When the trailer goes out of line it is not so bad, it is easily corrected, when the trailer pushes the cab out of the way is much hard to control
ABS has been required on truucks and trailers for a number of years
These days their is a lot of electronics that work with trailers and can catch/correct jack knifing and rollovers, but they are expensive and need to be specified.
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The thought has occured to me from a laymans viewpoint, that it must be quite scary sitting right at the front of a flat fronted cab where presumably you are the crumple zone whle towing 40 odd tons. Especially so when there is no grip. Shouldn't think there has been much trade in laxatives at the truck stops these past few days.....
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I saw a lorry being towed out of a snowy lay-by.
The trailer was at an odd angle, and I got the impression it was stuck rather than broken down.
Can our resident HGV drivers tell us how much traction a lorry has in snowy conditions?
Is it more or less than an average fwd hatchback?
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Ive been down that A66 many many times. Once i followed a lorry just got past the snow gates. It came on the radio A66 now closed. The lorry i was following at the next hill the back end kept coming out . Hence i was really worried. I have a lot of bad memories on that road.
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As a driver of one such vehicle....
Artics now have most of the gizmos that cars have, including ABS and traction control. We also have, and have had for years, exhaust brakes, whereby a button on the floor is pressed and exhaust gas is sent back into the cylinders, thereby slowing the engine down. It can also be on a stalk on the steering wheel. These type of things are not recommended to be used in conditions like we currently have, as they can actually lock the wheels up whilst moving.
A 6x2 unit (3 axles and 41t +) gives less traction in snow if the mid axle is down. However this is also dependant on how much weight is on the front end of the trailer, within the limits as per UK regs. If running light, then the mid axle should be lifted. The unit will then sit down slightly, thereby pushing the drive axle down into the tarmac. This is how I run, although certain makes like the Renault Premium, will lift the mid axle automatically once it gets within the weight tolerance, others have to be done by pushing a switch on the dash.
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Harley, thats interesting, what are the anti-jacknife devices?
Curious as to know what electronics (other than brakes) can do to stop the wagon going?
Must have something to do with angles between cab and trailer, or speed differentials or something like that?
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The term jack knife is overused and always has been, but it sounds sensational so the media use it.
I've seen the aftermath of probably a dozen jack knife accidents on the road in my years at the wheel, luckily it hasn't happened to me.
As HM rightly points out a real jack knife is a terrifying and rare event, the tractor unit drive axle usually has to lock up or lose all it's grip under braking and the weight of the trailer pushing spins the tractor round smashing the cab into the side of the trailer so the tractor is facing backwards almost alongside the trailer...obviously at this point if the trucks still moving fast all hope is lost and there is no recovery once the spin gets this far out of control.
Remember the only driven wheels on an artic are the rear axle of the tractor, the vast majority of 3 axle tractors will only be single axle RWD, the front axle obviously steers and the second axle is either steered or might be fixed inline but undriven.
Very few tractors have double drive, where both rear axles are driven, you might find this on low loaders or those huge articulated refuse trucks that trunk bulk refuse to landfill (Brogborough M1 jct 13 for example).
On ice or snow the rear axle will spin very easily and if climbing a hill the tractor will track sideways as the wheels spin and the tractor rear veers towards the lowest camber (you will have felt this happening with a RWD car spinning its wheels) , obviously the trailer will stay straight...it might get to the point where the driver can go no further or the truck gets jammed against the verge or something and it appears the truck is jack knifed, but in fact the driver was just trying to keep it going, and has got stuck but across the road tractor and trailer at odd angle.
As regards grip...not a lot, my truck has 6 axles and only number three is driven...quite obviously traction isn't good.
Incidentally diff locks don't always help and using the diff lock makes the sideways slip more likely, sometimes better straighter progress is made just by letting the normal diff do it's thing allowing one driven wheel to spin and dig down to something grippy and the other driven wheel 'steers' the back of the tractor keeping it straight.
Edited by gordonbennet on 05/01/2010 at 21:41
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As per GB's post, jack knifes are caused by loss of grip on the tractor unit rear axle(s) - most usually under braking.
This used to be guarded against on older trucks by using a load sensing valve on the rear axle to restrict braking effort when the axle was lightly loaded. If the valve failed or its mechanism siezed up, then you could have trouble.
From about the mid 80's onwards, ABS began to appear on trucks which makes locking the rear axles less likely, and can also reduce the conservatism that used to be built into load sesning valve settings.
We had one medium speed jack knife incident on my father's fleet of trucks, which left a large trailer shaped dent in the back corner of the cab of the tractor unit.
