They are not visibly prominent, and many drivers won’t even be aware of their existence, but RTM components play a vital role in vehicle dynamics and safety.
Take suspension struts and shock absorber top mounts; as well as preventing vibration and noise from disturbing the passengers, their condition affects steering, handling and braking characteristics.
What is RTM?
In simple terms, a rubber-to-metal component is a vulcanised rubber block bonded to metal (or plastic), used to join components or structures that must be isolated from noise and vibration. In practice, reaching an effective compromise between durability, safety and comfort when designing each RTM component to suit a specific vehicle model and purpose involves many complex considerations.
This leads to unique rubber/additive recipes and product designs to tune dynamic properties whilst also meeting requirements such as high fatigue strength, resistance to ageing, temperature resistance, and recyclability. For this reason it is recommended to always use replacement parts from an original equipment manufacturer. Parts from other sources that appear visually identical may, for instance, be constructed of rubber with unsuitable characteristics e.g. hardness, or simply glued together rather than chemically bonded during vulcanisation.
The manufacturing process
Natural rubber comprises long polymer chains that can move independently of each other, allowing plastic deformation. To be suitable for RTM component construction it must be vulcanised to form an elastomer, which is elastic and dimensionally stable. When making RTM parts, the rubber is also bonded to the metal substrate during the vulcanisation process. The metal is first primed then given a topcoat consisting of polymer solutions and other ingredients. The rubber/additive blend is then mixed with sulphur, which acts as a catalyst, and moulded with the metal substrate under heat and pressure.
During the vulcanisation process, the metal interacts with the primer, the primer with the topcoat and the topcoat with the rubber. A very high modulus layer forms in the rubber next to the substrate, yielding a strong bond of only ten to twenty microns in thickness. Meanwhile, polymer molecules in the rubber form cross-links, reducing the ability of the polymer chains to move independently. This allows the rubber to deform under stress but return to its original shape when the stress is relieved.
Safety, comfort and consequential wear
Shock absorbers maintain tyre-to-road adhesion for good grip and braking. Worn top mounts reduce optimum contact, resulting in longer braking distances and compromised handling especially during critical avoidance manoeuvres. In addition, vehicle safety systems such as anti-lock braking and traction control only function perfectly if all suspension components are in top condition.
Worn top mounts impair ride comfort, generating noise and transmitting vibrations into the vehicle interior. They also create higher loads on new shock absorbers and other suspension components such as drop links, which then wear more rapidly.
Top mount fault diagnosis
Wear in the top mounts may manifest itself through increased vibration, longer braking distances or stiff or non-self-centering steering; symptoms that usually develop gradually and go unnoticed by the vehicle owner until a knocking noise prompts investigation.
Where possible without dismantling, visual inspection may reveal folds or cracks in the surface of the rubber or the rubber detaching from the metal. Following strut removal, a thorough examination can be made and the top mount height compared with a new item. Although elastomer rubber tends to settle under a static load, for a component in serviceable condition the height difference should be less than two to three millimetres.
It is recommended that top mounts are renewed each time the shock absorbers are replaced; no additional time is involved and it helps to ensure 100 per cent safety and performance on the road. Like shock absorbers, they should always be replaced in pairs. Workshops should check for top mount wear symptoms during a test drive at all service inspections and advise customers of any suspected faults. Such recommendations help to reinforce customer satisfaction and loyalty by demonstrating attention to detail and preventing additional time off the road.
The range comprises top mounts for more than 75 per cent of the UK vehicle parc.