Staying on the Straight and Narrow: Why trucks jackknife and how to avoid making the 6 o’clock news

If you have ever yearned to have your name in lights, or at least make the six o’clock news, just jackknife your truck on any freeway in any major city.

The public, rightly, has a special level of disdain for crashed trucks that keep them from their appointments. And nothing says “I screwed up” louder and clearer than a trailer splayed out across three or four lanes of traffic. 

The thing is, jackknifes and trailer-swing crashes are almost entirely avoidable. Statistically, they do not rank high in terms of fatalities. However, loss of life often occurs with secondary and tertiary collisions that follow when traffic piles into the initial crash.

While accurate and up-to-date Canadian numbers are difficult to dig up, the U.S. Federal Motor Carrier Safety Administration reports about 5,000 such incidents occur south of the border each year, representing about 5% of the country’s total number of truck wrecks. It’s reasonable to assume that Canadian stats would be similar.

Two types of crashes are commonly referred to as a jackknife — so called because of the resemblance to the blades in a partially folded pocket-knife.

A true jackknife is when the tractor is literally pushed out of the way by the trailer, causing it to swing left or right and slam into the side of the trailer.

A trailer-swing crash is when the rear of the trailer swings or slides sideways, but the tractor remains more or less pointed in its initial direction. Banked curves and canted road surfaces with high crowns can be contributing factors in such crashes.

In either case, the root cause is loss of traction.

You might be thinking, ‘Wait, anti-lock braking systems (ABS) became mandatory on new tractors in 1997, followed by trailers a year later. And we’ve had electronic stability control (ESC) in one form or another since 2017. How then is a crash resulting from traction loss even possible?’

I’ll come back to that, but let’s park it while we look at physics behind the jackknife-type crash.

A skidding tire leads the crash

While this might be difficult to get your head around, a skidding tire — ie., one that is not rotating — has much less traction than a rolling tire.

Here’s an interesting historical fact: Up until the mid 1980s, new tractors were often sold without brakes on the steer axle. This was one way of mitigating traction loss on the steering axle during a hard braking event or on icy pavement.

It was also possible, back then, to spec’ a truck with a manual front brake limiting valve. This was a driver-selectable valve mounted on the dashboard the driver could use to limit application pressure to the front brakes on slippery roads.

This feature still exists on tractors, but the valves are internal and function automatically. The valve limits the application pressure to the front wheels by about 50% in any brake application of less than about 50 psi. Beyond 50 psi application pressure, the valve opens and delivers full application pressure to the front brakes.

These limiting valves are designed to prevent the steering tires from locking up in a hard brake. If the wheels lock up (skid), the driver will be unable to maintain directional control of the truck. But the same thing can happen if the drive wheels lock up.

Locked-up drive wheels

During a hard brake, or any hard-ish brake on slippery pavement (icy, snowy, wet, etc.), the drive wheels will skid if they lock up. Under normal driving conditions, the tractor pulls the trailer. But when braking, the weight of the trailer pushes the tractor forward.

If the drive wheels are locked up — skidding, not rotating — the momentum of the trailer will push the eight locked drive tires forward while the two poor steering tires struggle to maintain their grip on the pavement.

The steer tires almost always lose this little tug-of-war.

The sliding drive tires offer little resistance to the push from the trailer. This usually results in the back end of the tractor accelerating against the resistance of the steer tires, causing it to spin around sideways and slam into the side of the trailer.

All this happens in an instant. It’s impossible for a driver to prevent a jackknife once the process has begun.

You can see a demonstration of this in the video below.

Locked-up trailer wheels

The same thing happens to trailer wheels when they lock up. They tend to slide sideways, from the high side to the low side of the road, depending on the lane position relative to the road profile.  

If the truck is in a curve when the trailer wheels lock up, the centrifugal forces acting on the back of the trailer will push the trailer wheel toward the outside of the turn.

I recall a situation from early in my driving career when rain had turned to freezing rain as if I’d driven through a curtain. The road surface had become a wet skating rink in an instant, leaving drivers with zero traction.

I was in a right-hand curve in the left lane on I-75 in Ohio. When I realized my traction had evaporated, I pulled the tractor over the left edge of the pavement where the tires could bite into some icy gravel. Braking cautiously, I got my speed down to about 10-15 mph (16-24 km/h).

But because of the radius of the curve, which was banked slightly, my trailer wheels were still on the icy pavement. I watched in horror as my trailer wheels, under even that slight braking pressure, slid down the bank in the road surface, straight toward a car-carrier trailer in the right lane beside me.

The only option I had was to take my foot off the brake pedal and allow the trailer wheels to resume turning. Once they had regained traction, the sideways slide stopped — with the trailer just inches from the car carrier.

