Stumbled across this video on YouTube showing a lorry doing an emergency stop (don't ask...).
youtu.be/0RW88PbA25w?t=35s
The technology is very impressive, but my question to the lorry drivers out there is can a typical HGV on British roads, at similar speeds, stop in the same distance? Does the video reflect an unloaded vehicle, and if this happened in real life, would everything in the back now be in a big pile at the front?
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The answer to all three questions is yes!
The point we sometimes forget is that if a major fault develops in the air brake system, the vehicle will stop like that of it's own accord as well.
If that lorry had been loaded badly or the roads wet the trailer could have come round and jacknifed.
The load, in theory, should always be strapped sufficiently to prevent it moving in those conditions.
In practice this doesn't happen.
As an example....
Imagine a 1 ton pallet placed in the centre of a 45' trailer and not secured at all.
In an emergency stop at only 20MPH the pallet will be the equivalent of 10Tonnes when it hits the headboard due to kinetic energy.
Pat
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>> can a typical HGV on British roads, at similar speeds, stop in the same distance?
Empty or lightly loaded dry road yes no problem, wet road empty not a hope, full load dry debatable.
>> Does the video reflect an unloaded vehicle, and if this happened in real life, would
>> everything in the back now be in a big pile at the front?
I can't get too much info from the video, speed etc, in real life a 55mph panic stop isn't the violent one, it's the 25 mph and below stop that is incredibly violent and where loads move.
Though the video might look good there is no real comparison in whats happening, a normal car running alongside the truck and braked at the same time would have shown just how slowly the truck brakes in comparison, dry is by far the best for trucks, the difference between car and most trucks increases in my experience as the surface deteriorates for whatever reason.
The road surface looks perfect too, that's highly unlikely on UK roads, and road truck tyres are designed for maximum mileage minimal rolling resistance.
The only trucks i've ever driven that could outbrake most cars of the time were drum braked tippers, 3 and 4 axle before ABS, Leylands especially...those trucks were tyred for off road so tended to improve more against cars as the surface got wetter, i haven't driven a modern tipper with disc brakes and ABS, i imagine they are better still.
Anyone remember the 'air brakes' warnings on the back of trucks, and 'disc brakes' stickers on cars.
Last edited by: gordonbennet on Mon 5 Dec 11 at 11:32
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>>Anyone remember the 'air brakes' warnings on the back of trucks, and 'disc brakes' stickers on cars.
Maybe there should be "winter tyre" stickers...
:-))
"(insert any description suitable ) ..on board " perhaps?
Last edited by: Humph D'Bout on Mon 5 Dec 11 at 11:42
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I am having some stickers for the rear window made up
"A Mondeo Estate is not Just for Christmas"
Poor Betsy.
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>>Anyone remember the 'air brakes' warnings on the back of trucks, and 'disc brakes' stickers on cars.
>>
I remember the warning on MK 2 Jaguars. Not a sticker but a nice little chrome badge( as Jaguar would) in the middle of the rear bumper.
I suspect you would demolish the rear bumper before you had time to read and react to it :-)
www.redsimon.info/simoncars/xxtails/slides/Jaguar%20MkII%203.4litre%20tail.html
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As mentioned, commercial vehicle tyres aren't optimised to give maximum grip.
One of the large unknowns faced when setting out commercial vehicle brakes is the axle loadings. If the Merc in the video has the commercial vehicle equivalent of EBD, then I can imagine it would make a massive difference to the braking performance.
For a tractor unit in particular, there are two large sources of uncertainty. a) the load on the trailer, and b) the ratio of the vertical force at the fifth wheel to the force along the direction of travel. Taking an awkward extreme, if the load is biased towards the rear of the trailer, there will be little load pushing down at the fifth wheel, but plenty acting along the direction of travel.
Also, the height of the trailer's centre of gravity can change significantly - from a low loader to a box van. As the trailers c of g height changes, so does the ratio of vertical to longitudinal fifth wheel force with respect to changing deceleration.
Prior to electronics, the only way that these changes were accomodated was via the use of a load sensing valve on the tractor unit's rear axle. By necessity, these had to be set very conservatively, as locking the rear axle of the tractor unit is the prime cause of jack knife accidents.
