Arrive Alive

Safe Driving and Preventing Jackknife Crashes

Preventing Jackknife CrashesWhat are jackknife crashes and how can we adjust our driving to prevent these crashes? We approached a few industry experts on driver training and crash investigation to gain some important insights!

What is a jackknife crash and how does it happen?

Jackknife crashes are, unfortunately, among the most common truck accidents due to the fact that the trailer is not fixed to the prime mover. Instead, the trailer platform is connected to the cab of the truck with a simple hitch to allow for free movement.

Although helpful turning, this free movement also causes problems when drivers are forced to apply their emergency brakes, the prime mover comes to an abrupt stop, but the momentum of the trailer pushes it forward until it swings around and jackknifes.

There are two types:

  • Trailer - When the trailer overtakes the prime mover 0r
  • When the actions of the prime mover (such as braking) cause the trailer to swing by.

A Jackknife collision occurs when there is a loss of traction on the rear wheels of a truck-tractor (also referred to as a “horse”) that is connected to a trailer, or trailer combination. It typically results in the truck-tractor going into yaw (rotating) to the point where the trailer and truck-tractor typically make contact.

While not the greatest cause of collisions, this is often one of the most devastating kinds on account of the sudden nature of this kind of loss of control, and often on account of the forces generated such as when a loaded trailer or trailer combination is involved. All Jack-knife collisions involve the rotation of the truck-tractor and a loss of control by the driver.

Which vehicles are most at risk of jackknife crashes

Which vehicles are most at risk of jackknife crashes?

Prime mover and trailer, trucks that are towing a trailer and even light rigid vehicles that are either towing a trailer, boat or caravan are all prone to a jackknife crash.

While the term “jack-knife” is often assigned to any combination of vehicles, the causes are often not the same. Light vehicles, such as cars towing boats or small trailers, can end up in a relative yaw orientation, but for reasons other than those that prevail in truck/tractor/combination events.

When a light motor vehicle and a towed unit (boat, caravan, or small trailer) are involved in an event where the towed unit has no braking, the jack-knife scenario normally prevails simply because of the “pushing action” of the towed unit on the main vehicle.

Truck/Tractor/Combination vehicles are involved in Jackknife scenarios for more complex reasons outlined below.

While any combination of vehicles involving a towing and towed relationship can technically end up in a relative yaw configuration as a result of loss of control, these happen to heavy commercial vehicles to a much higher degree, and far more often.

Where are we most likely to find these crashes?

Where are we most likely to find these crashes - are there specific areas or roads where we can expect to find these crashes?

Slippery roads, steep inclines and poor braking manoeuvres can also increase the risk of the trailer becoming unstable which could result in a jackknife

A Jackknife scenario can manifest on almost any road, under almost any conditions, and there might be a higher risk on wet roads, or at or near intersections where sudden stopping might be required. The true risk lies in the condition and maintenance of the towing and trailing vehicles.

Do road surface and weather conditions increase the risk of a jackknife crash?

Yes, it does if the road is slippery either from rain or if there is diesel or oil on the road surface, weather plays a role for example if the driver is blinded by the sun this would limit his vision and he could find that when he can see again there is a stationary or slow vehicle ahead which will require an emergency brake that might induce a jackknife - particularly in poorly maintained trailers

While wet or slippery roads - and therefore, a decrease in wheel friction forces - the condition of the vehicle, its maintenance, and the driver’s actions have a greater impact on the dynamics of a combination vehicle in a rapid deceleration event.

This aside, the Jack-knife scenario is much more prevalent in sudden deceleration or even rapid downward gear-shift choices.

What will be the warning signs for a truck driver that this might or is about to happen?

What will be the warning signs for a truck driver that this might or is about to happen?

There are two kinds of jackknife crashes and both involve loss of traction and weight pushing you. Most happen on wet or slippery roads but can happen on any road surface. ABS brakes have made huge improvements.

Either the prime mover can break traction or your trailer's wheels can break traction. Multiplying that with the weight of a load pushing - if not managed correctly - could result in the driver causing a jackknife. The key is prevention and to prevent losing traction in the first place by negotiating any adverse weather or road conditions at an appropriate speed.

