Or maybe it’s euro truck simulator that all this happened.
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.
There are lots of reasons why a truck jacknifes in a straight line.
Wet or icy roads will do it, load distribution will do it, faulty ABS will do it etc.
If the ABS on the trailer is not working properly and you brake suddenly the trailer will want to overtake the tractor and the only way it can do so is to go to one side or the other, having a good load mid trailer bed will help to prevent it to a certain extent, having too much at the rear will increase the chances of a jacknife even if the ABS is working, having the load on the front will have the same effect. I am in the position of pulling a 45 foot food grade tanker with no baffles, my load has a mind of it’s own and when I brake it just transfers all the weight to the front which make a jacknife more likely, it’s just a case of knowing how your truck will react in every situation. Most of you on here have been spoiled by having ABS, you have no idea how to react without it ? I had been driving about 8 years before I even hooked to a trailer with ABS 
Us old geezers know how to stop quickly without it … Ask the older bloke’s like Diesel Dan or Bassman
Hi, All
This is turning into an interesting debate. Many theories and reasons have been put forward as to the cause of “jack-knifing”.
I am of the opinion that a lot of modern jack-knifes are caused by the very things designed to rule them out,i.e. all the modern electronics.
Drivers accept these gizmo’s are there and POSSIBLY become convinced it can’t happen. Another factor ,I believe ,is boredom, mile after mile of motorway or dual carriageway dulls the senses so when something starts to build up reaction times are slower, altering a controlled braking experience into a panicked one.
It amazes me how many you see on M/ways and D/c’s.
I think, what could have possibly happened to cause that on that road, I accept that something must have started the sequence of events for it to finish up like that. But a lot of the ones i have seen ,I am sure need not have happened.
We might have done all we can to make sure that the vehicle is OK and the load is properly distributed but all that counts for nothing without concentration on the job in hand.
Incidentally , I agree that if all else fails and you do jack-knife, if you haven’t got an airfield around you there is nothing you can do to stop it.
That’s it I’ll get off my high horse and put my tin hat on !
Bassman
I thought technically that if cab comes round to meet trailer that is jack knife.
If trailer comes round to meet cab that is trailer swing.
Differing reasons for each of above happening and maybe if time and space allowed different ways of correcting the above.
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.
Yes it can do. No idea why you don’t think it will. All that needs to happen is the tractor unit braking efficiency to be a lot more than the trailer for whatever reason and the traction break loose on the drive axle tyres due to the higher forward momentum of the trailer taking the weight off it and you’re looking at a jacknife situation.
Conor:
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.Yes it can do. No idea why you don’t think it will. All that needs to happen is the tractor unit braking efficiency to be a lot more than the trailer for whatever reason and the traction break loose on the drive axle tyres due to the higher forward momentum of the trailer taking the weight off it and you’re looking at a jacknife situation.
I don’t think in good conditions a brake efficiency mismatch is enough of a reason the unit does the lion’s share of the braking even if everything is set-up as it should be, the same as the front disc pads wear quicker than the rears on a car.
My view on the science of it is that it occurs when the sideways friction between the road and the drive axle becomes so low that it can be overcome become by the forward motion of the trailer when the combination is the tiniest degree from the straight ahead position at the pin. At this point the drive axle is like a ball bearing that can travel in any direction that a force is exerted upon it.
So factors that normally affect the friction betwen the drive axle and the road are tyre pressure and tread, road surface ie wet/greasy and the force/weight of the trailer on it through either loading or pin location. One or a combination of these causing the friction to be lower than it should be means that harsh braking is the straw that breaks the camel’s back when it causes weight to shift from the back to the front axle of the unit taking the friction in the drive axle at the back down so low it’s past the point of no return.
Juddian mentioned pin location and I agree with that as well, I think there’s definitely a trade off where going too far forward either for close coupling drag reduction for fuel economy or to get weight on to the midlift on a 6x2 increases the risks.
Well done Weeto in trying to educate less experienced drivers, Shame you don’t know what you’re talking about…!
DrivingMissDaisy:
Well done Weeto in trying to educate less experienced drivers, Shame you don’t know what you’re talking about…!
Well, this just shows the ignorance of the majority of posters on here, not one of you has mentioned the biggest culpit that puts a modern ABS equiped truck into a jackknife!
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.
Come on just give us your scenario,looks like we’re all done guessing.
Mike-C:
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.Come on just give us your scenario,looks like we’re all done guessing.
You all seem to be the know it alls, work it out yourselves, but i will give you a clue!
Its has absolulty nothing to do with the braking system!
