How to Avoid Jackknifing

When a driver loses control of a large rig, like Hurricane Katrina caused here, the vehicle can jackknife, or fold into itself. Roy Causey inspects his car that was crushed by a tractor-trailer, after being hit by Hurricane Katrina in Biloxi, Mississippi. See more truck pictures.
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We hear about it all the time in traffic reports -- tractor-trailers jackknifing on the highway. Hearing those words so often, it's difficult not to get numb to them. But these kinds of accidents -- which can be fatal -- are often preventable.

Jackknifing refers to the dangerous situation in which a vehicle and its attached trailer get out of sync during towing to form an L or a V shape. The phrase gets its name from the shape the truck and the trailer create, which bears a resemblance to a knife whose blade folds into the handle.

To know how to avoid jackknifing, you need to understand why it happens. Although a few different factors could cause a vehicle and its trailer to jackknife, it usually comes down to a loss of traction. Traction has to do with how well a wheel's tires grip the road. It's essential to what makes wheels work. You may understand the concept of traction if you've ever had a vehicle stuck in the mud as its wheels furiously and fruitlessly spun but couldn't grip the ground.

The wheel isn't considered man's greatest invention for nothing. The genius of the wheel is that it uses friction -- the force that resists motion -- to work. In particular, wheels use static friction -- the force between two unmoving objects. Believe it or not, as a rolling wheel touches the ground, that contact point is technically static -- relative to the ground. The more static friction at this contact point, the better the traction. A vehicle's wheels depend on static friction for enough traction to both move and stop controllably.

Unfortunately, slick roads and improper braking wreak havoc on this grip, causing tires to skid along the pavement instead of rolling. Skidding tires are resisted only by sliding friction, which isn't as powerful as static friction (consider how it's harder to start pushing a couch across a carpet than it is to keep it sliding). In this way, slamming on the brakes could have an adverse effect, causing them to lock and leaving the skidding wheel without enough traction to stop. If the tractor or the trailer wheels lock, the loss of traction will allow the rig to swing sideways out of control into a tractor jackknife or a trailer jackknife, respectively [source: California DMV].

Now that we understand the forces that let it happen, we'll go over how to avoid jackknifing with safe driving.


Preventing a Jackknife Situation

To avoid jackknifing, brake before a turn rather than during a turn.
To avoid jackknifing, brake before a turn rather than during a turn.
iStockPhoto/Arpad Benedek

Luckily, even in dangerous conditions, jackknifing isn't inevitable. Preventing jackknife situations takes a few safe driving tips.

The first step in preventing a jackknife situation on the highway is to check your mirrors for trailer swing frequently. You should also do this every time you have to brake hard [source: California DMV]. If you notice that you're already starting to jackknife, it might not be too late to prevent your caravan from bending into an angle of no return. In this situation, experts recommend letting go of the brake which lets the wheel resume rolling and regain the traction of static friction [source: California DMV]. With a trailer jackknife (the trailer wheels lose traction), you can increase your speed to allow the trailer to fall back in line. However, if you are experiencing a tractor jackknife (the tractor wheels lose traction), and you think sudden acceleration could have caused it, let up on the gas pedal until the vehicle regains traction. Then, steer out [source: Byrnes].

In many traffic reports of jackknifing, the trailers are empty. This is no coincidence, and the reason points back to our friction discussion on the last page. The heavier a trailer is, the more it bears down on the road, meaning the more friction it has with the road and the better the traction. Compound this with the fact that over-braking is easier on light loads (because the brakes of a tractor-trailer are made for fully-loaded trucks), and it turns out that empty trailers are more prone to jackknifing [source: Pellerin]. So, although it might seem counterintuitive, you should watch mirrors for trailer swing, particularly when driving an empty trailer.

Proper braking is another essential. Instead of braking during turns, decelerate slowly on the long stretch before the bend or curve. Avoid slamming on the brakes, which could cause them to lock and will often result in a jackknife situation. Although, at times, there may be no alternative to hard braking, you can do your best to stay out of these situations. One way is to keep a safe distance from other vehicles on the road. When an emergency situation occurs, do your best to avoid braking and swerving at the same time. This might mean braking, letting go of the brake, swerving and then braking again [source: Wiley and Terrell].

It's important to note that jackknifing isn't a problem just for truckers -- drivers inexperienced with towing often set out on the highway with a trailer or a boat and end up losing control from heavy braking or other factors. For instance, an improperly balanced trailer with a rear-heavy load tends to sway and can cause the trailer to swing out into a jackknife [source: Kazmierski]. To avoid this, make sure to pack your trailer with weight spread out and on the bottom, giving it a low center of gravity.

However, the novice trailer-tower is even more likely to get stuck in a jackknife position when backing up. This kind of jackknifing has less to do with traction and more to do with knowing how to handle a trailer in reverse. One handy trick is to place your hand at the bottom of the steering wheel, or six o'clock position. While turned around to see where you're going, move your hand in the direction you want the trailer to go [source: Popular Mechanics].

Although these tips will help you as a driver, it's hardly comforting if you're concerned the driver of the enormous 18-wheeler beside you doesn't know them. Fortunately, we can take a little comfort in the development of sophisticated anti-jackknife technology, which has helped combat driver error.

Anti-Jackknife Technology

Engineers have attacked the jackknifing problem from a few different angles. Some technologies involve adjusting the design of brakes, while others involve devices that physically prevent the trailer from veering too far.

Because brake locking can easily lead to jackknifing, the addition of anti-lock brakes on a tractor or a trailer (or both) is an effective countermeasure. Anti-lock brakes do just what the name implies: They prevent your wheels from locking up. They do this by equipping a vehicle with sensors that are on the lookout for unusual heavy braking. When you slam on your brakes and risk over-braking the wheels into a dangerous lock, this trips the sensor into decreasing the brake pressure on the wheel [source: AIADA].

By doing this, anti-lock brakes keep your wheel braking while preventing it from freezing into an all-out lock. Although you may not necessarily be able to brake faster with these, you will be able to control steering better by maintaining traction in avoiding skids [source: FMCSA]. While this is going on, you'll feel a pulsing in the brake pedal. You can learn more about this system and its different types in "How Anti-Lock Brakes Work." Studies indicate that anti-lock brakes have proven extremely beneficial for large rigs that are prone to jackknifing -- even though anti-lock brakes are no better than normal brakes in cars [source: Harris].

A similar kind of technology addresses the over-braking problems of empty trailers that we mentioned earlier. A load-sensing regulator reduces the brake pressure to the wheel when the load is light [source: Kawabe].

Besides adjusting brake systems, other types of anti-jackknifing technology usually have to do with preventing a tractor and its trailer from folding too far into each other. This is usually an addition to the coupling device that connects a tractor and trailer called the fifth wheel. The inherent difficulty with this type of anti-jackknife device is that a tractor-trailer needs to be able to bend dramatically when making sharp turns.

A few different technologies attempt to overcome this obstacle. For instance, one idea is a swing-limiting device on the fifth wheel that can be engaged and disengaged manually by the driver at the appropriate times [source: STS]. Other designers have developed a prevention device that limits the possible angle of swing once the vehicle reaches a certain threshold of speed and manually disengaged it later [source: Ball]. The logic here is that the device won't be engaged during sharp turns, which should be taken at low speeds.

As engineers perfect this technology (and truck drivers and towers become more aware of safe driving techniques), we can expect our roads to get safer.

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More Great Links


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