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Suspension Tuning & Optimization: How Dampers Work

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How Dampers Work

12.10

00:00 - In the earlier springs module, we discussed how springs produce a force in response to displacement.
00:05 Dampers on the other hand, produce a force in response to the speed of suspension displacement.
00:12 Put another way, dampers produce a force in response to how fast they're moved rather than how far they're moved.
00:19 This is the fundamental difference between a spring and a damper and because of that, the working principle and the end use is very different.
00:27 Let's look at a graphic that'll help us visualise how the force response differs between a spring and a damper.
00:33 On the left side, we have a linear spring and on the right, a damper.
00:38 Both of these are attached to a common element that'll drive them at the same speed and displacement.
00:44 Underneath is a pair of plots that represents the force in the spring and damper respectively over time as we vary the input.
00:51 if we start driving the system with a slow but constant sinusoidal input, we can see the force response in both the spring and the damper.
00:59 As we begin to increase the speed of the input, we can see that the force response of the spring is the same as it was at slow speed while the force to drive the damper has increased.
01:09 Continuing to increase the speed even more, the damper force continues to increase while the spring force again remains the same as for the original speed.
01:19 This is because the amount of displacement isn't changing but the speed is.
01:23 So the takeaway here is that for elastic elements like springs and antiroll bars, we care about how far they've been displaced, bent, squashed or twisted.
01:33 For a damper we care about how fast it's been displaced.
01:37 Modern motorsport dampers use fluid dynamics to provide impressive performance and tunability.
01:42 There are a number of different technologies out there which we'll get to soon but fundamentally they all do the same thing.
01:49 They force a fluid to pass through small holes or restrictions.
01:53 With the fluid being forced to take a difficult path, we're impeding its movement and that produces a resistive force.
02:00 With this in mind, we can start to understand why the force is in response to how fast we displace the damper shaft rather than how far the damper shaft is displaced.
02:10 This is quite easy to understand if we simply move the piston through the damper body slowly by hand.
02:15 In this case, there's very little resistance but try to do it quickly and the force required increases steeply.
02:22 By forcing the fluid through these small holes, the kinetic energy imparted to the damper is converted into heat energy.
02:28 This results in the damper essentially absorbing some of the kinetic energy stored within the springs that would otherwise cause the suspension to oscillate and move.
02:37 This is why dampers get hot in operation and interestingly it's also one of the reasons the hydraulic fluid within them deteriorates, requiring it to be changed during servicing.
02:48 As the oil breaks down, the viscosity of the oil will reduce and in turn reduce our damping forces produced as it becomes easier for the fluid to pass through the restrictions within the damper.
02:59 As a quick aside, let's clarify what we mean when we say viscosity.
03:02 This is a measurement used to describe the reluctance of a fluid to flow.
03:06 The higher the viscosity, the thicker it is and the higher the damping forces that will result.
03:11 The fluid inside our dampers will be specified by the damper manufacturer and it's one way that the behaviour can be modified.
03:19 To understand how a damper is working, we generally characterise the behaviour on something called a damper dyno.
03:25 This allows us to run the damper at different speeds and we define how the damper is working by plotting the force in response to the different velocities it's driven at.
03:34 This is a generic plot of what the force response of a simple damper would look like.
03:39 We have damper force on the y axis and damper shaft velocity on the x axis.
03:45 This is what we would call a linear damper in that the force changes linearly with the speed the damper is driven at.
03:52 In this plot, the area in the top half is when the damper is in compression and the area in the bottom is when the damper is in rebound.
03:59 This isn't always the case so it pays to check the convention used by the person who plotted it to make sure you're not interpreting it the wrong way.
04:07 To be clear, this isn't a realistic representation of what a real damper force plot will look like.
04:13 In reality, there's usually a difference in the behaviour of compression versus rebound.
04:17 Besides the asymmetry between compression and rebound, a typical real damping force plot will also not be linear.
04:24 In road racing anyway, almost all dampers are designed to work with a digressive force response.
04:30 Looking at this plot, we can see what's meant by digressive.
04:34 At lower speeds, we see the same linear response before the force kinks and the rate of force increase reduces.
04:42 The dotted line represents the behaviour of the original linear damper.
04:46 The point at which the damping behaviour changes is called the knee of the damping curve.
04:51 A digressive damping curve is popular as it gives us higher damping forces at lower speeds with reduced forces at higher speeds.
04:59 If we didn't use a digressive behaviour, and if we had the low speed force response that gave us the transient handling traits we were looking for at low damper speeds, the damper would be providing far too much force at higher damper speeds.
05:13 We'll leave it at that for now but this is something we'll be discussing in more detail later.
05:18 It's also worth remembering that these examples we're showing here are idealised.
05:22 Our real damper performance would look quite different.
05:25 Here are some examples of how real dampers with similar traits might actually behave.
05:29 Something important to understand about plotting the damper behaviour in this way is that looking at the force versus velocity plot doesn't actually tell us everything about how each damper will perform on the car.
05:41 When testing dampers on a dyno, we're kind of doing things backwards.
05:45 We're effectively controlling the speed that they move at and measuring the resultant force.
05:50 When they're on the car, they're instead restricting the movement of the suspension based on the kinetic energy they're subject to.
