I am acting as an expert witness in an insurance claim court case.
My expertise is not in vehicle mechanics but security.
It has been suggested that the driver of a vehicle that had been parked up for seven hours would have to wait several minutes for the air pressure to build up in the braking system before he could drive away.
Can anyone please advise me if this is the case?
Thank you in anticipation.
jeffwilliams:
I am acting as an expert witness in an insurance claim court case.
My expertise is not in vehicle mechanics but security.
It has been suggested that the driver of a vehicle that had been parked up for seven hours would have to wait several minutes for the air pressure to build up in the braking system before he could drive away.
Can anyone please advise me if this is the case?
Thank you in anticipation.
Hi Jeff,
I have the feeling that you’re not going to get a clear-cut definitive answer because it might be that the truck in question did OR did not need several minutes [with the engine running] for the air to build up before it could move off after seven hours parked up.
There’s no correct answer ,my lorry is 19 yrs old and 17 yr old trailer only drop from 8.5 bar to 7 over 11 hrs ,at 7 bar I can drive away from cold starting ,basically any lorry with even the slightest of air leaks would have low air after 7 hrs and would need to build back up and probably in sequence before all systems have enough air although depending on which system is leaking would make a difference on if the lorry could engage a gear and possibly move off or not .
It all depends on the vehicle concerned, some makes only have a singe stage compressor, if an artic on air suspension all round where the whole vehicle had lost virtually all of its air then even revving fast as possible it could still take several minutes, 3 or 4 easily possible, to get enough air up to move off, and maybe another minute to get full air pressure.
Another make with a faster compressor could fill the tanks on less than half that time.
One with a slow compressor left on tickover only (say to keep noise down in order to steal the thing if one was so inclined) it might take 7 or so minutes at tickover to inflate, and yes i’ve had modern well maintained vehicles as bad as this.
This is assuming the vehicle in question had lost all or almost all of its air, it might have been the case the pressure was only just below the minimum required for moving off in which case 30 seconds fast idle might have been more than enough.
As Dieseldave alludes to, almost infinite variables here.
It would probably be best to visit a premises where an identical vehicle both tractor and trailer, same gross weight (presumably loaded), and year of manufacture of both tractor and trailer, could be checked and physically timed how long it takes to get enough pressure up, even then it would be pure guesswork anyway because no one would know how much air was already in the tanks when the vehicle in the case you are appearing for was first started.
Are you still able to actually drive off with low or no air, or will the truck not physically move?
idrive:
Are you still able to actually drive off with low or no air, or will the truck not physically move?
It won’t move without sufficent air in the system.So no you won’t be driving off with no air in it.
Sent from my SM-G950F using Tapatalk
So many…
Variables. New truck, old truck, new trailer, old trailer was it well maintained?
For example most of our fleet are 68 plate with some 18s as well as a few 67s. 99% of the time you can just drive away from starting up. Every now and then one will have low air. If it’s a bad leak they get booked off. Some drivers will go with a bad leak and some won’t.
With empty tanks though, you can’t move, you have to build the air supply up and depending how low the tanks are, you could be standing for 7 minutes.
jeffwilliams:
I am acting as an expert witness in an insurance claim court case.
My expertise is not in vehicle mechanics but security.
It has been suggested that the driver of a vehicle that had been parked up for seven hours would have to wait several minutes for the air pressure to build up in the braking system before he could drive away.
Can anyone please advise me if this is the case?
Thank you in anticipation.
Braking systems on trucks operate as follows. The brakes use brake pots that have springs connected to diaphragms. The springs ensure that the brakes are applied fully. To release the brakes air pressure is fed into the brake chamber to overcome the pressure of the spring. Therefore no or insufficient air pressure then the brakes are applied. This means brakes on a lorry work completely the opposite way around to a car where on a car the default position is released and brake fluid applies pressure to apply the brakes. The reason they’re done this way on air brake systems is because even properly maintained you can get leaking in the connector on the air line from the tractor unit feeding the air to the trailer and because of the amount of valves, regulators etc there are many points of failure. As the vehicle is fitted with a compressor the braking system having minor air leaks is inconsequential as the compressor and reserve tanks will always be able to overcome losses from minor leaks. If either the unit or the trailer had even the slightest air leak then they would likely need to wait a few minutes for air to build up after that period of time even if the unit and trailer are well maintained.
