Have you ever received a fresh set of coilovers and seen the graphs in the box? Perhaps you have come across them online and wondered what the graph is actually trying to tell you. Despite looking quite complex they are quite a simple graph to understand and can help in your understanding of how the damper works, and give indications about its health, durability and how to set them up.
Quite a common misconception with regards to damper dyno graphs is that they are the magic key to setting up your damper. The reality is that they are more of a guide than a specific answer, and used correctly can reveal information about your damper that would have otherwise been very difficult to find.
We took a trip down to Nitron Racing Systems recently where we were able to use their damper dyno to take a deeper look into the graphs and generate a further understanding into what the graphs mean and how they can be interpreted. If you prefer a good watch than a good read then the below video will inform you with plenty of damper dyno graph information as well as a few other questions answered. Grab a drink and have a watch and if you want some more information then come back and read on.
Different Types of Damper Dyno
There are a few different styles of damper dyno currently in use. The most poplar is the crank hub dyno. This dyno operates by mounting the damper between the two mounting points. The upper point is fixed in position, usually around ride height and then the lower mounting point is connected to a crankshaft. This crank shaft is then rotated at different speeds by a motor which alters the velocity of the damper compressing and extending it generating a force through the damper. These dynos are widely used due to the ease of their operation, generation of accurate results and they are a very cost effective option for testing dampers with. However, one slight drawback is that for each oscillation the velocity and the stroke in the bump and rebound direction is fixed.
The other type of damper dyno which is relatively new to the market is the electro-magnetic actuator damper dyno. This dyno is not operated by a crank hub but is instead directly driven by a high power actuator that can extend at different lengths and speeds on each individual stroke. This means that for the first compression stroke the dyno can move 40mm at 100mm/s and then for the rebound stroke it can move 30mm at 20mm/s. This technology means that the damper can be worked at more realistic speeds, velocities and forces to more accurately plot the behaviour of the damper. The software that powers the damper dyno that we were using in our video at Nitron also has the capability for data logged information to be uploaded so that the actual movement of the damper can be simulated from a real race. This is particularly interesting for high stress environments such as rallying or for the circuit world, endurance racing such as the Le Mans 24 Hours.
Data logged information taken from damper potentiometers mounted to each damper displays the travel and velocity of each damper at any given point in time. It records, kerb strikes, cornering behaviour and extensions and compressions over crests. Having the ability to replicate this information in a room for the full 24 hours means that the following years damper can be developed to an even higher level, ensuring that the damper is capable of handling the forces and abuse that its predecessor received in the previous year. It can also be worked even harder to ensure a factor of safety is accounted for in the design in case the race conditions happen to be worse than when the data was recorded. For situations such as rally, this dyno can be used to ensure that the damper oil is not going to over heat and begin to cavitate and break down, ensuring that the damper will be working to its maximum effectiveness for the entirety of a stage run.
Damper Dyno Calculations
The results we are effectively looking for on the damper dyno are the forces being controlled by the damper., the acceleration of the damper shaft and the velocity at each given time. First of all we take a look at what we know. On the crank hub dyno for example, we know two things. The rotational speed of the motor, and the linear displacement of the crank shaft that is attached to the damper. With these two pieces of information we are able to determine a linear velocity of the crank over each stroke. For this example we will take a rotational speed of 120rpm. This is a convenient speed for our calculations it equals 2 full rotations per second. For each rotation our crank is doing one full movement up and one full movement down. Now we will say that our crank has a linear displacement about its centre of 40mm. Therefore one full rotation travels a total linear distance of 80mm up and 80mm down. Therefore, our 1 revolution per second travels a total linear distance of 160mm.
With the fundamentals outlined we now need to find the maximum linear velocity and acceleration experienced by our damper. For the acceleration we take another look at the rotational speed of 120rpm. When something is rotating at an angular velocity, it is constantly accelerating due to it having a constant change of direction. When a linear movement is linked with the rotating part, a constant linear acceleration is also developed. This is because at the very top and very bottom of each linear stroke the moving part stops. Even if it is for an extremely tiny amount of time the part moving up and down must physically stop in order for it to change direction. This process of stopping and starting again means that it has to accelerate up to maximum velocity and then decelerate back to a stop and then accelerate once again and so on. Therefore, we can say that the linear part is constantly accelerating too.
