The roll stability secret

It’s safe to say that Roll Stability Control (RSC) is now a common optional extension of ABS and EBS technology on powered vehicles, not only in Europe, but also in the US and Japan. In fact, it is often part of a more extensive ‘ESC’ stability control package. RSS, also known as RSP, is the equivalent feature in the trailer world, and is now standard on all TEBS systems currently available.

Regardless of the type of vehicle used, roll stability systems are meant to avoid the worst-case scenario of a vehicle rollover. While the conditions leading to such an event can vary, modern electronic systems are intelligent enough to respond to differing circumstances in the most appropriate way – based on a complex set of data collected within the TEBS unit.

However, the relative benefit of having both prime mover-based RSC and trailer-based RSS in an articulated vehicle combination – specifically a prime mover/semi-trailer set-up – is still controversial. Global Trailer therefore asked Australian braking specialist Greg Byrnes to clarify the issue. 

According to Byrnes, the first point to understand is that in a typical prime mover/semi-trailer combination, the trailer will always roll before the prime mover itself. “This is simply a function of the fact that the trailer carries the load and generally has a much higher centre of gravity than the prime mover,” he says. “Of course, when the trailer rolls, it will often pull the prime mover over with it, unless there is a mechanical failure of the fifth wheel coupling between them.”

Prime mover and trailer-based systems are therefore both primarily about preventing a trailer rollover, which is most commonly triggered by misjudging the cornering speed of a curve or an emergency situation.

According to braking specialist Byrnes, prime mover-based systems can be quite effective at preventing semi-trailer rollover events triggered by specific conditions, whilst trailer based systems are more effective at responding to certain other types of rollover conditions. Maximum rollover protection is therefore realised when both the towing vehicle – i.e. the prime mover – and the trailer(s) are equipped with a stability system.

“However, considerable benefit still accrues from having a roll stability system on just one or the other, and trailers are generally the most accessible option in terms of cost and ease of installation – which also makes them a more viable candidate for retrofit,” Byrnes explains.

In scientific terms, a vehicle rolls over when the result vector (Fr) of two significant forces – the vertical force of the vehicle mass due to gravity (Fg) and the horizontal centrifugal force (Fc) due to cornering – fall outside of the vehicle’s support base, the tyres. This is commonly referred to as the vehicle’s rollover threshold, and the cornering speed at which it occurs as the rollover threshold speed. The resultant force vector will create a turning moment through the centre of gravity of the vehicle and around the outer contact point of the outer tyres that will eventually tip the vehicle over.

According to Byrnes, simple geometry dictates that the higher the centre of gravity, the lower the centrifugal force required to tip the vehicle over. “As a result, trailers invariably roll over at a much lower cornering speed than prime movers, due to their much higher centre of gravity. If the mechanical coupling between them remains intact, a rolling trailer will pull the prime mover over with it, but a prime mover will never pull a trailer over because it will never roll over at a speed lower than the trailer itself will roll.”

The vertical force at play in the example is fixed by vehicle mass and gravity. The horizontal force is a function of cornering radius – fixed by the road design – and cornering speed. In that scenario, the only factor which can be altered whilst cornering is speed, so roll stability systems work by automatically applying brakes independently of the driver to slow the vehicle down.

Note also that during cornering, the centre of gravity generally moves outwards due to body roll and tyre sidewall flex, reducing the rollover threshold, so suspension roll stiffness is an important factor in roll stability. Also, for a rollover to occur, the tyres must generate enough frictional force to prevent them from sliding sideways, which is typically the case for heavy goods vehicles cornering on dry sealed road surfaces.

The first rollover scenario we see in every day life generally involves a simple misjudgement on the part of the driver about the safe cornering speed of a curve. This is as opposed to an emergency avoidance manoeuvre – a sudden violent change in direction precipitated by some unforeseen circumstance.