ABS and ESC

As I mentioned earlier, you may be thinking, how is a jackknife even possible with the plethora of advanced safety systems on today’s trucks?

Those systems are invaluable and while they have reduced the number of jackknife crashes, they are not foolproof.

The technology was designed to reduce risk, not eliminate it. ABS helps prevent wheel lock-up during hard braking, lessening the probability of trailers swinging out suddenly. ESC actively reduces engine power and can apply individual brakes to steer the vehicle back on track. Traction control works to limit wheel spin when accelerating on slippery roads.

These systems, however, are preventive, not corrective. They can help prevent the conditions that lead to a jackknife, but they may not be able to recover if conditions are severe.  

On top of that, the safety systems — and the rest of the braking system — must be in good working order to function as intended.   

I can think of a dozen conditions that might reduce the effectiveness of such systems. These are probably the Top 3:

Malfunctioning or non-working ABS: The yellow ABS malfunction light located in the dash cluster and at the rear of the trailer is there for a reason. If it’s on at any time other than when you first key on the truck, there’s a problem that needs to be dealt with.

Improperly maintained foundation brakes: This includes out-of-adjustment slack adjusters – both can compromise brake performance.

Excessive brake application, and importantly, release timing: If the brake actuators are mechanically jammed or get hung up before releasing, the wheel could remain locked up long enough to cause a skid, even if the ABS has commanded the brakes to release.

Physics still wins in extreme conditions

Even the best tech can’t defy physics. There are situations where technology is simply overwhelmed. On black ice, for example, ABS and traction control may not generate enough grip to maintain trailer alignment. Or, when braking hard in a curve.

ABS and ESC can help control braking, but they cannot create traction where there is none.

And certain vehicle characteristics can contribute to inherent instability. Lightly loaded or unladen trucks are more prone to skidding because there’s less weight to generate traction — especially in a hard-brake situation.

Uneven loading can also contribute to the problem. If there’s a full load on the drive axles, but the rear of the trailer is empty, the trailer wheels could lock up before the drive wheels in a hard brake, possibly causing a trailer swing event.

And then there’s driver error. The two greatest threats to instability are driving too fast for conditions and not leaving adequate reaction time and stopping distance from the vehicle in front. 

When things go south directly in front of the truck, an inexperienced driver’s first instinct is to slam on the brakes — a sure-fire way to induce a skid. The proper way to brake in an emergency is to apply the brake slowly but firmly. This allows the wheel to slow more gradually (rather than stopping it instantly) while weight shifts forward onto the steer and drive tires increasing traction.

Of course, if you haven’t left adequate stopping distance, you may not have the luxury of time to apply the brakes properly. You might be already screwed.

Tips for staying on the straight and narrow

Here are a few scenarios almost guaranteed to trigger a jackknife crash if drivers are not paying adequate attention.

Remember that ABS is a driver-assistance feature, not your savior in times of need. Do you remember how to get a truck out of a skid without the benefit of anti-lock brakes?

Hint: Don’t hold the brake pedal to the floor. Release and reapply the brakes repeatedly to allow the rotating wheels to regain traction.

Off to skid school

Back in the late ’90s, as I was transitioning from from truck driver to journalist, I was invited to visit a skid school — The Michigan Center for Decision Driving in Kalamazoo, Mich. It had been closed for many years, but it’s now open again, operated by the Michigan Trucking Association.

It’s one of very few skid-pads for commercial vehicles open to students anywhere in North America.

That day was a transformative event, though I already had 20 years of experience. It came a bit late in my driving career, but I’ve never forgotten it.

The course demonstrated how jackknifes occur and why. It also taught students how to recover from such a crash — or at least try. Few of us were successful.

My take-away from the course was that recovery is difficult, even unlikely most of the time, so I was far better off avoiding a jackknife than ever trying to recover from a developing situation.

Cara Hunter, president of Transport Training Centres of Canada, our country’s only public skid school, has a more enlightened perspective.

“Many drivers won’t know how to react in an emergency situation, but if they have muscle memory from being trained in a jackknife scenario, they stand a much better chance of improving the outcome,” she told trucknews.com.

And a much better chance of staying off the six o’clock news and the front page of the newspaper.

TTCC’s skid school program, operated at a privately owned skid pad in Shannonville, Ont., is offered completely free to anyone who takes their A/Z Class 1 driving program. They also offer packages for experienced drivers and corporate customers looking to enhance their driver safety and development programs.

Back in 2010, CBC’s Rick Mercer visited TTCC’s Shannonville skid school and filed a story for the TV show he was doing at the time, The Mercer Report. It’s hysterically funny, of course, but eye-opening too. Check it out.