Caravans and their dynamics

A recent thread inquiring about a ball weight prompted me to post this. The reason being it is better to have a discussion about the dynamics of vehicles and vans separate to a post about a vehicle ball weight. Ross M speaks a lot of sense but that can and has been taken out of context. Here is an old explanation of towing vans and 5th wheelers.

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Reply By: Member - Ian H (NSW) - Friday, May 10, 2013 at 22:44

Friday, May 10, 2013 at 22:44
Thanks Rockape. Great article with good clean explanations.
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Reply By: Ross M - Friday, May 10, 2013 at 23:50

Friday, May 10, 2013 at 23:50
Thanks for the interesting article. Oh, it does me 'ed in, much simpler to believe a manufacturer cos they wouldn't mislead me.

One thing hinted about but not fully discussed in the article is the matter of pitch control and associated forces.
The article mentions quite a few issues with the horizontal movement of a caravan and the hint about pitch is also important.
It was interesting to read the comment about the length of vans greater than 5metres or so.

A modern vehicle with a rated ball weight of say 200kg and a 2ton van in tow, over undulating ground or through a dip, can generate huge down force or sudden uplifting force on the tow ball because the mass of the van is pitching.
This can easily bottom the suspension of a vehicle and it tries to also bend the chassis of the caravan near the front of the bodywork.
Usually most survive but some don't and they crack and fail, also vehicle towbars, although tested,certified and strong, are transferring the load to a chassis which becomes the weakest link.

However, all weights and ratings have been applied within the specified norms.

On flat ground Norm is with you, when the situation becomes more demanding Norm's ability to back up and retain the safety of the rig is less.
To me it is about using all of it wisely and having respect for the fact that the weights/ratings are MAXIMUMS and for some folk to use those figures as the NORM is bordering on a disregard for safety of all road users.

Ross M

AnswerID: 510769

Follow Up By: Warb - Saturday, May 11, 2013 at 07:08

Saturday, May 11, 2013 at 07:08
Hmm. Are we to believe that the team who designed my vehicle and stated the maximum downforce on the ball was 250kg had never thought of bumps? Or heavy applications of brakes? Or safety margins?

It is far more likely that in those cases where damage has occured the trailer has been massively overloaded. We all know that this is not unusual, half the caravans I see have the back of the towing vehicle dragging on the ground and the front wheel barelys touching the bitumen.

I have always assumed that the sheer terror caused by driving a vehicle whose steering wheel has almost no effect on its direction was used as a judge of ball weight by most caravanners. And also why they are unable to exceed 70kph.....

However I did, at the Rosehill show, overhear a couple of people who seemed to be judging the ability of their vehicle to tow a caravan based on it's tare weight rather than it's ATM. This is both illegal and stupid, because the tare weight is always far and away less than the actual weight when the caravan is loaded and equipped.
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Reply By: Member - Andrew & Jen - Saturday, May 11, 2013 at 09:47

Saturday, May 11, 2013 at 09:47
Thanks RA for posting the link to a very interesting article.

I only hope that the message that in many cases the dynamics of a large caravan/tow vehicle combination can become inherently unstable and unpredictable at or even near the speed limit will sink in.

There also seems to be an increasing number of vans being loaded at the rear end - 2 spare wheels, large full width box, and /or extra fuel/water - all of which increases the pendulum effect in a sway situation and with pitching.

I suspect that the recommendation that most large rigs should probably treat 90 kph as a maximum will probably set the hounds baying :-)

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Follow Up By: Member Bushy 04(VIC) - Saturday, May 11, 2013 at 12:00

Saturday, May 11, 2013 at 12:00
Andrew & Jen ,could not agree with you more, as we do 80-90 kph and then have some #### pass at a great rate of speed which not only has them wobbling all over the road but often forces us to slow down as well.
More often than not they will be the ones involved in an accident.
Some van drivers don't realize why we are called wobbly's by the truckers.
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Follow Up By: Rockape - Saturday, May 11, 2013 at 17:52

Saturday, May 11, 2013 at 17:52
I would like the van manufactures to have clear and concise rules on how a van is to be setup regarding weight distribution.

An example of this is my van which is 17 foot and with modest gear in the front boot. 2 Hoses/2 leads/gas bottle/Light camp table/2light camp chairs/small jockey wheel/ stabiliser handle and a broom and bucket.

