Brake testing South Africa

ECE90 Brake Testing (Pty) Ltd is a company that was established with the objective of providing a test and certification service for replacement brake lining assemblies. This being primarily to the requirements of the Economic Commission for Europe ECE Regulation 90 but also other national, international and industry specifications used in the brake friction industry.

Friday, October 13, 2006

Dynamometer test results

Dynamometer Test Results and interpretation:
W.S. Scholtz
dynamometer testing results

The Test Results reported comply with both the ECE 90 and VC8053 regulations. By looking at the call-outs 1 to 4 in Fig. 2a above the reader is referred to a point by point explanation of the various results that is presented.

In the centre column a value of 1016 Nm was inserted. This is a very important value and is used as a base point for the % Difference calculation that appears in the 3rd column. This value is obtained by fitting a set of Original Equipment (OE) brake pads in the dynamometer and then to test it to the relevant amount of stop applications to the various pressures and speeds as is stipulated in ECE 90 and VC8053 regulations. This is a test that only needs to be done once to obtain this value. By selecting the pad reference to be tested this value is drawn from a database of OE tested pads and is inserted into the test program. It follows that this value is different from reference to reference.

These values, as is recorded in column 1, are the average of stops as is done during the Speed Sensitivity Test. See Fig. 2b. The value of importance is marked by the grey dots and is the average torque taken over 3 stops for the relevant speeds as shown. The Type-O (40 km/h) average is shown but does not form part of the % Diff. calculations. Although it is a meaningful and interesting value it is not required by both Regulations. Of interest to the reader at this point is to observe that all these values are not far from the recommended OE value as discussed in 1 above. By observing this the reader should get a sense of the fact that the pad that was tested compares well with an OE supplied brake pad. See the Speed Sensitivity discussion below for a better understanding of these values and how they were measured. The core idea is to understand that they are the average torque’s taken over 3 stops for the relevant speeds as shown. (These stops refer to points 4 to 7 in the Test Sequence as presented above.)

The value in the centre column is the same as the value in 1 above, the OE requirement. The value, 650 in this case, is the Residual Performance Test result. (See point 12 of the Test Sequence.) The 64% reported is the percent calculation between the residual (hot performance) and cold performance Value. Regulations ECE 90 & VC8053 require that the hot performance of the samples on test not be less than 60% of the performance obtained when the pads were cold. The pad in this instance also passes the test. An interesting fact is that an OE pad does not have to pass this specific requirement!

This residual value after the 2nd Fade is for observation purposes only and not a requirement. It can be noted that both these residual values does not differ much which is also a good sign.

Fig. 2b

Refer to points 5 to 8 of the Test Sequence as described above. Also see point 2 of Fig. 2a where the results of these stops are reported.

The 1st Speed Sensitivity section of the test is a requirement in both regulations. These Type-O stops are plotted as in the graph above to supply a graphical indication of the test. For instance, at 40 Km/h, the average of the 3 stops, a Friction Co-efficient (Mu) of about 0.36 was achieved. At 80 Km/h the Mu was almost 0.4! (Mu, the Friction Co-efficient is simply put the value obtained by dividing the clamping force of the calliper by the driving force of the disk. Mu is therefor simply a ratio and has no “magic” attached. – The writer will soon publish a full article explaining Friction Co-efficient (Mu) in more detail.

This is an repeat of the stops done in 5 above but after the fade tests have been done as per point 11 of the Test Interpretation described above.

During the Cold Performance Equivalence Test 6 stops are made from 80km/h allowing the stop to zero. The stops are made at increments of pressure specified in bar, whilst the torque, reported in Nm is recorded. These stops are then plotted as per the blue line in the graph above.

The lines in red indicate the upper and lower limits as is specified by Regulation ECE90. It is required that the torque curve obtained be within the limits in the upper two thirds of the graph. It is clear from this graph that this test passes the requirement. It follows that if the brake pads being tested do not comply the graph will be plotted well below the bottom limit and typically curving downwards.

Fig.2c

Fade Test Section

This indicates the pressure (information only) at which the fade test was conducted as per calculations done by the test software. (See 10. of the Test Interpretation above) The lower this pressure is the better the test pad performs. In this instance, with a test pressure of 31 bar the test pad performs well and it is close to the test pressures reported for OE pads of the same reference.

During the fade test 15 brake applications are done at very short intervals from fairly high speed. The temperature versus the friction co-efficient is plotted together. During these stops pressure is applied which allows the pad to be tested to become very hot and as a result one can observe the behaviour of the test pad during high temperature applications. An inferior test pad will fade drastically, especially during the 1st 7 stops after which it will recover somewhat. This is also referred to as Green Fade (Please see the previous article “Burnishing of Brake Linings” for a full explanation of Green Fade)

After completion of the total Dynamometer test the Wear and Mass loss is calculated and reported in this position. Although this is not a wear test per say it does give one an idea of durability, especially when compared to other tests. There are specifically designed wear tests but is not within the scope of this article. Mass loss is of interest to the manufacturer of the brake pad and is specific to the friction material being tested. It is not an indication of pad performance as such and is of academic value only to the layman.

This graph shows a second fade test and is not required by Regulation ECE90 requirement of Safeline as a further indication of pad performance. Temperature and Mu is plotted for a further 5 brake applications.