Obviously once a jack knife starts, the driver has in reality become a passenger - I tend to think this is also true for many incidents where trucks skid on ice, especially artics. In a bad winter, we would usually end up pulling at least one of them out of a ditch (or worse!).
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Thanks NC for explaining that. Being these days a humble "rigid" driver I'm not clued up with how anti-jacknife technology actually works.
Out of interest, whilst exhaust brakes are downright dangerous on artics in icy weather, for the reasons mentioned, on a rigid HGV I find then very handy indeed. Much of my driving is spent on minor roads delivering to farms, and I find that (careful) use of the exhaust brake is actually better than using the foot brake when descending icy slopes. Obviously this is because I don't have an unbraked trailer pushing me, as an artic would in those circumstances.
If anyone cares to try the ultimate in "skater's waltz", I can highly recommend driving an articulated tractor unit sans trailer. Those of you who've watched truck racing on the telly will know what sort of antics they can get up to, and that's in the dry!
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I've found exhaust brakes to be rather variable. From the ones on the older Merc V8s which you could really feel the deceleration to those on the Rolls engines in the Constructor 8 wheelers which were really rather feeble.
We once had an exhaust brake stick on a laden truck - only one bank of a V6, reducing a 10 litre engine to a 5 litre slant three - very slow progress getting that back in to the workshops!
As I'm sure you're well aware, the really dull thing to do with an exhaust brake is to press the clutch while you're using it...... guilty!, but in my defence, I've only ever done it once, and the sudden loss of power steering as the engine stalled woke me up a bit!
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As I'm sure you're well aware the really dull thing to do with an exhaust brake is to press the clutch while you're using it...... guilty! but in my defence I've only ever done it once and the sudden loss of power steering as the engine stalled woke me up a bit!
I did that once on a Mercedes, when they first had those horrible EPS gearboxes. You might recall they had a nasty habit of slipping into neutral on the overrun, mine did this, I didn't notice and hit the exhauster, and as you say it tends to nip the nether regions up rather quickly!
Most useless exhaust brake I ever encountered was on an old DAF 65, I'm sure it was just a flap on the silencer which made it sound as if you'd got one!
Edited by Harleyman on 07/01/2010 at 00:14
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>>those horrible EPS gearboxes.
Thankfully, I've never had any dealings with the electronic gearboxes, the Eaton Twin Splitter was about the limit of complexity on any truck I've driven.
On the earlier Mercs, say up to 1981 or so, the gearbox patterns were mirrored, with first being close to you, and top over at the left hand end of the gate - very odd!
I know we had a few engines saved by the high speed warning buzzer which measured the gearbox input shaft speed, and we had one very expensive engine thoroughly trashed where the warning system had failed, and, confused by the odd layout, the driver went down a gear instead of up!
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There used to be (may still be) a bit of kit called The Hope Anti-Jack Knife Device, made IIRC in Feltham Middlesex.
I believe this worked by stiffening up the fifth wheel connection, so that an artic became more like a rigid while it was being driven in a straight line.
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Before that we had to use the old method of scraping as much grease as possible off the turntable.
Pat
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Before that we had to use the old method of scraping as much grease as possible off the turntable.
You do love a wind up Pat.;)
That is a problem on the few remaining articulated car transporters, the 5th wheel is so exposed to the elements it quickly runs dry and you can hear the tortured steel rubbing plate screeching on bends.
Never driven a vehicle fitted with a Hope, no hope in other words, expensive fitment if i recall so the main customers were the likes of the oil companies.
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That was meant quite seriously GB, we used to break a slat of wood off a white pallet and scrape as much grease as possible of the fifth wheel to make it less able to slip round.
You young 'uns have had it easy:)
Pat
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Thinking about it, modern technology could largely cure this problem.
Once you're going above, say, 40 mph or so, there's usually very little articulation between tractor unit and trailer, and it would be simpe enough to arrange some solenoids to push hardened pins between the fifth wheel and similarly hardened holes or bushes in the turntable which would limit how much articulation was available.
A bit of experimentation would be needed to set the right amount of slop to allow normal use, but not too much so that a jack knife situation could be caught before there was enough energy in the motion to overcome and shear the pins.
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In the past, jack-knifing typically happened when the rear wheels of the tractor unit locked under heavy braking while the vehicle was steering slightly, and the forward momentum of the trailer spins the tractor unit round.
Now that lorries have ABS and often anti-jack-knife devices as well, this is less likely. Also top weight artics have to have a total of 6 axles, meaning the tractor units have two rear axles, with the king-pin mounted further forward than on older types, further reducing the risk of jack-knifing. What happens now is more likely to be a general loss of control and folding up of the vehicle owing to extreme slippery conditions.