Generally speaking, experienced drivers develop a fairly good feel for how hard they can push a vehicle; and may drive a familiar vehicle close to its limits on familiar routes for many years without incident. “Of course, the only way of knowing exactly where the limit is, is to exceed it, and doing so is a rather expensive and dangerous exercise,” says Byrnes. “Plus, the limit is not fixed – the rollover threshold of a trailer can change every time it is loaded.”

Note too that centrifugal force – the force that pushes a vehicle over in a bend – is a squared function of cornering speed. That is, a relatively small increase in cornering speed can result in a significant increase in centrifugal force – e.g. doubling speed quadruples lateral force – so just a slight misjudgement of the entry speed on a curve can be enough to increase the centrifugal force beyond the rollover threshold. “On top of this are numerous other variables that the driver may not be aware of or have little control over,” Byrnes says, pointing out that this type of ‘steady state’ rollover often looks like a slow motion event when captured on video.

“It is generally the result of a relatively gradual increase in lateral acceleration as the driver turns into the curve, which peaks around the apex of the curve and then gradually decays as the vehicle exits the curve and straightens up. If the cornering speed results in the lateral acceleration exceeding the rollover threshold of the trailer, it will of course roll over,” he says, noting that the lateral acceleration rise rate described here as” relatively gradual” is to be seen in comparison to an emergency manoeuvre, for example.

Despite that deceptive ‘slow motion’ nature, drivers are often unaware that the trailer has exceeded its rollover threshold until it is dragging the prime mover over as well, by which time it is usually too late, as Byrnes explains. “Due to the torsional flex of most trailer chassis and inherent lash in most fifth wheel couplings, inside trailer wheels have usually lifted before there is any significant transfer of rollover force to the towing vehicle,” he explains. “So it is that prime mover-based roll stability systems often fail to detect critical trailer roll until it is too late to prevent. The peak lateral acceleration that caused the trailer to roll is likely still well below the much higher rollover threshold of the prime mover, and therefore probably hasn’t triggered a prime mover based system into action. In this scenario, a trailer-based RSS control provides the best chance of preventing rollover.”

The trailer TEBS or RSS control unit incorporates a lateral force sensor, which is able to detect when a nominal lateral acceleration trigger value is exceeded. This is generally around 0.35g, which – according to Greg Byrnes – is a conservatively safe value for most typical trailers. It can, however, be programmed to a lower value for particularly roll sensitive trailers, for example when transporting hanging meat or sloshing liquid.

“Depending on numerous variables – such as the design of the trailer, the load mass and height, the suspension roll stiffness etc. – 0.35g may be nowhere near the rollover threshold of a particular trailer at a particular time, so to avoid unnecessary intervention, the TEBS/RSS employs some clever trickery to test the trailer in order to determine if it is actually approaching a potential rollover condition or not,” says Byrnes.

Once the 0,35g trigger point is reached, the TEBS/RSS initiates a series of short low pressure brake applications – referred to as a ‘level 1’ RSS intervention. “These ‘test’ pulses are generally barely perceptible to the driver, but they enable the TEBS to measure the resultant wheel decelerations via the ABS wheel speed sensors. Because the cornering force is shifting the trailer load from inside to outside wheels, equal brake pressure, i.e. force, on each side results in different wheel deceleration rates,” he says. “The TEBS evaluates this difference and can determine with reasonable accuracy when the inside wheels are approaching lift off – that is, almost completely unloaded.”

At this point, the TEBS would apply maximum braking to the trailer in an attempt to slow it below the rollover threshold speed – referred to as a ‘level 2’ RSS intervention.

However, if the ‘level 1’ test pulses do not indicate that the trailer is approaching a rollover, a ‘level 2’ intervention does not occur, and the test pulses will continue until the lateral acceleration falls below the trigger value. This initial conservative trigger value is then assumed to be too conservative, so to avoid unnecessary test pulses in subsequent corners, it is incremented upward slightly with each ‘level 1’ only event until a ‘level 2’ intervention occurs. The ‘level 1’ trigger value is reset if the trailer load changes, as measured by the TEBS load sensing function, or the TEBS is power cycled, and this self learning process begins again.