First we must calculate the angular velocity of the crank which we can then convert into the linear acceleration of the damper. We know that the crank is rotating at 2 revolution per second currently but we need to convert this into radians per second for our angular velocity using the following equation.
Angular Velocity (rads/s) = Revs Per Second x 2Pi
Angular Velocity = 2 x 2Pi = 12.57 Rads/s
We can now use this to calculate the linear velocity at maximum speed using the equation:
Linear Velocity (m/s) = Angular Velocity (rads/s) x Radius (m)
Our radius about the crank centre is 40mm or 0.04m, therefore:
Linear Velocity (m/s) = 12.57 x 0.04 = 0.503 m/s
Now that we have our maximum linear velocity we can input this into the equation for linear acceleration:
Acceleration (m/s2) = change in velocity (m/s) / change in time (s)
We know from earlier that our moving part comes to a stop at the top and bottom of each stroke. Therefore our change in velocity is the maximum velocity minus the minimum velocity:
Change in velocity (m/s) = 0.503 m/s – 0 m/s = 0.503 m/s
We also know that at the current speed our crank is completing 2 full rotations per second. Therefore it is completing 1 full rotation every 0.5 seconds. Per rotation our damper has a full compression stroke and a full extension stroke. Therefore it accelerates from zero to full speed and back to zero again in 0.25 seconds. Now assuming that the maximum velocity takes place at exactly half way through each stroke we can confirm that the piston accelerates from zero to maximum velocity in just 0.125 seconds. Therefore our change in time for the change in velocity is 0.125 seconds. We can now complete the below equation to solve for linear acceleration.
Linear acceleration (m/s2) = 0.503 / 0.125 = 4 m/s2
The linear velocity calculated earlier is used for both of the graphs generated by the damper dyno. The acceleration figure is paramount for the calculations for the forces applied to the damper as Force = Mass x Acceleration. Its not as simple as that for the dyno equations however due to the damper internals resisting and controlling the force. Therefore, the damper dynos have a load cell installed to measure the forces being applied to make the damper moving at a known rate.
Damper Dyno Graphs
Now that we understand the fundamental physics behind what the damper is experiencing on the dyno we can build a better understanding of the graphs that we see on the screen.
The two main graphs produced by the damper dyno are the Force vs Velocity graph and the Force vs Displacement graph. The two are distinctively different to each other. The Velocity vs Displacement Graph shown below has lines spaced apart from each other on a diagonal path mirrored about the X axis. This style is shown above.
On this graph we can see a series of lines going upwards in a diagonal trace and a series of lines moving downwards on a diagonal line. On the x axis we have the linear absolute velocity of the damper and on the y axis we have the damping force of the damper. All lines start at zero velocity and zero force. This is due to the starting/stopping that we discussed in the calculations section. At the bottom of the compression stroke we have zero velocity and zero force as both items are at the point where they change their direction. As the damper accelerates and the velocity increases, so does the force required to increase the velocity. At full compression, the damper stops again back to zero velocity and force and the damper then begins accelerating in the opposite direction as the force is applied in the opposite direction (pulling instead of pushing). This is where the lower lines start back at zero and being tracing downwards and across for the rebound section of the graph.
The upper half of the graph are all lines representing compression strokes at different speeds and the lower half of the graph is representing the rebound strokes of the damper at different speeds.
Looking at the graph you will notice that there are multiple lines spaced apart from each other often in different colours. These lines each represent a different damping setting on the damper. The lower lines are the softer settings and the higher lines are the stiffer settings. This is because as you stiffen the settings on your damper, it requires more force to move the damper through a stroke at the same velocity. Therefore for the same trace of velocity, the force applied is higher and therefore the line sits slightly higher. The same applies for the lower rebound section of the graph but as the rebound settings are made stiffer the line gets lower on the graph due to the rebound forces being recorded as negative force.
The other main graph, Force vs Displacement, takes a more circular form and looks like the example shown below. – starting and stopping in a circular motion.
This circular graph perfectly represents the pattern of velocity we discussed in the calculations section where it starts and stops at the top and bottom of each stroke. On this graph the displacement is on the x axis and the force on the y axis. This time we have a display of negative displacement as well as the negative force. The negative displacement is where the damper changes direction of travel and begins the rebound extension stroke. The starting and stopping of the damper can be seen as the red line crosses the x axis. At maximum displacement, the force can be seen at zero. This is because at this exact point in the time the damper has finished accelerating and is about to begin decelerating. Therefore, no force is being applied as force only exists where there is acceleration occurring. Unlike the previous graph this only shows a snapshot in time of one full stroke cycle of the damper. This graph is mainly used for checking the consistency of the damper which we will look at in the next section.