The conservative setting was necessary because of the ad hoc way that tractor units and trailers are changed when in service. I can see that using an electronic system to reduce this excessive conservatism would result in a useable increase in braking performance.
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>>as locking the rear axle of the tractor unit is the prime cause of jack knife accidents.>>
It would seem to me that some form of restricting / controling, or even automated locking of, the rotation of the trailer around the 5th wheel would stabilise the unit under extreme braking.
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>> It would seem to me that some form of restricting / controling, or even automated
>> locking of, the rotation of the trailer around the 5th wheel would stabilise the unit
>> under extreme braking.
Experimented with many years ago it never became a common fitment though.
www.britishpathe.com/record.php?id=2309
Hope Anti Jack Knife device.
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By the way note the chains linking the tractor to the trailer in the newsreel film to limit the jack knife in demonstration runs, the first two runs down the hill with locked drive axle should have seen the full jack knife happen.
The chap driving could have been taken from the cover of 'Headlight' magazine from the same time, looked uncannily like Jimmy Morrison the man who trained me.
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Thanks GB, I wonder why it never caught on?
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Not sure Cheddar but i'm inclined to think cost, secondly it was around this time that we started entering the Common Market, and in the late 70's early 80's started to harmonise, for example with tachographs instead of log books and the steady demise of the Britsh three line truck air braking system, discussed recently.
I've often wondered if we'd stayed out of the CM whether the Hope or similar might have become a compulsory fitment.
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Interesting man who was involved view with Hope here from a poster called Dog Dog from the Pistonheads forum, hope it's ok to link.
www.pistonheads.com/gassing/topic.asp?h=0&t=644895&d=11302.39124&nmt=
I did wonder about inadvertant or premature locking of the device, but the poster mentions the system was progressive.
What i like about it was the lack of electronics to go wrong.
Seems Hope turned down several buyers, if he'd sold to a German company would there now be an EU directive on anti jack knife fitments?
Never drove a vehicle so equipped meself.
Last edited by: gordonbennet on Mon 5 Dec 11 at 14:06
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Snow in Scotland this morning and the 3 main routes, M8, M74, M73 were all blocked at various times by jack knifed lorries.
I really do think that in rush hours in the snow, they should be forced to park up but the reality is, there are so many variants that it would be impossible eg for starters - how much snow is snow? and where do they actually park up?
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>>
>> I really do think that in rush hours in the snow, they should be forced
>> to park up
Why? Is their journey less important than yours? And what's to say it wasn't swerving to avoid a mimsing car driver which caused them to "jacknife".
So we'd park up in rush hours, then at night in case we disturb your sleep... oh wait a minute, you want us to RUN at night so your commute isn't slowed down by a couple of minutes?
And heaven forbid the drivers should park up in the suburbs and lower the tone of the neighbourhood.... leave 'em out in a lay-by, literally without a pot to pee in?
Rant over, sorry but this sort of attitude bugs me.
.... the reason the word "jacknife" is in inverted commas BTW is that in a true jacknife situation the artic cab comes round and makes contact with the trailer, very nasty, often fatal but thankfully rare these days due to the set-up described by NC above. What is described as such by ill-informed traffic reporters are usually merely artics that have become stuck in the snow and have slid sideways due to lack of grip.
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For a severe case of load shifting under braking, see this sobering pic from vwvanman0 in this thread;
www.trucknetuk.com/phpBB/viewtopic.php?f=35&t=35749&p=842530#p842530
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>>Why? Is their journey less important than yours? And what's to say it wasn't swerving to avoid a mimsing car driver which caused them to "jacknife".
Absolutely not. Purely and simply for the fact that if a car or van slides, it does not close the road off usually, some brute human power can sometimes clear the road. When a lorry jacknifes across 3 lanes off the motorway it is going nowhere and neither are the hundreds of vehicles stuck as a result.
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If HGVs can stop like that... why did one run into my hire car? I didn't even stop. Memory of event not all there but I didn't stop on the Autostrada.
End result was closure of a main road from Milan Malpensa. Must have taken time to reopen. Luckily it was Italy and not the UK where it might have taken longer.
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Lorry stopping distances - Pat
