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Damper Dyno

How to check the condition of your car's shocks.

By Michael Knowling

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Shock absorbers, shockers, shocks, dampers. Call them any one of these and people will usually know what you're talking about. However, the most technically correct name is "damper", because its role is to damp the oscillatory movement of the suspension spring. It's designed to apply a controlled amount of (hydraulic) friction to the spring - therefore preventing it from oscillating indefinitely. Without this, you'd drive over a road bump and still be feeling the suspension bouncing for a good 50 metres down the road. And by delivering a resistance against spring movement, they also help to reduce a car's level of body roll - although this is not their primary function. A damper takes the superfluous mechanical energy stored in the spring and converts it into thermal energy - heat.


With each extension and compression stroke, a piston slides up and down inside the damper, forcing fluid to flow between its upper and lower chambers. These chambers are known as the pressure cylinder and reservoir respectively. The internal fluid has to pass through an assortment of orifices and tiny valves that open and shut depending on the direction of the fluid flow. These valves are designed to provide a measured amount of resistance to the flow of fluid (and therefore effect damping stiffness). Where fitted, a spring-loaded valve can open to allow additional fluid to pass through - but only when enough force is being applied. This force is generated by the speed at which the piston is moving. As a generalisation, as the stroke velocity of the piston increases, more valves come into play to resist the flow of fluid. Therefore the damper's rate is sensitive to the speed of suspension movement.

Unfortunately, the condition of a damper usually deteriorates relatively quickly because of the high levels of vibration, speed and load that it endures. If you're driving your car every day, you might not notice this gradual deterioration, because the car always feels just the same as when you drove it last time - but rest assured, it won't be as good as it once was. As an example, it isn't uncommon for car manufacturers to recommend replacing dampers every 60-70,000 kilometres. And while this might be a good suggestion for cars driven under "normal conditions", how do you know the condition of your car's dampers?

Damper Dynamometer

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If you're really serious about checking your car's dampers - or maybe you plan to modify them - this is the ultimate tool. This is a happy coincidence because if you're suspicious of your car's dampers, you can have them evaluated, revamped and modified all at the same time! (That's so long as they're the rebuildable type of damper - not the sealed throwaway type like most OE stuff.)

A damper dynamometer - such as this PC-controlled SPA example - works by driving the test damper through its compression and rebound stokes at selectable speeds. A mains-powered electric motor is used to push and pull the dyno's lower ram which, when connected to a test damper, transfers pressure to a load transducer at the top mounting location. This transducer gives a constant indication of the damper's resistance to movement though both compression and extension strokes at the various shaft speeds.

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Depending on the damper you want tested, it might be necessary to have a pair of adapters fabricated to mate the damper to the dyno.

The SPA unit photographed here is capable of managing a massive 1500kg damper load. It can be used to plot velocity-force, force-displacement and force-position graphs. Of these, force-velocity is generally the most useful and widely used data.

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On a force-velocity graph, the damper's bump (compression) rate is indicated by the line above the horizontal (nought) axis and its rebound (droop) damping is shown by the line below. The shape of these two lines gives a good indication of how much the damper will work against the movement of both small and large bumps. A damper will have a higher piston speed over a large bump - which is a very important consideration when making damper modifications. To give an example of how much shaft speed can vary, a road car generally experiences shaft speeds of around 0.1-0.3 metres per second, while a racecar that's running over harsh rumble strips tops out with shaft speeds of around 13 metres per second!

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Looking at these two shock dyno graphs, the first damper has a rapid build up of rebound and bump damping force at lower velocities that then flattens off at shaft velocities above 1.5 inches per second. The second (which shows the results of testing an actual racecar damper) gives much less rebound damping at low shaft speeds but ultimately builds up to a massive 750 pounds of force at an equally massive 14 inches per second shaft speed. You can see it also gives very little bump resistance in comparison to its rebound specs.

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A faulty or worn out damper will quickly show up when tested on a damper dyno. Depending on what's faulty, a sudden drop-off in damping rate will be seen, or alternatively, the damper will be around 25-50% down on original specs across the whole board. If this is the case, you can have it rebuilt to either stock or modified specs (so long as it's of the rebuildable type, remember!).

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As we've already established, a damper's rate varies as a function of velocity and displacement. And because altering the internal valves can control the flow of fluid within it, its force-deflection curve can also be adjusted. A damper's force-velocity curve can be changed by altering the internal piston type to any one of five different designs. Without going into the extreme complexities of things, a careful selection of piston type and valving can deliver the right balance between low-speed suppleness and high-speed firmness. The final product will depend upon numerous factors and, most importantly, the role of the car. But the good thing about using a damper dyno is that you can see precisely the effects of any damper modifications.

To give you an idea, here are just some of the considerations when building up a custom damper:

  • The more bump and rebound force that's applied, the faster the suspension's oscillatory movement can be ground to a halt - but it is possible to go too far. "Critical damping" is where the spring doesn't cycle at all - it just slowly returns to its original ride position without overshooting.
  • The ratio between bump and rebound damping for a normal road car is widely said to be around 30/70. However, this is a dangerous assumption to make. When building a custom damper, it's imperative to consider all aspects such as the engine location, type of suspension design and numerous other factors.
  • Dampers give reduced effectiveness when they're overheating (ie usually when they're driven fast over rough surfaces). This can cause the fluid to change in its flow characteristics through the damper.

So, you can see, damper modification really is a bit of "black art" that's best left to the experts...

Drive-On Test

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A drive-on test is by far the easiest way of determining the condition of the dampers already fitted to your car. You don't even need to remove them! Australia's Pedders Suspension outlets each has their own Boge drive-on test machine that comprises two motor-driven oscillating platforms, a pressure sensor under each, and a control unit with a rotating chart mechanism.

You simply park the car's front or rear tyres over the two steel platforms, leaving the handbrake off and the gearbox in neutral.

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The operator then winds an adjusting wheel to bring the machine's scribing needle to the zero line of a fresh paper result chart (an adjustment necessary to calibrate it to suit the weight over the axle). And now the measurement process is ready to begin...

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With a push of a button, the individual left or right hand side platform starts to vibrate up and down by a couple of centimetres, with this typically continuing for about 5-10 seconds. While this is happening, a pressure sensor located underneath the platform measures how well the damper can control the movement of the wheel (by sensing pressure against the pad). After the run is completed, the scribe needle plots around the circular baseline the information attained from the pressure sensor.

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It's from this zigzagged line that the operator can identify the condition of the damper. First of all, the maximum distance between the peaks is measured with a ruler and checked against a Pedder's reference book. If the distance exceeds that specified in the book, it's time to replace those worn out dampers.

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In addition to this very general result, an experienced operator will also be able to read more from the chart. This chart was drawn from a Subaru Liberty equipped with extra firm STi tarmac-spec struts. In comparison to the other chart (from a more conventional car) it shows a much "tighter" rate that would be more suited to hi-performance. But because it's so much more heavily controlled, it is also much harsher over bumps (a very true conclusion from my experiences - it's my car!). So while this process does give an indication of the specifications of the damper as well as its condition, it's not anywhere near as accurately defined as the information obtained from a damper dyno.

However, this really is a very quick and easy way of checking the condition of your dampers. Each end of the car should take no longer than about ten minutes to bowl over - and the charge will be correspondingly low too. Much lower than any amount of time on a damper dyno - that's for sure!

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