Smoothing the bumps
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Equipping a truck or trailer with suspension is inevitably a compromise between ride and handling; the ability to work carrying a vast range of loads; and, increasingly, the need to be kind to the road surface.
This balancing act is tricky enough for on-road applications: designing suspensions for on/off-highway work must give designers some sleepless nights.
A good example is the host of demands made by six and eight-wheel tipper operators when selecting the tandem-axle bogie suspension for their vehicles. The chosen system must offer acceptable ride and handling both on and off road, with and without a load; good axle articulation for good traction; high roll stiffness for tipping stability and cornering; ruggedness and durability; low maintenance requirements; low weight, and acceptable cost.
The three main alternatives for tippermen are metal, rubber and air. Though air has been chosen by a few hauliers, particularly for on-highway use, it remains the minority option for the timebeing, with most tipper operators going
Foden's rubber suspension is standard fit on its multiwheelers.
either for traditional metal or rubber.
The take-up of rubber varies greatly from chassis manufacturer to manufacturer, largely because not all of them are geared to offer rubber as standard or a line-fit option — a comment which applies more particularly to the continental builders. For this reason, if rubber is available it often comes at a price premium, sometimes a hefty one.
Despite this, it is estimated that about a third of sales are now on rubber, with an increasing number of hauliers and chassis manufacturers going the rubber route. So it is appropriate to take another look at the pros and cons of rubber suspensions, and at the various systems on the market.
perhaps the best known system is the Hendrickson Norde rubber suspension (called the Norde system before the company was bought by the American firm Hendrickson in 1985). It was orginally designed at North Derbyshire Engineering in the 1950s by fleet engineer Ron Wragg who was looking for a solution to spring breakage and maintenance problems.
Hendrickson refined the package, adding a simpler suspension beam and more compliant beam-end bushing. This, with the established Hendrickson axle attachments, allowed the use of major propietary axles such as Eaton and Rockwell with compatible bracketry in place. The suspension was marketed as the HN Series, and it is now replacing the Norde Mk VI design with most British chassis manufacturers, being adopted as a standard fitment on all ERF and Seddon Atkinson 6x4s and 8x 4s, and as a factory-fit option by lveco Ford (for Cargo 6x4s), AWD, Multidrive and Volvo.
Like all rubber systems, the HN suspension relies on rubber sandwiched between metal plates, with the resulting bonded springs being arranged in vee formation. The HN springs use the rubber in shear and compression. Among its claimed benefits are good ride and articulation, high roll stability and noise-free operation (important in environmentally sensitive areas).
Of more interest to fleet engineers is the lack of maintenance and the good spring life: with no greasing required, rubber suspensions systems are essentially maintenance-free; spring life is claimed to be three or four years and, because the springs fail in a progressive way, they should not cause roadside dramas or unplanned stoppages.
Hendrickson Norde marketing director Terry Green reports that the rather high bogie weight of some continental manufacturers has led to a renewed interest in rubber from the chassis producers (a weight saving of over 200kg is possible), and Green is confident that the HN Series will appear on other ranges over the next few years. He reckons that rubber could eventually meet the requirements for higher axle loads to compete with air systems.
The other proprietary rubber suspension supplier is GKN Tadchurch. While essentially similkar to the Norde design, the GKN system differs in that the rubber bolsters are transversely mounted on to the beam, rather than longitudinally. In this way the forces act differently on the rubber, producing shear and compression in bump; compression in roll, and torsion in articulation.
This is claimed to make the most of the material properties.
The main applications of the Tadchurch suspension ore as a factory-fit option on the Leyland Daf Constructor six and eight-wheelers, although it can also be supplied as an SVO option on Volvo multi-wheelers. GKN claims the same benefits for on and off-road performance as the HN design, as well as its maintenance-free properties. For the future GKN is also working on optimising the "road friendly" characteristics of rubber to benefit from the higher axle loads.
It aims to reduce weight and simplify the production of the Tadchurch product, GKN having taken the company over last year.
To date the only UK chassis manufacturer to have produced its own rubber suspension system is Foden, which offers its FF20 design as standard on its three and four-axle rigids.
The FF20 is very similar in principle to the HN system, using the rubber springs mounted in a vee-pattern in-line with the frame. Foden claims the same benefits for its system as Hendrickson Norde and GKN Tadchurch.
As the recommended suspension for normal on-off highway applications, the FF20 system is fitted to the vast majority of Foden`s tippers, although it still offers steel or air alternatives.
For very heavy-duty off-road work, where variable loads are to be carried over the worst kind of terrain and optimum axle articulation is required for traction, Foden recommends its KW6-50A multi-leaf suspension. Its main drawback is a weight penally of around 300kg when compared with rubber.
A more recent alternative is the AirTrac air-bag suspension developed by fellow Paccar company Peterbilt in the USA. Air is not recommended for tipping on uneven surfaces, due to the reduced roll-stiffness, and Foden also suggests that operators specify cross-axle and inter-axle diff locks if site work is contemplated. Air systems often offer less axle movement than rubber systems, particularly when lightly laden, which could reduce traction on the rough.