GRP SUSPENSIONS - A NEW POSSIBILITY FOR BUSES

A contribution to the 30th Meeting of Bus and Coach Experts, Scientific Society of Mechanical Engineering, Győr, Hungary, August 1999
HARRIS, Leighton Polymath Engineering, UK
MAYER, Rayner Sciotech, UK
MIHÁLFFY dr., Pál and
MIRKAI, László AUTÓKUT, Hungary 

Abstract

Glass reinforced plastics is a suspension material, whose properties have yet to be fully utilised in vehicle suspensions. The successful development of replacement springs for taxis and semi-trailers illustrates how improvements in ride/handling and reductions in noise/vibration can be obtained. The extension of such designs to bus suspensions is considered.

Background

The most common suspension materials comprise steel, air, rubber and most recently glass fibre reinforced plastics (GRP). Of these GRP has the most appropriate blend of properties including high working strains, good fatigue performance and inherent damping properties. In a current Eureka-funded collaborative project, a novel flexible manufacturing route has been evolved which enables new types of suspensions to be considered. The process is also low cost thus enabling GRP suspensions to be manufactured competitively for niche market applications.

As a consequence suspensions can now be designed with complex geometry, which allows greater scope for packaging the suspension within the available space envelope. This is of particular importance for city buses and trams where low floor concepts dictate that space is restricted. While steel is a homogeneous material and the bending and torsion stiffness values are determined by its geometry, in the GRP the arrangement of the glass fibres can be varied and the optimal properties for the particular application can be achieved (e.g. high bending and low torsion stiffness).

Design & manufacturing attributes

The geometrical shape is only limited by the requirement of being able to preform the glass fabric pack. This is then placed in a closed mould tool and catalysed resin injected under low pressure around 1-2 bar. The component is moulded direct to final shape thus eliminating any need for post forming to shape (scragging). Polyester resin is used rather than epoxy in order to be able to mould thick sections (up to 60 mm so far) and withstand the exothermic temperature rise of ca. 100 C when the resin gels.

This precision moulding technique allows the bore of the taxi eye to be moulded within a few hundreds of a mm thus ensuring a reproducible interference fit between the bush and the eye. As the eye is of the closed type, a rubber bush can be used with an inner, but no outer sleeve, thus maximising the volume of rubber within the available space.

Outcome of vehicle testing

Two replacement spring types have been evaluated - a flexible rear spring for the black London taxi and a single leaf design for a 9 tonne semi-trailer axle.

For the taxi, the challenge was to improve the ride/performance and durability. This has been achieved by systematically varying the vertical to lateral stiffness and evolving the rubber bush, which is now of the bearing bush type and utilises a high hysteresis rather than a general purpose rubber. The inherent damping properties of GRP material provide a benign and durable environment for the bush.

The challenge for the semi-trailer was to develop a mechanical suspension as a replacement for an existing 3 leaf steel suspension which could be classified as road friendly. This required using a single leaf geometry to eliminate interleaf friction and low friction materials for the wear end. The fully laden and instrumented semi-trailer was evaluated on the wide variety of road surfaces available in and around Budapest. The suspension is able to cope equally well with motorway, pave and pot holes and the benefits are summarised in table I.

Table I.
Comparison of single leaf GRP suspension with 3 leaf steel suspension on a tri-axial semi trailer
Vertical spring rate 3 leaf steel  1200 N/mm
single leaf GRP  1320 N/mm
Decrease in average frequency of sprung mass vibrations* 0.4 - 0.7Hz
Average damping ratio (no interleaf friction) 15%
Reduction accelerations  chassis  85%
suspension  80%
Reduction in road contact force 5 -10%
Noise reduction 1.2 dBA
*The reduction is a consequence of lower dry friction and elimination
 of blocking the suspension at low amplitudes 

Evaluation on shaker rig

An additional set of tests were undertaken by mounting the complete trailer in AUTÓKUT's shaker rig in order to characterise fully the suspension. The methodology is based on that developed by the OECD-funded project DIVINE and has proved to be very powerful in tuning the GRP suspension. 

The principal force peaks and damping are contrasted in table II. for the three suspension types. The GRP spring is appreciably better than the steel spring and compares favourably with the air spring because its unsprung mass peak is much smaller and its sprung mass not much higher.

