How come every single day traffic reports include several Jack kniffed lorries and caravans?
How do so many occur, and why do they take so long to be cleared up? and why do they nearly always take up 2 lanes on motorways.
Why on a single road (A303)today, do lorries struggle to do 40, until the dual carriageway when they suddenly find 50+ is easy, only until they reach the end of the dual carriageway when they revert to 35-40 AGAIN.
Finally WHY when I filled up at Sainsbury's in Staines was diesel 96.9, but when I got to Bristol,at Sainsbury's (A38 towards Bristol airport off M5) it was only 92.9. Is this the cheapest diesel around??
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1 Jack knifing
This is where the trailer (or caravan) tries to overtake the towing vehicle under braking. As it always swings round to do this it always blocks two lanes.
2. Lorry speeds
The legal limit for hgvs is 40mph on single carriageway roads, but is higher on dual carriageways/motorways.
I dont know what the lowest deisel (yeuch) price is, but unleaded is only 91.9ppl round here:-)
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Jack Knifing is where the drive axle of the tractor unit locks up under braking - the tractor unit then behaves a little bit like a car doing a handbrake turn, BUT, once it has begun to turn, the extra push from the trailer then forces the "handbrake" turn to completion **extremely** rapidly.
Once the jack-knife has initiated, the tractor unit isn't pointing in the direction of travel, and the driver has no control other than to stamp harder on the brake, more often than not, the jack-knifed truck occupies most of the carriageways available to it.
The tractor unit cab ends up crushed against the side of the trailer, looking from above a little bit like a blade being opened on a penknife, hence the term jack-knife.
Jack knifing is more likely to occur if;
The drive axle of the tractor unit is over-braked
The tyres on the drive axle are in poor condition
The trailer is under-braked
Because of the huge weight change between a loaded and unloaded trailer, which cannot be fully compensated via load sensing valves; and the ad-hoc manner in which tractor units are coupled to trailers, jack knifing incidents are inevitable.
Number_Cruncher
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Finally WHY when I filled up at Sainsbury's in Staines was diesel 96.9, but when I got to Bristol,at Sainsbury's (A38 towards Bristol airport off M5) it was only 92.9.
Cos staines is posh, and Bristol is a rat ole.
Is this the cheapest diesel around??
yes
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TourVanMan TM < Ex RF >
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Jack knifing is now very rarely down to braking, more likely poor tyres on cab rather than the trailer too. Most if not all lorries are fitted with HGV version of abs to stop it happening under braking. Happens a lot in winter if you remember news headlines correctly too.
Reason why? It is easier to explain if the lorry is doing a slight bend, the trailer is at an angle, the cab part slides and cannot regain grip because of the weight pushing it on (In a car releasing the accelerator would normally be enough to bring the steering back) and running out of room to be able to get the cab steering back again.
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The How and Why that often mystifies me is concerning overturned vehicles. How does this happen so regularly? Even heard about an overturned steam roller recently.
Oz (as was)
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If its on the same stretch of road Continually I would hazard a guess that the camber of the road has a great deal to do with it.
Other likely factors are poor loading of vehicle (too much weight too high) and some trailers are getting higher if you notice. Poorly adjusted ride heights, or even poor driving (too fast at bends etc). Or clipping kerbs etc at too high a speed also.
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>>>> Cos staines is posh, and Bristol is a rat ole.<
Staines posh? Blimey, that must be a first!
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The simple reason why they jack knife is that they are out of balance in some way. This normally happens under braking, but not always. If a front tyre blows on the unit then the sudden turning of the wheel and change in direction can cause a jack knife. I very nearly jack knifed one just by changing down the gear box in the wet, this was enough to cause the drive wheels of the unit to slow and loose grip just as I was approaching a corner - not my most fun moment!!!! In the snow and ice it is very much more likely and driving them becomes much more "fun", jack knifes can occur by accelerating too hard in these conditions as the drive wheels spin and again loose grip.
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TU, I think you are right about being able to lose control of the drive axle by acceleration as well as braking. It's surprisingly easy to break traction when the trailer is lightly loaded. [Difficult to resist the urge to play when negotiating deserted wet roundabouts while on night trunking duty :-)]
Having said that, I think the natural response of backing off the throttle when grip is lost on the drive axle is usually enough to keep everything in line.
While ABS reduces locking up of the rear axle, truck brakes aren't as fast to apply and release as car ABS.
Number_Cruncher
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Sorry but hgv brakes apply and release much faster than car. But its a lot more weight they are trying to stop so braking time takes much longer.
HGV brakes are much more fierce in operation than car if used in anger.
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>>Sorry but hgv brakes apply and release much faster than car.
Interesting, please explain the difference, and tell me why.
Number_Cruncher
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The basic difference is that hgv brkes are air operated release.
That is the brakes are on unless there is sufficient air pressure to release them. Application of the brake cuts off air supply and they are on. The harder the pedal is pressed the more the air is restricted, the more they are on.
If you were to flick the handbrake lever on at anything over 5 mph you would be lucky if the seat belt held you (exageratted but thats how it feels).
Air pressure works far quicker than hydraulic. Its also a reason why lorries use 2 air lines (1 service and 1 emergency) because if an air line failed in use on the road it would stop like you wouldnt believe unless witnessed.
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You are confusing the abruptness with which the spring brakes apply with the speed of their application.
Air brakes are quite slow for a number of technical reasons
1) The pressure difference across the system is low 140 psi or so, compared with thousands of psi in hydraulic systems
2) The volume of air that has to pass through the system prior to brake application is much larger than the tiny volumes of hydraulic fluid that moves in a car system
3) For trailer brakes in particular, the speed at which a signal travels along an air system is about 5 times slower than for a hydraulic (different acoustic wave speeds)
4) Typically air brakes work through load sensing and relay valves wheras hydraulic brakes have a much more direct "signal" path.
I agree with what you say about abruptness of handbrake/spring brake application, but, sorry, you are quite wrong about speed of application.
Number_Cruncher
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We are going to have to diagree then.
My training days on air brakes are a long while ago, but the basics is that the air pressure is to keep the brakes off, not on. It can be slower to release in certain circumstancces than hydraulic, but is way quicker to apply.
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>>but the basics is that the air pressure is to keep the brakes off, not on
Yes, but that's only true on spring braked axles, the other axles work conventionally.
Number_Cruncher
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