Line of roll centre

Submitted: Sunday, Jun 30, 2013 at 11:22
ThreadID: 103020 Views:2479 Replies:3 FollowUps:5
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In a recent article 'Caravan Stability' it refers to a 'Line of roll centre' can somebody explain this to me in simple terms? I am considering purchasing an independent suspension on a fifth wheeler vs a fixed beam.
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Reply By: Member - Frank P (NSW) - Sunday, Jun 30, 2013 at 12:22

Sunday, Jun 30, 2013 at 12:22

It would be difficult for anyone to explain caravan stability better than Collyn Rivers did in his article, but if I understand your question correctly perhaps I can address it.

Caravans, trailers and fifth wheelers (let's call it a caravan) have a "roll centre". This is the "pivot point" about which the caravan will roll sideways if subjected to a sideways force. The force could be a gust of wind, a passing truck blowing or sucking the side of the caravan, or cornering forces.

If I recall correctly from the article, a beam axle has a higher roll centre than independent suspension, and some types of independent suspensions have higher roll centres than others.

As I understand it, the higher the roll centre and all other things being equal, the less the caravan will tend to roll when subject to a sideways force. The tendency to roll is affected by other things as well. With cornering forces, the further the centre of gravity is above the roll centre the more the caravan will tend to roll. With sideways wind loading, the further the centere of pressure is above the roll centre the more the caravan will want to roll.

Because the caravan is joined to the tow vehicle at the hitch, and the hitch may be above or below the natural roll centre, the trailer cannot roll around its natural roll centre. Instead it rolls around a LINE joining the hitch to the roll centre. This is the "line of roll centre", or the "roll axis". Compared to what the caravan wants to do naturally, this line modifies the way in which it will roll when subjected to a sideways force.

Collyn says that this will be taken this into account in a well designed caravan.

The various types of suspensions have advantages and disadvantages. Some are better for off-road, some are better for highway. The fore-and-aft position of the axle(s) and load distribution affect the outcome. Which design is is the best compromise for your application is an engineering issue that is beyond my limited understanding.

But I hope that I have correctly explained the concept of "line of roll centre".

If anyone thinks I have got it wrong or can explain it better, please don't hesitate to post.



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AnswerID: 514022

Reply By: hooks - Sunday, Jun 30, 2013 at 15:35

Sunday, Jun 30, 2013 at 15:35
Thanks Frank, I understand your explanation. I will need to qualify these issues with the design of the particular trailer that I purchase. Do you have any experience or knowledge with independent suspensions in general? I understand that 5th wheelers are more stable than caravans and may in fact resist the sideways and cornering forces better.
AnswerID: 514029

Follow Up By: Member - Peter H1 (NSW) - Sunday, Jun 30, 2013 at 16:54

Sunday, Jun 30, 2013 at 16:54
I have a 5th wheeler with AL-KO suspension [no springs, no shocks] which is a rubber filled axle and the inner axle mooves in this.
I am very happy with this set up, most of the time i don't know it's there and road trains don't affect it.
Just done 3000k [Sydney to Laura FNQ and now in Cairns, off tomorrow for Karumba] handled lovely.
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Follow Up By: Member - Frank P (NSW) - Sunday, Jun 30, 2013 at 18:24

Sunday, Jun 30, 2013 at 18:24
You're welcome, Hooks.

I don't have much general experience or expertise in independent trailer suspensions, just personal experience with my rig which I've owned for 5 years.

I have a Kimberley Karavan. These are narrower than a standard caravan, high-clearance off-road with independent trailing arm coil spring and shock suspension. The suspension travel is long and compliant to best suit rough terrain, at which it excels.

According to Collyn Rivers' article that type of suspension has about the lowest roll centre, so coupled with its width (or lack of) you would expect it to roll around a fair bit when subject to sideways forces. We're talking about highway now. It does roll a fair bit under hard cornering, but it doesn't sway or snake. Who corners hard when towing a caravan anyway? BUT, if you did a swerve-to-avoid manoeuvre I think you'd be in as much trouble as anyone else doing the same with a caravan in tow. There are limits to what you can do and you should drive with that in mind.

Also, because it is relatively low the centre of pressure is also low, so sideways wind gusts don't affect it much at all.

Prior to the Karavan I had an Avan camper - same width as a standard caravan, but very low when packed up. Alko axle (like Peter H1's) but with shocks. You could not tow anything steadier - except maybe a 5th wheeler :-)

The Rivers article suggests that a beam axle and leaf springs might be least prone to roll, with independent trailing arms most prone. In between there is independent swing arms (triangular arms pivoted on the base of the triangle in or near the centre of the trailer with the stub axles at the apex and sprung with leaf or coil springs near the stub axles - the middle diagram in the section entitled "Roll Centre" in the article).

You need to discuss with your 5th wheeler manufacturer the relative merits of these as to how they may affect your trailer.


