Stability - do other factors beside loading and weight matter?

[Lutz]

In view of the fact that so many users choose not to abide by the 85% weight ratio recommendation, but prefer to make more use of the possible maximum allowable towload, I think it would be more appropriate to carry out towcar tests at the manufacturer's specified limit or at 100%, whichever is the lower. After all, if the towcar is any good at 100%, it will be even better at 85%.

 

[NigelHutson]

 

[NigelHutson]

But many do use the 85% as a guide. And conversly, any car that is hopeless at 85% will be even worse at 100%. I did mention that I didn't want to start the debate regarding percentages thank you, as it's something that's already been done to death on this and every other caravanning forum. Whatever the percentage, it's all done on a level playing field so to speak (it just happens to be the recommended 85%).

 

[Dustydog]

We've all known for years the only body that has attempted anything scientific on caravan / trailer handling is the University of Bath whose boffins have worked closely with Bailey Caravans.
It's worth reading their website for more interesting info
http://www.towingstabilitystudies.co.uk/stability-studies-research.php
They conclude that SPEED is the biggest dominant factor in stability.

 

[NigelHutson]

Hi Dusty,
Speed is more than likely a factor in a loss of control, BUT speaking from experience the same caravan behind one vehicle is rock steady at 70+ (a lot) whereas it's all over the place at 60 with another (at MIRA test tracks). Like many things, stability (or lack of) is probably down to a combination of things rather than one single factor.
Nigel

 

[Lutz]

But many do use the 85% as a guide. And conversly, any car that is hopeless at 85% will be even worse at 100%. I did mention that I didn't want to start the debate regarding percentages thank you, as it's something that's already been done to death on this and every other caravanning forum. Whatever the percentage, it's all done on a level playing field so to speak (it just happens to be the recommended 85%).

I wasn't arguing about the pro's and con's of the 85% recommendation. I just believe that one should face the fact that many do exceed 85% and it would be well worth knowing how their towcar behaves under such conditions. What's the point of testing a car at 85% with good results, then actually towing at a higher but perfectly legal percentage, only to find that it falls short of expectations?

 

[Parksy]

Surely if a combination is tested at 85% and is stable a correctly loaded combination with a higher towcar / caravan weight ratio would also be inherently stable. If this proved not to be the case I'd have thought that the safest and best option would be to reduce the percentage back to 85% where good results have been achieved?
One thing that I've often wondered about is whether or not stability and / or traction control devices fitted to modern towing vehicles could mask the onset of a snake?
My truck, a 1997 LWB Mitsubishi Pajero 2.8 TD with automatic transmission but no stability aids whatsoever, tows well and gives a definite 'seat of the pants' warning when rutted motorway carriageways cause the normally docile Abbey 620 that I tow to dance around a wee bit.
I've spoken to drivers of vehicles fitted with air suspension and electronic stability, of which I have no knowledge, notably Disco owners but I'm unsure which model, who have had some very scary moments for no apparent reason and with little or no warning.

 

[Lutz]

You're right in saying that most outfits that are good at 85% should still be acceptable at 100%. However, it is possible that there is an odd exception, for whatever reason, and it would be helpful if any such exceptions were pointed out. I'd be very disappointed if I've bought a towcar on the basis of it being recommended in a test only to find that that once I've got it home and tow for the first time at, say, 95%, it feels a bit 'twitchy', though not necessarily unstable. Would I go back and sell it and buy a new car? Probably not, but I'd feel deceived by the test recommendation.
Besides, quite independent of stability, there's also the issue of performance. An outfit which will still accelerate acceptably at 85% could be borderline at a higher weight ratio.

 

[NigelHutson]

Hi Lutz,
As the main sponsor of the Tow Car event is the Camping and Caravanning Club, and their recommendation is not to exceed the 85% ratio, that is why 85% is used. That is made perfectly clear when the results are published. If someone decides to exceed that, then that's their decision and not one that would be endorsed by many organisations (whether rightly or wrongly; that debate will continue until new evidence for or against is produced). You'd be surprised how "twitchy" some vehicles feel at 85% (including some very popular tow cars), and some are verging on being down right dangerous. The good ones however, I'm pretty confident would still be good above that limit. I'm sure your experience with tow cars has shown that some are better than others

Steve,
There was an issue with the Discovery 2 with rear air suspension and adjustable tow bars. It was discovered (excuse the pun!) that the tow ball was set too high, and when the suspension lifted, it gave a nose-up attitude on the caravan. In most cases, dropping the tow ball resolved the issue. Now whether towing nose-up or nose-down makes a significant (or any difference) is another long-standing debate, BUT in the case of the Discovery 2, nose-up did seem to be one of the causes of issues.
I haven't towed with a Discovery 2, but have done so with the 3 and 4. The 3 was excellent, the 4 is superb with regards to stability. One of the problems with towing with a large 4X4 is that some people think they're invincible, and probably pay less attention to loading and speed. That's not knocking 4X4's in any way, as there are some super tow cars amongst them (equally there are some rather poor ones too). Indeed, both of my last two cars have been bought on the back of doing the Tow Car event, and both have been large 4X4's.

