wrapped with enough twists of copper wire becomes a magnet when current goes through the wires. The more current, the stronger the magnetic field.

Now, how quickly does it take electrical impulses to go from A to B? Hardly any time at all. This means that an active magnetic suspension system will detect what’s going on with the road surface, the speed, how fast and hard you’re cornering and all those other factors that contribute to handling, and will increase or reduce the current going through the coils in a magnetic shock system almost instantaneously. This means that the fluid in the shocks becomes hard or soft as needed.

Designs for magnetic dampers are being worked on all over the globe and should be able to move from more luxury vehicles (such as the Cadillacs and Ferraris that had it early on in the picture) to common everyday vehicles.  The boffins will have to work out how the increased energy needs will work in electrical vehicles, but regenerative braking and harvesting the energy absorbed by the shocks themselves will go some way towards this.

The response speed isn’t the only advantage that the new magnetic systems have over the hole-based ones. Wires don’t wear out as quickly, whereas moving parts do, as we all know.

Speaking of moving parts wearing out, you can use the boing-oing-oing-oing factor as a test when you are checking out a second-hand car.  Shove down as hard as you can on the back end without denting the boot. If you get an oing-oing-oing after your initial boing, the shocks are shot. Walk away and look at another vehicle – or start having a chat to the team here at Private Fleet – if you want to avoid the mechanic with the sucked in cheeks and the shaking head.

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