According to Byrnes, the second common risk on our roads are emergency avoidance manoeuvres, which usually involve an unplanned, sudden, rapid, and violent change in direction precipitated by some unforeseen circumstance. “The  shear violence of the manoeuvre can often lead to instability and rollover,” he says. “These events are typically characterised by a very rapid rise in the lateral acceleration, far more rapid than in the ‘quasi’ steady state scenario just examined, so that an immediate response is necessary.”

The trailer, however, may well have passed the point of no return by the time the ‘test braking pulses’ of the RSS unit confirm the rollover risk. Roll stability systems are therefore designed to recognise this scenario by measuring the rise rate of the lateral acceleration, and will initiate maximum braking immediately to mitigate the risk.

“Even though the eventual peak lateral acceleration experienced may still not exceed the rollover threshold of a prime mover, a prime mover-based RSC control will recognise the potential risk posed to a coupled trailer and initiate braking even before the full effect of the directional change is felt in the trailer,” Byrnes says. “When the directional change exhibits the same rapid increase in lateral acceleration in the trailer moments later, the trailer-based RSS will respond in kind, and maximum braking of both prime mover and trailer ensures maximum deceleration and the best possible chance of preventing rollover.”

A trailer-based RSS control alone will still respond to this scenario in the same way and reduce the risk of rollover, Byrnes explains, but it can only apply the trailer brakes, and only after the trailer follows the prime mover through the same directional change. Because the prime mover undergoes this rapid increase in lateral acceleration first, the earlier intervention of a prime mover based RSC control and it’s ability to apply prime mover braking as well as trailer will enhance this rollover protection capability further.

“Note that this scenario is probably a somewhat less common cause of heavy goods vehicle rollovers than the earlier described ‘quasi steady state cornering’ case,” says Byrnes. “And, since trailer based RSS systems are often more effective in that previous, more common scenario, it could be argued that trailer based systems are the more cost effective rollover mitigation measure, particularly as they are more easily deployed.”

Byrnes points out that every roll stability system works by slowing down a vehicle to a speed below the rollover threshold, so they are most effective when they have access to enough braking power to slow the vehicle quickly. However, if the entry speed to a corner is significantly above the rollover threshold speed, the available braking power may not be enough to slow the vehicle adequately, and a rollover can still eventuate.

“In real world terms, for a truck and trailer combination, roll stability systems probably deliver a safety margin of around 10 per cent,” says Byrnes. “That is, they can likely prevent a rollover if corner entry speed is up to 10 per cent or so higher than the vehicle rollover threshold speed. However, the exact margin will vary with numerous variable circumstances, so drivers should never intentionally increase their normal cornering speed with the expectation that RSC/RSS will always keep them upright.

“In other words, roll stability systems can often save a driver from a small misjudgment or unforeseen event, but they can’t save an idiot from gross culpable negligence.”

In the particular case of multi-trailer combinations as they can be found in Byrnes home country of Australia, the effectiveness of roll stability systems is presently somewhat of an unknown. “TEBS/RSS on each trailer functions independently of others to detect impending rollover of just that trailer. If only one trailer approaches a rollover condition, the RSS on that trailer only will activate and apply the brakes on that trailer only. Braking one trailer only out of two or three or more will obviously not decelerate the combination as quickly as braking the only trailer of a single trailer combination, so RSS effectiveness is likely somewhat reduced compared to single trailer combinations,” Byrnes says.

On the other hand, if a leading trailer triggers RSS control, effectiveness could be enhanced by braking following trailers as well to increase the deceleration rate, and there are methods available for enabling this function, according to Byrnes. “That said, it is often the last trailer in multiple trailer combinations that tends to roll due to rearward amplification (sway), so this option may not add appreciable value in such cases.”

Nevertheless, RSS may still be partially effective in reducing the likelihood of rear trailer rollover due to uncontrolled sway, as braking it alone against the forward pull of the rest of the train probably tends to straighten it up. Having a roll stability system is certainly more effective than not having it, but just how effective it is on multi-trailer combinations is yet to be determined.

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