Why a pig and not a dog?

Submitted: Friday, Apr 12, 2019 at 13:31
ThreadID: 138153 Views:2878 Replies:11 FollowUps:11
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Of course, when it comes to eating, I'll always go for the pig (ham, pork, bacon... - I've had reliable advice that dog meat tastes horrible).

But when it comes to caravans, particularly larger, heavier ones, why do manufacturers invariably opt for a pig-trailer configuration (wheels in the middle) instead of a dog-trailer configuration (wheels at each end)?

For trucks carrying heavier loads (eg. dirt), the choice is a dog-trailer, and I've found myself wondering, particularly as caravans are becoming available in increasing lengths and weights, should this decision be reconsidered?

Some advantages as I see it:
1- the wheels of the trailer carry the entire load of the trailer, without placing heavy loads on the extreme rear point of the towing vehicle.
2- the front axle can be configured so that the wheels closely follow the wheels of the tow vehicle (ie. reducing cutting-in on corners).
3- no "see-saw effect" which can push the rear of the tow vehicle up and down on rough roads (or even concrete freeways, as we've found with our dual-axle non-load-sharing caravan), reducing comfort for the occupants.
4- better stability when parked (detached from tow vehicle).

Perhaps a dog-trailer is harder to reverse, but surely if one can learn to back a pig-trailer, one could also learn to back a dog-trailer (and plenty of truck drivers can do it, so it is possible).

Or does the stability of the combination (trailer and tow vehicle) become a bigger problem when the weight of the trailer exceeds the weight of the tow vehicle (a situation that is permissible with pig-trailers eg. 2t cars rated to tow 3t or 3.5t caravans)?

Why are caravans invariably built as a pig-trailer configuration rather than a dog-trailer configuration?
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Reply By: Duncan2H - Friday, Apr 12, 2019 at 13:54

Friday, Apr 12, 2019 at 13:54
What a great question..

Anyone remember the Leyland Brothers trailer for their West -> East crossing?

https://editorial.pxcrush.net/caravancampingsales/general/editorial/simpson-desert-1966-02.jpg?width=1024&height=683&aspect=pad


AnswerID: 624906

Reply By: Keith B2 - Friday, Apr 12, 2019 at 15:13

Friday, Apr 12, 2019 at 15:13
Here's what Collyn Rivers says in his recent book on caravan stability:

Dog-trailers Dog-trailers have two or more axles. One axle (at the front) pivots, or has so-called Ackerman steered wheels (as used at the front of cars and trucks). Either way enables the trailer to be steered by the vehicle’s drawbar. Such trailers have long since proved stable. Many load-carrying commercial versions are used to this day – but the dog configuration is now rarely used in caravan form.
In practice, dog trailers shimmy slightly on rough roads but are not prone to ‘wagging the tail’ of whatever tows them.
AnswerID: 624909

Reply By: Ron N - Friday, Apr 12, 2019 at 16:13

Friday, Apr 12, 2019 at 16:13
The simple reason caravans are not dog configuration is cost. Dog trailers require a turntable or a beam axle with steering knuckles.
Either arrangement adds a great deal of cost to constructing a trailer or caravan.

In addition, the design adds a serious amount of weight to the van or trailer tare .

Some of the larger commercial/mining vans or trailers use a dog trailer axle arrangement, and nearly always with a turntable.
But these trailers or vans are invariably high cost units, and larger than most "consumer" vans.

I used to own a couple, they were ablution vans, built in dog trailer arrangement - but they were big dollars, and also quite large, requiring a small truck to pull them around. I had an F350 and a 5 tonne Mitsubishi truck for that.

Then there's the possible need for an upgraded drivers licence for articulated trailers.
As a dog trailer has two articulation points, in theory, it is an "Articulated Vehicle" under licencing laws - and you should have an Articulated Vehicle licence to ensure you're compliant with the law.

However, surprisingly, I can find no reference under any State laws to needing an Articulated Vehicle licence, when towing an articulated trailer.

I have seen several homebuilt articulated trailers on the road, running on Holden wheels and axles, but I wondered if they were actually road-legal.

