Capital Equipment News December 2015

TRANSPORT

DEFINING TRAILER SUSPENSIONS - air or spring?

By Pierre Sanson

T here are obviously many schools of thought when it comes to trailer sus- pension systems. Both have their par- ticular applications and advantages but at the end of the day the economies of scale and applications will determine the outcome. Leaf spring suspension is a simple form of spring commonly used for suspensions in wheeled vehicles. It is one of the oldest forms of suspension, dating back to medi- eval times. It takes the form of a slender, arch-shaped length of steel of rectangular cross section. The centre of the arc pro- vides location for the axle, while tie holes are provided at either end for attaching to the trailer body. For very heavy loads, the leaf spring can consist of several leaves. The leaves that become progressively shorter are stacked on top of one another. Such a system is known as a multi-leaf spring. The right type of spring to suit the appli- cation is important. If the trailer’s springs are too rigid, its wheels will often leave the road when hitting bumps and disturb the load. This also increases tyre wear. A good suspension design keeps the wheels on the road as much as possible. The ratings on the spring packs are therefore paramount to the application. A leaf spring package consists of hangers, u-bolts and a metal plate to which hang- ers are fitted. Leaf springs require a cer- tain amount of maintenance. They should be checked for wear regularly. A worn or cracked leaf spring can cause untold dam- age to a trailer and its contents. Generally,

the leaf spring configuration on any trailer has its place in the transport industry, espe- cially for heavy loads and most rough terrain applications, but keeping it well maintained is the key. The introduction of air suspensions in trucks has over the years been extended to trail- er suspensions. Although it has involved re-configuring the trailer design, the ad- vantages have been enormous. The most significant has been the increased payload as a result of a lighter construction for the suspension system. In some cases the sav- ing in tare weight can be as much as 300 kg per rig. Air suspension results in a lower vibration level in the trailer under all conditions. It not reduces damage, but also ensures that there is uniform road contact by the wheels at all times. This no doubt reduces tyre wear as well. The softer ride offered by air suspensions protects the load from direct shock when travelling on uneven road surfaces. This is beneficial to transporters of delicate goods. One of the major advantages is that all the airbags on the trailer are interconnected, thus ensuring that all axle loads are equal- ised under all conditions. The same size brake cylinder and slack ad- juster lever lengths are used on all axles of the trailer. The trailer with air suspension is less sensitive to the angle it runs at due to the differences between fifth wheel heights.

Long haul drivers using air suspension trailers will generally find increased com- fort and less fatigue at the end of a jour- ney, as well experiencing less stress on the prime mover and a reduction in fuel costs. So, with less moving components required for air suspensions, the maintenance factor is greatly reduced, thus contributing to the economies of owning such a trailer and re- duced operating costs.

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