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I always thought it was to provide an extra thing for the driver to stand on as they climb in and out. Especially short portly ones. ;-)
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Surely it would take an almighty impact to move an HGV any significant distance?
My guess is it's to stop the truck rolling away if the parking brake fails.
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>>if the parking brake fails.
Possible, but extremely unlikely. On most trucks, the parking brake is a spring brake, which is held off by air when you take the handbrake off. Effectively, the handbrake is designed in a fail safe way - the only feasible way that ti would come off is if the lever were released in the cab.
The handbrake itself is a very simple valve with a locking device to hold it on - it's not like a car handbrake that can be partly applied - it's an on/off device - although the handbrake usually doubles as a secondary brake which can be partially applied if the footbrake fails.
It's also not a cab access trick - cab steps and grab handles have moved on in leaps and bounds since the days of trucks that GB dreams about. ;-)
I've never bothered with turning the wheel in myself, but, I think it's to avoid being shunted into the road.
Edited by Number_Cruncher on 14/06/2008 at 19:57
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It's also not a cab access trick - cab steps and grab handles have moved on in leaps and bounds since the days of trucks that GB dreams about. ;-)
Its a good job my skins thick, bit like me head..:)
As an aside, one of the many reasons i still like those older trucks is that you had to be a driver to drive one. Kept a lot of the numpty's away.
No speed limiters, proper crash gearbox's (change gears as fast as you wanted on some, and when you wanted), good old school diesels with oodles of torque from tickover in some cases especially Cummins. Common courtesy from other drivers.
Proper cafes, big mugs of tea. No speed cameras, very few unmarked plod, so if you kept your eyes open and checked behind often you were ok.
Waking up in the morning (lying across 2 boards stretched from door sill to door sill, mind you that is going back a bit) and scraping the ice off the inside of the windows (sometimes the digs were full and you got caught out).
That part wasn't so good, or lighting fires under the fuel tank when the diesel froze.
Wouldn't have missed those days for the world..:)
Getting all misty eyed here.
Turning the wheels in layby...hmm
I too think a lot is old school practices, my first trucks didn't have spring brakes, so if the air ran out, which it would during the night the 'dead man' would come off, and the handbrake on old Fodens was a brake drum on the rear axle behind the diff.
Dare say you can imagine how good that was with no diff lock.
But good old British trucks had a much better fail safe than the other also rans..:)
we had the 3 line air system, which meant if the service (main) brake line snapped or failed somehow, with the 'dead mans handle' you had full braking on the trailer and on the truck front axle to bring things to a safe stop through a completely separate set of air lines.
Thanks to the euro masters we now only have a 2 line system.
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"Surely it would take an almighty impact to move an HGV any significant distance?"
That's what I thought.
Maybe it's just me but I've noticed it fairly recently, and at first I thought it may be lorries parked up for the night but I've seen it at all times of the day.
"My guess is it's to stop the truck rolling away if the parking brake fails."
But the places I've seen them do this are pretty flat!
I wondered whether it's some sort of signal to other truckers??
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maybe it just very old truckers who started driving before air brakes came about......old habits and all that :-))
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"Surely it would take an almighty impact to move an HGV any significant distance?"
Empty or part loaded LGV, 20 tonnes maybe. Would move a long way if hit by 44 tonnes at 60mph.
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But is this likely in a layby that is parallel to, and separate from, the carriageway? Not like a hard shoulder.
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With a couple of assumptions, here's an estimate of how far a parked truck might be punted by an impact from a runaway truck.
m1=38000; % Mass of moving truck (kg) m2=38000; % mass of parked truck (kg) % Speed of moving truck (mph) v1_mph=[10 20 30 40 50 60 70]; v1_ms=v1_mph*(1609.3/3600); % converts to m/s % Just after the impact, the 2 trucks move as one, at v1 KE=0.5.*(m1+m2).*((v1_ms).^2); % Assume a mu of 0.7 and locked wheels only on the parked vehicle B_Force=m2*9.81*0.7; % Equate kinetic energy with work done D=KE./B_Force; % Tabulate speed (mph) and distance travelled (metres) [v1_mph' D']
ans =
10.0000 2.9101
20.0000 11.6402
30.0000 26.1905
40.0000 46.5610
50.0000 72.7515
60.0000 104.7622
70.0000 142.5929
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so this is where stephen hawkings spends his free time!!!
Edited by welshlad on 14/06/2008 at 21:13
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Pointless, incomprehensible to all but a few, but brilliant as ever, NC.
It's people like you that make this forum worth visiting.
Bet you're the most boring bloke in the world to actually talk to, though. :)
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>>Bet you're the most boring bloke in the world to actually talk to, though. :)
Achingly tedious! Although on some of the projects I've worked on, I've faced stern competition on this front.
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Here's odd for you.
When sober, I thought my quick calc was OK.
After some beers, I realise the [truly inexcusible] flaw - momentum isn't conserved in my calc. I'll correct it now.
I thought that judgement was supposed to be degraded by drink?
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Sorry about my earlier erroneous posting - this is a better effort;
m1=38000; % Mass of moving truck (kg) m2=38000; % mass of parked truck (kg) % Speed of moving truck (mph) v1_mph=[10 20 30 40 50 60 70]; v1_ms=v1_mph*(1609.3/3600); % converts to m/s
>>% here's the correction
>>% Just after the impact, the 2 trucks move as one % at a speed given by conservation of momentum VPI=(m1.*v1_ms)./(m1+m2);
>>KE=0.5.*(m1+m2).*((VPI).^2); % Assume a mu of 0.7 and locked wheels only on the parked vehicle B_Force=m2*9.81*0.7; % Equate kinetic energy with work done D=KE./B_Force; % Tabulate speed (mph) and distance travelled (metres) [v1_mph' D']
ans =
10.0000 0.7275
20.0000 2.9101
30.0000 6.5476
40.0000 11.6402
50.0000 18.1879
60.0000 26.1905
70.0000 35.6482
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Just me perhaps, but I thought the sober version was more elegant.
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