The problem is that it's difficult to get a low enough tare with a dog trailer, utilising Holden wheels and hubs, to end up with any modest amount of load capacity.

You need very heavy axles, wheels and hubs, to enable the construction of a dog trailer/van arrangement. This is also a major additional cost factor. Most of the smaller dog trailers use 3 ton axles and hubs as a minimum in their build.

The towing stability of dog trailers/vans is improved over trailers/vans with axles in the centre.
But drawbar length affects their stability. Too short a drawbar means they will "tailwag" with sharp steering movements.

Backing an articulated trailer is beyond a lot of people who don't have semi-trailer experience.
You only have to see the number of people who can't back caravans or trailers with centre-mounted axles.

Cheers, Ron.
AnswerID: 624914

Follow Up By: mike39 - Saturday, Apr 13, 2019 at 11:45

Saturday, Apr 13, 2019 at 11:45
Actually, there are plenty of "dog Trailers" on the road, usually a smaller "SUV" towed behind a motorhome.
Have also seen vehicles as large as a Patrol or Landcruiser tabletop behind a motorhome providing quite a decent payload capacity.
How the trailer braking arrangements in these situations are handled may be a challenge.
mike
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FollowupID: 898485

Reply By: Gbc.. - Friday, Apr 12, 2019 at 16:57

Friday, Apr 12, 2019 at 16:57
There is a market for pig trailer converter dollies, esp in the US. Dog trailers are heavy and you get no extra towing concession so you spend a lot of your payload on running gear.
AnswerID: 624915

Reply By: RMD - Friday, Apr 12, 2019 at 18:59

Friday, Apr 12, 2019 at 18:59
Tim-c
You say, “perhaps a dog trailer is harder to reverse” but many many people who travel Australia cannot back the normal trailer much at all. Having to back a double articulated trailer to a specific spot is far beyond most people who already can’t even back a single vehicle very well.
As an apprentice I was the only person in my workshop Who would back double articulated trailers and reverse parallel park them into that workshop for maintenance. Others who often drove semis and manoeuvred vehicles every day would not have a go. I just had plenty of practice that is all.
The drive throughs in caravan parks are provided for a very good reason.
AnswerID: 624921

Reply By: Peter_n_Margaret - Friday, Apr 12, 2019 at 20:10

Friday, Apr 12, 2019 at 20:10
Saw this one in Germany a few years back.
The other major advantage is that it has a ball weight of close to zero.

Cheers,
Peter
OKA196 motorhome
AnswerID: 624923

Follow Up By: Ron N - Friday, Apr 12, 2019 at 22:26

Friday, Apr 12, 2019 at 22:26
Peter, that's an interesting rig. But no mention of it, on the Bimobil website.

They specialise in "custom constructions", and I'd suspect that rig above is a custom build, to a clients request.

Bimobil - All models

Cheers, Ron.
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FollowupID: 898482

Reply By: Ron N - Friday, Apr 12, 2019 at 22:37

Friday, Apr 12, 2019 at 22:37
King Caravans specialise in commercial and custom-built caravans, mobile ablutions, mobile classrooms, mobile laundries, and mobile offices - and they seem to like the dog trailer style of construction.

They will probably build a dog-trailer-style caravan to your specs.
However, I get the impression King specialise in some very basic construction techniques, and most of their products just look like dongas on wheels.

King appear to be catering to the mining, exploration, and oil & gas industries, and these companies are only interested in some pretty basic designs.

King Caravans

Cheers, Ron.

AnswerID: 624925

Reply By: Member - Rustygq - Saturday, Apr 13, 2019 at 11:58

Saturday, Apr 13, 2019 at 11:58
Years ago I towed a DOG trailer behind a rigid tipper. When I first started a mate who already towed one, said "Whatever ya do don't look in the mirrors." Thats because they have a mind of there own an swish about a bit, you learn not to chase them but let them do their own thing.

Regards reversing. They have an air operated lock on the turntable and then are quite easy to reverse.

Personally I think there would be too much weight constantly trying to drag the arse of the 4x4 sideways. Having done hundreds of thousands of Ks towing a dog trailer, its not something I would contemplate with a caravan.

