Tow Car Tests in General

[Zafiral]

Remember that the max tow weight for the car is also for a flat trailer which will behave differently to a caravan.

Some cars (Renault especially) have a max tow weight that is driver only. Fill the car and your tow weight is vastly reduced.

 

[Lutz]

Yes, Zafiral, you've made a good point. That's why the 100% legal weight ratio limit in Germany only applies to caravans. For flat trailers the limit is 120%, always assuming that the car manufacturer has specified limits which allow such high towloads (otherwise, of course, their lower limits apply).

 

[ProfJohnL]

Remember that the max tow weight for the car is also for a flat trailer which will behave differently to a caravan.

Some cars (Renault especially) have a max tow weight that is driver only. Fill the car and your tow weight is vastly reduced.

Thank you Zafiral

I am aware of that as I have a Renault, but my point is still valid, there is nothing in the manufactures specifications that defines the type of trailer you may tow, only the maximum weights.

In respect of the weight limitations of typical of Renaults method of specifying, my spread of tests would reveal those limitations for all too see.

 

[Gafferbill]

Do manufactures design their cars with towing in mind?

.... I do not think they do.

I think their attitude is: -

If you must attach a caravan, then this is the maximum noseweight that a caravan can safely apply to our car if it is to be driven safely.

Apart from that it is down to you the driver!

This has meant that caravan design has been almost totally restricted to a balanced trailer type.

That is the axles are mounted slightly aft of centre to comply with the noseweight restrictions.

Automotive engineers know how to design proper trailers, they are called semi-trailers and a large part of the trailer weight/load is transferred to the towing unit.

This creates a much more stable vehicle and when hitched is effectively one vehicle.

A car towing a balanced trailer will always be two vehicles joined together and for me this is the important bit..

The second vehicle has no steering.

The steering is done for the second vehicle by the lead vehicle or tow car.

The steering is achieved as the caravan effectively follows the tow car.

Several laws of Physics and Aerodynamics say that sometimes the caravan does not want to follow the tow car.

This will manifest itself in lateral forces at the towing hitch.

If the tow car fails to resist these lateral forces, its rear axle will loose grip with the road surface and instability will result which could be uncontrollable.

Scientists with more brains, money and resources than me have carried out research that shows that the heavier the tow car in relation to the caravan the more stable the outfit is.

It seems to me that having a tow car that is a MINIMUM 15% heavier than the caravan it is towing is a wise precaution in what after all is a leisure activity.

This for me is how what John L calls

"a silly arbitrary figure" of 85% came about.

Progress has allowed weight ratios to be closer with the introduction of self sensing stability braking devises such as Alko's ATC.

If you must tow over 85%.. then make sure you have such a system.

 

[Lutz]

When the weight ratio law first came into force in Germany it specified an 80% limit for all caravans. After a trial period of 2 (or maybe it was 3, I can't quite remember) years, they raised the weight ratio limit to 100% for caravans fitted with an approved stabiliser because the data gathered in the meantime showed that the number of accidents did not increase appreciably by raising the limit. The 80% weight ratio limit was only continued for caravans without stabiliser. For this reason I think it is perfectly legitimate to question whether a blanket 85% recommendation which originated in the dim and distant past, is appropriate or not.

 

[xtrailman]

You are a card lutz, so germany increased the limit to 100%, no more accidents resulted so that makes it safe?

Very scientific.

Our limit in england has also been raised, it was 75%, now its 85%, we british take a more cautious approach, and prefer to air on the side of caution.

Which is one of the reasons that we have a national speed limit of 70mph, rather than no limit.

 

[Lutz]

At least there is a plausible explanation of how the 100% limit was arrived at. No-one, on the other hand, has come up with an good explanation of why 85% and not 80% or 90% was selected as a recommendation. One can therefore only assume that its origins were totally arbitrary, which makes it difficult to take seriously.

 

[machighlander]

Our limit in england has also been raised, it was 75%, now its 85%, we british take a more cautious approach, and prefer to air on the side of caution.

your limit in england ray ?, then to say we british, hmm.

 

[Garfield]

I agree with Lutz in that a quantifyable stability test would be very useful ie the heaviest caravan that a car can tow at 60 mph before instability occurs regardless of 85% or manufacturers limits. Or the highest speed loaded to 85% before instability occurs, though I can immediately see some plonker using the above as an excuse to go too fast or overload.

