Evaluation by dynamic measurement

The reason for an evaluation by dynamic measurement

The driving behaviour of an A-type is very pleasant, most road surfaces are taken without difficulties. On normal flat surfaces, one gets a feeling of driving a car with a very sofisticated hydraulic suspension. Small uneven surfaces are unnoticeably flattened out. Remarkably, whatever brand of tyre or shock absorber is mounted, the road responsiveness is almost always the same.

This nice experience is however very often not confirmed when an A-type is undergoing a suspension test whereby the so called road contact, and/or an derived characteristic called phase shift, is expressed by a limiting value. Most European countries, all adhering to the same legislation, have not added this kind of road simulation testing in their yearly car safety testing. The few who did, have not taken into account that, not all suspensions can be tested correctly on such test bench.

The question was kept alive : what is the added value of this simulation test for the A-types, when it is very clear that the heavy construction of the suspension is totally different from the modern cars with a vertical lightweight suspension.

The measurement method

A road surface can be flat or very rough (e.g. cobblestones). A vibration table is used to move the tyre up and down over 6 mm. To commence the measurement the table is first vibrating at a frequency of 24,5 Hz. Then the table is running free bringing the frequency down to zero. The dynamic weight is counted with the static weight as reference.

When starting this project we have gathered a lot of information regarding the A-type suspension form many Citroen A-type drivers, within my region as well as abroad. I also saw different viewpoints from drivers after they visited the yearly safety testing station.

We were astonished to see that no clear clarification was given for the cause of the difficulties with the suspension testing.
Some answers stated that the correct shock absorber should be applied, some explained that the most flexible tyre had to be chosen. Of course both components are dominant in the road handling behaviour. And on top of that the shock absorber decided by Citroën to replace the “batteur” and “frotteur” is no longer produced, since many years now.

Two values can be taken into account

– the value for road contact (agreement from the Shock absorber manufacturers)

– the value for shock absorber damping (initiated for lightweight cars) which is expressed as a phase shift value. This should be minimum 35°;

The testing, with the purpose to investigate the behaviour of the A-type suspension, has been executed by two students for their end-therms at the  high school*.

On top of those extensive testing, the writer obtained extra knowledge due to the testing by the tetsing station of an affiliated car with lowered suspension.

For practical measurment reasons a running chassis was taken by the students, however without a body, instead loaded with cans filled with water to come to a comparable weight and weight division.

Because it is known that tyres and shock absorbers are the most important variables, several different brands for each were put under investigation.

Because the testing was not meant to compare brands, the names of them will not be published. This was also the case for the students to make a thesis without brand names of the used components. But do not worry, the results are very revealing !

The bachelor students did hundreds of test runs, with most variants thinkable. Some comparison tests were performed on other lightweight cars (Fiat Panda and Morris Mini) too. These show no problems to meet the criteria, the reason is that they have vertical mounted suspensions;

Most impressive were the findings of the bachelor students :

– all 125X15 tyres readily available in the market react more or less in the same way to the frequency sweep of the test bench. The 135X15 tyre is a bit more flexible.

– the oil filled shock absorbers tested and which are sold nowadays are performing well after the air within is mixed in the oil by “pumping” them many times before they are put to the dynamic suspension test. Of course when the car is standing still for a long time the performance diminishes slightly. Driving remains very acceptable.

The gas filled shock absorbers, introduced a few years ago, are not tested on the dynamic tester of the high school. Experience gained by A-type drivers learn that these absorbers perform as expected, increase the absorption of high frequentcies and do not need the “pumping” before testing.

– Even with good tyres and shock absorbers the limits for phase shift are too strict.

So herewith we would like to thank the bachelor students greatly. We also would like to highlight that their end work was chosen to be the best presented amongst all from the other students ! The jury members, some representing known and large car companies, really were astonished by the outcome of the testings performed and showed their great interest.

On top of the tests performed by the bachelor students, we also got an inside in the test results of vertical mounted (Mac Pherson) suspension on nowadays regular cars. Visiting an MOT station, we clearly noticed that the dynamic test has been developed for cars sold late 1980. All cars showed results far above the limit of 20 for road contact, but also the limit of 35° for phase shift. Large margins were noted of 50 to 55 ° over the limit. Knowing that the cars checked were at least 4 years old, some even 9 years but still giving results largely above the limit.

To remember : the test bench for dynamic testing of suspensions was obviously developed with a view to judge the degradation of vertical mounted supensions and shock absorbers. A large margin was given in order to leave sufficient room for defining the safety level and at the same time to allow a normal degradation of the applied shock absorbers.

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