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AnswerID: 624930

Follow Up By: Peter_n_Margaret - Saturday, Apr 13, 2019 at 18:28

Saturday, Apr 13, 2019 at 18:28
I would suggest that any dog trailer that 'swishes about' is in need of a wheel alignment or other attention. Cars towed behind other vehicles on A frames tend to be quite stable.

In 1969, I towed a Cooper S on an home made A frame behind a Mini Deluxe from Adelaide to Canberra at 100kph. The bumper bars were about 350mm apart. The combination was quite stable. The only difficulty was the lack of brakes going down through the mountains :).
I made no attempts at reversing the combination :)

A dog trailer is inherently stable (if set up correctly) whereas a pig trailer is inherently unstable. Chalk and cheese really.
Cheers,
Peter
OKA196 motorhome
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FollowupID: 898495

Follow Up By: Ron N - Saturday, Apr 13, 2019 at 20:48

Saturday, Apr 13, 2019 at 20:48
There are many factors that makes dog trailers tailwag excessively. Here are some.

1. The amount of overhang between the trucks drive wheels and the drawbar pivot point. A longer overhang between the drive wheels and the drawbar pivot point increases the amount the front of the trailer drawbar moves when steering corrections are made. This is exacerbated if a short trailer drawbar is used as well.

2. The height of the C of G of the load being carried. Some of the tall, dry freight vans, suffer from a high C of G when fully loaded, and thus suffer from more upper side-to-side sway, with steering corrections. This side-to-side sway then translates to trailer tailwag.

3. The style of suspension. Air bag suspensions are notorious for increased side-to-side sway, because they lack the roll resistance of mechanical (spring) suspensions. Anti-sway bars and heavier shock absorbers are often fitted to try and improve this suspensions performance.

4. The amount of bump steer suffered by the truck (the prime mover). Some trucks have poorer front suspension geometry than others, and suffer more from bump steer (unwanted steering movement created by bumps in the road).
Wear in the trucks steering mechanism also reduces the steerings resistance to road input, as it does when the vehicle is new.

5. The width of the spring packs. Some spring packs aren't sufficiently wide enough to prevent excessive side-to-side axle movement. This problem is not confined to trucks, it affects many vehicles.
I can recall Holden fitting wider rear spring leaves to the HR Holden to reduce the side-to-side rear axle sway, that was a problem with the HD Holdens handling.

6 Wear in suspensions overall, can contribute to tailwag and sway. Worn suspension pin bushings are common with high km vehicles.

Trailer tailwag is a multi-faceted problem and many technical studies have been undertaken to try and pin down the major factors affecting the handling performance of combinations.
Many road train combinations today still suffer from poor handling and excessive tailwag, not helped by constantly varying load concentrations, in many cases.
In all cases, the load positioning in your trailer is crucial to good handling behaviour.

Cheers, Ron.
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FollowupID: 898497

Follow Up By: 9900Eagle - Sunday, Apr 14, 2019 at 04:10

Sunday, Apr 14, 2019 at 04:10
Ron's correct, but we can't compare truck dog trailers of 30 years ago with the one's of today.

The Australian design rules and modern trailer design have nearly eliminated that giggle headed movement, they include distances like overhang, length of drawbar and positioning of axles, many also have esc with manufacturers testing their trailers. Unlike caravans if a trailer doesn't perform word gets round very quickly in the transport industry.

Just follow any modern truck and dog combination and watch, of course if the road is bad they will wobble around a small amount but settle quickly.

From memory in the 80's Mossman mill carted sugar to Cairns with trucks and dogs. They were an absolute nightmare on the Captain Cook Highway.

