When a car is on a lift at a service center, finding the source of the problem is not always easy: the steering is fine, the shock absorbers are dry, the ball joints are in order. Visual inspection does not reveal any defects, even rocking the suspended wheel does not help. The only way to detect the malfunction is a special suspension testing stand. And only then does an experienced mechanic begin to suspect the bushings. And indeed, the suspension arm bushings are to blame. But why did they fail if their service life is far from exhausted, even considering the quality of our roads?
Suspension arm bushings, like silent blocks, are designed similarly. Their task is to provide an elastic connection between suspension elements and the body, damping shock loads. This protects parts from damage and adds comfort for the driver and passengers.
The rubber filler in bushings and silent blocks can be of two types: solid rubber or hydraulic fluid. Let's start with the more complex ones — hydraulic bushings. They were created to increase comfort and fully justify themselves during measured driving on smooth surfaces. But on uneven roads, during sharp acceleration or braking, the load vector on the inner metal sleeve changes compared to calm movement. The central sleeve shifts not only parallel to the bushing axis but also experiences longitudinal (axial) and angular (cardan) forces.
Due to such loads, hydraulic bushings and silent blocks can crack, and the fluid leaks out. The result is deteriorated handling: the car seems to drive on ice. A simple solution is to replace the original parts with those where the hydraulics are replaced with regular rubber. For example, the German company Wulf Gaertner Autoparts AG produces MEYLE-HD arm bushings without hydraulic chambers, filled with solid rubber, for many prestigious European cars. They did the same with silent blocks: in MEYLE-HD versions, even if the original had hydraulics, there is no fluid.
But simply replacing the hydraulics with rubber is not enough. The shape and design of the rubber element made of natural rubber must provide comfort comparable to a hydraulic bushing. From a reliability standpoint, a simple replacement also does not solve all issues. If the rubber forms folds when bending, it quickly deteriorates in those places.
There is another common problem with all rubber bushings. When the car is on the ground, the suspension arm is in its normal position. When lifted on a lift, the wheel drops down, and in this position the arm is often replaced. Lowering the car to the ground, the arm returns to its place, and the rubber in the bushing twists. If the wheel then hits a deep pothole, when driving out of it and compressing the suspension to the stop, the rubber can tear away from the metal sleeve. To avoid such problems, MEYLE uses computer modeling of rubber elements and a bushing calibration procedure.
When creating the fully rubber part of MEYLE-HD bushings, computer modeling is used. This allows calculating a shape in which the rubber does not crumple into folds but compresses and stretches evenly. After all, if the rubber is "chewed," it quickly cracks.
Calibration is a complex but effective process. If you simply pour liquid rubber into the arm bushing or between the sleeves of a silent block, it shrinks when cooling, which on bad roads can lead to rubber peeling off the metal. Therefore, at MEYLE, the outer sleeve is initially made 5 mm larger than the required diameter, and after pouring the rubber, it is crimped. As a result, the rubber in the bushing is slightly compressed, which improves its adhesion to the metal and creates a reserve for stretching in areas opposite to the direction of the central sleeve's displacement.
So even such a simple part as a rubber bushing can be refined to increase reliability. If we talk about more complex suspension elements — joints and ball joints — there the possibilities for improvement are even wider. But we will tell about that next time.
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