Most jack knives today are weather related. Years ago, there were problems with converting to an ABS-equipped vehicle with significantly improved brakes. Companies would keep prime movers for 4 - 5 years on average and trailers for about 10 years on average. Drivers could therefore have a prime mover with ABS and a trailer with no ABS. A very dangerous combination on slippery surfaces.

There are a variety of elements that a driver could be aware of, that might alert him to the possibility that his vehicle combination is prone to Jack-knife dynamics. These may include - in order of importance:

  1. Maintenance. If the truck and trailers are not properly maintained, brake force balancing could deviate, causing abnormal braking on some axles. It is normally the imbalance of braking friction at the truck-tractor drive wheels that is the most likely cause of Jack-knifing.
  2. Retardation choice. While you can decelerate (slow down) a truck-trailer combination by “gearing down,” as you would with a regular vehicle, this increases friction only on the drive wheels, increasing the risk of a Jack-knife dynamic.
  3. Trailer brake anomalies. If a truck driver detects slipping, skidding, skid-skidding, lock-up on trailer wheels, or any form of lock-up on the drive wheels, there is a chance of poor brake force balance being the cause. If the braking forces are not properly distributed across all axles, Jack-knifing becomes a greater risk.
  4. Driving style. If a driver is actively thinking about the brake balance, wheel friction, drive wheel traction, load, and road conditions, Jack-knifing can typically be avoided.

How should the truck driver adjust his driving to prevent a jackknife crash?

How should the truck driver adjust his driving to prevent a jackknife crash?

To a certain point, jack knives are recoverable if he can regain traction and remove the weight pushing factor. That might be letting off the brakes, counter steering if necessary and accelerating. There is however a point of no return where regaining traction isn't possible or traffic doesn't permit it.

  • Watch your mirrors frequently to see if your trailer is swinging or swaying. Adjust your speed or steering direction carefully to stop the trailer’s erratic movements.
  • Be aware that an empty trailer is lighter and, therefore, more likely to lose contact with the ground while in motion and start to jackknife.
  • When entering a turn, brake before entering the turn. Smaller cars can brake or accelerate during a turn, but this behaviour can be hazardous for the large prime move with trailers. Ideally, only brake while driving in a straight line.
  • If your trailer is starting to swing widely, avoid slamming the brakes. Taking the foot entirely off the brakes can help, assuming it is safe to do so.
  • In some situations, trailer sway can be corrected by increasing speed gradually, adding more to the prime mover pull strength, thus straightening out the trailer. Typically, increasing speed will not help if it is the tractor or caravan that is shaking or swaying. apply all of these tips, too.

If a truck driver is going to take any steps to avoid a Jack-knife dynamic, the following actions should become an integral part of his natural driving culture:

  1. Always operate a roadworthy vehicle. If the driver detects any brake abnormalities, notices any abnormal wheel or brake wear (on any axle), or notices that stopping or slowing is becoming less effective, they should have their vehicle tested or seen to.
  2. The driver should always load his trailers with a specific interest in the balance of the brake forces across all axles. Placing the load over the rear-most axle is better than placing it over the fifth wheel (forward on the trailer) because the greatest braking force (for the trailer) must be at the trailer wheels.
  3. The driver should avoid any harsh or sudden braking as far as possible. Due to the high masses, heavy loads, or dynamic load movements of commercial cargo, there are a variety of force events that could destabilize the truck/trailer combination. The driver should always take active steps to slow down with the wheel brakes (as opposed to engine braking - unless it is safe to do so) and avoid any sudden or harsh brake application while doing so.
  4. The driver should ensure that all his tyres are equally inflated, of equal trad-depth, and of a similar kind (at each wheel position, where double wheels are installed). Any imbalance in braking, friction, or wheel diameter can affect brake force balance.

Braking the "Jackknife Myth"

Unlike cars, a tractor-trailer has several braking options-the steering axles, trailer axles, or drive axles. A properly trained driver should be able to quickly apply the needed brakes to best avoid a jackknifing incident. Furthermore, an experienced trucker will tell you that, ideally, a driver should never be in a situation that requires emergency braking techniques.