There is no definite answer to the time they’d take to do it as it depends on many variables not just including the brakes but the suspension as well as it is also operated by air and therefore the load of the vehicle comes into play too. If the vehicle was more heavily loaded then it would likely lose air pressure more than an empty one as the suspension on the rear axle of the tractor unit and all trailer axles use air bags instead of springs which are fed by the same supply that operates the brakes.
Then there’s the amount of air in the system when the vehicle was parked. On a fairly new lorry with a fairly new trailer lightly loaded which had recently been parked up somewhere where they wouldn’t have needed to do much manoeuvring then they would have more pressure in the system than a vehicle heavily loaded which had had to do some manoeuvring to get to the final position where they turn the engine off. The more manoeuvring you have to do in the minutes leading up to engine shutdown such as getting into a parking space in a truck stop where you may need to do a few shunts the less air pressure you’re likely to have when you stop from multiple gear changes, operations of the clutch and the brakes.
If you wanted to research this to get an idea of the percentage of lorries that would need to build up air before being able to move the best thing to do is to go and wander around a lorry park around 4-6am when everyone is starting up after being parked several hours overnight. You’ll no doubt see that most of them spend a few minutes idling as they build up air, some sitting there for several minutes revving the engine to get the air pressure up. You’re usually quite lucky if you can turn on the ignition, fire the engine up, chuck it in gear and set off straight away after being parked for several hours.
Again as said it is impossible to say one way or the other. Even if one had been able to run an experiment on the particular vehicle to see what happens and it did not lose air, on the day of the incident it could have done. Passing traffic is quite capable of rocking even a fully laden vehicle on its suspension; as is the wind. Over time that rocking will deplete the air.
jeffwilliams:
I am acting as an expert witness in an insurance claim court case.
My expertise is not in vehicle mechanics but security.
It has been suggested that the driver of a vehicle that had been parked up for seven hours would have to wait several minutes for the air pressure to build up in the braking system before he could drive away.
Can anyone please advise me if this is the case?
Thank you in anticipation.
Clearly it is possible that the vehicle would need to have the engine running for a few minutes before being drivable.
Low air, preventing release of brakes, may have many causes and the design/state of the air compressor may be variable, but it is easily conceivable that a vehicle can need a few minutes before being movable.
At the same time, it may be possible to turn the key, engage gear and move practically instantly.
Both situations are quite credible.
jeffwilliams:
I am acting as an expert witness in an insurance claim court case.
My expertise is not in vehicle mechanics but security.
It has been suggested that the driver of a vehicle that had been parked up for seven hours would have to wait several minutes for the air pressure to build up in the braking system before he could drive away.
Can anyone please advise me if this is the case?
Thank you in anticipation.
If a driver has said this in a statement they’ve made about an incident, for instance, then it would realistically have to be accepted.
Trucks cannot move without sufficient air in the system the older the vehicle (and trailer) gets the more they tend to leak air away and the larger the vehicle the longer it has been parked up the more time the air takes to build to move away.
I would be prepared to say that the average articulated truck parked up for seven hours would not be able to move away immediately.
Franglais:
jeffwilliams:
I am acting as an expert witness in an insurance claim court case.
My expertise is not in vehicle mechanics but security.
It has been suggested that the driver of a vehicle that had been parked up for seven hours would have to wait several minutes for the air pressure to build up in the braking system before he could drive away.
Can anyone please advise me if this is the case?
Thank you in anticipation.Clearly it is possible that the vehicle would need to have the engine running for a few minutes before being drivable.
Low air, preventing release of brakes, may have many causes and the design/state of the air compressor may be variable, but it is easily conceivable that a vehicle can need a few minutes before being movable.
At the same time, it may be possible to turn the key, engage gear and move practically instantly.
Both situations are quite credible.
I wouldn’t say they are equally credible though I think that after seven hours only a minority of the newest artics and trailers would move instantly.
jeffwilliams:
I am acting as an expert witness in an insurance claim court case.
