Been driving for 38yrs and have seen a few jacknifed trucks sitting around in that time. I’ve never experienced it personally or come close to it; and never want to but -
What are the circumstances that cause a jacknife. Yes I understand some of the technical stuff of mass etc.
Is there a way of getting out of it once it begins.
It wasn’t something covered in my artic training and after seeing a jacknifed truck on a perfectly straight piece of motorway it got me wondering.
One cause is the trailer brakes locking up while the units brakes don’t, especially locking up on one side only.
Locked up wheels skid with less friction on the road than hard braked wheels. If it’s applied un-evenly as well, then the trailer can step out. Keep braking hard and the trailer will continue on round into a jack-knife.
If this starts to happen, letting the brakes off will allow everything to come back into line for another try at stopping in a straight line.
Part of the trouble is that we’re so used to ABS keeping everything in line now, we don’t really have this possibility in mind anymore, so can react too late when it starts to happen.
An artic jack-knifed on a straight motorway is usually as a result of the accident, not usually the cause of it.
For eg, if you start to nod off, or really loose concentration, or get distracted by something, and run off the road. Your first reaction would probably be to hit the brakes hard. If both front wheels are off the road, they’ll almost certainly lock up. You then loose any possibility of steering. When you then catch something (like a barrier or ditch) with the font of your unit it’ll spin into it, resulting in a jack-knifed unit.
a trailer with all the weight at the back is a time bomb, or if the trl brakes arn’t as good as the unit ones but anytime you’ve got no weight over your drive axle, when you turn, the trl will want to go straight an can just flick the back end of the unit around, its worst if you brake and turn at the same time as the front wheels on the unit will also force the back end of the unit around, this is a rallying technique to get the car sideways! obviously the faster you’re going an the slippier it is, the risk increases. if it starts to come around you only hope is to come off the brakes (easier said than done if your gaining on something!) and apply enough but not too much opposite look, if your lucky and it starts to come back start taking the opposite look off otherwise when it all grips again you’ll still have the opposite lock on and get fired off in that direction. if the units got a trailer only hand brake pulling that could also pull the combination straight again, but it ain’t going to be pretty!
i’d imagine most will happen very fast with little / no time for the driver to react
limeyphil:
Speed limiters cause jacknifes, You can’t pull yourself out of it when you are on the limiter.
thats only in the type of jack knife you see in films, were the trailer comes swinging around like a water skier, when the only time i’ve had this start to happen was in the snow, although if the trailer brakes were over keen and locked up then (pretty rare these days though) it could come around, but for this to happen you’ll have to be brake firmly, so chances are you’ll be under the limiter anyway and of been braking for a reason, so getting back on it might not be an option. once you let the brakes off the trailer would start too come back by itself as the unit wont be braking which is what forces the trailer around in the first place.
quote=“limeyphil”]Speed limiters cause jacknifes, You can’t pull yourself out of it when you are on the limiter.
[/quote]
…Not good advice, imho.
If you start to jacknife take your feet off all peddles steer out of it…But you will get a million and 1 different ideas on this…But that has been my experience
The reason you Jacknife is because you are trying to slow down, the last thing I would of thought for you to do is to speed up.
quote=“limeyphil”]Speed limiters cause jacknifes, You can’t pull yourself out of it when you are on the limiter.
…Not good advice, imho.
If you start to jacknife take your feet off all peddles steer out of it…But you will get a million and 1 different ideas on this…But that has been my experience
The reason you Jacknife is because you are trying to slow down, the last thing I would of thought for you to do is to speed up.
[/quote]
You’ve contradicted yourself.
A jacknife starts when you slow down, as you have stated.
But then you say, The last thing you would want to do is speed up.
Since the EU banned the anti jacknife brake which opperated the trailer brakes from inside the cab without applying the unit brakes, then the only way out of a jacknife is to accelorate until you are out of it.
If you get into a jacknife when over running the limiter, you have no chance.
Thanks Brussels.
I have never had one, never needed one. but if the tail is too heavy it will wag the dog.
Crap brakes on trailer, good brakes on unit may cause a jacknife, not enough weight over drive axle may cause it too, and simply erratic driving may cause one.
Limeyphil should be careful with his sensible advice as that brain donor who killed his boss tried to blame Jeremy Clarkson in court when he crashed
As someone defending Top Gear said.
“If Jeremy told you to sell your house and buy a Lamborghini”. Would You?
True jacknife is caused by the unit doing more braking than the trailer, which results in the trailer pushing the unit round. This can be caused by all sorts of things - using an exhaust brake on ice is always a classic…poor brakes on the trailer, especially if the ones on the unit have just been relined…anything which creates the above scenario.
The kind refered to above where the “tail wags the dog” isn’t true jacknife, it’s trailer swing, hence the confusion about the best way to solve the problem.
IMHO (and only IMHO) you are best to speed up for trailer swing, and take all your feet off all the pedals then try to steer out for jacknife.
HOWEVER, when I had a near-fatal (for me) jacknife on a straight, nigh on level, dry stretch of the M40 in a non-limited lorry with an empty trailer, I can assure you that NOTHING I could have done would have stopped it coming 'round.
I managed to hold it in a straight line for a short while after the “trigger incident” (later discovered to be a failure on the handbrake valve causing the whole lot to lock up - obviously the brakes on the French customer’s trailer were crapper than the ones on my unit) so no other vehicles were involved, but I can’t even take credit for that since I acted on instinct, not training…As I think you’ll find most people do when it actually happens to them. I’d had my artic licence a week at the time, btw.
When this actually happens to you at speed, trust me, you ain’t going to be thinking about what you’re doing, you’ll be too busy filling your pants as the embankment rushes up to kill you.
Ps. For those helping me with my “Crash” thing for T&D, the above is the proof that I’m comign at this as someone who’s “in the club” and therefore able to treat the subject sensitively. A fuller description can be found elsewhere on here if you search for it.
Here we go again, as I’ve said before, anyone who thinks you can drive out of a jacknife is talking ■■■■■■■■, it happens in a split second, all you better hope for is that it comes to the passenger side, I have been in one as a passenger and this was an M/T truck, just glad we wern’t loaded.