A truck driver can proactively prevent an accident that could result in jackknifing by doing the following:

  • Maintaining the correct stopping distance. Increasing the following distance between the truck and the cars ahead will create a large stopping distance in which the rig can slow down before full braking.
  • Braking over a longer distance. By gradually slowing down, a truck driver can decrease the momentum of the trailer and keep it from pushing itself to the side.
  • Braking ahead of time. Rather than starting to brake in the middle of a turn, truckers should begin to slow down before the turn to decrease inertia and centripetal force.

For the problem with Jack-knifing to be fully understood, we need to first visit and understand some of the design features of commercial vehicles - especially tractor/trailer combinations. The thing we need to be considered first is brake force bias - the amount of braking that each of the many axles should be contributing to the total braking effort. On tractor/trailer combinations all wheels do not contribute an equal amount of braking friction. If this were the case, there would be challenges, which would include Jack-knifing.

Let’s address the brake bias issue from the front to the rear…

The front wheels cannot ever “lock up” because any loss of steering would result in a very serious situation on account of the size and weight of commercial vehicles. The manufacturers design the front wheels to have only about 10% of the total braking capacity. This means that the front wheels are much less likely to “lock up,” allowing the driver to steer the vehicle, even under heavy braking. This is because a locked-up wheel is simply skidding across the ground, and cannot make any contribution to steering while skidding.

The drive axle is typically where more of the weight is concentrated - especially when an empty trailer is attached. As a truck applies brakes, the momentum of the load also increases, resulting in a greater downward force on these wheels. Since they are also at the most vulnerable point of the combination (the 5th wheel), they need to carry most of the “braking burden.” So, the drive axle typically carries about 36% of the total braking capacity of the combination.

The rear-most axle (in this example) carries about 24% of the braking capacity of the combination. This is because the rear wheels are also not allowed to “lock up” since this can cause a “trailer swing” (causing the trailer to skid around and “overtake” the truck).

While there are more complexities to the brake bias of a combination, especially where ABS (Anti-Lock Brakes) and other electronic and mechanical safety systems are installed, this could be considered the “bedrock” of the issues to consider in Jack-knifing incidents.

But it goes deeper…

When we look more closely at the rear axles (the trailer axles, in this example), we will see that the axles typically move up and down, relative to the trailer, as loads are added or removed. This movement is very important as it can be linked directly to the load on the trailer.

A device, called a Load Sensing Valve, is typically installed on the trailer (it can also be on the truck), and attached to the axle or axles via an actuation arm, which is connected to a sensing arm. As the truck is loaded, the suspension springs will compress, which will lead to the sensing arm being rotated and the valve condition changing.

Through this mechanism, the amount of braking that is available on the rear axle can be “adjusted” to the load on the trailer. The heavier the load gets, the greater the braking power available at the rear axle. When a load is removed, the axles will “drop” which causes the valve to change and the braking force to reduce.

This is very important since a faulty, disconnected, or broken valve can cause the trailer brakes to assume a state of maximum braking, even when the trailer is empty. This, in turn, means that the trailer wheels can lock up sooner than they should. This will result in a loss of friction at those wheels, resulting in greater momentum (mass times velocity) being transferred to the truck, through the 5th wheel.

This means that the truck wheels, which have the greatest braking capacity, could lock up since there is hardly any weight on the drive wheels. When this happens, the rear of the truck-tractor (of the horse) can be forced to “spin out” while the trailer pushes forward in a near-straight line. When this happens, you have what we call a Knife-Jacking.

This is why the best way to avoid or eliminate Knife-Jackings is to properly maintain the vehicle, and to avoid harsh or sudden braking at all costs - especially on any smooth surfaces, like wet roads, snow, ice, or loose gravel.

A word of appreciation to the following:

Derek Kirby, MasterDrive, Stand Bezuidenhout, Forensic Road Transport and Risk Expert.

Also view:

Truck and Goods -In Transit Insurance in South Africa

Safety Tips for Truck and Bus Drivers

Safe Driving and Towing a Trailer Safely

Understanding Brake Failure 

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