My expertise is not in vehicle mechanics but security.
It has been suggested that the driver of a vehicle that had been parked up for seven hours would have to wait several minutes for the air pressure to build up in the braking system before he could drive away.
Can anyone please advise me if this is the case?
Thank you in anticipation.
If your expertise is not in vehicle mechanics, then hopefully you won’t be giving evidence to the court on vehicle mechanics!
It’s certainly perfectly possible (and not at all unusual) for air to be lost from a system to the point that it takes several minutes (or something of that order) before it can be moved.
The point at which a vehicle will move is also not the point at which it is fully charged.
A certain amount of charge is lost whenever the handbrake is released, and if a vehicle is charging very slowly or struggling to maintain pressure, a driver may wait until the air has had a decent period of time to charge amply, rather than trying to pull off at the earliest possible moment.
if you are an expert,then why are you asking obviously answerable questions on here when anyone with a modicum of trucks would give you the same answer as everyone else above…expert in what pray tell??
Own Account Driver:
Franglais:
jeffwilliams:
I am acting as an expert witness in an insurance claim court case.
My expertise is not in vehicle mechanics but security.
It has been suggested that the driver of a vehicle that had been parked up for seven hours would have to wait several minutes for the air pressure to build up in the braking system before he could drive away.
Can anyone please advise me if this is the case?
Thank you in anticipation.Clearly it is possible that the vehicle would need to have the engine running for a few minutes before being drivable.
Low air, preventing release of brakes, may have many causes and the design/state of the air compressor may be variable, but it is easily conceivable that a vehicle can need a few minutes before being movable.
At the same time, it may be possible to turn the key, engage gear and move practically instantly.
Both situations are quite credible.I wouldn’t say they are equally credible though I think that after seven hours only a minority of the newest artics and trailers would move instantly.
I stand by the two situations being credible.
I did not say they were equally likely.
dieseldog999:
if you are an expert,then why are you asking obviously answerable questions on here when anyone with a modicum of trucks would give you the same answer as everyone else above…expert in what pray tell??
He said in his post he’s an expert in security. I’m with you, someone who is not an expert in vehicle mechanics should not be giving evidence as an expert witness on questions on vehicle mechanics.
Mind you at least he’s had the decency to ask instead of just trying to bluster his way through giving any evidence. I gave as thorough answer as you can reasonably do on a forum as I thought it was worth giving a quick overview to how the brakes work so hopefully that and the fact that every other comment says it is situation normal to sit there in a morning building up air at the beginning of the day is enough to get the job done.
Nosey bugger in me though is intrigued to know what its all about.
What date was it, have you checked the weather for that 7 hours, if its windy, it may have used more air as the trailer rocks on its air bags, is it an automatic or manual gearbox, was the air suspended seat locked off? the clutch will not normally operate or the gear selector without sufficient pressure.
Thats a lot more variables to consider
Conor:
dieseldog999:
if you are an expert,then why are you asking obviously answerable questions on here when anyone with a modicum of trucks would give you the same answer as everyone else above…expert in what pray tell??He said in his post he’s an expert in security. I’m with you, someone who is not an expert in vehicle mechanics should not be giving evidence as an expert witness on questions on vehicle mechanics.
Mind you at least he’s had the decency to ask instead of just trying to bluster his way through giving any evidence. I gave as thorough answer as you can reasonably do on a forum as I thought it was worth giving a quick overview to how the brakes work so hopefully that and the fact that every other comment says it is situation normal to sit there in a morning building up air at the beginning of the day is enough to get the job done.
Nosey bugger in me though is intrigued to know what its all about.
^^^^^^^^^^^^^^^
i woder how to connect brake pressure to getting a stanley down your curtains,or your back door hinges ground off.
a trucks a bit big for a ram raid.
the scallys over here tried a ram raid n the mobile shop,but they ran out of petrol before they caught up with it. 
I think it was a tricky question and all of you failed. Build the air up and go? What about doing your vehicle checks? 
fuser84:
I think it was a tricky question and all of you failed. Build the air up and go? What about doing your vehicle checks?
Ah! Good start.
Hoodie in lo-viz black?
DNA proof gloves?
Boots with smooth soles?