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FollowupID: 898503

Follow Up By: Gbc.. - Sunday, Apr 14, 2019 at 05:43

Sunday, Apr 14, 2019 at 05:43
Not to mention that a mini being towed isn’t a dog trailer at all. No centre pivot turntable. You cannot spin a towed mini in its own wheelbase backwards to tip the main body....... every design has its own pluses and minuses.
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FollowupID: 898504

Follow Up By: Peter_n_Margaret - Sunday, Apr 14, 2019 at 10:07

Sunday, Apr 14, 2019 at 10:07
I think you will find that the caravan in the pic I posted has no turntable either.
Still a dog trailer in my book.
Cheers,
Peter
OKA196 motorhome
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FollowupID: 898507

Follow Up By: Ron N - Sunday, Apr 14, 2019 at 11:23

Sunday, Apr 14, 2019 at 11:23
Peter, there's one of those Bimobil dog-trailer-style caravans for sale in the link below, and if you do a Google translate, the first sentence in the sales description says the van has a turntable.

Bimobil turntable caravan for sale

That particular van model is called the Bimobil AX 575, and there's quite a few European websites that have articles about that particular van.

Not especially cheap at around 70,000 to 75,000 Euros new, but nothing from Europe (and Germany in particular) comes cheap.
Cheers, Ron.
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FollowupID: 898509

Follow Up By: Peter_n_Margaret - Sunday, Apr 14, 2019 at 17:17

Sunday, Apr 14, 2019 at 17:17
OK, thanks.

It also says "So there is no lurching, no weight on the trailer hitch and it is on the road like on rails."

Cheers,
Peter
OKA196 motorhome
1
FollowupID: 898519

Reply By: Hoyks - Sunday, Apr 14, 2019 at 14:41

Sunday, Apr 14, 2019 at 14:41
Suspension weight would be a biggie, so too engineering in articulation.

Two axles close together only have to deflect a small amount to cope with traversing a ditch/drain/gutter at an angle. You need a much more complicated suspension system to allow for the suspension travel or you have to design a van to cope with substantial torsional that will twist its body.

Twisting of the body leads to bent cupboard hinges, lose fittings and the body falling to bits, all things customers aren't real keen on.

Then there is overcoming the 'We've always done it that way' argument. Very hard to break into the market with something that is significantly heavier and more expensive than you competitors.
AnswerID: 624941

Reply By: Gbc.. - Monday, Apr 22, 2019 at 02:24

Monday, Apr 22, 2019 at 02:24
This was posted as stolen in northern nsw today. Carries 3000l of diesel. Looks to be a great design for a low ball weight, high atm trailer. If you spot it, call the police please.

AnswerID: 625077

Follow Up By: Hoyks - Tuesday, Apr 23, 2019 at 09:36

Tuesday, Apr 23, 2019 at 09:36
One hell of a long range fuel tank!
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FollowupID: 898716

Follow Up By: Ron N - Saturday, Apr 27, 2019 at 13:05

Saturday, Apr 27, 2019 at 13:05
What a pair of scumbags!! When they catch them, they should hand them over to the locals for a bit of "rough handling"!

Break and Enter - Mungindi

Theft from isolated farm sheds is a huge problem - but industrial/commercial theft is always ongoing, too.

The W.A. Police caught a bloke locally, who had rented a factory unit, and filled it with stolen goods, such as thousands of litres of fuel, power tools, hand tools and even two stolen Bobcats!!

They only caught him after he broke into a local toolstore, which had a monitored alarm system.
The Police arrived and promptly grabbed one of the thieves outside the toolstore, equipped with a 2-way - and his scummy mate was found hiding inside the toolstore, equipped with the other 2-way.

Cheers, Ron.

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FollowupID: 898804

Reply By: tim_c - Friday, May 10, 2019 at 13:41

Friday, May 10, 2019 at 13:41
Thanks everyone for taking the time to reply.

A common response was the increased cost and weight of a dog configuration (compared to the pig configuration), though as I see it, two axles is still two axles, whether they are close together in the middle, or one at each end.

However, someone also mentioned the problem of rigidity, which I can certainly understand. Particularly in off-road 'vans - the axle articulation would need to be much greater for axles placed further apart, and this would undoubtedly increase torsional (twisting) stresses on the caravan body, requiring it (and/or the chassis) to be further reinforced, adding weight, to prevent twisting of the caravan body which would cause a whole range of issues, including the cupboards popping out of place.

Thanks again for all the responses.
AnswerID: 625474

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