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Dynamic Brake Tester Many well-specified cars now have sophisticated systems to provide safety by over-riding the driver's inputs. ABS anti-lock brakes were the first to be widely adopted, and have been followed by ESP (Electronic Stability Program), while the latest Lexus LS460 has grill mounted radar to apply brakes if the driver gets too close to the car in front. Unfortunately, these capabilities often work in opposing ways - for example ABS functions by reducing brake effort to allow the wheel to continue to rotate instead of locking up, thus maintaining useful contact with the road surface. ESP can sense both wheel-spin and yaw, and reacts by reducing power of the engine and applying brake effort to the spinning wheel to retain traction. Yaw is countered by application of brake effort to the opposite rear wheel, so pulling the vehicle straight. Put simply, modern cars can apply their brakes without the brake pedal being depressed, and also can reduce brake effort despite increased brake pedal force. These different safety systems mean that customer complaints regarding brakes that "pull" to one side are unpredictable, or do not feel efficient, cannot be simply cured by bleeding. On board diagnostics are effective in identifying fault codes, but cannot identify problems for which the parameters have not been pre-programmed. Comptek was asked to produce a tester which can measure, store and display real values of speed, pedal-pressure and actual hydraulic pressures within the four calipers (on wheel-cylinders on drum brakes). These parameters allow qualified personnel to reproduce the conditions under which the complaint of problem presented itself, and also to repeat the test exactly duplicating wheel speed at the start of the test, and brake-pedal force which are the two variables dependant upon driver input. Total braking effort can be calculated by deceleration, while imbalances and interventions due to ABS, ESP etc. can be examined in minute detail by expansion of the graph or reading the raw data in Microsoft Excel format. The following data illustrates just how useful this equipment can be for detecting intermittent faults. The following is real data recorded from a vehicle with a brake fault. "Chart 1" demonstrates caliper pressure and brake pedal pressure data recorded over a period of time. Click on each chart for a larger, clearer version. Chart 1 - Click to enlarge
From looking at the larger version of the graph it is easy to see that caliper 2 is not behaving as it should; sometimes its trace demonstrates expected caliper behavior, other times it can be seen that the same caliper sees no pressure. From examining the data more closely a better understanding of what is happening can be realized. See charts 2 and 3. Chart 2 - Click to enlarge
Graph 2 is a section taken from Graph 1 at a point where the brakes seem to be behaving as they should and demonstrates all four calipers working correctly. It can be seen that as pedal pressure increases (the operator is pressing his foot down on the brake pedal), caliper pressure increases accordingly, as would be expected. Chart 3 - Click to enlarge
Graph 3 is taken from a point in Chart 1 where there appears to be a problem with caliper 2. By examining this graph, it is evident that the brake pressure in caliper 2 does not rise with pedal pressure as would be expected. By also examining the raw data recorded in table 1 it is possible to see exactly where this fault is occurring; the highlighted section indicating the lack of expected brake pressure. Table 1 is again an excerpt from the complete table covering the full run: Table 1
The fact that caliper 2 sometimes seems to experience brake pressure and sometimes not, gives tangible, dynamic evidence of an otherwise almost impossible to detect brake fault. The fact that the unit records data live, whilst the vehicle is being driven allows the operator to determine and record evidence of a fault which may not be detected or simulated statically in a workshop. |
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