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14/03/2008
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Shock-absorbing elastomers sit at the heart of a revolutionary suspension system. Lou Reade reports
An innovative suspension system that was developed for a new design of bicycle is on the verge of being adopted by a range of other industries.
The system uses elastomer-filled bushings – rather than springs or hydraulics – and takes advantage of a phenomenon called the Mullins Effect, where the molecules in the elastomer ‘line up’ under stress, causing a damping effect.
It has been developed and patented by Australian company Rias Technology. Rias is an acronym for ‘rheopectic impact absorption and suspension’. (A rheopectic material is one that becomes thicker when sheared.) The suspension system is dubbed ‘Riasorb’.
Aldo Contarino, the company’s technical director, says this arrangement has several advantages over traditional designs: it is lighter, more robust, is both maintenance- and lubricant-free and can work at extreme temperatures. It is also more likely to avoid catastrophic failure, and is already being assessed by customers in the aerospace, automotive and medical industries. He expects the first licensee to sign up by June of this year.
“Understanding this effect has allowed us to overcome the limitations of conventional engineering thinking, and design a new effective suspension,” he explains.
The elastomer – whose formulation can be altered widely to match the needs of different applications – is designed into a bushing to supply torsional damping. A critical factor was to use a lobe design – which controls the torque angle – inside the bonded bushing. The polymer is contained within this volume and its shear can be accurately controlled and predicted.
“Understanding the material behaviour and torque angles, we could deliver a more uniform strain of the elastomers and deliver greater recovery,” says Contarino.
The bushing design allows the elastomer to be strained in a more uniform fashion, he says.
“The molecular chain is better aligned than would be the case with a linear design because it is rotated around the core,” he says. “The limited torque angles on the hub design change the angle of shear through the elastomer – so that shear occurs through a larger cross sectional area.
“The temperature gained in the shearing process is used to recover the elastomer. The speed of recovery is determined by the time of impact.”
Lobe and casing design play a critical role in allowing the elastomer to be strained more evenly and utilising the non-linear response of the elastomers. This will occur most effectively in a limited torque angle design.
On your bike
The damping system was originally designed to meet the needs of a new type of bicycle developed by the company – dubbed the Streetsurfer, and still the only confirmed commercial application.
Streetsurfer has a four-wheeled ‘bogie’ in place of a conventional front wheel. This, says Contarino, allows a different type of riding experience – more akin to windsurfing than cycling. But to get the desired performance, a new type of suspension system was needed. The orginal suspension was based on springs and hydraulics.
“It was heavy and cumbersome and not capable of rotating through the complex, ever-varying angles required to steer the cycle,” says Contarino. “There were also problems controlling rebound, oil-seals leaked continuously under high frequencies and there was dust build-up and impact damage.”
The main issue was that the shock components’ inner and outer tubes were creating static friction, or stiction – a common issue with telescopic suspension components. Also, extra pivots were needed to control the direction of the suspension and shock absorbers also proved to be challenging creating wear and tolerance problems that were compounded with the multiple linkage problems.
“Incorporating elastomers into the suspension system overcame every issue,” says Contarino. “The damped qualities could be exploited using the patented torque angle designs.”
Contarino says the company is pursuing opportunities in the cycle, automotive, industrial and aviation industries.
“In a shock overload situation the reduction of impact time would enhance the integrity of the supporting structure – and improve the fatigue limit – while avoiding catastrophic failure by controlling the force applied to the structure,” he says.
Energy is absorbed through torque of the elastomer and dissipated as low level heat. The claimed benefits are: improved performance and safety; reduced maintenance; and better load-carrying capacity.
The first alternative applications are a handlebar and seat post suspension for the cycling industry – launched this month at the Taipei Cycle Show. Another in-house development is the Long-board, which uses Riasorb in the ‘trucks’ to produce a skateboard sensation that can manoeuvre within a tighter turning circle than a standard skateboard “in a motion very similar to snowboarding”. This is now in the prototype phase.
In mid-scale unmanned aerial vehicles (UAVs), for example, it could be a cost effective replacement for existing designs – which tend to follow large-scale aircraft concepts.
An ongoing research programme with Curtin University of Technology in Perth aims to broaden understanding of the elastomer characteristics, and fit them into FEA models for a broad range of applications.
“A number of companies in a range of industries – with whom we are in confidential negotiations – are evaluating the technology with a view to licensing an application,” says Contarino.
Critical elements
Three elements are absolutely critical to the system’s performance, says Contarino: limiting the torque angle; the elastomer formulation; and the process to bond the elastomer to the metal within the bushing.
1) Limiting the torque angle reduces and controls stress risers, and takes advantage of the rheopectic characteristics of the material. The material wraps around the core when strained, as can be seen on FEA models.
2) Varying elastomer formulations provide characteristics of abrasion resistance, elongation, optical clarity, moisture resistance, chemical resistance, tensile and rebound control (to name a few).
3) The process that bonds the elastomer to the inner core and outer housing is crucial, and determines the controlled failure in an overload situation. In this way, the likelihood of catastrophic failure can be reduced or eliminated.
See patent numbers WO 2005/115825 A1, WO 03/010015 A1 and WO 98/57839
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Author
Tom Shelley
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