The Ball weight was 225kg which is ridiculous for a van that size. Most of the reason is the front kitchen that causes my problem. I have had to place a box on the back and re distribute the load to the rear to reduce the ball weight to about 165kg.

It is up to us to make sure our vans are within spec but when you start off with a badly balanced van things can just get worse.

I guess in the future they will start booking owners for overloading and boy doesn't that hurt.

I would love to weigh some of these vans new to see what their real tare is and compare it with what tare is plated on them.
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Follow Up By: Member - Andrew & Jen - Saturday, May 11, 2013 at 20:13

Saturday, May 11, 2013 at 20:13
Hullo RA

I understand your issue and it is a vex question as to what to do.

Unfortunately, while easing the problem associated with excess ball weight, you have inadvertently created another one. You don't mention whether you used (or considered) a WDH and if so, did that help?

Given your extensive knowledge and experience, I might well be at risk of teaching my grandmother to suck eggs :-)

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Follow Up By: Rockape - Saturday, May 11, 2013 at 20:49

Saturday, May 11, 2013 at 20:49
I used a wdh on my Troopcarrier but with my new Ranger Ford says don't use one. I guess the reason is it puts to much strain on the chassis. The chassis is double plated to 5mm but that is only up to axle hump.

Yes, it would be great to keep the weight close to the axle to reduce pitching but on test towing around 800k it was very stable behind the new vehicle with no wdh. In fact I find less pitching with no wdh. Much of the weight distribution has been to the front part under the bed and I also keep the tank full which is just behind the axle.

I ran down to Rocky at between 90 to 95 kph then on the way back I sat on 100kph with a max of 110 to test the whole rig with no indications of handling problems at all. I have had the van and troopcarrier up to close to 130kph when overtaking and again no problems at all.

I don't advocate the high speeds but I do like to see what is acceptable and what isn't.

Andrew, no you can always teach an old fella new things and we never stop learning. What you said is quite valid and one of the reasons I believe we should have greater rules governing the van manufacturing sector.The van I have is fine, if I buy another one it will be with a mid kitchen and I will want to witness real tares and ball weights.

My lovely daughter once said to me "get back in your wheel chair you decrepit old bastard" That was when I was in my early 40's. Guess what she is in her mid 40's now and I don't miss her one bit. LOL.

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Reply By: Steve M1 (NSW) - Saturday, May 11, 2013 at 13:35

Saturday, May 11, 2013 at 13:35
Whilst on the subject of caravan dynamics, do any of you fellas have any idea what effect, if any, a rear air-foil/spoiler on the rear of my caravan roof might have? The van is a single axle, 1680 kg tare van with 16" wheels on control-rider suspension. The sceptic in me feels it's all for show but perhaps it may have some effect on the van's stability or maybe helps stop the thing bouncing around? The foil is a bit like a fig 7 (looking side-on) only the vertical bit of the 7 points this > way and not that < way and is mounted on the back edge of the pop-top section. So it catches the air up to a height of about 80/90 mm travelling along the roof and directs it downwards to the rear of the van.

AnswerID: 510798

Follow Up By: Ross M - Saturday, May 11, 2013 at 17:31

Saturday, May 11, 2013 at 17:31
Steve M1
I don't think it has anything to do with stability issues but may have a direct effect of spoiling the forming vacuum at the rear of the vehicle and therefore keeping it cleaner and keeping dust out of the van and also possibly providing some beneficial effect on fuel consumption if the vacuum at the rear is being reduced ie less rearward suction applied.

After a 140km/h run on a dirt road is the rear clean and is the fuel use less? Just joking.
Ross M
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Follow Up By: Member - Legendts - Sunday, May 12, 2013 at 06:41

Sunday, May 12, 2013 at 06:41
Steve, I think you would find that all the obstructions on the van roof would negate any benefit the rear van spoiler might give. As to creating downforce to act as a stabiliser, again you need clean airflow for it to benefit you and speed, neither is effective for a van. A benefit if it was to push airflow down the back of the van (which I think your description implies) may keep the rear of the van clean as mentioned by Ross M but that is all it may do. I had a play on an F100 canopy once and it took a lot of playing about to try and get it to work, in the end I gave up as no real benefit was gained either way. Just my two bobs worth.
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Follow Up By: Steve M1 (NSW) - Sunday, May 12, 2013 at 12:14

Sunday, May 12, 2013 at 12:14
thanks for the input guys.

