Automotive E-ssentials

Physical testing of buses

The number of M2 and M3 category buses tested at our TÜV SÜD laboratories located in Czech Republic are increasing. The whole certification process does not only include the necessary documentation, but the buses must also undergo a series of physical tests, both in terms of strength and driving. Certainly, some of the most demanding tests are the rollover and lateral stability tests. Until now we have organised this type of test for our customers in external laboratories, but now we are also able to carry them out under our own roof in the newly established laboratory at the Bezděčín workplace.

The existing premises for testing bus rollover offered only limited options. In particular, we have experienced limitations on the dimensions of test samples and since the test rooms have been outside, we were also dependant on outdoor conditions. For customers, this meant that tests could only be performed under good weather conditions, usually only in the summer months. “The construction of a new bus rollover test laboratory is a big step forward. Now we can offer these tests almost all year round and we can test all categories of passenger vehicles thanks to the great variability. In addition, due to changes in Regulation ECE R66 – bus rollover, we expect greater interest in these tests, especially in the category of smaller van-type buses. This test is already mandatory on new models if they carry 16 or more seated passengers,” says Jakub Jelínek, TÜV SÜD Czech Profit Centre Manager.

ECE R66 physical tests

The body strength test is one of the key tests concerning the protection of passengers in the event of a rollover. The principle of this test is the inclination of the bus to an unstable position and the subsequent rollover of the sample from a height of 80 cm above the ground. If the bus is equipped with passenger restraint systems, its operating mass will be supplemented by a fixed weight for the passenger (34 kg per passenger) and 75 kg for the driver during the test. A bus without restraint systems is supplemented by the driver’s weight only. Before the test, the impact side of the bus in the worst-case scenario, the less rigid side, is carefully selected.

Typically, this is the side with fewer doors and windows. Increasing the number of doors and windows means adding extra support columns to the chassis, thus reinforcing the whole side. This physical test is very demanding on the structure. During an impact, the chassis has to absorb a lot of energy, for example, around 50 kJ for small buses. To illustrate, that is the same energy as when a 100 kg weight falls to the ground from a 50 meter height. With large 13-metre buses, the energy value is in the hundreds of kJ. There are also very significant structural deformations due to the high impact energy values.

Most commonly, plastic junctures form on the columns, and only rarely do the columns fracture. The result of this test is deformation of the body structure, which must not penetrate into the passenger compartment. This space is located 150 mm from the vehicles inside lining. In addition to measuring the amount of deformation of the main columns, tests adjusted to customer requirements can also include, for example, bolt forces, acceleration by using accelerometers, or tension gauge components. Video tracking is another important test. Here, the movement of points on the test sample are being evaluated by using high-speed camera recording. The advantage of this measurement is a more accurate description of the deformation process, which can be very well validated in a numerical simulation based on this data.

Combining virtuality and reality

TÜV SÜD is one of the few companies that can offer customers both physical and virtual tests. There is a great interest in the finite element method, especially among smaller manufacturers who do not have much experience with Regulation ECE R66. Very often, the design must be optimised before the desired result is achieved complying with the regulation. This virtual procedure can be very effective compared to physical tests. Unlike physical tests, it is possible to test several construction variations in a short period of time and prepare one final variation for the real test. The virtual test procedure is now also recognised as homologation and is sufficient for a certification. The CAS Centre is accredited for this type of test thanks to the successfully validated real test model.

Modern technology is an advantage

The laboratory is one of the most modern testing facilities for R66 type tests. Its chief merit is that buses with a length of up to 13 m (two axles) can be tested inside the laboratory in an enclosed area where the test is not limited by weather. Many competing test facilities are outdoors, and they are only able to run the tests in the summer months. In addition to small and two-axle buses, the laboratory will also be prepared for physical tests by rolling over large articulated buses with three axles. However, these tests will be performed outside in front of the laboratory. Testing articulated buses is one of the unique advantages of the new test centre. This flexibility is made possible using three mobile platform systems.

These platforms will be computer controlled with very precise synchronisation. Customers have the advantage of being able to monitor the platform tilt angle in real time during the test. They will be able to view this data along with the live image on their mobile device via Wi-Fi. There will be a total of three mobile platforms with a tipping area of almost 2 x 3 m. The bearing capacity of one platform will be 13t with a maximum tilt angle of 55°. These specifics are more than sufficient for most commercial buses. The unstable position of most means of transport is between 30° - 40° and in exceptional cases, for example, buses with a centre of gravity very low to the road, the position is up to 52°. Synchronisation is also very important under each axle for separate platforms. Here, high demands were put on accuracy and individual platforms should not deviate by more than 0.1°. This is ensured by powerful servomotors and precise linear guidance with trapezoidal screws. The issue with this test is difficulty in determining the unstable position of the sample. In the past, this problem was solved only visually by subtracting from the test video recording, but this method is greatly inaccurate. Therefore, the new test facility will be improved by adding contact switches under the wheels, which can accurately identify when the last wheel is disengaged from the platform, and the platform itself will evaluate the unstable angle of the sample.

Bus stability

Another advantage of the new test facility is its flexibility. The tilting platforms will be used not only for rollover tests, but also for equally important stability tests according to Regulation ECE R107. The stability test is a provision to prevent the bus from rolling over on sloping roads. In stability tests, the platforms are modified vertically, and the bus is then able to drive onto them under its own power, as opposed to the rollover test, where the bus must be lifted up. The test is non-destructive, and the sample is tied to the platform. Then, according to the regulation, the bus is tilted to 28° without visible loss of tire contact with the platform. This test is difficult to perform for manufacturers because the bus is loaded up to the total effective weight (i.e. the maximum permissible axle load) with all passengers (68 kg per passenger). In addition, because of the suspension and tire deformation, the body tilt angle is much larger, even up to 7° compared to the platform tilt. During the stability test, it is possible to have active chassis stabilisation. If the wheel does not lose contract at 28°, the test is successful for the test sample. Often, however, tests also determine the real value of the unstable position.

Other laboratory activities

Since the portfolio of services in the automotive industry is very wide, the new workplace at Bezděčín will not only focus on the abovementioned tests. It will also be possible to determine very precisely the position of the centre of gravity in any vehicle on the platforms, ranging from cars to utility quads. The tilting platforms can also be used for cyclic testing of the engine running on an inclined plane. Furthermore, there will be an assembly pit in the laboratory, used mainly for driving tests, where the buses will be fitted out for testing the brakes, driving stability and other aspects. The laboratory will also be used by our colleagues from emission measurement, who will use the assembly pit to check exhaust systems and connect measuring equipment.

For more information please contact our expert Petr Záruba