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Many HGVs have 1 or more axles that they can raise, presumably to save tyre wear when running empty. Do these lower automatically when the vehicle is loaded or is it up to the driver to do it manually? I 've seen some flatbeds loaded with goods, for example steel, with an axle still raised. Are the operators saving money improperly, as this will increase the axle loading which is what does the damage to the roads, or does the system keep the vehicle within its axle limits?
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I believe it's due to saving wear on the unused axle when the trailer is empty, however IIRC it can alter the characteristics of the the handling & when the trailer is turning, where as if you'd have all three axles down the it can make the trailer easier to turn around in the yard for example.
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Its not what you drive, its how you drive it! :-)
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tyres as far as i am aware, they arent cheap anymore and 750x16 8 ply are long dead.
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If you wait a little while, I'm sure TU (or DVD for that matter) will be along at some point to clarify.
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The older wagons were lowered and raised manually by a switch in the cab. The more modern ones have weight/load sensors on them, which lower them automatically when a preset (not user set) limit is reached, this is why you may see some wagons driving around with a load on and the axle raised. It is also possible to ease them up even if you have a full load on, there is a button in the cab that allows you to do this to gain extra traction for a short period of time, on slippery ground and hills etc. They are raised to save on tyre/brake wear - personally I prefer twin steers to lifting axles, but each to his own.
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Simple innit.
More axles more weight can be lawfully carried.
Dvd
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>>Are the operators saving money improperly, as this will increase the axle loading which is what does the >>damage to the roads, or does the system keep the vehicle within its axle limits?
No generally speaking it is at the discretion of the driver whether his lift axles are up or down. Once he has left the yard the boss cannot see the truck anyway and why would a driver risk being caught with overloaded axles rather than just dropping his lift axle down.
A lot of loads can be legally and safely carried without all of the available axles touching the floor, as a fair amount of products carried can be bulky in volume rather than just weighty. The reason people have lift axles that they don't appear to be using is for the times that they have got a full heavy load on and they need the extra axles to stay legal with the weight they are carrying.
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There have been instances where Customs & Excise have had a word with drivers about running loaded with axles raised, this is because a 6 axle artic pays less road fund than a 5 axle artic, so in theory should always run with the axle down - this is an on going issue with container hauliers I believe (or was a couple of years ago) as technically even with an empty container on the back they are running loaded as the container is the load regardless of it's contents.
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My company has tri-axle trailers that will automatically lower one axle if the load dictates. If a motive unit is fitted with a third axle, it is at the discretion of the driver if said axle is lowered, again this is dicated by how heavy the load is. I think overall it must be for comfort, stabilty and perhaps durability as the same load can be carried on a twin axle unit pulling a tandem trailer.
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I've often wondered the reason for raising the axel,so appreciate the answers.
Now has there been a survey done as to whether or not lorries are more prone to jack knifing with the axel raised with a full load, I would expect so but not sure
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In my company its only the units that have lifting axles. With the mid-axle raised n pulling a tri-axle trailer it can carry 38tonnes, with the mid axle down it can carry 44tonnes. So you can still pull 38 tonne with the mid axle raised, but it must be lowered to carry 44tonnes if it doesnt automatically go down.
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>>Now has there been a survey done as to whether or not lorries are more prone to jack knifing with the axel >>raised with a full load
I can't answer your question about a survey but there are many factors involved when a truck jacknifes and to be perfectly honest I don't think it would make an awful lot of difference whether or not the vehicle had some axles raised.
A jacknife normally occurs when the trailer has sufficient forward momentum to push the tractor unit round upon itself. Why does this happen? Well there can be many reasons but some of the main causes are insufficient trailer braking ability, the driver turning his tractor unit too sharp from one direction to another, a problem on the tractor unit which causes the tractor unit to rapidly decelerate (ie a locked wheel), or perhaps a more common occurance of one truck crashing into something in front causing the unit to stop and the trailer to push its momentum forward and turn the unit into a jacknife.
And another fact that may amaze you, as many trucks jacknife empty as they do loaded.
>>There have been instances where Customs & Excise have had a word with drivers about running loaded >>with axles raised, this is because a 6 axle artic pays less road fund than a 5 axle artic, so in theory should >>always run with the axle down
Yes I can see your point TU, but you probably know as well as I do that in the real world this is impractical and obviously the Customs & Excise bods don't have any actual experience of driving these things. Just as an example to illustrate my point, the company that I work for (a recovery firm) has a DAF 95 480 6x2 with a rear lift axle towing a low loader trailer. The unit is plated up to around 80T gtw, and the trailer is somewhere towards that mark too. I can't tell you exactly without looking at the plates, but you can legally put 44 ton of load on the trailer bed. Now my point is that it is near impossible to drive the unit with an empty trailer with the rear lift axle down. This is because there is just no traction and all it does is spin the wheels constantly. How anyone could argue that by having that axle on the floor all the time is better for the road surface is beyond me.
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The biggest cause of jacknifing is when the rear axle of a tractor unit locks up under braking. Having a mid-lifting axle down will stabilise such a situation, and make a jack-knife less likely.
ABS on the tractor unit helps the situation, but, if the rear axle of a tractor unit loses traction under braking, a jack-knife is the very rapid result. Breaking traction under acceleration is a more benign situation, and can usually be cured by lifting off the throttle a bit, which is an easier thing to do than lift off the brakes during an emergency.
At the design stage, it's quite a juggling act to set the braking distribution on an artic - although load sensing valves can help adjust the braking ration depending upon the loading condition, there are always some combinations of tractor unit and trailer, loading conditions and braking regimes which leave very small margins against lock-up.
Lift axles are always specified to save tyre wear - how drivers choose to use them on the road is another matter!
When the 38 tonne rules were first brought in during the 80's, the artics in my father's fleet remained at 32 tonnes. My father argued that as our trucks spent a fair amount of time on-site the extra axle on a tri-axle trailer would scrub tyres even more quickly than the twin axle trailers we used, and owing to the relatively poor rates we were earning on these routes, the extra money per tonne would probably be wiped out by extra tyre costs. Perhaps we should have investigated lift axles - they weren't quite as popular then, because most suspensions were steel as opposed to the almost universal air suspensions in use today.
Number_Cruncher
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