SUSPENSIONS
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TECHNOLOGY
• It comes as no surprise to learn that the heavier the loads on an axle, the greater the rate of wear on the road surface. It is this basic relationship that the Department of Transport has in mind when setting weight limits for commercial vehicles; particularly when increased axle weights are under discussion.
Much of the thinking on road wear dates back to research in America which resulted in the "fourth power" law, stating that road wear is proportional to the static axle weight raised to its fourth power. Using this law, it follows that when the maximum weight that a truck axle could legally impose on the road surface in Britain was increased by 3.2% in 1983 — from 10.17 tonnes (10 tons) to 10.5 tonnes — the road damage inflicted (per maximum-weight axle) rose by some 13.5%.
Similarly, the EC 11.5-tonne axle limit which reaches the UK in 1999, implies a potential 44% increase in road wear.
In many applications, especially on semi-trailer bogies, substituting wide-single tyres for twins further aggravates the road damage factor — by as much as 50%, says the Transport and Road Research Laboratory, and the reduction in tyre "footprint" area can increase the rate of wear under that axle by as much as 150%.
Suspension characteristics are also critical in determining the punishment a laden axle inflicts on the road surface. In general the harder the springing the greater the hammering of the road by the tyres, although the degree of damping is also relevant ... and it is this fact which has led to the concept of a "road-friendly" suspension. If the suspension can limit the effect on the road, higher axle loadings can be more easily tolerated. Spring rate (measured as vertical deflection under a set loading) is directly related to the frequency of the suspension, which is why legislators concerned with limiting road damage are pre-occupied with suspension frequency: the softer the spring the lower the frequency, and the more road-friendly the suspension.
Research by the TRRL and others has established that in general air suspension is the most road-friendly, but because air springs are not suitable for all applications, the proposed regulations allow for "equivalent" low-frequency suspensions. With air suspension performance as a road-friendliness criterion, the Department of Transport has set a maximum acceptable frequency of 2Hz (two cycles/sec), with an attendant imposed-axle-weight concession which is proposed initially for the tandem bogies of rigid six-wheelers.
In combination with a specified minimum damper (shock absorber) performance, from 1 January 1993 a 2Hz-or-less bogie suspension would qualify for a 19-tonne bogie loading. This implies a one tonne GVW advantage over the 25-tonne GVW limit which is due to come into effect on that date for other six-wheelers.
As is usual with UK transport legislation, with only 13 months to go the DTp is dragging its heels over finalising the requirements, giving manufacturers insufficient time to plan and tool up for the new suspension systems.
In parts of the EC the tandem bogies of 6x4 and 8x4 truck chassis (with a typical axle spacing of 1.35m) are already allowed to impose 19 or more tonnes on the road surface, as long as the axle loads are nominally equalised — in the case of leaf springs using balance-beam geometry.
If Britain's legislators followed suit air-suspension makers would stop rubbing their hands, and the development work put in by specialists like Hendrickson-Norde in developing simpler, lighter "air suspension equivalent" designs would have been largely wasted.
However, manufacturers and trade associations in close touch with the DTp have said in recent weeks that the 2Hz frequency requirement does look like being adopted unilaterally by the UK, first as a condition for 26-tonne six-wheelers and later for 32-tonne eight-wheelers.
Those who condemn the 2Hz frequency requirement and the accompanying demand for a minimum of "20% damping ratio" tend, perhaps inevitably, to be champions of leaf-sprung bogies. They contend that the prescribed test method is unrealistic. Under this procedure, which is carried out in a stationary, fully laden vehicle, the front axle suspension is locked solid so it cannot affect the rear bogie's behaviour.
The rear of the truck is then lifted and dropped from an arbitrary height of 80mm. While the test seems a realistic enough method of determining the characteristics of the suspension package, repeatability of results over a series of seemingly identical drop-tests has proved difficult. This could cast doubt on those suspensions whose recorded frequencies and damping ratios fall close to the proposed thresholds.
Some steel-suspended bogies, notably softer four-spring designs, have proved capable of meeting the 2Hz requirement during isolated drop-tests. What's more, because frequency is directly dependent on spring rate, it is theoretically possible to reduce the "bounce" frequency of a non-road-friendly leaf-sprung bogie by adopting longer, and hence softer, leaf springs.
Unfortunately, longer springs often won't fit in two or four-spring tandem bogies. Most multi-wheelers are short-wheelbase tippers, mixers or refuse collectors with short rear overhangs which limit the space available for longer leaf springs. In any case, the axle spacing of any bogie, except for a widespread trailer tandem, imposes its own restrictions on spring length. While soft springing cushions the road against higher cyclical dynamic loadings, a price must be paid in suspension roll stiffness and damping characteristics.
Under the DTp's proposed requirements for road-friendliness, damping as well as spring rate is covered. The damping ratio must be no more than "20% of critical" — a mathematical definition related to the number of spring oscillations, or bounces, which occur before the jolted suspension stops vibrating. "Critical" (100%) damping is defined as that which would prevent any oscillation at all; after being deflected upwards when it hit a bump the vehicle wheel would return to its original position quickly without "overshooting".
Most of the current four-spring bogie suspensions capable of meeting the 2Hz frequency standard will need heavier dampers than they have today. Interleaf friction is apt to make traditional multileaf springs, like the cheapest single taperleaf types, too bouncy.
Any legislation framed around spring damping should take account of damper deterioration. For many years tippers managed without shock absorbers; apart from adding cost, dampers were seen as being vulnerable to damage and wear under harsh site conditions.
However, the roll stiffness of soft "2Hz or better" four-spring suspensions will need to be enhanced, not to comply with the law, but to ensure lateral stability, particularly on the rough ground. Stiffer, and therefore heavier, anti-roll torsion bars are a likely solution. When it comes to roll stiffness, air suspension designers have boxed clever in recent years, making the axle tubes double as anti-roll stabilisers. This idea could not be easily adopted on more traditional suspensions.
Other alternatives to air suspension, which avoid its relative expense and complexity, are now under development. Rubber is the medium chosen by Hendrickson-Norde and GKN-Tadchurch. The now familiar steel/rubber sandwich springs, pioneered under Dunlop's Metalastik brand name absorb suspension loads partly in compression and partly in shear.
To soften spring rates to the required 2Hz frequency HN has kept its rubber chevrons disposed longitudinally; GKN has them arranged across the vehicle.
On GKN's proposed road-friendly design the likely loss of roll stiffness is countered by widening the chevron angle from about 90 to nearly 120° so under roll forces the rubbers are put into more direct compression. The Hendrickson HN420 design makes use of an auxiliary conventional torsion bar.
EI by Alan Bunting