Two Types Of Tractor-Trailer Jackknives, And How To Deal With Them

by Brett Aquila

Editor's Note:

A recent forum discussion covered the do's and don'ts of engine braking (Jake Brake), and inevitably turned to the subject of jackknifing, and how to avoid it:

"A good driver will not jackknife if forced to go down a snowy hill with 'high' engine braking because he feels the truck and trailer. It's a bad idea of course, and serves no purpose, but can that person do it? We're not supposed to drive on black ice either, but can you do it confidently and safely?"

Original Conversation - Engine brake: when to use?

A trailer jackknife tends to happen when you're empty on slick roads. You hit the brakes, the trailer brakes lock up, and the back end of the trailer starts sliding to the left or right. The tractor often keeps pointing straight ahead and has full traction.

Now trailer jackknives tend to happen rather slowly and are normally pretty easy to get out of if you can let off the brakes a little and lightly touch the throttle. Once you let off the brakes the trailer tires can begin finding a grip and touching the throttle will pull the trailer back in behind you. I've had this happen on slick roads with an empty trailer in strong crosswinds without hitting the brakes. I've also had this happen when hitting the brakes with an empty trailer on slick roads.

As soon as you notice that trailer is moving to one side or the other you let off the brake, touch the throttle a little, and you're usually fine. Of course if you aren't in a position to be able to let off the brake and touch the throttle a little you're in big trouble. Once that trailer loses traction and starts coming around it's not going to stop coming around until you allow it to. Fortunately because the trailer is so long it comes around rather slowly.

Tips For Getting Out Of A Trailer Jackknife -

Truck driver's quick tips for getting out of a jackknife situation when your trailer brakes lock up and the trailer starts coming around.

A tractor jackknife tends to happen on slick roads when you're loaded heavy, your tractor and trailer are not quite lined up in a straight line, and you hit the brakes. It can also happen if you hit the throttle too hard on slick roads causing the drive tires to break loose. Maybe you're going around a curve. Maybe your trailer tandems are out of alignment a little bit. Anything that makes your tractor point in a slightly different direction than your trailer...and I mean slightly....can cause this.

The drive tires on your tractor will lose traction and the rear of your tractor spins around to one side or the other as the weight of the trailer pushes your tractor around. The trailer stays straight ahead and often times does not lose traction. The tractor spins around and slams into the side of the trailer.

A tractor jackknife normally happens so fast that by the time you realize what's happening your tractor is facing the wrong direction on the highway and you get smashed into the side of your own trailer. Because the wheelbase is so short on the tractor it whips around way more quickly than a trailer jackknife. The only time I had this happen in a scary way was when I was climbing a hill on slick roads with a light load and the drive tires broke loose. I heard my engine wind up and suddenly my tractor was on a 45 degree angle toward the snowy field to my right.

From the time I was going along quietly until I was facing that field was less than one second. Fortunately the moment I heard the engine rev I knew exactly what it was and instantly let off the throttle and started steering left. The entire tractor and trailer went about halfway into the hammer lane, which was empty fortunately, but I saved it and continued on.

What Happens In A Tractor Jackknife? -

When the drive tires break loose on slick roads, or when the drive tires lock up, the trailer starts to push the tractor, causing it to spin sideways. Tractor jackknives are nearly impossible to pull out of.

Now I'll say this.....having the drive tires break loose like that when you're on the throttle like mine did isn't nearly as bad as having them break loose when you're on the brakes. I was able to save a tractor jackknife caused by too much power to the drives on slick roads. But you normally won't be able to save a tractor jackknife caused by over-braking on slick roads because:

1) The trailer is "pushing" the tractor harder if the drive tires break loose when you're on the brakes than it does when you're on the throttle so the tractor tends to spin around more quickly if you're under braking.

2) There is a delay in the air brake system that you don't have with hydraulic brakes. Hydraulic brakes react almost instantly. Air brakes react more slowly. So even if you react almost instantly to a tractor jackknife caused by heavy braking that 1/2 of a second or so can mean the difference between saving it or wrecking it. Here's a quote from air brake section of the High Road Training Program regarding this delay:

"With air brakes, there is an added delay – the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light /medium trucks), the brakes work instantly. However, with air brakes, it takes a little time (one-half second or more) for the air to flow through the lines to the brakes."

That same delay that exists when you step on the brake also exists when you're letting off it. From the time you release your brakes until the pads leave the drums is about 1/2 second. That's an eternity when you're in the middle of a tractor jackknife. Now there is no delay with Jake brakes themselves, but there will be in your reaction time when turning them off.

So there's almost no way you're going to save a tractor jackknife, especially if it's caused by over-braking (Jakes or foot brake) on slick roads. You're going to smash your face off the side of your own trailer and run over anything in the path ahead....unless of course there's a cliff ahead of you in which case the last thing you'll see is your trailer landing on top of you as you fall backwards and upside down to the bottom of the ravine.


Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.


Tandem Axles

A set of axles spaced close together, legally defined as more than 40 and less than 96 inches apart by the USDOT. Drivers tend to refer to the tandem axles on their trailer as just "tandems". You might hear a driver say, "I'm 400 pounds overweight on my tandems", referring to his trailer tandems, not his tractor tandems. Tractor tandems are generally just referred to as "drives" which is short for "drive axles".


Tandem Axles

A set of axles spaced close together, legally defined as more than 40 and less than 96 inches apart by the USDOT. Drivers tend to refer to the tandem axles on their trailer as just "tandems". You might hear a driver say, "I'm 400 pounds overweight on my tandems", referring to his trailer tandems, not his tractor tandems. Tractor tandems are generally just referred to as "drives" which is short for "drive axles".


When a violation by either a driver or company is confirmed, an out-of-service order removes either the driver or the vehicle from the roadway until the violation is corrected.

by Brett Aquila

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