What is a “truck jackknife”?
Ever use or seen a pocketknife? The term “jackknife” comes from a pocketknife folding configuration. The collision term “semi-truck jackknife” refers to a truck accident where a truck with two separate parts (a cab and a trailer) folds in on itself at the point of separation. The cab and trailer swivel where they are linked together, at the fifth wheel, forming a 90+-degree angle of an “L” or “V” shape.
What causes a truck to jackknife?
In most cases, it comes down to a loss of traction. Traction has to do with how well a wheel’s tires grip the road. wheels use static friction — the force between two unmoving objects. When the rolling wheel touches the ground, that contact point is static. 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.
Slick roads and improper braking causes 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. 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.
Types of Surfaces
Slippery Surfaces take longer for trucks to stop, and it will be harder to turn without skidding when the road is slippery. You must drive slower to be able to stop in the same distance as on a dry road.
Wet, Snow Packed and Icy Roads significantly increase stopping distance. Speeds must be adjusted for safe operation and control for surrounding conditions.
Bridges: When the temperature drops, bridges will freeze before the road will. Be especially careful when the temperature is close to 32 degrees F.
Melting Ice: Slight melting will make ice wet. Wet ice is much more slippery than ice that is not wet.
Black Ice. Black ice is a thin layer that is clear enough that you can see the road underneath it. It makes the road look wet. Any time the temperature is below freezing, and the road looks wet, watch out for black ice.
Coefficient of Friction
The coefficient of friction is a rating of the grip or “traction” between a road surface and a tire. The value of the coefficient of friction is a fraction, which must be between zero and one. The lower the value of the coefficient of friction of the roadway, the more slippery the roadway will be.
For example, an icy surface may have a coefficient of friction in the range of 0.1, while a clean, dry asphalt surface may have a coefficient of friction of approximately 0.8.
Type of Roadway Surface | Coefficient of Friction
- Dry Asphalt | 0.80
- Wet Asphalt | 0.55
- Hard Packed Snow | 0.20
- Ice | 0.10
Speed Not Reasonable-Prudent for Conditions: a tractor-trailer traveling at the speed limit on dry surfaces is sometimes difficult to stop efficiently. In poor roadway conditions, the truck braking efficiency is reduced. If the driver slams on his brakes at a higher speed, chances increase drastically that the trailer in tow will swing out of control.
Slippery weather conditions and roads: Loss of traction with the road can cause weight to shift against the center of gravity causing both the truck and trailer to slide. Due to the excessive weight and overall length of commercial trucks, even small movements can cause the truck to jackknife.
Methods for Preventing Semi-Truck Jackknife Situations
- Be wary of light loads. Heavier loads provide more traction.
- Even brake pressure must be applied—but stopped short of locking down the brakes.
- Maintain slower reasonable speeds based on existing conditions.
- Spread your braking over the longest possible distance, braking progressively and reducing your speed gradually.
- Give yourself increased following distance – Time and Distance are your friends!
- Avoid having to brake or even decelerate on a curve.
- Be very cautious about using your engine brake or retarder.
- Jackknifing starts as a skid, so do your utmost to avoid skidding.
- Keep both tractor and trailer properly maintained.
- Modern anti-lock braking systems automatically adjust the braking force to prevent wheel lockup.
Semi-Truck Jackknifes Are Preventable. Drivers must always be prepared and alert. Speeds and following distances must be reasonable and prudent for conditions, including weather, roadway conditions, traffic volume, time of day (night/day), and terrain (hills curves).
For more truck driver safety tips, visit Truck Driver Safety Blogs