The suspension of any vehicle has traditionally been designed to
Page 22
If you've noticed an error in this article please click here to report it so we can fix it.
provide reliability, stability and good ride and handling, which benefits driver and load alike.
Many of these parameters are in conflict. For example, a "soft" suspension is good for ride but requires large suspension deflections. With mechanical springs this can result in unacceptably high load platforms and, if not controlled, excessive roll. Too much roll stiffness can lead to traction problems on uneven surfaces, and so on.
Such problems are avoided with air suspensions; hence their growing popularity. They usually provide a "soft" suspension for good ride with reduced platform height and reduced vehicle roll.
All that has been defined and agreed throughout the EC is that "the drive axle" should be fitted with twin tyres and air suspension, or a suspension recognised as being an air equivalent. This is what most people refer to as "road-friendly suspension". It allows weights across the EC (except the UK) to rise from 1 January 1993 as follows: The DTp has accepted the three-axle rigid EC weights for international transport, and looks set to accept it for domestic transport too.
The new tandem-axle bogie capacity has also been accepted for international transport, and is likely to be accepted for domestic transport as long as the drive axle(s) do not exceed 10.5 tonnes, The problem here is that only the equally split bogie is practical for the UK: the unequally split 10.5/8.5 tonne cannot be tyred sensibly.
This penalises the UK operator, who is forced to use heavier, more expensive equally split bogies until 1999 when the UK will be forced to accept 11.5-tonne drive axles.
So far the four-axle rigid has been accepted in DTp proposals at 32 tonnes GVW, provided the rear bogie is "road friendly". The 38-tonne four-axle artic has not been accepted, but a limit of 35 tonnes GCW has been accepted for international transport with no suspension requirement. This is also proposed for UK domestic transport, but again only with road-friendly suspension on the drive axle.
The first point to note is that not all air suspensions are the same. As you might expect the latest designs tend to be the most road friendly. To keep things simple, we will assume that all air suspensions are road friendly.
Taking a single axle, some have two bags on trailing arms: some have two bags on quarter elliptical springs; and the most modern designs incorporate four bags and radius arms.
For bogies with air suspension there are added difficulties. In general, air suspension bogies do not have as much articulation as two-spring bogies. This presents problems for on-off road vehicles, such as tippers or refuse vehicles: clearly these types of operation cannot be jeopardised on gross vehicle weight so some sort of compromise is required. Ideally this would be a suspension that offers the road friendliness of air, but has the articulation of a two-spring bogie.
The reason for fitting twin tyres to drive axles is to limit the ground pressure of the drive axle (normally the heaviest axle of any vehicle or combination). This reduces rutting compared with a heavily laden wide-single tyre with a higher tyre pressure.
The equivalence definition is still a proposal but is only for single drive axles exceeding 10.5 tonnes. This proposal states that the suspension should provide the vehicle with a natural body bounce frequency of two cycles per second.
If this complex jigsaw of axle, suspension, vehicle and combination weights is to be pulled together then legislators in Europe and the UK still have much to do.
For example, from 1 January 1993 international transport will be allowed to use 26-tonne three-axle rigid vehicles. As things stand, if a Belgian 6x2 (26 tonne) truck with a 19-tonne rear bogie loaded at 11.5 tonnes/7.5 tonnes on air suspension arrives at Dover on 2 January 1993, it will not be permitted to enter.
Even though the truck complies with the new EC rules — 26 tonnes GVW and air suspension with twin tyres on the drive axle — its 11.5-tonne drive axle and 19-tonne unequally split bogie will not be permitted.
To overcome the difficulties and confusion, the DTp could do British hauliers a great favour by making the following changes from 1 January 1993:
Cl Accept 11.5-tonne drive axles on twin tyres — with air suspension if necessary. 0 Allow equally split bogies at 19 tonnes. CI Allow unequally split bogies at 19 tonnes with 11.5-tonne drive axles — again with air suspension if necessary. 0 Quickly develop a technical definition of road-friendly suspensions, without the influence of political expediency. Then permit the use of road friendly equivalent suspensions at the same weights as air or equally split bogies.
Cl Once the 11.5-tonne drive axle has been accepted, follow up with 40-tonne artic and drawbar combinations on five or six axles.
This would improve vehicle productivity and reduce road damage, environmental pollution and road congestion, particularly with a limit of 44 tonnes on six axles. With such solutions we could be nearer to achieving one of Prime Minister John Major's objectives: "Keeping Britain in the mainstream of Europe."