05:56 This in itself will affect what velocities and therefore damping forces the dampers are subject to.
06:03 With that in mind, the way these plots should be used is as a reference or datum by which we compare the behaviour of one damper to another or one setting to another in a simplified way.
06:13 The fact remains that there are many nuances in the damper behaviour that we can miss by just looking at the force vs velocity plot.
06:21 The number of external adjusters on a given damper indicates something about the level of sophistication of the internals.
06:28 While a basic damper will probably have none or maybe one adjuster, high end examples can have as many as 5, enabling us to modify both the high and low sped compression and rebound behaviour.
06:40 Although we'll be going into detail in the next module, for now just know that for the most part, assuming a motion ratio of roughly 1:1 between the wheel and damper, the damper speed, which is commonly considered the threshold between high and low speed is around 25 mm per second or approximately 1 inch per second.
06:59 Anything below this is referred to as low speed damping and anything above is high speed damping.
07:05 We'll be breaking down these thresholds in detail in the next module.
07:08 In the case of a damper with no adjusters, the damping behaviour is of course fixed.
07:12 Unless it's of the type that can be disassembled and modified internally.
07:16 When only a single adjuster is present, this often gives us the ability to begin tuning our damping without having to pull everything apart.
07:25 A single adjuster would most often give us the ability to adjust the low speed rebound only.
07:30 Which would affect this area of the force versus velocity plot.
07:34 It's important to understand that while a single adjuster may primarily adjust the low speed rebound behaviour, it'll also tend to have a more minor effect on the high speed rebound as well.
07:45 In some cases a single adjuster may also have some effect on the compression behaviour.
07:49 Ultimately you may need to contact your damper manufacturer to understand the overall effect of the adjusters if it's not clear.
07:57 It's also worth recognising that different damper manufacturers will label the direction of the adjustments in different ways.
08:04 Some will use something like a plus and minus symbol to indicate the adjustment direction.
08:07 Others will use terms like hard, stiff, firm or soft.
08:12 Again, if it's not clear, consult with your damper manufacturer on the direction and effect of each adjustment.
08:18 In dampers with a second adjuster, this commonly gives us the ability to adjust low speed compression.
08:24 The effect of this sort of adjustment can be seen here on the force vs velocity graph.
08:29 Once we step up to 3 adjusters, this normally means low speed rebound and compression plus a high speed compression adjuster.
08:36 Sweeping the high speed compression setting will usually have an effect like this on the damping force.
08:42 In the case of 4 adjusters, this will generally allow for both low and high speed rebound and compression adjustment.
08:50 By adding a high speed rebound, we can now affect the damping force like this.
08:55 Some high end dampers will also have a 5th adjuster which is normally termed a blow off adjuster.
09:01 This can be thought of as a very high speed compression adjustment.
09:04 It usually affects this area of the damper force plot.
09:07 We'll leave it there for now but we'll be having a close look at the purpose of each of these adjustments in the next module.
09:14 Dampers vary widely in sophistication and as mentioned earlier, there are a number of different technologies used for their internal workings.
09:21 The most common configuration is a combination of needle valves and shims that results in the desired damping behaviour.
09:28 The needle valve works by varying the size of the orifice to make it easier or more difficult for fluid to pass through them.
09:35 These valves are generally used as part of the low speed rebound and compression circuits.
09:40 Winding the low speed adjusters in and out will bring the needle closer and further away from the seat.
09:46 Increasing and decreasing the low speed damping forces respectively.
09:50 Shim stacks on the other hand, work by deflecting once a certain internal fluid pressure in the damper is reached.
09:56 These will be closed while the dampers are operating at low speed then begin opening at higher speeds where the internal pressure is greater.
10:04 There are a huge range of high speed behaviours we can achieve by using different configurations of shim stacks.
10:09 We can vary the thickness of each shim as well as the number of them.
10:14 We can use different diameter and shapes of shims as well.
10:17 The order the shims are stacked in also gives us another way to fine tune their opening behaviour.
10:23 If a damper has high speed adjustment, this usually means applying a different pre load force to the shim stack which results in changing when the high speed circuit becomes active.
10:33 One important distinction for damper tuning is that external adjusters are quick to make changes to.
10:38 Whereas changes to the shim stack itself will require at least partial disassembly of the damper.
10:44 One final aspect of damper behaviour that's worth looking at is the use of pressurised nitrogen gas.
10:50 Which is used to reduce cavitation within the hydraulic fluid as well as providing space for the fluid displaced by the damper shaft during compression.
10:58 Cavitation is a phenomenon that occurs when the pressure of the fluid is lower than its vapour pressure.
11:04 When this happens, small bubbles form inside the fluid.
11:07 These bubbles tend to reduce the effectiveness of the damper and can also damage the internal components when they collapse on or close to the internal components.
11:16 Depending on the design of the damper, the nitrogen can be stored internally within the main body of the damper or in many cases in motorsport, externally in a piggyback style or remote canister.
11:27 In summary, dampers produce force in response to the velocity they're displaced at.
11:31 There are a range of different internal layouts and workings but they fundamentally all work by forcing hydraulic fluid through restrictions to produce a force.
11:40 The rebound and compression behaviour of practical dampers is usually not symmetrical and the level of sophistication of the damper will determine how many adjusters it has.
11:51 Dampers can have no external adjusters or as many as 5.
11:55 We've only touched on the bare basics of practical damping behaviour here.