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Putting a damper on things....or no shocks please? A look at a damper dynamometer


Tony Cotton spends an afternoon watching his Vauxhall Junior dampers being tested
The dynamometer

A few years ago, Graham Easter penned an article about dynamometers. An engine of unknown power worked against a resistance and by measuring the energy absorbed the power and torque of the engine was plotted. Imagine that the power of the engine was of no interest, in fact replace it by an electric motor.Then measure the forces that motor imparts on something of interest. A damper. The principle is the same - a fluid (water in the engine dyno, oil in the damper) is worked and the effect is measured. So we've gone from an engine dynamometer to a shocker dynamometer. In this case it's an SPA. SPA are, of course, the UK distributors of Penske dampers, and known as quite handy fabricators.

The load cell Temperature band strapped round damper and pressure measuring device (right) - a tyre gauge just empties any pressure

Ray Rowan invited me to see his SPA shocker dyno, which he's had for many years. It's a box the size of bedside table with a powerful electric motor (around 7kw) which moves a piston up and down. The damper is attached to this piston and by programming the dynamometer the stroke of the piston can be changed so that the damper can be given a work out over its operating range. Another adjustment allows the speed of the motor to change, thus affecting the speed at which the damper is worked. It may be obvious to engineers (though it certainly wasn't to me at first) that the graphs produced by the shocker dyno are merely illustrations of how that damper behaves when subjected to the piston moving in the way it does on the test. Therefore it isn't possible to readily compare 2 dampers if the test conditions are different, and the graphs produced aren't like those for an engine on a dyno which should be much the same on any dyno (though they never are). To complcate matters even further, we all know that oil's viscosity changes with temperature and that friction warms it up. The same damper will behave differently for 40 seconds on a hillclimb car at Harewood in March and for 24 hours on an LMP2 at LeMans in June. So there's also a temperature band which straps to the damper and a setting to generally bash it about a bit until it warms up to a set temperature.

All of this motion is captured by a load cell at the top, and the program draws a series of graphs based on the sample runs. The same data is used for several styles of graph - they show the same information, just differently presented. In the rest of this article, the same dampers were run with the same settings on the dyno, so they are generally comparable (except for the Ohlins at the end).

The first graph (below left) shows what resistance the damper is putting up to the motor at different points in the testing cycle, and plots the force at each position. Positive movememt is bump, negative rebound. The graph shows 8 different runs with the low speed rebound getting progressively stiffer, a "click" (actually a flat on the adjustment nut) at a time with the red line being full stiff.

By convention, the middle of the stroke is taken as the zero position and the most negative value is the bottom of the cycle with the damper "fully" extended. The quotes are there because it's only fully extended within the stroke used for this test. Starting in sector "A" with the force at zero, the damper is at the bottom of the stroke. As the piston moves up, the linkage inside the machine produces an increasing speed - if piston speed is plotted against time it's a good approximation of a sine wave. Hence in sector A the damper starts off providing little resistance but it very quickly builds up as the piston moves up the stroke. What's important to remember, though, is that in principle (and we'll come later to why this is wrong) the resistance to movement of the damper at a given speed is the same wherever it is in its stroke. What changes the resistance is how fast the piston is moving. At some point in the graph the low speed damping gives up its role to high speed damping in bump. This continues while the piston is proceeding in an upwards direction at its highest speed past the mid point of the stroke (position = 0) and into sector B where it begins to slow. As it slows the resistance decreases, and since the damping is supposed to be speed dependent, the curve in B might be expected to be a mirror image of A. It might be on an F1 unit, but on the dampers in Mono, even the slightly exotic ones which cost well north of £1,000 each, it isn't. The reason is that most materials, damper oil included, have what's called hysterisis. Put crudely, it's a sort of memory, that once they're doing one thing they want to carry on doing it. This means that the graph is not quite symetrical. This will be a bit clearer in the next graph. Sector C starts with the damper stationary at "top dead centre" and as it is pulled down and accelerated, the damping increases and the high speed valves take over. The effect of the low speed damping being changed by the adjuster is far more pronounced on rebound (sectors C&D), as it should be since that's what it's designed to do. Maximum damping force, on this test, more than doubles from 50 pounds at the lowest level of damping to 120 at the highest when the damper is passing its mid point at maximum speed. (At the risk of labouring the point, maximum speed in this test - if the speed of the piston moving increases, the damping will amost certainly increase at the maximum.) Finally, in sector D, the damper decelerates back to the bottom of the stroke, nearly symetrically with sector C.