 

[Lutz]

I quite understand your point Nigel, but it's a bit like doing a tyre comparison and then only carrying out the test at a comparatively modest standardised load rather than at the respective maximum allowable. The result of such a test would be equally meaningless.

 

[NigelHutson]

There will never be a perfect test, and we all realise that. However, using the maximum towing limit would also present problems in that some of the larger 4X4's can tow several tonnes (sometimes much more than the gross weight of the car, let alone its MIRO), and then there are some vehicles which can't even tow the 85%. How would you differntiate between those? The car that cannot tow 85% may seem fantastic when compared to others towing 100%, and the ones towing 100%+ may seem poor. But then again, those that can tow 100%+ may be brilliant if it was dropped to 100%. However, if they're all towing a similar ratio at least you get a seat of the pants feeling (usually backed up by the GPS/computer data).
Oh for a perfect world! We do what we can.
Nigel.

 

[Lutz]

There will never be a perfect test, and we all realise that. However, using the maximum towing limit would also present problems in that some of the larger 4X4's can tow several tonnes (sometimes much more than the gross weight of the car, let alone its MIRO), and then there are some vehicles which can't even tow the 85%. How would you differntiate between those? The car that cannot tow 85% may seem fantastic when compared to others towing 100%, and the ones towing 100%+ may seem poor. But then again, those that can tow 100%+ may be brilliant if it was dropped to 100%.
Nigel.

I don't want to dwell on the subject but please allow one last comment. You ask how one would differentiate between a car that has a towload limit of less than 85% and big 4x4's that can tow well over 100%. Differentiation is already there. The two already can't be compared directly so there isn't any need to differentiate any further. If I have a small caravan to tow I wouldn't be looking at getting a big 4x4 nor would I look for a small car with a low permissible towload if I want to tow, say, a 1900kg twin axle. But I might contemplate towing that same twin axle with a medium sized 4x4 with a 1900kg kerb weight if it's any good in preference to an even bigger heavier one that's only going to cost me more fuel and be dragging unnecessary deadweight when not towing.

 

[ProfJohnL]

The debate about whether the 85% recommendation is satisfactory or not will rumble on.

Whilst I disagree with the way the guidance of 85% has become a caravanning mantra, often to the exclusion of the other major factors in towing stability, I do concede that in the absence of any other practical method of estimating an outfits towing suitability, something is better than nothing. But it is far from perfect, and we should strive to find a better and more reliable system.

It's first imperfection is the ratio uses the notional kerbweight of the tow vehicle. Kerbweight is not a required measurement in any legal document, and most manufacturers do not quote it, but some do estimate or give a range dependent on the model and its accessories. This is rarely accurate. It is perfectly possible to have up to a 10% error in the real value for your car.

It would be better to use the UnLaden Weight, or Mass In Service. which are specific to each car.

Secondly different organisations either use the actual weight of or its MTPLM for the trailer These are not the same. Logically it should be the actual weight of the trailer in tow, but realistically most of us will not know our trailers real weight from journey to journey, so it is sensible to use the MTPLM, but non the less its another error factor.

So for any quoted ratio there is a considerable margin for variance. so its far from exact.

Now consider the number of caravans that have experienced sufficient instability to cause the driver to react or worse, a significant number must have adhered to the 85% ratio, so it goes to show that reliance on 85% is ill founded. It suggests that the guidance should be reviewed, and that could mean revised downwards!

 

[ProfJohnL]

Surely if a combination is tested at 85% and is stable a correctly loaded combination with a higher towcar / caravan weight ratio would also be inherently stable.....................

Hello Parksy
I don't think your assumption in your sentence can be taken as gospel.

Even though this topic has settled around weight ratios the following concepts in principal can also be applied to speed.

Assuming all other factor like road and weather conditions etc. remain the same then I hope we can all agree if the real weight ratio (or speed) is increased the outfit will tend towards instability and at some threshold point it will become unstable.

I think it is reasonable to assume that the transition between control to uncontrolled likely to have an inverse square type relationship where doubling the feature creates 4 times the loads and forces that try to invoke instability, so as you approach the threshold it more rapidly tends to take you towards and through it.

The problem is you cannot predict that point. so you don't know without trying it. For some outfits it may be a very low ratio (or speed) even less than 85%, for others it may be 86% and for some it may be greater than 100%.

