What is jackknifing?

Jackknifing is when a truck and trailer fold in on themselves to create a ‘V’ shape. Let’s take a look at how this occurs and the best ways to avoid it.

Jackknifing is a type of trucking accident when the truck and trailer fold in on themselves to create a ‘V’ shape.

The word comes from a pocketknife where the blade hinges onto the main body of the tool. Jackknifing can be incredibly dangerous for the driver, other road users and the environment in which the incident happens.

It’s also costly due to the time lost to an accident and potential goods and vehicle damage. So let’s take a look at how jackknifing generally occurs and the best ways to avoid it. 

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What’s the difference between jackknifing and trailer sway? 

Trailer sway can lead to jackknifing if the trailer is not brought under control but it’s different from jackknifing. It generally happens in windy conditions when the tractor unit is driving parallel to the road and the trailer is moving from side to side behind it.

If the cab also isn’t moving straight ahead, for example if it’s turning, the trailer can swing towards the way the tractor is turning and cause a jackknifing incident. It’s important to recognise trailer sway and bring it under control before turning the vehicle to minimise the risk of jackknifing. 

Common causes of jackknifing 

During adverse weather conditions such as rain, sleet, snow and hail, it’s more likely that a trailer will skid out of control. This means small corrections or instances of trailer sway can be more dangerous. 

Poor traction can be a result of light loads, slippery surfaces and engine braking. Trucks and trailers are designed to be used at full capacity so when the trailer is empty or lighter than usual the brakes can be very strong. Hard braking can cause a trailer to skid and without its usual load to weigh it down and make it stick to the road, it can result in a jackknifing incident.

Whilst engine braking can be a more efficient way to improve the lifespan of your brakes, this slowing force is only applied across the driving axle. Gentle braking is the best way to slow down, in anything other than dry conditions, as the brakes will be applied across all axles on the trailer. 

Speed is also a key culprit of jackknifing, it’s especially important to pay attention to the vehicle’s speed when descending a steep incline or approaching a bend. If the trailer is not slowed enough, its momentum can continue and cause the trailer to travel beyond the truck.

All jackknifing incidents can be extremely destructive and place the driver in immediate danger if the cab gets spun the opposite way. The enormous weight carried in trailers is incredibly powerful if it becomes out of control at speed. 

Bends in the road make it more likely to jackknife if the truck and trailer aren’t both driving in line and parallel to the road. Given the potential weight of a trailer, any minor deviation can quickly result in an accident. 

Source: Essex County Fire and Rescue Services

How to avoid jackknifing 

The first way to prevent jackknifing is to ensure both the truck and trailer are properly maintained. Any tyres with flat spots or insufficient tread can lead to traction loss and make it harder to control the vehicle. Brakes and suspension are also an essential part of truck and trailer maintenance, that should work evenly across all wheels. 

Challenging weather conditions are unavoidable, so extra care should be taken when the roads are slippery. The following tips should be useful in all situations but in difficult conditions make sure to give the vehicle more time to slow and be mindful of trailer sway in windy environments. 

Jackknifing tends to be as a result of skidding, so knowing how to avoid and recover from a skid is essential. At high speeds, things can get out of control very quickly, so when approaching descending slopes or curves in the road, slow the vehicle down by gently applying the brakes.

Progressively brake on the approach to a bend and avoid braking or decelerating during the turn. Let off the brakes and apply gentle pressure to the accelerator to stop the driving wheels from losing traction. 

Remember, the heavy trailer behind has a lot of momentum, so slow down more than you think to make sure it’s under control before approaching turns. If the trailer starts to skid to one side, try to gently accelerate to bring it back behind the tractor unit. Braking will mean the trailer has more forward momentum than the cab. 

Uneven loads can also lead to an inconsistent distribution of traction, making it more likely the trailer will skid. Along with daily vehicle checks, ensure the vehicle is loaded correctly to distribute weight evenly over the axles. 

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Trinity Francis
Trinity Francishttps://www.trinitygfrancis.com/
Freelance automotive journalist and motoring writer focusing on all aspects of automotive content, with particular attention to emerging trends, industry innovations, tech and consumer advice.

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