Table II.
Comparison of Eurospring & Divine results of semi-trailers 
on shaker rigs
Suspension type   Air   GRP   Steel 
Frequency response [Hz]
Sprung mass 1.5 2.5 3.1
Unsprung mass 12.0 17.0 na
Peak response [kN]
Sprung mass 4.0 8.0 30.0
Unsprung mass 20.0 2-3 3.0
Damping ratio as function of axle drop [%]
40 mm 11.0 7.0 7.0
80 mm 7.0 20.0 10.0
120 mm 8.0 10.0 n/d
Dynamic load coefficient
Worn concrete 0.05 0.05 nm

Characterising buses

There are generally four categories - city or transit, suburban, the inter-city bus or tour coach and, particularly in the USA, the school bus. The city bus operates within city limits and is characterised by low operating speeds, short stage journeys, low ride platform, many entrances, provision for standing and virtually no luggage space. The suburban bus is designed for medium speeds, relatively longer inter-district stages, with a single entrance, some in-bus luggage compartments and overhead racks. The inter-city bus or touring coach is capable of sustained high speeds, usually has a high ride platform to provide the maximum space for storing luggage under the floor, over head luggage racks, air conditioning, luxurious seating, and a wash room. The European school bus is usually adapted from the suburban bus, but in the USA they almost always consist of a 50 passenger bus body, with special provisions for safety, mounted on a long wheel base truck chassis. In Europe there is also a growing tendency to use mini-buses for all types of duty; these have 10 to 20 seats and are purpose built on commercial van chassis

Virtually all inter-city coaches use air suspensions, but although there is a growing use of air suspensions for the other types of bus, there is still a very high proportion of buses in service using standard leaf or coil spring suspensions.

The bus designer, the fleet operators and the passengers all want the maximum comfort, absolute safety, and the greatest reliability, at the lowest possible cost whilst doing minimum amount of damage to the roads and producing the lowest environmental impact.

Passenger comfort

The best ride is always associated with low spring rates, optimum (usually near the minimum) damping and maintenance of evenly distributed tyre loads. Unfortunately, for an acceptable working space and safe handling characteristics, some compromises have to be made, and so vertical suspension frequencies in the range 1.5 to 2.5 are normally specified. To meet the highly variable static axle loading in service - from the driver only to fully laden condition - a variable rate suspension is an advantage. This feature is obtainable with mechanical or air suspensions, but both have some limitations on their capabilities.

Suspension frequencies below 0.5 Hz are primarily generated by terrain profiles for which the suspension cannot maintain a level vehicle. The passenger comfort zone is usually defined as the disturbance region from 1 Hz to 20 Hz and the accelerations at frequencies in the range 4 to 8 Hz are the most important when calculating the ISO discomfort parameter. Engine and gearbox vibrations on the mounting are in the range 7 to 15 Hz, the excitation deriving from the operation extends to the audible frequencies (several hundred Hz). The suspension must effectively isolate the passenger from all these inputs.

Benefits of GRP suspensions

The suspension is fundamentally used as an energy absorber, so the more energy absorbed the better the isolation of the spring mountings from the input at the wheel. Although the vertical stiffness of the system can be specified to effectively absorb vertical movements of the wheels, it is very much more difficult to tune the suspension to absorb the inputs over the whole range of loading directions and disturbance frequencies in the comfort zone.

One of the great advantages of using composites is that they are not only excellent energy absorbers, but the fibre lay up and the component configuration can be tuned very effectively to provide the optimum ride/handling characteristics.

In general single leaf composite springs usually replace multi-leaf steel springs. It is well known that Coulomb damping - in this case interleaf friction - is very bad for the vehicle's ride characteristics, because produces high acceleration peaks and in case of small amplitudes blocks the suspension. Parabolic steel springs (consisting of one leaf or several separated leafs) have negligible own damping, so they need hydraulic dampers and the safe operation is depending on their good condition.

Fibre composites however have comparatively high and nearly linear damping so can provide the dynamic damping characteristics that are ideal for a good ride.

Benefits in comparison with air spring suspension are the following:

Single wheel bump

At disturbance frequencies of 1 to 3 Hz - close to the natural frequency of the suspension - the coherence from wheel to wheel on the same axle is likely to be high, so both wheels will move up and down in phase. In this mode the specified vertical stiffness of the springs is the dominant factor for ride comfort. However when one wheel only is deflected - the single wheel bump case - other factors come into play. Steel leaf springs, and especially the steel trailing links, have very high torsional stiffness when compared to the equivalent composite component. However in single wheel bump, the spring or link must deflect torsionally to allow the wheel to move vertically. Consequently the single wheel bump rate for steel leaf springs, or for steel lever guided air spring suspensions, can be significantly higher than the ideal spring rate. The comparatively low torsional stiffness of composite springs means that single wheel bump rates very close to the designers' optimum requirements can be achieved.

Advantages of mass reduction

Composite springs are much lighter than steel springs. Typically for a taxi, a 3.5 kg composite spring replaces a 10 kg steel spring, while at the other end of the spectrum, for a railway freight wagon (23.5 ton axle load), a 46 kg dual rate composite spring replaces a 145 kg steel spring. Work at Leeds University has shown that on a typical car suspension, reducing the unsprung mass by 38% gave a 10% improvement in the ride comfort parameter.

