Rubber is the ideal material with this because of its ability to accomplish multiple critical functions simultaneously: sealing the pressurised cushion of air that softens our ride.
Typically the engineering definition of a plastic material is “any materials that can stretch to at least 100% of its original length, and return to its original condition without long term deformation”. Although the term “rubber” originated from true natural rubber derived from trees, today the term is utilized to recommend to a host of different engineering materials, almost all of which are synthetic, and all sorts of which exhibit the hallmark versatility of natural plastic.
Although engineers may utilize many other options to achieve any of these purposes, rubber grommets often performs with better elegance and lower total cost than the options, and certainly with the highest degree of flexibility. In addition, rubbers can be molded into extraordinarily complex configurations, and can be bonded to practically any substrate material to form a composite component, greatly boosting the engineer’s ability to tailor a component’s function.
One reason that most engineers know so little about rubber is the complexity. Rubber is among the most complicated material that an industrial engineer can draw upon, and its very complexity gives rise to its flexibility. The first level of complexity is the molecular nature of rubber itself: rubber polymers own the highest molecular weights and longest chain lengths of all substances. This large size and length allows rubber molecules to fold and flow with extreme freedom, and it is this microscopic movement that translates into macroscopic deflections that are ten-times better than any other materials.
Another level of difficulty arises with actual rubberized formulations themselves, which are much more complex blends of ingredients than other engineering materials. For example, metals are usually alloyed from perhaps 2 to 4 elements; plastics are usually blends of 3 or 4 materials. By simply comparison, a typical plastic formulation is usually constructed of 10 – 20 total ingredients, all of which must be carefully selected and apportioned to modify the final properties.
Typically the final and defining complexity of rubber is its thermosetting nature. To produce a rubber component you must heat the plastic for sufficient time to cause an irreversible substance reaction that involves many of the ingredients, a reaction that transforms the rubber’s properties to make it permanently flexible and useful. In the case of metals and plastics, only phase changes occur, “thawing and freezing” of the materials, in a sense; this makes for reasonably predictable actions among the few constituents that are blended together for these materials. Since rubbers are composed of so many different ingredients and involve chemical responses between many of these ingredients, there is a amount of complexity and unpredictability that can defy analysis. You will find simply too many variables at play!
A good especially challenging subset of applications involve dynamic biking of rubber. Dynamic bicycling requires rubber to frequently flex through a mobility, for which rubber is typically well-suited; but recurring cyclical flexing can create fatigue cracks which could in the end cause the failure of the rubber. For powerful cycling applications, it is very important define the dynamic requirements: the expected frequency spectrum; the anticipated deflection amplitude or loading that will be transmitted; and whether start up or shut-down events will pose particular challenges (due to the driving machine passing through its critical frequency). The design of dynamic applications shoves the rubber art to its greatest limits, and requires the best care in characterizing the applying and in developing the optimal plastic formulation to meet the challenge.