Every new trailer driver has had this moment: you're backing up, the trailer starts to turn, and before you know it the truck and trailer are folded into a sharp V. You turn the wheel the other way, but the trailer just keeps going. That's a jackknife, and it's the most common way a trailer back-up goes wrong.
This article explains why jackknifing happens, why it seems to happen so suddenly, and how to stop it before it starts.
What a jackknife is
A jackknife happens when the angle between the tow vehicle and the trailer gets so sharp that they fold toward each other, like the blade of a pocket knife closing. When backing, it usually means the trailer has swung so far to one side that you can no longer steer it back, and continuing to reverse only makes the angle worse. Go far enough and the trailer's front corner hits the truck's bumper or tailgate, which can damage both.
The word is also used for a different, much more dangerous situation on the highway, when a tractor-trailer's wheels lock under hard braking and the trailer swings around. This article is about the far more common kind: jackknifing while backing at low speed.
The hitch angle
The key to understanding jackknifing is the hitch angle: the angle between the centerline of the truck and the centerline of the trailer, measured at the hitch.
- When the rig is straight, the hitch angle is zero.
- When you back around a turn, the hitch angle grows as the trailer swings to the side.
- When the angle gets large enough, the trailer is past the point where steering can bring it back while still reversing.
In reverse, the truck is pushing the trailer from an angle. The bigger the angle, the more of that push goes into swinging the trailer further sideways instead of pushing it backward. That's why the angle doesn't just grow, it accelerates.
Why it happens so fast
At a small hitch angle, the trailer turns slowly and responds well to corrections. At a large angle, it turns quickly and corrections barely help. There's a point, sometimes called the point of no return, past which even turning the steering wheel all the way the other way can't straighten the trailer while backing. The truck simply can't get its back end far enough to the side to catch up.
This is why a jackknife seems to come out of nowhere. For the first few feet everything looks fine. Then the angle crosses a threshold and runs away from you. The mistake didn't happen at the moment of the jackknife. It happened a few seconds earlier, when you kept the wheel turned too long.
The main causes
- Holding the wheel too long. You turn the wheel to start the trailer moving, then keep it there. The trailer turns, then keeps turning, then folds. This is the most common cause by far.
- Turning too much. A big steering input makes the hitch angle grow fast, giving you less time to react.
- Backing too fast. The angle changes in proportion to how far you've traveled. At higher speed, you cover that distance before you notice.
- Not watching the trailer. Focusing on the target space instead of the trailer means you miss the angle building up.
- A short trailer. Short trailers react faster and jackknife sooner. See Short vs Long Trailers.
How to prevent a jackknife
Follow the trailer
The single most important habit is this: once the trailer starts turning the way you want, start turning the steering wheel back the other way so the truck follows the trailer. Your first input points the trailer. Every input after that is about keeping the truck lined up behind it. If you remember only one thing, remember this.
Make small inputs and watch the response
Turn the wheel a little, wait to see what the trailer does, then adjust. Small inputs keep the hitch angle small, and a small angle is easy to control.
Go slowly
Slow down to idle speed or less. Speed doesn't change how far the trailer turns for each foot you travel, but it changes how much time you have to react. At a crawl, you have time to catch the angle before it runs away.
Watch both mirrors
A growing hitch angle shows up in your mirrors. The trailer gets bigger and moves further across one mirror while it disappears from the other. If one mirror is full of trailer and the other shows none, the angle is getting large.
Set up well
A good setup means the trailer only needs a small turn to reach the space. Pulling well past the space and starting with the trailer slightly angled toward it keeps the maximum hitch angle smaller for the whole maneuver.
What to do when it's starting
If you see the angle getting too sharp:
- Stop. Don't keep backing and hope. More reversing makes it worse.
- Pull forward. Driving forward straightens the rig, because in forward the trailer always follows the truck. Pull forward until the truck and trailer are much straighter.
- Start again with a smaller input. Use what you learned. If the angle got away from you, you probably held the wheel too long.
Pulling forward is not a failure. It's the correct response, and experienced drivers do it constantly. The only real mistake is continuing to back once you know the angle is too sharp.
Jackknifing and equipment
Some modern trucks have trailer backing assistance systems that steer the truck automatically while you control the trailer's direction with a knob, or that warn you when the hitch angle gets too large. They can be a big help, but it's still worth learning the skill yourself, especially if you might tow with different vehicles. Also be aware that some trailers, such as those with a long tongue or a high-sided body, can strike the truck at a smaller hitch angle than you'd expect, so know your own rig's limits.
Practicing in The Trailer Parker
The Trailer Parker shows you the hitch angle as you drive. The hitch-angle warning turns amber when the truck and trailer are getting too sharp, and a red JACKKNIFE alert appears when you've gone too far. Use the amber warning as your training signal: when it appears, either start steering back to follow the trailer or stop and pull forward.
A good exercise is to try a session in the drone or 2D overhead view, where you can see the hitch angle directly, and practice catching it early. Then try the same lot in the cab view, using only the mirrors. Every pull-up counts as a direction change, so a clean park with no jackknife scores better too: three or fewer direction changes earns the full 20 points for corrections.