Lower unsprung mass, together with the reduced number of leaves, the excellent dynamic damping characteristics, the lower single wheel bump rate, and the generally "softer" load reaction characteristics of composites, are some of the reasons why composite springs give such a significant improvement in the ride comfort factor. This can be observed subjectively as well as quantitatively. Very experienced LDV test drivers gave Sherpa panel vans fitted with composite springs a subjective rating of 9, compared with a rating of 7 for the same vans fitted with steel springs.

Even if an air suspension is preferred for the control of ride height or other considerations, the use of composites for the location spring or trailing link can provide significant advantages for ride comfort.

Noise and vibration

The high material damping coefficient for glass fibre composites also provide excellent noise and vibration attenuation. In many buses whilst stationary at the bus stop, the vibrations gradually grow until the windows rattle!! This "idle boom" is often caused by engine / gearbox vibrations transmitted through the driveline, into the wheel and axle, up through the suspension, and into the body.

Steel components "ring", a graphic demonstration of their good sound propagation characteristics. Composite materials however are made up of many layers of fibrous material in a low stiffness matrix. This results in an acoustically dead material, ideal for significantly reducing the noise and vibrations transmitted to the vehicle body. Sound reductions of 3 dBA have been measured in panel vans tested on typical class B roads. Moreover the appropriate combination of GRP springs and rubber mounts can be tuned to minimise extraneous vibration.

Safety

The fibrous reinforcing structure also acts as a very efficient crack and damage arrestor. Consequently composite materials not only have superb fatigue properties, they are also very damage resistant. At the end of its life or in the event of damage in service, a composite spring will typically exhibit surface delaminations, or small shear cracks. Even when the damage is visually very evident, the spring stiffness may only be reduced be a few percent, and springs will normally survive for many thousands of miles before the reduced spring stiffness is apparent. Even if the spring is left on the vehicle until complete delamination failure occurs, the wheel will deflect vertically to the bump stop, but the axle will still be fully located.

This "fail safe" characteristic permits the removal of passive safety components, specially fitted to prevent the catastrophic axle loss that occurs when steel components fail. Of course the frequent safety inspections specified for all bus operations will easily identify any signs of delamination at its early stages. On the other hand, incipient failure of steel springs or links can be very difficult to establish without special NDT equipment.

Reliability and maintainability

The suspension system is a primary factor in vehicle reliability. The substantially longer life, improved ride, and reduced vibrations, exhibited by suspensions incorporating composite springs, will significantly improve the reliability of the vehicle. Unlike steel springs, composite springs do not rust, rot or deteriorate over their life. They do not need oiling, greasing, painting, or cleaning. They are precision moulded components and, also unlike steel springs, their operating characteristics such as spring rate, damping characteristics and sound attenuation properties, stay the same over the life of the spring. They do not sag or yield, so do not require packers or adjustments to maintain the vehicle ride height.

The rubber bushes fitted to most suspensions, are often the weak link in the system, and can require regular - and costly - replacement. The reduced torsion loads and lower dynamic wheel loading, inherent with composite springs, substantially reduces the loads on these bushes, significantly improving their life.

Life time costs

Composite springs or suspension components are usually more expensive than their direct steel counterparts. However even the initial capital cost can be significantly lower if less sound proofing materials are needed, special safety devices eliminated, and the lower tare mass is considered. A road friendly composite leaf spring suspension - which can rival an air suspension for ride comfort - is obviously an order of cost cheaper than an air suspension. In terms of life cost the comparison is even better. The light weight of composite components can often eliminate the need for special lifting equipment or holding fixtures, and always results in much simpler installation and maintenance procedures. Reducing the failures due to overloading or vibration damage, and the improved durability of all the vehicle components, means lower maintenance costs and smaller spares inventories.

Conclusion

It is evident that composites offer significant advantages to the suspension designer, the vehicle builder, the fleet operator, and the passengers of all types of bus. They give the opportunity to provide simple, road friendly suspensions, without the need for very heavy, complex and expensive air spring systems. Even when an air spring suspension is specified, composite links, or suspension frames, will provide substantial benefits in passenger comfort, noise reduction, reliability and life cost.

Acknowledgements

The work reported in this paper is the collective output of some 16 partners in EU 888 Eurospring project. We acknowledge the assistance of our colleagues particularly Michael Litton (Culzean Fabrics), Stig Black (EM fibreglass), Ramin Rezakhanlou (Reading University) and Paul Griffiths (Meritor HVS) and funding organisations throughout Western & Eastern Europe.

References