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Dynamometer Testing

Dynamometer Brake Testing

It is mandatory that all replacement brake lining assemblies in South Africa comply with a minimum specification as is published in Notice No. R443 appearing in Government Gazette No. 33211 dated 28 May 2010. This specification is also known as VC8053.

The above specification is regulated by the National Regulator of Compulsory Specifications (NRCS).

This specification is the United Nations Economic Commission for Europe Regulation (ECE Regulation 90) with some National changes. The performance requirements for the brake lining assemblies for heavy motor vehicle remains unchanged however there is a specific cold performance requirement for disc brake pads fitted to light motor vehicles in place of the comparative performance to the originally fitted brake pads as required by ECE Regulation 90. The vehicle fade test specified in ECE Reg. 90 has also been replaced by a test conducted on an inertia dynamometer.

Test Design

It is logical to conclude that dynamometer tests will be designed using the above specifications as a guide. These specifications are specific about the required testing and go as far as to supply the pressures, speeds, temperatures and torque’s required to do the testing.

A test report should supply the required results in a format that would be recognised by engineers world-wide. Such a report is supplied by ECE-90 Brake Testing (Pty) Ltd and is discussed in some detail below.

Dynomometer Testing Tips

Test Interpretation

Dynamometer Test Overview – A short overview of the Test Sequence will be presented below, followed by a more detailed explanation of each stage supported by presenting actual portions of the complete test sheet that is shown above. This will assist the reader to have a better understanding when interpreting a test.

  1. Brake pad bedding-in procedure. In order to do a test the friction surface of the pad need to be bedded-in first of all. This is to ensure a proper contact surface between disk and pad. (For the same reason one should drive careful for the 1st 100 km’s when a vehicle was fitted with new pads. Also see article on Burnishing) There are proposed bedding-in procedures in existence but this portion of the test is not regulated as such. It is therefore up to the manufacturer to specify a specific and suitable procedure. For this reason it is common that a full dynamometer test is used for bedding-in.
  2. Test Preparation. After the bedding-in cycle the pads are removed from the calliper, weighed and measure for thickness. The pads are then replaced when the weight and thickness measurements as well as the other relevant information have been entered into the control program. (See Fig. 2 above, the General Information call-out.)
  3. 3 stops at 30 km/h are then done to heat up the brake pad samples but are not reported on the Test Sheet.
  4. 3 stops at 40 km/h are done at the predetermined pressure. The average torque is inserted under Test Results Type-O (40 km/h). (VC8053 & ECE90 requirement) – See plot in Speed Sensitivity area
  5. 3 stops at 80 km/h are done at the predetermined pressure. The average torque is inserted under Test Results Type-O (80 km/h) (VC8053 & ECE90 requirement) – See plot in Speed Sensitivity area
  6. 3 stops at 120 km/h are done at the predetermined pressure. The average torque is inserted under Test Results Type-O (120 km/h) (VC8053 & ECE90 requirement) – See plot in Speed Sensitivity area
  7. 3 stops at 160 km/h are done at the predetermined pressure. The average torque is inserted under Test Results Type-O (160 km/h) (VC8053 & ECE90 requirement) – See plot in Speed Sensitivity area
  8. 3 stops then done to calculate maximum pressure values for the Cold Performance Equivalence Test as well as the pressures for the residual performance test. These are not reported.
  9. 6 stops are then done at 80Km/h but at increasing pressures for the Cold Performance Equivalence Test. – An ECE90 requirement only.
  10. A further 3 stops are done to determine the pressure to be used for both Fade Tests.
  11. 1st Fade Test – 15 repeated stops at regular intervals (eg. 45 sec. intervals from 120km/h to 60km/h.)
  12. 1-off residual performance stop to determine the hot performance of the brake pads.
  13. 2nd Fade Test – 5 more applications done at similar intervals as 1st fade (A Safeline only requirement.)
  14. 2nd residual performance stop (a Safeline requirement)
  15. Steps 4 to 7 then repeated – This is done to see the change in performance after the pads were exposed to a high heat cycle (This is done for information purposes only and is not a requirement)

Test Completed and Printed

0212_dynamometer_02a


Fig. 2a Dynamometer Results Data Sheet

Test 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.

1. 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.

Another value, ideal pressure in Bar, is also calculated during the OE “finger printing” test procedure. This value is also drawn from the database of OE references and used by the test program but is not supplied on the test report. It follows that this value is different from reference to reference.

The value (1119 Nm) in the 1st column is the average obtained of the 3 Type-O Stops at 80Km/h. (See 5 of the Test Sequence) In column 3 in this case a 10% Difference to OE is reported. According to Regulations ECE 90 & VC8053 a difference of ±15% is allowed for after market brake linings. The pads therefore pass this portion of the test well within the requirement.

2.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.)

3.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!

4. 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.

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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.

5. 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.

6. 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.

7. 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.

8. 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.

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Fig.2c

Fade Test Section

1. 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.

2. 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)

3. 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.

4. 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.