The next graph (far right) is the same data, but as the main interest is how force relates to the velocity of the piston, force is plotted against piston velocity. Sectors A and B are above the line and sectors Cand D are below the line, and as force is the main interest rather than whether it's positive or negative, the plot is of the absolute value. This has the additional advantage that the hysterisis can be seen as the gap between the 2 lines for each setting. It is negligible for the softest setting, but quite severe on the stiffest. However, the graph with 8 traces on looks a bit busy so for a "quick and dirty" look at the traces a simplified version which averages out the traces is available; the smaller one below.

Simplified force/velocity graph

Why Use a Dyno?

Like most workshop testing and computer simulation, the main purpose of the shocker dyno is to give objective, measurable results and to do it quickly.The dyno won't give a definitive setting for the shocker, but it will give a very good indication of where to start. Whilst the right setting for the damper tested could perhaps be obtained in a morning at Mallory Park, all of these graphs were done in a 15 minute period while drinking tea. Even more convincing is the very adjustable Ohlins damper shown below. As can be seen, the bump setting barely change but rebound can be set to anything from FVJ soft to at least twice the maximum rebound stiffness of that unit. The snag was that there were 30 different positions on each of the 3 adjusters, (27,000 variations) and on some of them the first 20 or so positions did virtually nothing and the last few had a massive (but not equally spaced) effect. Trying them out on the track would have been time consuming and potentially disastrous. Actually, remove "potentially" because these were a second hand set Ray had in stock, which he acquired when the previous owner reportedly accidentally set them so stiff that the rebound "grabbed" the car over a few bumps and pulled it onto its floor, at which point the steering stopped working.

 
Ohlins damper   Depressurised damper

Another example is the depressurised damper - the gas pressure of one of my FVJ rears has leaked out. The red line shows the test results on the same settings as the blue line after re-pressurisation. Lest you've forgotten why dampers are gas pressurised, it's to put the damping oil under pressure and stop it from cavitating as the piston drags through it. The effects of cavitation are clear, but since the deterioration was gradual the driver did not notice it. On other traces the cavitation is not only clear, but it happens at different points of the curve on different runs.

Naturally, there's a lot more to it than I've covered here, and it's certain that there's a lot of gains to be made from correctly adjusted dampers.

I would like to thank Ray Rowan for his patience in demonstrating the dynamometer, and the SPA/Penske website for the information on both the dynamometer and their shockers. For the avoidance of complaints, many makes of dampers are available, mostly excellent and with different cost/peformance benefits and neither I nor the club endorse any one over another.

Damper Adjustment - a brief summary

A damper literally damps the spring action, slowing its movement down in bump and rebound with the objective that the tyre keeps in better contact with the road.

A racing damper will usually have adjustment for low speed rebound (which usually also affects low speed bump) and separately for high speed bump and rebound. The latter may need to be adjusted by internally changing the valves of the damper, leaving the low speed as the only user adjustment. The low speed adjustment helps determine how quickly weight is transferred through the springs and accordingly whether the tyre is loaded sooner or later.

When the grip is low, for example the wet, the tyre doesn't like sudden changes, so the bleed on the low speed is increased making the damper softer, and the tyre loads up slowly. Conversely, when the grip is higher, the tyre's grip can take the more instant loading and so more damping leads to sharper turn in and a more stable platform. The driver will usually describe the stiffer car as "crisper" or "having better feel". The stiff set up also loads the tyre quicker under braking as the weight transfer is to the front. Sometimes under braking the tyre bounces - high speed bump can be increased to reduce this. However, low speed damping can be too much, and then the grip is reduced on change of direction, there's a lack of overall grip after initial turn in, and the car can lack traction, spinning its wheels in slow corners. Reducing low speed damping helps cure this.

Another reason for increasing low speed rebound is an apparent lack of stability. Increased low speed rebound can enhance stability, though if done to excess it can contribute to higher tyre wear. Softening the rebound on the front allows weight transfer to the rear under acceleration; softening the rear gives more weight transfer to the front under braking and turn-in.

Changing springs isn't something that the average Mono driver does regularly, but given that the car starts off with a reasonable balance, then the rule of thumb is that a stiffer spring requires softer bump and stiffer rebound. A softer spring requires more bump and lower rebound.

If you have the luxury of separately adjustable bump and rebound and are brave enough to tweak them, this table shows the effects which Penske expect. But remember they are the usual effects "all other things being equal", and as with everything in racing (and indeed life) reality has a habit of butting in.

Usual Effects : Front adjustment Rear adjustment
Bump Middle and Entry Middle and Exit
Rebound Middle and Exit Entry and Middle

Tony Cotton

Disclaimer: The above represents only the unofficial view of the writer and not of the Monoposto Racing Club in any way whatsover. Subheadlines and captions are not originated from the named author. We are unable to reproduce results due to copyright reasons. If any pictures are copyright and the owner wishes them removed please email us.