UKC

Noticed many times on articulated vehicles that there is often an axle on the tractor unit on which the wheels are clear of the ground. Interested to know why this is. Can anyone tell me please?

In reply to FesteringSore:

Do you mean the front two wheels on the cab are rolling, then the next set behind them are off the ground and still...and then all the rest of the wheels (under the load) are rolling?

Dunno.

My nephew asked me this on the way down the M1 yesterday and I was a bit stumped.

In reply to ThunderCat:


That's it.

In reply to FesteringSore:

When a 6 x 2 unit is carrying lighter loads they don't need the extra pair of wheels and you can operate the tractor unit as a 4 x 2. They can be lowered when heavier loads are carried.

In reply to Bellie:


Ah, wondered if it was something like that. Thanks.

In reply to FesteringSore:

http://en.wikipedia.org/wiki/Axle#Lift_axle

In reply to Bellie:
There is also the saving of tyres when doing tight manuoevres (sp? Someone correct me!) With three axles close together at least two will have to scrub hard in the turn. Watch even a two axle trailer doing a tight turn, now imagine this to a lesser degree on every turn. Adds up to excessive tyre wear.
The third axle is usually lifted when the load is low, below the legal axle load limit, and down when the limit would be exceeded on just two axles.

In reply to Bellie:


I can now appear to know what I'm talking about next time nephew is in the car with me. cheers!

TC

In reply to FesteringSore:

I thought it was to do with fuel consumption. When light loaded they lift the wheels to reduce drag.

In reply to FesteringSore:

The annual Vehicle Excise Duty the owner has to pay is calculated based on the weight of the tractive unit and the number of axles it has (more axles mean the load is spread between more tyres and less damage is done to the road) this can be a significant factor when choosing a new vehicle.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/2...

In reply to FesteringSore:
Last week I had the pleasure of driving one of those vehicles around the Highlands in the sunshine. A marginally shabby 26t 6x2 Volvo of some age with a lift axle.

You can legally go to 26 tonne only with the third axle down. Normally, you don't really know for sure how much the vehicle weighs when partially laden so the axle is best left down until coming back empty unless you have something really light.

Manoeuvrability is improved with the axle up. However, on most of these vehicles tail-swing increases which is important to watch out for in town situations. For car drivers or cyclists the idea of just nipping past the rear of a truck as it turns can be an even worse plan than usual in those circumstances.

Because the lift axle keeps weight off the drive axle and therefore reduces traction, occasionally, on rough or soft or icy ground, drivers will find that lifting the axle gives them improved traction and gets them out of trouble. This can be a difficult judgement on some ground since you can also get bogged deeper if it's too soft.

Some air suspension systems look after themselves most of the time but others seem to need reset for every load change.

For busy motorways or for big descents it is important to remember that an extra pair of brakes is handy!

Back to the maths (and aircon) next week.