Review: The Dynamics of Caravan Stability

My Article Rating: My Rating 5/5

An excellent presentation on the dynamics and forces incurred when towing. I have not previously considered the concept of yaw inertia. I now understand how high yaw inertia would give a false sense of security until such time as the bleep hits the fan. I would appreciate the authors comments in relation to whether chaotic instability is directly related to towing speed, or for example the square of speed, as for wind forces. ExploreOz, I thank you for your initiative.
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Reply By: Caravan & Motorhome Books - Monday, Jun 24, 2013 at 13:04

Monday, Jun 24, 2013 at 13:04
Currently overseas with very limited internet but can confirm speed is indeed a major determinant re the "critical speed". Returning to Australia on July 12, so can post a full response after then if you don't mind.
Regards, Collyn
AnswerID: 513718

Follow Up By: martin c1 - Monday, Jun 24, 2013 at 18:06

Monday, Jun 24, 2013 at 18:06
Thank you I look forward to your reply. I hope your time away is enjoyable and rewarding.
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Reply By: Nomadic Navara - Tuesday, Jun 25, 2013 at 00:00

Tuesday, Jun 25, 2013 at 00:00
For further reading, try This link

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Follow Up By: martin c1 - Wednesday, Jun 26, 2013 at 14:45

Wednesday, Jun 26, 2013 at 14:45
Thanks Peter, I have downloaded it and will see how well I can follow it.
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Reply By: Caravan & Motorhome Books - Wednesday, Jul 17, 2013 at 15:34

Wednesday, Jul 17, 2013 at 15:34

Thank you for your kind comments – the article has resulted in a fair amount of direct email so will cover the main points raised there as well.

There is empirical (practical) evidence that chaotic instability is related to speed (see also below). Certainly the energy is related to the square of the speed, but mass distribution and yaw angle play a major role. It has been postulated that the onset can be mathematically calculated. If this proves to be correct, we have a major issue with our seemingly ever more massive end-heavy caravans and inexorably increasingly lighter tow vehicles.

I cannot safely suggest what that speed is likely to be, as unlike Europe, to the best of my knowledge (and disturbingly) no facility exists in Australia to measure the radius of gyration and yaw inertia of local product. I can only say that I would not personally buy any new caravan much over 18 ft and would (unless an EU-like design) tow above 80 km/h.

A few people have commented that, apart from the importance of minimum hitch overhang I do not cover tow vehicle dynamics (in this article).

This is because that too is equally complex – and whilst any full analysis must take that into account, the major concern with some local product is by far that of massive yaw inertia. I have covered this topic in my 2010 article on Vehicle Dynamics (currently archived in the Article section of my own website). It is being currently being updated and will eventually be a preface to my main article. (There is already a great deal known, but here again I appear to be the only technical writer attempting to do this in non-mathematical form - Ollley is a superb reference, but is nevertheless heavy going.

A couple of email respondents raise the issue that my comments regarding the front of the trailer rolling around the tow ball depends also on the height of the rear roll centre of the tow vehicle. This is indeed so but as most big tow vehicles have beam axle rear suspension, that height is likely to be more or less at that tow ball height. It will be much lower if there is independent rear suspension. Here again I stress only to see Roll Centre and Roll Axis is very rough approximations.

Re the Bath University paper, it does need to be born in mind that the tests involved light short caravans known to be exceptionally stable (both from Bailey), and with centralised mass. The actual tests involved only the distribution of some 220 kg (plus a few with an additional 50 kg) of personal effects within the ‘vans – not of ‘van design. Even then, the dynamics effects are worrying – with initial testing speeds and methodology having to be modified for driver safety. If they can truly be extrapolated for our larger end heavy product there really does seem cause for concern particularly if towed by lighter vehicles.

Work done on WDH usage in the USA (in the late 1978 by Klein, Johnston and Szostak) initially assumed that jack knifing (infinite yaw gain) is unlikely at trailer yaw angles of less than 20 degrees and that a possible performance metric is hitch angle response as a function of speed. The paper notes that this had to be reduced to 10 degrees when one test resulted in an unrecoverable jack-knife. Many tests worryingly showed that jack knifing occurred at speeds of as low as <80 km/h at some at 70 km/h.

This paper notes that load levelling (with a WDH) ‘is not a satisfactory solution to the problem of heavy hitch loads’. (This is because it can cause oscillatory shifts from understeer to oversteer due to changes in the relative slip angles of the tow vehicle’s front to rear tyres.) This paper goes as far as suggesting that, if people really must use megavans, apart from the safer fifth wheeler approach, the solution is the 'B-double' dolly solution. This was tried for a time in the 1970s-1980s but (unlike with commercial vehicles), may not have been adequately developed.

I have received a few complaints that the article requires some physics understanding.
This is I fear so if you need evidence that what I state is true. This is essentially a very complex issue - that can otherwise only be presented on a 'take it or leave it level'. That I will not do as long experience shows it inevitably ends with 'that's just your opinion mate' responses.

I might mention that this ExplorOz article is now beginning to attract interest in the USA – particular amongst Airstream owners.
Collyn Rivers
AnswerID: 514919

Follow Up By: martin c1 - Monday, Jul 22, 2013 at 11:12

Monday, Jul 22, 2013 at 11:12
Thanks for your further information. I like your presentation, keep up the physics. Speaking of which, it should not be too onerous to calculate yaw inertia from basic principles.
FollowupID: 794318

Follow Up By: Caravan & Motorhome Books - Monday, Jul 22, 2013 at 11:49

Monday, Jul 22, 2013 at 11:49

Calculation of yaw inertia (and radius of gyration) is certainly possible (and I will include the equation in my techo version on my own website) but necessitates the magnitude and position of all mass to be known - which is close to impossible. But easy to measure it - and I showed how it can be done in my original Caravan World article.

The Bath uni investigation also arrived at this view - and built their own test rig. Is not hard to do. I find it incredible that to the best of my knowledge not a single local manufacturer has done this.

Incidentally, have readers seen the three-axle 30 ft or so 'van (with overhung spare wheel/s) in latest Caravan & Motorhome magazine?
FollowupID: 794321

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