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Active-roll suspension technology could allow a truck to corner like a sportscar—at a price.
Handling can also be improved with more affordable systems that are already on the market...
Trains that tilt to go around curves faster are in the news again, so why not tilting trucks? High trucks could certainly benefit from a bit of active roll control, and not only is it technically possible—it's already been done.
That doesn't mean we'll see tilting trucks on the road in the near future. The huge cost of this technology might make sense in a train costing millions, but not in a truck costing thousands.
Then there's the power needed to oppose the roll, which would typically he around 120hp for an LGV This demand could be satisfied by an accumulator system, but the energy reserve still has to be generated, and that takes fuel.
A half-way house approach that does not consume power is variable damping Koni and Sachs have both developed dampers that do this with internal orifices altered either by integral electric motors or by connection to the pressure in an air spring. Wabco offers a system which adjusts damping by the electronic levelling of the air suspension.
All these devices are much more refined than the two-position electrical switching to select soft or hard damping that is sometimes available on cars, but they in turn are less refined than the oleo-pneumatic suspension developed by the Motor Industry Research Association. For each wheel it incorporates two spring chambers at different pressures with a fluid damping orifice at the entry to each. Neither electronics nor power absorption are involved; the cost is kept down to about £300 per axle—and dynamic forces at the Wes are said to be cut by a third.
This could be a strong argument for allowing higher axle weights when there is a proven road-friendly suspension but these semi-active systems cannot by themselves stop trucks from overturning on corners.
Certainly, stiffer and progressive damping slows the development of sway. It can also arrest the alarming leap in roll angle in a swerve if the spring frequency should coincide with the rate of change in the steering direction, or with liquid side-surge frequency in a tanker (known as roll resonance).
Support force
But dampers do not provide a support force to oppose roll. To do that the springs have to stiffen on the side taking the lean and soften on the opposite side. This is just what an anti-roll bar does: its drawback is that it spoils the suspension's flexibility to ride single-side bumps. Travel behind an airsprung trailer and you'll see that if one side hits a bump, both sides jump together—the axle tube acts as a massive anti-roll bar that virtually prevents one side moving without the other.
To avoid such crude action Sachs is working on a divided anti-roll bar with a progressive hydraulic lock between the two halves. When running unladen, or in a straight line, the bar's cross connection is free, leaving each wheel's springing to handle its own bumps. Beyond a certain cornering force, however, the two halves of the anti-roll bar are hydraulically connected. It's a neat idea, but the design team could not resist the wonders of complex electronic controls instead of something as simple as a pendulum.
There's also that problem of powering the hydraulics and sealing in the extremely high pressures.
While we wait for active anti-roll systems and tilting trucks to come on to the market, the rollover threshold of many heavy trucks remains worringly low.
A typical speed at which big trucks negotiate roundabouts and corners in urban areas (with an inside radius of, say, 30m) is about 25 km/h (15mph). That implies good judgment by drivers, because on such a curve the risk of overturning gets serious at 32km/h (20mph).
Vehicles more than 4.5m high risk overturning on sharp corners at anything above 30km/h unless they have roll-stiffening equipment. An equivalent static tilt test is between 14° and 16° with lateral g-force of about 0.3 (which, fortunately, a driver would find uncomfortable).
The unpredictable risk arises in swerves, especially if the roll and steering frequencies happen to be in resonance.
There are a few old-fashioned methods for enhancing stability, and they don't cost much. The first is to fit really effective dampers, preferably with load-sensitive or progressive action. The second is to lower the load; the quick ways to do this are low-profile tyres or smaller wheels).
Combining a low centre of gravity with a high roll centre obviously enhances stability, so look for suspensions with high roll centres (an upper A-frame in the axle location, for instance). Even more effective are wide spring bases (springs outrigged at wheel-track or, giving the same effect, independent suspension). Doubling the spring base quadruples the roll stiffness.
To help the judgment of the human in control, do not insulate the driver so thoroughly from vibrations and steering feedback that he loses any feeling for what the vehicle is doing. In fact, there is something to be said for a cab suspension that rolls a bit more in corners than the body to act as a sort of built-in early warning system.
Until tilting trucks arrive, drivers will be on the ragged edge of rollover more often than they realise.
El by John Dickson-Simpson