Lorries with liftable wheels

[RaspberryFool]

On the way into work this morning, I passed a lorry with a pair of its wheels lifted off the road. I have often wondered why they do this, then suddenly remembered somewhere I could post this question as somebody would, surely, be able to answer it.

I have assumed that it must be save the cost of tyre wear. However, if that is the case:
1) Wouldn't the other tyres wear down more quickly as they have to take an increased load?
2) The cost of producing the mechanics necessary to lift the wheels must be significant. How long would it take before the amount of savings by reducing the tyre wear compensated for the extra cost of the liftable wheels?

Can someone satisfy my curiosity?

[didds]

maybe this?

https://truckanddriver.co.uk/big-lorry- ... axle-lift/

Volvo Trucks is launching a new function – Tandem Axle Lift – which makes it possible to disengage and raise the second drive axle. This gives better road grip and up to 4 per cent lower fuel consumption when the truck is driven unladen. The solution is ideal for heavy-duty operations with loads being carried one way and returning empty, such as hauling timber, bulk cargo or operating in the construction industry.

didds

[RaspberryFool]

Thanks didds - that must be the reason.

I'd still be interested to know how much this adds to the cost of the truck and how long it takes for the 4% saving in fuel to compensate for it ...

RF

[staffordian]

I imagine fuel costs are a pretty significant proportion of overall costs for most hauliers, so that 4% would soon add up.

Tyre wear would be reduced too, because when you have a pair of rear axles, whenever the vehicle negotiates a turn, the rear tyres scrub sideways. This must be quite significant because another relatively recent innovation is for the rearmost axle of a rigid six or eight wheeled truck (and even some articulated trailer wheels) to be steerable, which significantly reduces this scrub.

[DrBunsenHoneydew]

Every little helps at a typical 10 mpg.

[bungeejumper]

Very common, I saw four this afternoon. They tend to be the kinds of trucks that (a) carry very heavy loads frequently and (b) may find themselves driving considerable distances back to base while empty. Building material flatbeds and concrete mixers are favourites. AIUI, the cost savings from taking an axle off the road can be as high as 12%, which can make the difference between being price-competitive and, well, not.

I believe there's another reason. If you take an axle off the road, you're also taking the related braking equipment with it. And actually that can be a good thing. Having a truck with too much braking power for the reduced (unladen) weight of the vehicle is not going to be good for the driver's nerves, so lifting the wheels tones down the G forces a bit.

Can be more than just one axle, of course. Try this one for size:

BJ

[tjh290633]

All our local dust carts have this feature (they are Dennis models) and when I see them just after that are starting, the middle axle is always raised. I assume that the load on the rear axle is monitored and, when the axle load reaches a certain level, then the axle is lowered.

I must look next Monday and see what it's like when it gets to us, about 3 hours later.

TJH

[pochisoldi]

An empty trailer has a lower brake/deceleration force than a fully laden trailer
A tractor capable of hauling 44T must have more capable brakes than a truck capable of hauling a lesser load.

This means that in a heavy braking situation, a tractor capable of hauling 44T with two rear driving axles on the tarmac hauling an empty trailer will jackknife more easily than the same tractor hauling the same but fully laden trailer.

Example: https://www.youtube.com/watch?v=8U7FmTgBTkQ (For those of you watching in black and white: twin rear axle tractor+ long unladen 4 axle trailer overcooks it on a roundabout, brakes hard and jackknifes)
If the driver had lifted the axle, the tractor would have taken longer to stop, and the trailer wouldn't have overtaken the tractor so easily (if at all).
(Yes the truck driver is hooning, and the curvature of the roundabout increases the tendency to jackknife, but translate this into sudden braking due to an idiot in a micra suddenly cutting across a truck in lane 1 moving at 56mph in order to exit the motorway before the chevrons start, and it starts to makes sense)

PochiSoldi

[madhatter]

I used to wonder if the extra stuff to get maintained and go wrong could be worth what I assumed would be saved by wear on a pair of lightly loaded tyres until one occasion when I was stuck behind an unloaded lorry on which both rear axles were down. We both slowed on approaching a roundabout, quite gently, but one axle just locked up and the tyres dragged along the road and kept dragging until we stopped.

This naturally left a great long skid mark and probably flat spotted the tyres, yet the driver was almost certainly unaware of it. It was long enough ago that anti-lock technology would have been unavailable, even on cars or indeed sensors on wheels.

I recall thinking that it probably explained many of the long, apparently seriously urgent lorry tyre marks I often saw on the road surfaces, and at least some of the chunks of shredded tyres visible on the hard shoulder, for surely if it happened enough, a tyre would burst?

It suddenly seemed quite a good idea to raise an axle after seeing that.

[Itsallaguess]

Aren't they just a bit like birds, and will lift a wheel every so often to give it a bit of a rest?

They're more intelligent than we realise, these lorries -

https://imgur.com/VyeF4tO

Cheers,

Itsallaguess

[madhatter]

Too much information dude.

I did actually try to think of another way to put it, but could think of nothing as apt, so I supposed it might pass under the radar of a certain sort of poster.

I plainly supposed erroneously.