Here's a dumb question...

Submitted: Thursday, Jan 08, 2009 at 00:58

ThreadID: 64907 Views:2703 Replies:4 FollowUps:2

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Off-track

Dangerous thing - I got to thinking in relation to a recent thread about large caravans of 3.5t and specifically their towball weight and dynamic instability. Is there any reason why there are no articulated caravans, much like a dog-trailer?

Pros - no towball weight as all the weight is carried by it's axles.
More stable as the axles are at either end instead of the middle.
Better braking as on normal trailers the drawbar tends to nosedive under braking.
Better approach/departure angles.
Cons - many people wouldnt be able to reverse it.

Might be a dumb question but hey...

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Reply By: Member - Lionel A (WA) - Thursday, Jan 08, 2009 at 07:07

Thursday, Jan 08, 2009 at 07:07

O-t, Ive often wondered the same thing.

Maybe height would be an issue as the front axle would need a large sweep area.
Another could be the additional weight of the steering axle and drawbar assembly.
The only other problem I could see would be levelling the van, where current vans can pivot on the center axles, the dog-style would or could mean lifting a complete axle.

Cheers.....Lionel.

AnswerID: 343173

Reply By: Dave B (NSW) - Thursday, Jan 08, 2009 at 07:21

Thursday, Jan 08, 2009 at 07:21

Geez, a lot of people can't reverse a normal caravan, can you imagine the circus with a dog trailer.
You would need 40 acre drive through sites everywhere.

Dave

'Wouldn't be dead for quids'

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AnswerID: 343174

Reply By: Member - Graham H (QLD) - Thursday, Jan 08, 2009 at 10:35

Thursday, Jan 08, 2009 at 10:35

I read an article on this in a Caravan book a few months ago and apparently the reason why was that they dont track very well.

Now I know you will say then why is there so many behind trucks.

I dont know but that was what was said in the article.

The braking system on trucks can be adjusted so that the trailer comes on earlier and holds itself in line and whilst caravan controllers can do the same ,not as well apparently.
I know our bus brakes have more on the rear wheels than the front and boy do they pull up good.

It also said something about steering geometry etc but I cant remember the details.
Perhaps someone else read it and can elaborate.

The opinion was it would be more trouble than it was worth.

AnswerID: 343197

Follow Up By: Member - Matt (Perth-WA) - Thursday, Jan 08, 2009 at 14:12

Thursday, Jan 08, 2009 at 14:12

Heavy and Light vehicle braking systems are substantially different in design requirements.

The PRIMARY brakes in light vehicles are the front brakes due to large amounts of mass transfer forward under spike brake conditions.

Where the PRIMARY brakes on HV are the rear brakes due to less mass transfer due to longer wheelbase, load over the rear axles and the size and capacity of the rear brakes. (ie twin bogie and duals=4 times the traction of the front) but the trailer also plays a huge part in traction. When the truck brakes mass transfer would normally push down on the front and lift the rear but the trailer transfers its mass onto the turntable decreasing the normal reaction and increases the weight on the rear axle.

The trials that were carried out for the implementation of ADR 38 (commercial trailer braking systems) found that timing did little to the attitude of the trailer under braking and while it is important...the braking capacity (effective retarding co-efficient) of the trailer was the most important factor.

If the trailers braking capaity is sufficient it will enable the combination to brake well...ie reduce truck front wheel lockup and nearly eliminate truck rear wheel lock up (jacknife).

My opinon of a dog type setup on a non-commercial vehicle would, in theory, control mass transfer and provide a setup that non-commercial vehicles are designed for. Having a front primary braking system, a dog trailer does not change the mass transfer on the towing vehicle. Where a pig trailer (typical caravan) does change the mass transfer of the tow vehicle and pushes down on the rear...hence the need for vehicle leveling and stabiliser systems for safe towing.

In practice and without seeing the report you mentioned...the logistics of engineering a dog type trailer setup for a non-commercial vehicle would be a PITA with large amounts of engineering and very hard to meet the ADR requirements.

Thats without the other types of problems mentioned like driveability and manouverability.

I agree with you, I think it all just gets into the 'too-hard' basket.

All the best

Matt.

FollowupID: 611011

Reply By: Steve - Thursday, Jan 08, 2009 at 14:23

Thursday, Jan 08, 2009 at 14:23

I would think a tandem axle would be a good compromise between the two. Hope so because we recently bought our first tandem. Definitely better on the towball and they are more stable. Apparently not quite as easy to manouvre but I can't say I've noticed any difference so far. I supp[ose the main disadvantage is the added weight of the extra axle and in any case, all larger vans are tandem, anyway.

AnswerID: 343218