I don't have any obstructions on the roof at all as it is a pop-top and has no need for roof vents and I don't have fancy do-dahs like aircon or ensuite on this van. :)) I've cut that stuff out and am back to a solid no frills off-road van. The only thing on the roof is one of those small pressure hatches which is flush with the roof surface, when not in use.

Oh well, at least the back of the van will need less cleaning if nothing else. I'm tempted to take it off tbh but if it keeps me a nose in front of the occasional Ferrari, I might have a re-think.
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Reply By: Bob W5 - Sunday, May 12, 2013 at 07:17

Sunday, May 12, 2013 at 07:17
Hi Rockape, Thanks for a very informative post. After reading your link I've got a much better understanding about hooking up and towing my 16ft van. Before reading I put my hand up and say I'm one of these who would load up walk around my rig scratch my head and say, yeah.. ahh..mmmm, all is good.. We never stop learning. Sometimes we've got to take the time to keep learning...
AnswerID: 510841

Reply By: AdrianLR (VIC) - Sunday, May 12, 2013 at 09:18

Sunday, May 12, 2013 at 09:18
By coincidence I was looking at camper rollover reports just last week as googling a possible (very) future camper replacement pulled up a couple of images of my prefered unit following a roll. I came across another one of Collyn's articles that has a very simple demonstration of weight distribution using a broomstick:

Caravan dynamics

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Follow Up By: Rockape - Sunday, May 12, 2013 at 12:02

Sunday, May 12, 2013 at 12:02
Thanks for adding that additional info. Takes a while to read but it is good value and hopefully everyone that tows a camper or van will read it.

I know what you mean about toolboxes. Mine at least only weighs 13 kg when full of my little treasures.

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Follow Up By: AdrianLR (VIC) - Sunday, May 12, 2013 at 14:16

Sunday, May 12, 2013 at 14:16
Thanks Rockape, I hope others read it as well.

My background is science rather than engineering and I tend to approach articles written by "retired engineers" with caution (no specific offence meant to Collyn!) for two reasons. Quite often the explanation given will be based on knowledge of an unrelated field ("I know about flowing water, therefore I will apply the same rules to flowing electrons", for example). Secondly, complex phenomena are "approximately" explained. This explanation is sufficient for the majority of situations, but is totally inaccurate when you move outside a narrow range of conditions because it was actually the wrong explanation of the observation in the first place.

I admit that I started reading the article I linked to with the same scepticism. What changed my attitude was Collyn's admission that he'd pretty much blindly accepted the 10% rule up until a few years' back. He could have just as easily written the article by saying something along the lines of "and I did the analysis and concluded that 10% is about right". Instead he went back to basics and analysed the full extent of the situation - what changes when things are outside the norm. I found it amusing that he hints at the end if the article at some of the historical confrontations he's had on forums (and probably elsewhere) where his views were questioned. In this particular analysis of the subject I can't find any reasons to doubt his conclusions.

BTW, the camper that started me thinking is a single axle, sub-5m hybrid. I suspect that its rugged appearance lulls owners into a false sense of security but doesn't negate the laws of physics. There is probably no fundamental problem with the design.

While I'm in a contemplative mode, I wonder about the influence independent suspension on campers has on stability at the edge. Again, a strong marketing aspect (IS is more modern, complex etc than a solid axle therefore must be better) but does it introduce an extra independent leverage point? For top-heavy vans, can the wheels oscillate (particularly as some dampers are at silly angles) and exacerbate the likelihood of random motion of the whole unit?

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Follow Up By: Rockape - Sunday, May 12, 2013 at 14:57

Sunday, May 12, 2013 at 14:57
As you can see I am no rocket scientist except when I am in my armchair.
One of the reasons I selected the van I have is I followed one for around 150k a good few years ago and I thought to myself. Gee that thing is tracking well and not moving around at all. Speed was around 95kph.

About 1 year later I spoke to a retired truck driver with the same van at Takaraka and enquired as to what he liked about the van and if it had any problems with it. He relayed pretty much the same story to me about following one and how well it tracked.