Interpreting the Graphs
Force vs Velocity
Now that we have developed an understanding of what each graph is displaying we can now look at how we are able to interpret each graph and take away some useful information from each of them.
In the below image you can see that towards the left hand of the graph (at the low velocity sections) the lines are all very close to each other. This means that for each adjustment that we made on the dyno, the low speed section showed a very little change in damping force. This is because we didn’t adjust the low speed damping for the making of this graph and only moved the high speed adjuster. At the point between the low speed and mid range sections you can see a kink in the lines. This is known as the Knee. Through the mid range section of the graph you can see that the compression lines begin to open apart and have a consistent space between them through the mid and high speed range. This is displaying the different positions of the high speed adjuster and shows that each adjustment makes a clear difference to the damping forces at each speed. All lines from top to bottom also display roughly the same sized gap between them meaning that each adjustment has a similar effect over the last. This is a sign of a healthy adjuster and a good damper offering different stages of adjustment within a designed window for an expected spring rate.
If the graph displayed some lines very close to each other, almost touching through the mid range then this would suggest that the damping adjustment wasn’t giving enough variation and would need re-valving or re-shimming internally. You might also notice that the gaps begin closing up very slightly as the velocity increases to a very high speed. This is because the damper is moving at such a high speed that the mechanical internals can struggle to keep up with such a high rate of acceleration and are not responding to the changes in speed at the exact time of the speed which can cause a slight delay in response and therefore lowers the force of the damping resistance slightly.
With the low speed compression adjuster being adjusted we would expect to see the lines at the lower end of the graph begin to open apart from each other as the adjuster alters the damping resistance at the lower speeds. Using the two adjusters allows the user to develop a damper with a high amount of adjustability throughout the entire range of the damper. Looking at the lower lines on the graph for the rebound adjustment you can see that the lines at the lower speeds are much more spread apart than those of the compression section. This is because the dampers that we used for this test were 3 way dampers so only had 1 adjuster for the rebound which adjusts both low and high speed at the same time. Even with the low speed being adjusted you can see that all lines still start out very close to each other at the very low speed section of the graph. This is because the velocity is so low through the damper that even on the stiffest setting, the force is quite low to get it to move due to the velocity being so low.
On this graph you can also see that the rebound section had a much lower total velocity than the compression side. This is because the particular damper we were using allowed the operator to set the speeds in the bump and rebound direction to represent a realistic simulation for the damper movement. If we were using a conventional crank hub dyno then the velocities would be the same for bump and rebound on the graph.
Having an even gap between each point of adjustment isn’t always the norm though particularly at very high levels of motorsport. This is because some purpose built dampers might have much closer lines in the centre of the adjustment range so that the engineers have much smaller increments of adjustment in the operating window to really fine tune the damper to work exactly as they would like it to.
In terms of interpreting this graph for tuning your dampers and noticing any damping windows that would affect you on circuit, there are 3 main speed groups to be aware of that apply to real world damper forces. Speeds between 0-50mm/s are considered low speed movements and are based on vehicle body movement such as low to mid speed cornering on entry and exit. Speeds between 50-150mm/s are experienced during heavy braking, and fast direction changes such as a a fast chicane section. The higher speeds of 150mm/s and above are experienced during kerb strikes or landing after a jump. Therefore, you can identify an issues out on circuit within these categories and can look at the graph to see how the damping can be adjusted in these windows using the low or high speed adjuster.
Force Vs Displacement
The force vs displacement graph shows us a single snapshot in time of a single stroke and is a very useful graph for diagnosing the health and consistency of a damper. The main thing we are looking for on this graph is a smooth consistent line forming an oval or circle. If the graph had any major downward peaks or drop offs this would show that there was a drop in damping force and that the damper became less effective or ineffective during that stroke. This can happen due to aeration or overheating of oil or due to low quality oil where the viscosity is able to change due to the heat going through the damper which alters the damping forces inside the damper causing it to be less effective. Another reason could be that the seals inside have failed and there is no consistency to how they are compressing the damper oil and therefore not working the damper properly. An example of this is shown below.
