I only ask after being stuck on the A1(M) for an hour this evening because of a jack-knifed lorry.
The temperature was around freezing point and there was very little lying snow - the road was it's usual black colour.
I heard on the radio of one or two other jack-knifings around the country.
There always seems to be a few when we get some wintry weather.
So what causes a lorry to jack-knife?
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Usually the drive axle loosing traction and the trailer pushing the tractor unit. It can be caused by the brakes being applied while the lorry is "bent". Current drivers feel free to put me right if required.
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The term is wrongly used to describe an articulated vehicle that has drifted sideways by wheelspin usually when attempting to get grip on an incline, or has had an over steer slide on bend or roundabout that couldn't be caught...very easy to happen and the actions of the trailer push it further out.
Exacerbated often by using diff locks when stuck the driven wheels just spin uselessly and polish the snow into black ice, then whichever slight camber is present gravity will see the driven wheels drift.
You then end up with the steering wheels and the trailer wheels in lane one and the driven wheels in lane 2 or 3, it gets to the point the driver eventually has to give up when the driven wheels meet an obstacle.
By not using the diff lock usually one wheel will spin and get grip then the other, but the non slipping wheel will help keep the rig straight.
Diff locks are really meant for when one wheel encounters soft or slippery ground but the other has purchase, or as in HarleyM's usual line of work for digging deep in the cow dung to find something solid to grip, you can use diff locks in snow but need a 'feel' for when to knock them off.
A real Jacknife is a truly terrifying occurence that once started has little chance of recovery, usually caused when the driven wheels lock up under heavy braking, the truck then pivots on the locked driven wheels spinning the cab round and smashing the cab side into the still relatively straight trailer, the vehicle is locked and still travelling forward (cab backwards) with no control whatsoever.
Not saying that there might be one or two real jack knife's in this weather, but with modern braking systems they don't happen as much as traffic reports would have you believe.
Well thats my tuppence.
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A great, very comprehensive reply GB.
Something the average motorist may not consider is that a lorry cab unit weighs around 8 tonnes whilst its fully loaded trailer can weigh 36 tonnes. You can then imagine how a trailer can push "through" regardless of what the cab's trying to do.
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...The term is wrongly used to describe an articulated vehicle that has drifted sideways by wheelspin usually when attempting to get grip on an incline...
The road was closed, so I've not seen the site of the one on the A1(M), but that would fit as there are a couple of Northbound inclines on that stretch.
So if it's not clouted anything, it's likely the rig will have come to a fairly gentle stop?
And how is it recovered?
A chain on the front of the tractor unit and dragged out straight by something with enough grunt?
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>> And how is it recovered?
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>> A chain on the front of the tractor unit and dragged out straight by something
>> with enough grunt?
It's been my experience that a few shovels of road salt and a bit of rocking can work wonders, obviously best if the driver stops trying once it's obvious what happening before the road's blocked, but we all hope to find a bit of purchase and get going again, it'll probably be my turn again soon.
Other than that a gentle pull from a 4x4 on decent tyres aided by the trucks own drive will get most stuck trucks moving unless we're in serious trouble, usually we're only just stuck and once moving it's surprising what a truck can get through, once stopped it's a bind to get going again.
A mate phoned me from the A2 today, been there for hours, his description of other drivers who insisted on jamming themselves right up truck's backside's when needing some room to get a rocking action going was most colourful.;)
Last edited by: gordonbennet on Tue 30 Nov 10 at 22:50
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I think the press and agencies now use the term to describe any articulated rig thats not in a dead straight line in the normal place on the road surface, and the not the cab jammed tight against the side of the trailer that GB described above.
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I think you're right Z. Same as an aircraft that's gone off the end of a runway always being described as an overshoot. The correct term is overrun the runway; overshoot is the old term for 'go around'.
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As per GB's reply, a true jack-knife is when the drive axle locks up and loses sideways grip under braking.
It can be made much worse by having poor tyres on the back axle, and by having a sticking or badly adjusted load sensing valve on the rear axle - as the ratio of unlanden to laden axle weight is large on a heavy goods vehicle some kind of sensing valve is absolutely necessary to adjust the braking in proportion to the load being carried.
If you take a bare tractor unit for an MOT, VOSA use a large clamp applied to the fifth wheel to force the back of the truck down onto the brake testing machine's rollers to open the load sensing valve and allow the drive axle brakes to operate fully during the test.
A true jack-knife is now rare because trucks are now fitted with ABS.
Incidentally, with the exception of the loaded trailer pushing harder, the root cause of this type of accident is exactly the same as the loss of rear axle grip I've been rattling on about for years on motor cars....
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Why do lorries jack-knife? - Old Navy