Own Account Driver:
Conor:
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.Yes it can do. No idea why you don’t think it will. All that needs to happen is the tractor unit braking efficiency to be a lot more than the trailer for whatever reason and the traction break loose on the drive axle tyres due to the higher forward momentum of the trailer taking the weight off it and you’re looking at a jacknife situation.
I think there’s definitely a trade off where going too far forward either for close coupling drag reduction for fuel economy or to get weight on to the midlift on a 6x2 increases the risks.
You put that far better than i could and explained weight transfer factor simply.
Juddian:
Own Account Driver:
Conor:
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.Yes it can do. No idea why you don’t think it will. All that needs to happen is the tractor unit braking efficiency to be a lot more than the trailer for whatever reason and the traction break loose on the drive axle tyres due to the higher forward momentum of the trailer taking the weight off it and you’re looking at a jacknife situation.
I think there’s definitely a trade off where going too far forward either for close coupling drag reduction for fuel economy or to get weight on to the midlift on a 6x2 increases the risks.
You put that far better than i could and explained weight transfer factor simply.
Not really, the ABS’s job is to stop wheels locking up on all axles including the drive axle, hence no loss of traction.
Even when the wheels on each side of the truck are running on different surfaces, like compacted snnow on the near side and dry tarmac on the off side, that vehicle will still stop in a straight line under hard braking!
when for whatever reason traction on the drive axle is not enough the momentum of the trailer could push the unit round (like on the over run coasting or somthing)I think that was how it was explained to me in 1973
weeto:
Juddian:
Own Account Driver:
Conor:
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.Yes it can do. No idea why you don’t think it will. All that needs to happen is the tractor unit braking efficiency to be a lot more than the trailer for whatever reason and the traction break loose on the drive axle tyres due to the higher forward momentum of the trailer taking the weight off it and you’re looking at a jacknife situation.
I think there’s definitely a trade off where going too far forward either for close coupling drag reduction for fuel economy or to get weight on to the midlift on a 6x2 increases the risks.
You put that far better than i could and explained weight transfer factor simply.
Not really, the ABS’s job is to stop wheels locking up on all axles including the drive axle, hence no loss of traction.
Even when the wheels on each side of the truck are running on different surfaces, like compacted snnow on the near side and dry tarmac on the off side, that vehicle will still stop in a straight line under hard braking!
Ignoring the more sophisticated systems like EBS/ASR/EBD purely on its own all ABS does is monitor wheel speeds and then remove the brake application with a pulse effect when it detects one or more wheels are locking up. Generally they will still lock up momentarily even when it’s all working as it should it will still leave plenty of tyre rubber on the road indicating a wheel has locked albeit it momentarily. This also has a benefit to stoppping distances and increased control of the situation under harsh braking like steering.
I think the primary reason jack-knifes have decreased is the effect of ABS preventing locking up on the unit drive axle. Trailer ABS will have some part but the system is typically less sophisticated and only monitors wheel speed one axle usually the middle so it’s theoretically possible to lock the other two without ABS intervention at all.
As many people have posted in response to you what ABS does not do is prevent the weight transfer from the rear axle to front of the unit lowering the grip the drive axle has. In a car they always say ‘don’t brake on corners’, why this is said is because braking takes the weight off the back axle and because you’re on a corner the back end comes round. It’s the same effect on an artic except you don’t need a corner you’ve got a trailer so all you need is a tiny tiny angle off straight, a badly loaded trailer or a sharp steering input and off you go.
If you want to test this get an empty trailer open the back doors and fork some 5 ton lump on the back and close the doors. Then go and get unit with the legal minimum 1mm tread on the drive axle that’s been inflated to 130psi to save on fuel then hitch up and slide the fifth wheel right forward. Go out for a run on some smooth tarmac roads in that sort of light drizzle after a long dry spell and do a few emergency stops on the limiter and let us know how the ABS gets on. 
