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Tuesday November 21st 2017

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Standardized braking test – Discussion and Conclusions

Difficulties During Testing

Because there were different braking areas along the track, the conditions for each braking test were slightly different. Therefore, the adhesion coefficient and consequently the rate of braking changed over each braking event. If more time had been available the brake events would have been performed in just one braking zone to increase accuracy of results.

Weather conditions at MIRA at the moment of the test:

  • 6ºC
  • Damp and dry depending on the area of the track
  • Wind: variable 0-5m/s

The test track stayed with the same conditions during the whole test.

The test driver estimated the effort needed to reach the desired Brake Line Pressure for each break event and did seldom meet the target pressure exactly. The actual values were used for the analysis.

 

Analysis of the Data

All the tests were conducted with the vehicle transmission in neutral position. Therefore, only the braking effect of the brakes (and aerodynamic drag) was considered. For a more realistic result, involving the engine brake effect, the car would have needed to be left in gear during the brake events.


Tests 1, 3 and 4 plots involving Rate of Braking vs Brake Line Pressure should project back through the origin (0,0). They should behave like that because when there is no brake line pressure there should be no braking force on the brakes. With no braking force the rate of braking should equal zero, hence the trend line should pass through the origin. However, in the test there were forces acting against the vehicle which made it come to a stop. These forces include: internal friction forces, rolling resistance in the tyres and aerodynamic drag coefficient. All these forces would eventually bring the vehicle to a halt. These forces can be seen in the trend line of rate of braking as very small values just before the braking event begins.

When comparing the results of the Case Study with the Test Data it can be seen that there are not dramatic differences.  In terms of rate of braking it can be seen that both the Case Study and the Test Data plots follow the same trend. This close relation can be explained by the fact that the track conditions were almost optimum and that the vehicle had been equipped with new brake pads and discs recently. Therefore the efficiency of the system was very close to 100%. If the track was in a poorer condition, such as wet, the results for the braking rate for the Test Data would have been drastically worse.

If the car had been loaded to its maximum permissible weight the rate of braking would have decreased. This is because the momentum in the vehicle would have increased and therefore, with the same braking force the car would have needed a longer distance to stop.

When the braking distribution achieved in the Case Study is compared to the Test Data (1 and 2) it can be seen that both behave very similarly. This is, again, due to the good conditions encountered during the test. The braking distribution from tests 3 and 4 was very different from the one in tests 1 and 2. This is because tests 3 and 4 only used one axle braking, whereas tests 1 and 2 used both axles braking. Therefore the braking distribution between 1 and 2 differs from the braking distributions from 3 and 4.

The calculated values of adhesion were consistent throughout the test. The overall adhesion values were reasonable for the slightly damp track conditions. The European Braking Directive states that the ABS Adhesion Utilization should meet the value of 1.2, however the Adhesion coefficient calculated was of 1.05 which is under the legislation. This can be explained because of the damp track.

The mean adhesion coefficient was calculated to be 0.788. It can be seen that this adhesion coefficient corresponds to a rate of braking of 0.72. In other words, this means that the rear wheels would lock first. However, it can be seen that the actual rate of braking achieved was much higher (0.92) until the rear wheels locked. This can be explained by the fact that this braking event took place in an area were the tarmac was very rough, so it provided higher adhesion coefficient that the mean value.

If the test 1 was carried with the booster disabled and the vehicle loaded to its maximum permissible weight (18394N) the vehicle would have passed the Braking Directive R13H requiring 2.44m/s2 (0.248g) rate of braking with a maximum pedal force of 500N.

The legislation requires a minimum of 0.248g, therefore the vehicle passes the Braking directive.

conclusion

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