It has the coromal independent which is very simple but seems to work well. Not plugging coromal but I have had no problems with this little van at all except for the way to high ball weight.
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Follow Up By: Member - Andrew & Jen - Monday, May 13, 2013 at 10:08

Monday, May 13, 2013 at 10:08
Hullo Adrian

Thanks for the link to this article - read it with interest.

I couldn't quite see the point of the picture at the end (Dodge Ram with van) as it didn't seem to have any connection with vehicle dynamics. It was certainly a big rig with a 5.5 tonne tri-axle van.

I suspect that the broken A-frame was mainly the result of the very stiff suspension in the rear of the Ram (assuming the loading was within manufacturer's spec). We have a F250 with a F350 rear end (springs and dual wheels) and use an Air Safe hitch which has an air bag and shock absorbers. This has produced a significant improvement in the ride and a lessening of the rapid vertical acceleration/deceleration of the hitch, thus reducing the stresses in the A-frame.


NB - this hitch must not be used with normal 4WDs, only trucks with high load, stiff leaf springs
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Follow Up By: Nomadic Navara - Tuesday, May 14, 2013 at 00:46

Tuesday, May 14, 2013 at 00:46
Quote from FollowUp 2
"My background is science rather than engineering and I tend to approach articles written by "retired engineers" with caution (no specific offence meant to Collyn!) for two reasons. Quite often the explanation given will be based on knowledge of an unrelated field"

Collyn was once part of a team developing vehicle suspension at GM Vauxhall before he emigrated to Oz.

Another good read is THE DYNAMICS OF TOWED VEHICLES . This is a study by the University of Bath School of Mechanical Engineering. It was sponsored by Bailey caravans in the UK. It contains a lot of complicated maths but they are there to support the findings. If you gloss over the maths you can get a lot of useful info from it.

They don't take a liking to friction type sway control devices. Read the paper to find out why.

Retired radio and electronics technician

Lifetime Member
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FollowupID: 789134

Follow Up By: Collyn R - Wednesday, May 15, 2013 at 17:58

Wednesday, May 15, 2013 at 17:58
Thank you for kind comments re that article.

It is currently being updated to take in new information and is likely to be published as an Explor-Oz article shortly. Will also be on my own website in a more technical form.

I regret that pressure of time precludes my answering most posts on this issue - but I will follow the thread.

(Am not retired, and have no intention ever to do so - tried it about 12 years ago for about 12 months but found it boring!)
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Follow Up By: AdrianLR (VIC) - Wednesday, May 15, 2013 at 19:58

Wednesday, May 15, 2013 at 19:58
Wow! You just never know who's watching:) Thanks for the reply Collyn.

I grew up surrounded by engineers (including my Grandmother who was a civil engineer in the 1920s) and interact with many. It's a standing joke that you all "only think in straight lines" and aren't all that flexible! This is a very good thing when building a bridge, less so when dealing with "organic" situations (such as those that involve the vagaries of people). On a serious note, it is worrying that universities find it difficult to attract engineering students. Without them, great ideas often stay just that.

Staying off topic....and to the soundtrack of the Twilight Zone.....yesterday I took an old electronics primer by Forrest M Mims III off the bookshelf to help my 16yo understand circuits and right next to it was an ETI magazine from 1975. I may have started to build the Flash Trigger in that issue and guess that's why I still have it. For anyone who's got this far reading and is wondering why I'm rabitting on, the Editorial Director was one Collyn Rivers.

All the best, and thanks for not retiring Collyn!


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Follow Up By: Rockape - Wednesday, May 15, 2013 at 20:27

Wednesday, May 15, 2013 at 20:27

Just a simple thank you.
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Follow Up By: Collyn R - Thursday, May 16, 2013 at 13:30

Thursday, May 16, 2013 at 13:30

Re engineers.

Ask a scientist to define 'pi' and you'll get an answer along the lines of 'it's 3.04159...the ratio between radius and circumference etc.

A practical engineer may say, ' It's a tad over three and a bit mate - but lets call it six-nine for safety.

Re the quantitative versus qualitative approach - it's not that clear cut in our home at least - as my wife is a psychologist now working on ways of achieving quantitative neurological changes via largely qualitative techniques.

(ETI has its own Wiki page (Google 'Electronics Today International').

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