Edited by Sofa Spud on 08/01/2010 at 11:18
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Once you're going above say 40 mph or so there's usually very little articulation between tractor unit and trailer and it would be simpe enough to arrange some solenoids
That could be worth following up NC and sounds promising, lucrative possibilities there..
As you know i drive a modern car transporter which has the appearence of an artic but is a lorry and trailer combination with oversized pin coupling behind the rear unit wheels at normal hitching height for such, about 2ft above the ground.
If you've ever seen a swb landie towing an oversize trailer you'll have an idea how unstable they can be.
One of major players in transporter bodies pioneered this type of design, using a standard tractor unit fixed under the prime mover body, the design makes for economical construction/unit replacement and clever maximising of mixed loads and is very versatile, and all transporter body makers now have similar designs.
The stability problems were very bad in early designs of this configuration, 2axle truck and 2 axle trailer, depending on weight distribution etc sways became common resulting in several rollovers.
(Previous designs had seen either proper artics and although very stable the issue with the peak made them quite a handful and needed a very skilled driver, or in the case of lorry and drag the prime mover would be at least as long/heavy as the trailer and therefore controlled the trailer far better).
So the new designs have a clever stabiliser bar on one make comprising of a half moon brake disc surrounding the towing eye, and a type of brake caliper fitting with fairly normal brake pads kept under constant pressure against the disc.
If you see one of those turning a corner you'll hear the stabiliser set up creaking/grinding as the pads move over the disc.
The company concerned still use this sytem on their new vehicles, it works.
I use another companies product with no stabiliser set up.
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.You young 'uns have had it easy:)
ever have that feeling that someone's taking the mickey, i've got exactly that feeling.;)
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You young 'uns have had it easy:) Pat
What did you do with Scammell couplings Pat ? ;-)
(Please don't say, "chucked a bucket of cold water on 'em to stop 'em doing it")
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Well, I can see I'm never going to convince you two but I'm genuinely surprised that you've never heard of it,
It was common practice among drivers and much frowned upon by the workshop who always insist upon far too much grease anyway.
Our theory, right or wrong, was that with less grease, the sooner you could feel the trailer starting to move through the 'seat of your pants'
Did it work? I don't know, but no-one I knew around that time jacknifed!
What it did do though, was gave us confidence that we'd done 'something' and that in turn, reflected in the more confident way we tackled the bad roads which is never a bad thing.
It may well have been a 'Fenny' thing but I do remember a night on Aust Services when five of us all dropped our trailers, pulled forward and did this because it was snowing badly, I was the only 'Fenite' there!
Pat
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It may well have been a 'Fenny' thing but I do remember a night on Aust Services when five of us all dropped our trailers pulled forward and did this because it was snowing badly I was the only 'Fenite' there!
Pat forgive me but was the idea behind this so you had a few feet of clear track to get moving again in't morning, if so impressive.
The removing of excess grease i really had never heard of and personally always preferred the feel or knowledge of a properly greased and serviced truck where everything moved freely, especially freshly greased king pins/steering joints, you could always tell when they'd been done....that did give more feedback.
Was the degreasing a habit gained before trailer ABS became standard so you'd be able to feel a trailer with locked wheels slipping out of line?
As an aside do you remember in the early 80's when DOT inspectors especially at MOT centres would check for any brake pressure being fed down the airline when the exhaust/any auxilliary brake was pressed.
IIRC there had been a fatal when a truck was descending a town hill on his exhaust brake alone, the truck in question (MB if i recall) was putting 10% brake pressure to the trailer when the exhauster was depressed, and the oversensitive trailer was sweeping sideways down the hill with the driver oblivious.
Any truck they found showing any air line pressure similarly was GV9'd instantly...rightly so.
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No, it vertainly wasn't to get clear trach for traction GB, a few rocking motions on parking up on ice was all that needed.
>>>>Was the degreasing a habit gained before trailer ABS became standard so you'd be able to feel a trailer with locked wheels slipping out of line?
<<<<<<
That certainly was it, and ABS was a thing of the future!
I do remember the DOT checks and early exhaust brakes that would be all or nothing.
Mine used to come on in the Dartford Tunnel without me doing anything. It used to cause some concern to the cars behind me who mysteriously had hung back when I got out the other end.:)
Pat
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No it vertainly wasn't to get clear trach for traction GB a few rocking motions on parking up on ice was all that needed.
Come on Pat don't be such a tease why did you drop the trailers?
We used to drop loaded trailers over the weekend to give tractor unit springs a 'rest'.
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pat bemused and confused>>>>>
To scrape a good layer of grease off the turntable of course!
Pat
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To scrape a good layer of grease off the turntable of course!
doh.;)
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