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FollowupID: 792907

Reply By: The Bantam - Monday, Jul 01, 2013 at 23:58

Monday, Jul 01, 2013 at 23:58
You have already taken the biggest step you can toward towing stability and that is moving away from a pig trailer with a single axle group in the middle to an articulated combination.

There is a reason why the heavy transport industry has mostly moved away from pig trailers and that is because they are fundamentally unstable.....there is a reason why they call em pig trailers.

If you have a fundamentally unstable combination, it is not surprising that people try to do all sorts of things to improve the situation...hence all the weight distributing hitches, sway controllers and independent suspensions.....getting concerned about roll center is one of them.

Because you are looking at a fundamentally more stable articulated combination or fifth wheel trailer as the yanks call em you do not need to be as concerned with things like roll centres and stuff as you would with a single axle group pig trailer.

As far as independent suspension on trailers.......well..its almost unknown in heavy transport and a properly set up beam axle realy has a lot going for it.

Unless you are concerned with ride height or ground clearance, as ya self is the independent suspenion worth it.


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Follow Up By: hooks - Tuesday, Jul 02, 2013 at 06:24

Tuesday, Jul 02, 2013 at 06:24
Thanks Bantam,
I am most impressed with this forum, there is a lot of experience out there. I need a vehicle and trailer that can venture off road occasionally and the independent suspension may be the way to go. It may also help will corrugated dirt roads. However I take your point re a properly set up beam axil. All things being equal which system to you suggest would be the better? Ground clearance may be an issue sometimes .
FollowupID: 792999

Follow Up By: The Bantam - Tuesday, Jul 02, 2013 at 10:15

Tuesday, Jul 02, 2013 at 10:15
That would depend on the engineering and workmanship behind either system.

Unfortunately most beam axle and leaf spring systems on light trailers ( trailers under 4.5 tonnes) are less than well set up.The springs are mostly too short, cheap, off the shelf items and the spring rates are too stiff and the progression of that rate mostly does not enter the builders mind....As for things like shock absorbers and any other suspension refinements they are mostly an afterthaught.

one big advantage most multiaxle leaf spring systems have is they are a load sharing suspension with walking links between the springs.
This means that each axle and each wheel continues to carry an equal share of the load.
As each wheel rides over a bump or obsticle, the links walk and each wheel continues to carry a more or less equal share of the load.

This has several implications.

Many multi axle independent systems are not load sharing.

There are some very nice independent suspension systems available for light trailers, but there are also some that are ...well....

As soon as you step away from single axle independent systems I believe the advantages of independent syspension deminish......certainly a better design is needed to have a good advantage.

One question to ask about any independent trailer suspension is......who designed and built it.

There are trailer builders knocking up independent suspension systems of their own in the back shed or even worse having them made in China.
On the other hand there are some major independent suppliers that make some very nice independent trailer suspensions.

Another question to ask about an independent suspension is....are spare parts easily available.

I now a bloke who baught a very nice high end camper trailer with independent suspension......unfortunately they hit a large pot hole at speed and ripped one side out from under the trailer.
The problem was ( among other drama) the trailer manufacturer while still in business had changed the suspension system they where using and the manufacturer of who made the original was not in business any more.
This made things very complicated with the insurance.

If it was a supplier with good spares support they would have been able to get a trailing link assembly shipped to Birdsville....have it fitted and tow the thing home.

On the matter of load sharing and independent suspension, As far as I can see the only way to have load sharing suspension AND independent suspension is to run an airbag system as a great many heavy vehicles and trailers run these days.

Now ground clearance
There is no way around it, more ground clearance means a higher centre of gravity and a higher roll centre.
In addition off road suspensions require more articulation, that means more height required to accomodate it.
Running off road will also require significantly larger wheels, again more height, higher centre of gravity and higher roll centre.

If the trailer is to be used only on road, the certain way of improveing trailer behaviour all round is to keep the wheel size as small as possible to support the load and keep the deck of the trailer as low as possible.
The smoother the road, the smaller the wheels can be and the lower the deck height can be you realy want to off road this thing.

Beam axles and clearance.
If you are going off road, you probably want wheels the same size as the tow vehicle.....that will almost certainly be a beam axle vehicle.
The trailer has thinner axles and no diff, so where is the axle clearance problem.

While there are some very nice independent suspension systems, it occurs to me that a great many of the independent systems are designed more for profit making than actual efectivness.

Bet ya the independent suspension is considerably more expensive than the leaf sprung option.

AND is it the salesman that is talking to you about roll centre.

As you can see, there are a great many issues more important than roll centre on a trailer.

FollowupID: 793012

Follow Up By: hooks - Wednesday, Jul 03, 2013 at 06:21

Wednesday, Jul 03, 2013 at 06:21
I have a lot to think about. The ALKO brand of independent suspension has some good reviews. Have you got an opinion on brand names for each system?
FollowupID: 793075

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