If an outfits threshold were 90%, and it were tested at only 85% then it may be given the thumbs up. But a new owner basing their choice on your assumption may think its good for 100% when in fact its into its unstable area of operation.

Now reintroduce all the other variable factors and the whole issue of testing at part loads starts to look a little suspect.

 

[Lutz]

It would be better to use the UnLaden Weight, or Mass In Service. which are specific to each car.

Unladen weight is also not documented anywhere and is, in any case, even less than kerbweight as it is a "dry" weight without any fuel, engine coolant, oils, etc.

 

[Dustydog]

Nigel.

What do you think of the boffins findings from the University of Bath?

 

[NigelHutson]

I don't want to dwell on the subject but please allow one last comment. You ask how one would differentiate between a car that has a towload limit of less than 85% and big 4x4's that can tow well over 100%. Differentiation is already there. The two already can't be compared directly so there isn't any need to differentiate any further. If I have a small caravan to tow I wouldn't be looking at getting a big 4x4 nor would I look for a small car with a low permissible towload if I want to tow, say, a 1900kg twin axle. But I might contemplate towing that same twin axle with a medium sized 4x4 with a 1900kg kerb weight if it's any good in preference to an even bigger heavier one that's only going to cost me more fuel and be dragging unnecessary deadweight when not towing.

Hi Lutz,
In 2009 exactly that happened when the Volvo XC60 took not only the heavyweight class, but was the overall champion for some of the reasons that you've highlighted. The problem is that generally these mid-weight models as we have tested them are either OK at 85% or poor, but have never been what I would regard as good. Equally there have been some heavyweight ones too that have been diabolical, although sensibly David tones that down somewhat when writing the article.
Personally, I would always aim for as heavy tow car as possible towing as light weight caravan as possible, but even then there are a whole host of other factors to consider.
We tow the caravans under ideal circumstances, when the ballast is all at floor level and secured into place. However, I was asked to give a dempnstration to a guest at last year's event on the last day when the caravan was empty. There was no ballast or cushions in the thing. I used the Jaguar XF, which was the overall winner. When the caravan was ballasted, it was superb, and I mean superb. However, when empty, the outfit was nowhere near as stable.
Nigel.

 

[NigelHutson]

Nigel.

What do you think of the boffins findings from the University of Bath?

Hi Dusty,
I must admit that I haven't read the study in great depth, but I do think that what much of what they say is common sense and good practice. Basically what I believe that they're saying is that good caravan design, sensible loading, and inappropriate speed are the biggest determining factors in caravan stability. Common sense? I think so.
As I suggested to Lutz, my personal preference is for as heavy tow car as possible coupled to as light a caravan/trailer as possible. Now I know that this isn't always possible, so there have to be compromises. I'm very fortunate in that I don't need to use my tow car every day (I'd be the first to admit that I wouldn't be able to afford to do so), so I do sympathise with those who do. Also my choice of tow car allows me the luxury of beain able to tow ANY caravan, and being only the two of us means that I could if I so wished have a small, lightweight caravan. However, someone with a mid-sized Company car and a family cannot do that, so again compromises have to be met. There are plenty of good tow cars in the mid-sector together with a good choice of caravans. I think that one of the problems we have with both cars and caravans is that we all want the bells and whistles but don't really need them. Those toys only push weights in one direction. I'm as guilty as anyone on that.
When I look back at my early caravanning days (either with my parents or later) our first caravans didn't have fridges, had a two burner hob and a grill, and there were no electrics to speak of. We always had great holidays and breaks which just goes to show that the bells and whistles are really toys to make life that bit easier, but most of us wouldn't want to do without I suspect.
Nigel

 

[Gafferbill]

......my personal preference is for as heavy tow car as possible coupled to as light a caravan/trailer as possible

1958 Morris Minor towing 2012 Hobby Landhaus

.......is there a conflict here?

 

[Lutz]

One shouldn't forget that few of us can afford to own a dedicated towcar, so while there's no denying that there's nothing better than a heavy towcar together with the lightest possible caravan combination, the cost and environmental impact of running around in an unnecessarily heavy car the rest of the time while not towing will also figure in the decision making process.

 

[NigelHutson]

......my personal preference is for as heavy tow car as possible coupled to as light a caravan/trailer as possible

1958 Morris Minor towing 2012 Hobby Landhaus

.......is there a conflict here?

Bill,
You'd be surprised what an improvment the Moggie is over the Austin 7 we used to use!

 

[NigelHutson]

One shouldn't forget that few of us can afford to own a dedicated towcar, so while there's no denying that there's nothing better than a heavy towcar together with the lightest possible caravan combination, the cost and environmental impact of running around in an unnecessarily heavy car the rest of the time while not towing will also figure in the decision making process.

As I alluded in my last post pretty much.