85% Of Kerb Weight. Law, Guide Or Myth?

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For decades, now, many, including much of the towing industry, have worked on the basis that, for safe towing, we should, always, aim to stick to a trailer or caravan that weighs no more than 85% of the kerb weight of the towing vehicle. Over and over again, we hear; Well. The Caravan & Motorhome Club recommend it, so it must be right, but what, exactly, does it mean? Is there any scientific basis behind it, and, if not, what factors are important, and what weights should we, actually, be using?

Increasingly, trailer manufacturers are coming out and challenging the validity of the ‘85% rule’, and, it would appear, with good reason.

There are, it seems, a number of inherent flaws in the Caravan & Motorhome Club recommendation. Some of them are genuine flaws, whilst others are more down to errors of interpretation, than anything. A classic example of the latter is that many quote 85% as the figure recommended for anyone towing. Of course, this is incorrect, and the clubs recommend up to 100% for anyone who is more experienced at towing. The 85% hypothetical figure is for those new to towing, only, and, also, relates only to caravans, due, in the main, to their unique towing characteristics.

Is It Law?

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Another very common misconception is that the 85% is law, and must not be exceeded, under any circumstances. This is not true, and never has been. Indeed, and, more worryingly, some cars have a legal towing capacity that is, actually, lower than 85% of kerb weight, meaning that, using the latter, will, actually, render the driver illegal.

Whilst most are aware that the 85% is not a legal figure, there is a much more widespread belief that 100% is a legal limit. This is not, entirely, surprising. At the time of writing this, the web sites of the Caravan and Motorhome Club, Camping & Caravanning Club, National Trailer & Towing Association*, UK Tow, and just about every professional source I can find, clearly state that, on a post 1997 licence, the MAM of the trailer / caravan cannot exceed the kerb weight of the towing vehicle.

This is completely incorrect, but, to date, only the Camping and Caravanning Club have made a partial correction, on one part of their web site. The truth of the matter is that, from a purely legal perspective, there is no problem with towing a trailer or caravan that is in excess of the kerb weight of the towing vehicle, even on a standard B licence (provided it is, also, within both the physical towing capacity of the car, and the licence parameters of the driver).

Government sources did, originally, state that, on a post 1997 licence, trailer MAM could not exceed vehicle kerb weight, but this is no longer the case, and has been removed from all official Government sources (although not, it would appear, the majority of other industry sources).

Technological Developments

The 85% Guide (we will refer to it, as that, for now, as that is the term that is familiar to most) was conceived decades ago, before manufacturers, actually, specified an official braked towing limit for their vehicles. It was, also, put in place prior to the introduction of a significant number of technological developments, both in terms of vehicle, and trailer manufacture.

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We have seen significant advancements in things like suspension and brake design, ABS, ATC, stabiliser hitches etc. Towing, as a whole, is a lot safer than it was 30 or 40 years ago, provided we are mindful of other key factors, such as speed, loading, driving style etc.

Unfortunately, there has been no change in the recommended safe towing limit, since the mid 80’s, in order to compensate for this.

Why Was It Introduced?

The guide was, originally, introduced, in order to provide a measure by which those new to towing could gauge what was a safe figure to tow, with their chosen vehicle. Again, there are a huge amount of misconceptions as to why this was necessary. These range from; “it needs to be lighter, so that the car can brake more easily” to; “It needs to be lighter, so the car can pull it”.

The facts, however, are that a heavier car, relative to trailer, is preferable, as it is, generally, believed it helps to control any snaking that might occur. Caravans have very different handling characteristics, from other trailers. They are high sided, non aerodynamic boxes, with high centres of gravity, and low internal density.

This means they are more prone to snaking, caused by cross winds, overtaking lorries, etc, than other trailers. If this happens, then the theory is, that a heavier car, relative to the caravan, will help to prevent the ‘tail wagging the dog’. There is, of course, an element of common sense in this, however, in the modern world, with all the technological advances listed above, a correctly loaded caravan, with the right tyre pressures travelling within the speed limit, is far less likely to experience issues than it would have done thirty years ago. Of course, that is not, in any way, to suggest that it cannot happen, simply that the risks which were around when the guide was devised, are, now, significantly reduced.

How is it Calculated?

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This, in my personal opinion, is where the 85% guide really falls down, as it takes no account, whatsoever, of actual weights involved.

Contrary to popular misconception, you do not apply the 85% to vehicle towing capacity, or gross weight. You apply it to kerb weight.

So, we take the maximum gross weight of the caravan, and compare it to the kerb weight of the car.

That can give rise to some rather ridiculous inconsistencies. Let’s take a look at one, potential, scenario.

  • A car has a kerb weight of 1,600kg, and a max gross weight of 2,200kg
  • A caravan has a net weight of 1,500kg and a gross weight of 1,700kg.

The C&MC rule takes the maximum gross weight of the caravan, and compares it to the kerb weight of the car. In this case, gross weight of caravan / kerb weight of car equals 1,700kg / 1,600kg, which equates to a towing ratio of 106%. This would be rejected, out of hand, as a bad match, by our friends at C&MC.

OK, but that assumes the caravan is full, and the car is empty. So, what if it is the other way around? Net weight of caravan / gross weight of car equals 1,500kg / 2,200kg which works out to 68%.