I have posted this experience before but it is an extreme example of how vehicle type rather than weight affects stability, I towed a boat that I had just bought 70 miles home using my mk2 Golf GTI (970kg kerb wt 1200kg manufacturers max tow) I called at a pulic weighbridge near where I bought it to find that the boat weighed in at 1550kg! a 160% weight ratio. I decided to proceed home with caution but occasionally I got up to 60mph with no sign of instability. I then boght a BMW 530 (1989 1499kerb weight, 1600kg manufacturers max tow) which frequently hit 70mph with the boat on the back with no problems.

We then used my freinds 1992 Discovery to tow the boat over the pennines to the lakes, the outfit would start to snake badly at anything over 55mph, ie the heaviest vehicle was the most unstable.

A final point I would like to make is that I believe that an outfit should be stable to at least 60mph WITHOUT any stabiliser system fitted and the stabiliser is there as a safety net in emergency, not essential for normal driving.

 

[xtrailman]

garfield

With the dis being high of the ground, compared to the golf and bm, you was probably getting a aerodynamic effect, under the car, causing instability. I would be more interested in how all three performed with a caravan or horsebox on the back.

 

[Gafferbill]

Garfield .... would that plonker be youself?

Before you take offence....... I am only using the facts you have given us!

 

[DCW27]

Hi There

How can you take the magazines towcar tests seriously when they do not even check the kerbweight of the towcars they are testing?

David

 

[ProfJohnL]

Hello Gaffer,

I do not recall stating that 85% "a silly arbitrary figure" Arbitrary yes but not silly.

However that aside, you make some good points in your posting.

I tend to agree that towing performance, is not the top concern of car manufacturers, but as test for towing now form part of the type approval process, they cannot ignore the possibility their products will be used to tow.

Commercial articulated vehicles do use an inherently more stable arrangement with their 5th wheel set between the two main axles of the tug. This does impart a greater load on the driving and steering wheels of the tug, but even this arrangement can end up jacknifed so even this arrangement can behave, as you put it, as two vehicles.

I have been at pains in the past to express the desirability of keeping the trailer weight as small as possible. But the benefits are not as apparent as one might expect.

It certainly feels right that a heavier car is likely to control a towed outfit better, but it is not always so. If reports on this forum are anything to go by, then there are substantial numbers of incidents where relatively heavy tow cars have come to grief. What we don't know is what other factors may have played a part in this apparent divergence from what seems good sense.

But what it points to is that the car being heavier than the trailer does not necessarily impart a significant stability benefit, so a 15% margin in the ratio may not be enough, or there may be no significant difference if that margin is eroded or even reversed. The problem is I have with the 85% is there have been no facts and figures produced to substantiate the choice of 85% as the target figure.

I am all in favour of improvements to the braking systems of trailers and the introduction of satiability control systems, but the in my view the biggest effect on safety of an outfit is the way the driver drives.

 

[Lutz]

Outfits could probably be made inherently even more stable if the noseweight were increased significantly, to something in the order of at least 200kg, but that would revolutionise the caravan and car industry. Caravan chassis would have to be designed to take such a load, as would the car chassis. The cost of modifying caravan designs may not be prohibitive and only those who tow a caravan would have to pay the premium. But for cars, the picture is completely different. The car's underbody would have to be reinforced (naturally at additional cost) and if this were done across the board, it would penalise every owner, whether the car is ever put to towing use or not. I cannot see this as being feasible or acceptable.

 

[xtrailman]

Bath uni, did mention stability was improved with an increase in nose weight.

Which makes it odd, when some 4x4 owners with low nose limits of 80kg, shout their praise.

Odd also lutz, when you disagree with the caravan councils, recommendation of 7% for the nose weight, as this would often give a figure over 100kg, going towards confirming what you have said, re a nose weight of 200kg.

John l, as with gaffer i read your post, in the first paragraph, you twice mentioned a figure, firstly 85% figure, and then secondly, followed "this silly arbitrary figure".

I know i'm nit picking but you do appear to have denied all knowledge of your own statement?

"I think most people on this forum know my views about the 85% figure, so I will not repeat them in detail here, but I do agree with the OP that that the magazine reviews are perhaps short selling the abilities of some towing vehicles by sticking to this silly arbitrary figure."

