A Seasonal Reminder About Jackknifes
Winter is the season of cold temperatures, snowy roads, and limited visibility when the wind starts blowing. It is also the season of jackknifing trailers. Experienced truckers know that the risk of jackknifes is a normal part of the trucking job. They also know that jackknifes can occur on dry roads just as easily as snowy or icy roads.
With winter fast approaching, we thought it would be a good idea to remind drivers about jackknifes. A jackknifing trailer and a tractor jackknife presents a dangerous situation with a considerable risk of not ending well. At the very least, jackknife accidents can damage equipment extensively. A worst-case scenario results in serious injury or death.
The Physics Behind Jackknifes
We all know jackknifes occur. But what causes them? What are the physics behind jackknife accidents? It all boils down to two things: kinetic energy and momentum.
Kinetic energy is stored energy in motion. When a truck and trailer travel down the road, some of the energy that comes from friction between tires and road surface is transferred the rig. That means every tractor-trailer rig possesses kinetic energy stored in both the tractor and trailer – as long as it remains in motion. Here’s the problem: energy can neither be created nor destroyed. It can only be transferred.
When a tractor-trailer begins to slow down, that kinetic energy has to go somewhere. Under normal circumstances, it is transferred through the wheels and into the ground by way of friction. In a jackknifing situation, this isn’t what happens. The primary culprit is momentum.
The principles of momentum dictate that kinetic energy will continue moving in the same direction unless an equal or greater force interferes. This is exactly what happens in a jackknifing situation.
The tractor begins slowing at a rate that is too fast to accommodate the transfer of all the kinetic energy stored in the trailer through the wheels and into the ground. The same can happen with a trailer to the tractor. What cannot be transferred continues moving in the same direction, which is forward. Thus, the back end of the tractor swings out as the energy of the trailer continues moving in the same direction.
Unequal Traction and Friction
Physics aside, the most common cause of jackknife accidents is unequal traction and friction between the tractor and trailer. On dry roads, jackknifes are almost always caused by tractor brakes locking up while a truck is moving at a high rate of speed. Because there is unequal traction and friction between tractor and trailer, the tractor is trying to stop faster than the trailer. This inequality triggers the physics discussed in the previous section.
On snowy or icy roads, a tractor’s brakes do not have to lock up to initiate a jackknife. All it takes is for the tractor to lose a bit of traction. If the back end of the tractor begins to swing out because of snow or ice on the road, the same principles of kinetic energy and momentum can cause a jackknife.
Of course, the risk of jackknifing on snowy or icy roads increases when brakes are used too aggressively. As any truck driver knows, it is a lot easier to lock the brakes on snowy roads as compared to a dry pavement.
Whether you understand the physics behind jackknifing or not, you should know how easy it is to get in trouble if you are not paying attention. The simple answer is slow down and make gentle braking a habit; which means to look further ahead and start earlier when slowing down. Never drive faster than 30 MPH in slick conditions and never faster than you can see and react to what happening ahead. Take it easy, pay attention, and adjust your driving to weather conditions. Do everything you can to avoid jackknifing accidents and we will all be better off.