Damper Dyno Plots Explained

[BLiNK Motorsport]

Recently we have placed great emphasis on the importance of having your dampers Dyno Tested. We currently race cars which are fitted with budget after market coil over suspension units and it was obvious to us that these were variable in quality and the time they lasted between servicing, hence we decided to purchase our own damper dyno so we could test the dampers we fitted to the cars.

 

When we refer to budget this includes all suspension units in the sub £1500 bracket for a set of four dampers. Expensive dampers are also worth testing as they are not immune to faults but their consistency is far higher.

 

We feel it is very important to match the dampers across each axle and, quite frankly if they are not matched then corner weighting your car is a semi pointless exercise as weight transfer rates will be inconsistent.

 

Here is a copy of an information sheet we provide with all the dampers we supply, We have produced it here to try and provide a wider understanding of how a damper dyno works and the graphs it produces.

 

 

Our damper dyno is a 5hp crank style dyno that is powered by a three phase motor supplied by a variable frequency drive. The drive is controlled by a DAQ attached to an intel Computer and uses a P.I.D. alogorythm to maintain velocity regardless of load:

 

 

 

 

How the Test is performed.

 

The damper is cycled in the dynamometer giving a sinusoidal motion, the damper is tested through a 1.5” stroke (adjustable from 0.5" to 4") up to a maximum damper shaft velocity of approx. 7”/Sec (adjustable up to 20"/sec)

 

The damper shaft velocity is at is minimum when the crank is at Top Dead Centre and Bottom Dead Centre. It is at its maximum velocity at the 3 o’clock and 9 o’clock positions. There are therefore 4 individual cycles in one revolution of the crank, these cycles are as follows:

 

(1)    From TDC to the 3 o’clock Position; The damper shaft moves from zero velocity to its maximum velocity in the rebound stroke (Called Rebound opening as this shows the rebound shim stack valves opening when graphed).

(2)    From the 3 o’clock position to BDC; the damper shaft moves from its maximum velocity to zero velocity in the rebound stroke (called Rebound Closing as this shows the rebound shim stack valves closing when graphed).

(3)    From BDC to the 9 o’clock; the damper shaft is accelerating from zero velocity to its maximum in the compression stroke (Called Compression Opening as this shows the compression shim stack valves opening when graphed)

(4)    From the 9 o’clock Position to TDC; this shows the damper shaft slowing down from its maximum velocity to zero in the compression stroke (Called Compression Closing as it shows the Compression shim stack valves closing when graphed)

 

 

Whilst the crank is cycling the following readings are obtained from the Damper Dyno:

(i)                 Force – measured from a load cell attached to the damper.

(ii)                Displacement – this is set by the position of the connecting rod on the rotating drum

(iii)               Velocity - Calculated from the Displacement and hall effect sensor on toothed wheel.                    (iv)               Damper body temp. obtained via a contactless IR sensor.

 

 

The results from which can be graphed in several different ways using the Damper Dyno Software, a description of which is given below:

 

Force Vs Displacement

 

The graph shows zero displacement at the 3 o’clock & 9 o’clock positions on the crank (damper shaft is moving at its maximum velocity). TDC & BDC are at the extremities of displacement (damper shaft is at Zero velocity). The rebound forces are the negative portion of the graph (Bottom) and compression forces are the Positive side of the graph (Top).

 

 

 

Force Vs Velocity

 

Where the Force Vs Displacement graph is extremely good for noticing any mechanical faults with the damper and matching pairs across an axle the Force Vs Velocity graph gives more of an indication as to how the damper will perform on the vehicle.

A damper will only produce a force when the shaft of the damper has a velocity – in this respect it is different to a spring. When the damper is stationary there are no forces produced by it.

The Velocity of shaft movement can be split into Low Speed Shaft Movement and High Speed Shaft Movement’ both cover different characteristics of how the damper will perform.

Low Shaft Speed Movement covers what is known as vehicle transients – pitch and roll and is what the driver will ‘feel and describe’ as handling characteristics and generally governs the rate of weight transfer around the vehicle under any driver input change.

High Shaft Speed Movement covers very fast damper reaction such as hitting a pot hole or running over curbing at a race track – it will have a large effect on how ‘comfortable’ the car will feel.

 

There are several types of Force Vs Velocity graphs used:

 

Force Vs velocity:

 

This is shown as an ‘S’ shaped graph where the negative velocity is compression of the damper and Positive velocity is the rebound stroke. (Please note that this can vary across different dyno software).

 

 

 

 

 

Force Vs Absolute Velocity:

 

This is where all velocities are shown as positive; Compression is shown as Positive Force and Rebound is shown as negative Force. It is like folding the Force Vs Velocity graph over on itself. 

 

 

 

 

Force Vs Absolute Velocity (CC-RO):

 

This is the same as the above but only shows half of the damper crank rotation where the Compression shim stack valves are closing and the rebound shim stack valves are opening (CC-RO). 

 

 

 

 

Force Vs Absolute Velocity (RC-CO):

 

As above but shows the other half of the crank rotation where the Rebound shim stack valves are Closing and the Compression shim stack valves are opening (RC-CO). 

 

 

 

 

Average Force Vs Absolute Velocity:

 

The dyno software averages the compression and rebound forces from both of the opening and closing phases and plots them against the absolute velocity – this is not an accurate plot but is good for seeing general damper behaviour – it can mask poor hysteresis performance of the damper where there are large differences in forces between the opening and closing phases of the shim stacks. 

 

 

 

Here is an example of an Average Force Vs Absolute Velocity Plot of a well performing damper showing the 'adjustment sweep' on the adjuster:

 

 

 

 

 

Gas Pressure

 

All dampers are filled with gas and oil; the gas is usually an inert gas such as Nitrogen. The purpose of the gas is to pressurize the oil in the damper to prevent cavitation of the oil on the low pressure side of the damper piston.

Twin tube dampers have this gas contained in the outer reservoir tube in a bag and the gas is at low pressure. Monotube dampers have a floating piston with the gas kept on one side of the piston at a high pressure. It is important that we only analyse the damping forces produced by the damper so the gas pressure is removed from the graphs. This is done by taking a force reading at mid-stroke of the dyno.

 

 

Force at Zero Velocity

 

In the real world the damper shaft needs to have a velocity to produce a force, however a damper dyno does not produce a real world scenario and the damper shaft is only at zero velocity for a minute amount of time whilst being tested. During the test the pressures each side of the damper piston do not have enough time to equalise and a force reading is produced at zero velocity.

 

 

 

We hope this gives a brief insight into how a damper dyno works - at a later date we will explain how the dampers are matched and what the shape of the dyno plots indicate.

 

More reading on how the damper dyno plots assist:

 

http://forum.wscc.co.uk/forum/index.php/topic/93372-selecting-the-correct-springs-dampers-for-my-car/

[AdamR]

Great stuff, cheers Fraser!

[Greenstreak-Andy D]

V interesting read Fraser. I look forward to the next instalment!

[Arm]

Brilliant. Helps fill in a few gaps with my understanding of dampers.