Couple of points; at one time 60/70s when three line braking was the norm and before the age of spring brakes, the deadman or emergency brake was capable of operating the trailor brakes on their own this was very handy especialy on down hill stretches because you could brake the trailor alone causing it to drag which had the effect of keeping you in line.Some units, Ford Transcontinental, Volvo 88/89 and Scanias also had a facility which when you used the exhaust brake it also applied the trailor brakes very handy especialy on long down hill stretches (this on the Ford at least could be switched on and off ) . At the time a lot of the trailors didn’t have load sensing valves thus the brakes were either on or off and depending on the brake chambers ( Type 20/22s ) no air meant no stop. When spring brakes started to become the norm (no air no go ) the powers that be decided that when the deadman was applied it should work on all axles therby taking away what had been a usefull tool in the right drivers hands. The theory being that if you lost your air on the trailor you could still stop it with the unit , but it didn’t matter any more because you lost your air you stopped end of. In the 70,s I worked for a company that tested an anti jackknife device for a company if I remember rightly called " Hope " the two trailors concerned had different devices but one I remember consisted of a pin behind the fifth wheel pin which was attached to a drum and two steel cables which ran down the trailor chassis to a hydraulic valve which if it detected a sudden swinging movement locked the trailor up so effectively the artic became a 45ft rigid ! the main problem was that if the trailor had already started to swing that was the position it locked in until the brakes released. A company called Saddlers who ran a trunk from Liverpool / Glasgow smashed a brand new ERF up when the driver was pushed into a lamppost when it locked him into a semi jacknife position and the driver kept his foot hard on the brake. Sods law says that trailors don’t get as much attention as units and yet can end up doing more milage so brakes and their systems don’t get looked at as often if I was pulling a strange trailor for any distance e,g. across Europe I found 10 mins underneath with a spanner and adjust the brakes saved me a lot of hassel.,
fuse:
when for whatever reason traction on the drive axle is not enough the momentum of the trailer could push the unit round (like on the over run coasting or somthing)I think that was how it was explained to me in 1973
Thats likely if your going round a corner, but you wouldnt be coasting round a corner would you! thats asking for trouble.
weeto:
Mike-C:
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.Come on just give us your scenario,looks like we’re all done guessing.
You all seem to be the know it alls, work it out yourselves, but i will give you a clue!
Its has absolulty nothing to do with the braking system!
I’m really struggling here. They don’t jacknife on their own, they don’t jacknife when you break hard. For the benefit of those like me who do not know it all please tell us how they do jacknife ? This thread has gone from you dispelling an urban myth (which you still havn’t done) to you wanting to think of new drivers and help educate them (which you still havn’t done) to you telling us we’re all know alls. So dispell the myth, help educate new drivers and put all the know alls in their place by just telling what it is.
Mike-C:
weeto:
Mike-C:
weeto:
Following on from another post, So, can some explain this myth? Since when on straight road will my artic jackknife if I break “to hard?”, seems to me there a lot of misinformed drivers out there telly stars included!
Modern trucks don’t jackknife on there own, I can only think of one major scenario were this is likely to happen, but hard braking? I don’t think so.Come on just give us your scenario,looks like we’re all done guessing.
You all seem to be the know it alls, work it out yourselves, but i will give you a clue!
Its has absolulty nothing to do with the braking system!I’m really struggling here. They don’t jacknife on their own, they don’t jacknife when you break hard. For the benefit of those like me who do not know it all please tell us how they do jacknife ? This thread has gone from you dispelling an urban myth (which you still havn’t done) to you wanting to think of new drivers and help educate them (which you still havn’t done) to you telling us we’re all know alls. So dispell the myth, help educate new drivers and put all the know alls in their place by just telling what it is.
Cruise control! and if you want to see the evidence i will tell you were to find it, were trucks ended up jackknifed which wast not caused by braking.
Oh just to add to this re a Brake induced jackknife, even VOSA has an official line on it!!
Interesting that Norfolk.
‘‘Ford Transcontinental, Volvo 88/89 and Scanias also had a facility which when you used the exhaust brake it also applied the trailor brakes very handy especialy on long down hill stretches’’
I recall taking my lorry for MOT one year around '83 when all of a sudden that system had been banned and meant an instant fail.
I seem to recall it all came to a head when the driver of a (Merc?) applied the exhaust brake down a hill (in Maidstone i think) and the automatic slight application of trailer brake was enough to lock the trailer wheels which caused the trailer to swing out killing some poor woman.
I’m not aware of ever feeling that automatic trailer braking, wouldn’t have wanted it TBH, some trailer as you rightly say were instant locker uppers and needed a steady hand at the tiller.
Where i worked at the time with bulk tipper trailers very prone to locking up they fitted manual pressure limiting valves to the trailers, 3 position, empty, half loaded, and fully loaded, they worked very well if you used your loaf.
As you say, the 3 line system was excellent and a superb failsafe having a fully independent extra trailer braking system.
Thankfully we got EU standardised, praise be, lost a fully independent secondary braking system and went from a 12.5 hour day to a 15 hour day…all in the interests of safety…
weeto:
Cruise control! and if you want to see the evidence i will tell you were to find it, were trucks ended up jackknifed which wast not caused by braking.
Ok fair enough, its not something i’d considered. But i’ve an open mind.
I wouldn’t mind seeing some evidence.