What does a jack knifed lorry mean?

[oceanauk]

Blimey I bet this is a dumb question but I hear it all the time and wonder what it means? I assuming it means a lorry has turned over or something. But why this phrase and where did it come from? Thanks

[SaxyJag]

It's when an articulated lorry ends up with the tractor unit/cab at too tight an angle to the trailer - so tight that it can't be driven out of the situation.

[oceanauk]

Thank you!! Having a blond day!!!

[sddsddean]

As said, because the truck http://www.hankstruckpictures.com/pix/trucks/p at_mcclure/jack_knife02.jpg
looks like a jack knife http://www.knivesplus.com/media/RG-813.jpg

[carfurn]

hi their its not a dumb question, a jackknife is a type of knife, you may have herd of a flick knife or pen knife. when an articulated vehice goes out of control possibly due to heavy braking the forces acting on the trailer could push the unit so hard that it causes the unit to turn around to face the opposite direction. when stationary it would resemble a jackknife in the half open position. this is more likely to occur on an unladen vehicle. due to the lack of traction and the trailer bounce. a bit technical i no. www.carfurn.co.uk

[Toureman]

A jack knifed truck can quite easily be driven out of a jack knife situation if it is not damaged too much.

The photo answers show a jack knifed articulated truck and the simularities to a jack knife.

When you brake hard or even gently in snow weight is transfered from the rear wheels to the front wheels, this means the rear wheels do not grip the road or snow as much as the front, this causes the rear wheels to slip sideways as the weight of the trailer pushes on the back of the tractor unit causing it to skid and finish up in the jack knife position.

Once the truck has stopped and if there is enough room, simply drive it forwards putting on opposite lock to the direction of the initial skid and the truck can just be driven away.

[oceanauk]

Wow!! Thank you everyone!!

[dayoff]

this is caused when the trailor is heavy and you brake hard the unjt stops but the trailor keeps pushing thus pushing the unit out of the way and as its conected it swings around until it collides with the still moving trailor abs is helping prevent this but it still happens