 

[James3]

I have had a variety of different caravans and cars over the years and have tried to always be very careful on correct loading of the van, nose weights etc. I have towed between 87-92% but have never managed to acheive the 85% figure. I have not noticed much difference when towing with these figure. However I have always towed with the maximum nose weight permitted on the car. On the odd occasion when the nose weight has been lower than the maximum allowed I have noticed a signifcant difference in the stability of the outfit. There was one time when we towed back from the S of France in fairly windy conditions and I noticed that the outfit towed brillantly, so much so that I checked the nose weight when I got home and found it to be 3Kg higher than the maximum. I would add next time out it was back to the normal maximum figure.

But for me whilst the 85% is a good guideline it is not the most important thing and my key thing would be the nose weight.

 

[Gafferbill]

Hello Gaffer,

I do not recall stating that 85% "a silly arbitrary figure" Arbitrary yes but not silly.

However that aside, you make some good points in your posting.

I tend to agree that towing performance, is not the top concern of car manufacturers, but as test for towing now form part of the type approval process, they cannot ignore the possibility their products will be used to tow.

Commercial articulated vehicles do use an inherently more stable arrangement with their 5th wheel set between the two main axles of the tug. This does impart a greater load on the driving and steering wheels of the tug, but even this arrangement can end up jacknifed so even this arrangement can behave, as you put it, as two vehicles.

I have been at pains in the past to express the desirability of keeping the trailer weight as small as possible. But the benefits are not as apparent as one might expect.

It certainly feels right that a heavier car is likely to control a towed outfit better, but it is not always so. If reports on this forum are anything to go by, then there are substantial numbers of incidents where relatively heavy tow cars have come to grief. What we don't know is what other factors may have played a part in this apparent divergence from what seems good sense.

But what it points to is that the car being heavier than the trailer does not necessarily impart a significant stability benefit, so a 15% margin in the ratio may not be enough, or there may be no significant difference if that margin is eroded or even reversed. The problem is I have with the 85% is there have been no facts and figures produced to substantiate the choice of 85% as the target figure.

I am all in favour of improvements to the braking systems of trailers and the introduction of satiability control systems, but the in my view the biggest effect on safety of an outfit is the way the driver drives.

Now this post I can agree with!

"a silly arbitrary figure"... used by yourself in you post of 9 Mar 2010 08:35 PM in your first paragraph.

 

[ProfJohnL]

OK OK OK

Yes I did write it, Perhaps with the benefit if hind sight I should have been a little clearer.

85% is a perfectly good figure, as is any other for a given set of circumstances, but most outfits do not match those circumstances so it is silly to choose one figure for which there has been no credible justification, as such it an arbitrary figure.

 

[Gafferbill]

Lutz.. it does not matter if the extra weight you are talking about (which I agree is beneficial) is applied to the noseweight of the caravan or added to the rear axle load of the tow car.

Cars are already designed to carry extra weight in this manner, hence the scientist's advice to put heavy items in the tow car over the back axle.

 

[Lutz]

In the majority of cases, the 7% formula for noseweight is not achievable. For a car with a 75kg noseweight limit, a caravan with an MTPLM of 1100kg would already result in the 7% being exceeded. However, as the majority of people tow quite comfortably with a noseweight well under 7%, (the law only requires 4% minimum), the formula has lost much of its significance. The recommendation is therefore to make full use of the lower of the available limits for car and caravan respectively.

 

[xtrailman]

I managed a smile reading JL's reply.

I don't really think that we are that far apart with our views.

But remember that the 85%, and to a lesser extent 7%, are guide lines for the NOVICE caravaner, which is why i get involved when the guide is rubbished.

At the end of the day we will all drive and tow what we want, but the beginner should be given advise that does not place them in danger.

 

[Lutz]

I don't have a problem with the recommendation as such either, but I think it needs to be treated at face value. There is no reason to suppose impending disaster is inevitable if one does not stick to it. Some outfits do feel less 'twitchy' than others on the limit and my original post was simply to suggest that the testers publish their reviews accordingly. If an outfit is stable enough at 100% just think how super it must be at 85%! There is room for differentiation.

 

[ProfJohnL]

Hello Ray,

I can agree fully with your last paragrapgh, but as we know, both the 85 and 7% figures do not always work.

We have had postings from people on this forum who strive to get the ratio to exactly 85% and who then go on to find that the outfit is unhappy. Too much emphasis is placed on this figure, and it overshadows or even completely obscures the importance of other important factors. New caravanners may be overwhelmed by the undue importance the figure is given, to the point where other equally or arguable more important factors are forgotten.

We have also had people who places so much faith in the 85% figure which has no legal authority and religiously do not exceed it yet quite happily report they then go on to speed whilst towing! Totally illogical!