When looking at the graph it can be separated into 4 different sections. The top left section is the phase of acceleration in the compression direction; the top right is phase 2 and is the deceleration of the compression stroke. The bottom right is phase 3 and is the rebound direction acceleration and the bottom left is the deceleration of the rebound stroke. Therefore, any irregularities or drops offs in these sections can immediately help to diagnose any issues and where in the damper these issues might be coming from.
Another very useful way these graphs can be used is by overlaying multiple graphs from a period of time on the dyno. For example if the damper is run for an hour we can take several graphs of even time spacings throughout that time period and overlay them. If the below graph was created it would show that the damper has become less effective throughout the time being used, most likely due to heat or aeration altering the viscosity of the damper oil.
If the rings get smaller and smaller over time this shows that the damping force gets less and less as time goes on. For endurance races rallying this information is very important as it can show how the vehicles handling is going to change over a known time period throughout the race. If the damper can not be altered to work properly for the time period then alternative action can be taken so that the driver is aware of how the car’s handling is going to alter throughout the race and roughly how far in this will be occurring.
This graph also helps to diagnose shock issues by showing distinct spikes in the plot. If a graph displays a spike like in the image below (present on the compression acceleration stage) it can suggest that the damper has a bent shaft, causing it to bind in compression and require a sudden amount of higher force to keep it moving through the stroke. An example of this is shown below.
Digressive, Progressive and Linear Shocks
One characteristic that is very clear to see on a damper dyno graph is whether a shock is digressive, linear or progressive. We will be delving deeper into the benefits and purposes of each type of damping in another article but they are worth mentioning in this article due to their very different natures displayed on the dyno graphs.
All 3 styles can be seen on the below graph and show how digressive, progressive and linear shocks would be displayed on a dyno. The digressive shock has an initially much steeper curve, having much higher damping forces at lower speeds and then begins to level out and reduce the gradient of the curve at the higher speeds. The linear damping is where the line increases at an almost constant rate with force and velocity increasing with an almost constant relationship to each other. The progressive shock is the exact opposite of the digressive shock and has much lower damping forces at the lower speeds and then has an increase in gradient as it has a much higher stiffness for the higher velocities through the damper.
Each style of damper has its own benefits and applications where they are best suited and open up different tuning windows to get the damper working correctly. In general, circuit cars tend to prefer a digressive or linear shock as the digressive shock has higher damping forces at the lower speeds that are common during cornering, braking and accelerating so offer more control for the driver, they then have the lower force at high speeds for kerb strikes and landing after crests to prevent the shock locking and becoming too stiff in these instances. Achieving similar characteristics is possible with a well valved linear damper too especially with 3 way damping where the shock curve can be tuned using the high and low speed adjusters. Progressive shocks are more common in the off road and rally world as high damping forces are often required in the high speed sections due to landing after jumps or large crests where resistance is required from the shock to prevent bottoming out or similar due to the softer springs associated with off roading.
Setting Up New Dampers
So now the burning question remains of: How can I use the damper dyno graphs to setup my dampers for my car? Well the short answer is that you cant. The graph can show you which adjuster position will be soft or stiff and roughly where in the range it will sit. You can also use it to make an intuitive decision based on the stiffness of your spring and the stiffness of the spring that the damper was valved for. Therefore if you bought dampers supplied with 14kg springs and you are looking to install 18kg springs then you would sit the damper at a slightly higher stiffness on the rebound section so that you will be able to control the extension of the damper more easily. However, ultimately, setting up a damper relies heavily upon driver feel and many other factors about the car such as tyres being used, spring rates, vehicle mass, weather conditions, surface temperatures and one of the most important and subjective matters, driving style.
Therefore, the best way to setup your dampers is to go testing at circuit in ideal conditions and fine tune your settings throughout the day based upon data and driver feedback to fine tune settings and get the car handling at its absolute best for you.
That said, the damper dyno graphs do offer an insight into setting up 3 or 4 way dampers to a degree of accuracy using the graphs as a guide. This is because you are able to see the different forces at the low and high speed sections of the shocks and can set a curve based on these read outs. For example, a stiffer low speed setting can be used to get good control over a stiffer spring at the cornering force loads and then a softer setting can be used on the high speed damping to make the curve fall off and take on a digressive nature towards the high speeds of the graph.