Clearly, these are two completely different scenarios, however, according to the C&MC, they are EXACTLY THE SAME! Really? Of course, it is unlikely you will ever be travelling with a car that is fully loaded, towing a caravan that is completely empty, but is that, really, any less realistic than the C&MC version of towing a fully loaded caravan with a totally empty car? I suspect not.

Just for the purposes of comparison, let’s assume that both the car and the caravan are loaded to 50% capacity. The calculation, then, becomes 1,600kg / 1,900kg, or 84%. Ooh. Good. Spot on then. Again, all hypothetical, but it does expose the inconsistencies in the theory, and pushes it, still, further, into the realms of; ‘myth’.

Another way of looking at this is to take the example of a couple of potential tow cars. The Renault Grand Scenic 2 Litre and the Land Rover Defender have an almost identical kerb weight. The latter has a type approved towing limit of 3,500kg. That of the Renault is 1,300kg which, as it happens, is, almost 300kg less than 85% of kerb weight. In spite of this, the 85% Guide would have us believe that these two cars are equally competent at towing a caravan. Whilst I would not, for one second, advocate the towing of a three and a half tonne caravan with the Land Rover, it does, once again, highlight the potential issues with the guide.

Meanwhile, Back In The Real World

OK, so it appears that the C&MC guideline is both outdated, and riddled with inconsistencies, but what do we use, instead? I guess the simple answer is; a mixture of manufacturers’ towing limits and common sense. Another of the many misconceptions surrounding towing weights is that the manufacturers’ specified towing capacity, simply, represents the amount the car can tow up a 12% incline. This is not the case. That is, indeed, a factor, but these figures are the maximum amount we can legally tow. It is naive, in the extreme, to assume this is based, purely, on what it can tow up a predetermined gradient, with no recourse to suspension, braking capabilities etc. The manufacturers’ braked towing limit is the amount the vehicle can legally and safely tow, taking due consideration of engine capabilities, suspension, brakes etc. That said, we would be foolish to completely ignore the unique challenges afforded by towing a caravan, if, indeed, that is our accommodation of choice, and, in most cases, the ‘happy medium’ is, as you would expect, likely to be found somewhere in between the C&MC guideline, and the manufacturers’ specified towing limit.

How Do Towing Capacities Work, And What Can We Tow?

The commonly held belief is that we can tow, up to the limit specified on the car’s VIN plate. This is, to a large extent, true, however, it is a little more involved than that. A typical VIN plate will include four weights (see below).

VIN

The top weight represents the maximum allowed gross weight of the car.

The second weight is the maximum gross weight of car plus trailer (Gross Train Weight).

The last two weights are the maximum allowed load on each axle.

It is a widely held belief that the difference between lines one and two is the maximum amount the car can tow. In the case of the plate, above, this would be 1,695kg (4,200kg – 2,505kg). In some cases, this may well be the case, however, what this, actually, represents is the maximum amount the car can tow, when it is, itself, fully loaded. It is, perfectly, possible, that the actual towing capacity could, in fact, be more. The only thing that is legally enforceable, in UK law, is the Gross Train Weight. Provided this is not exceeded, you will be towing within the law. Hence, in the example above, if the car were only loaded to 2,305kg (under loaded by 200kg) then the trailer could be loaded to 1,895kg, and still remain within the required Gross Train Weight.

Also worth pointing out, at this stage, that, whilst driving licence restrictions** (where applicable) work on plated maximum weights, vehicle towing limits are based on actual weights, so, in this example, provided the actual weight of car plus trailer does not exceed 4,200kg, it will be legal.

Conclusion

There is absolutely no doubt we should, always, take whatever steps we can, in order to ensure we are driving / towing both safely and legally, and driving with an appropriate weight ratio is an important part of this, of that, there is no doubt. The question, here, is what, if any, place the 85% guideline has in this equation, in the modern world.

Clearly, there are far more important factors to consider, when towing, including, but not restricted to;

  • Correct distribution of load and nose weight
  • Correct tyre pressures
  • Use of a stabiliser hitch, such as an AL-KO or Winterhoff hitch
  • Adherence to the legal speed limit, when towing
  • Appropriate driving style, avoiding heavy braking, accelerating and cornering
  • Use of ATC, if fitted

All of these will help to minimise the risk of the kind of snaking the 85% guide was introduced to help us control. (A case of prevention being better than cure).

The important thing is to apply common sense (something the guide fails to do, as it doesn’t address actual weights of car and caravan, only kerb weight of car and gross weight of caravan).

Rather than blindly following an outdated, and, largely, illogical, guide, common sense dictates we look at what both car and trailer, actually, weigh and work on that ratio.

Obviously, the heavier the car, relative to the trailer, the better, but let’s keep things realistic, when working out what we can, and cannot tow.

*    Since writing this, the NTTA have responded, as follows, although, as of August 2017, they have, yet, to update their web site.

“Good afternoon Mr. Young

Thank you for taking the trouble to point out the inconsistent answer on our FAQs page.

I am inclined to agree with you, especially as the confusion relating to kerb weights was withdrawn by DVLA earlier this year. There was a kerb weight restriction that applied to those drivers who passed their driving test after 01/01/1997 but confusingly, this did not apply to those who gained their licence on or after 19th January 2013 as the EU Third Driving licence Directive 2006/126/EC was implemented.

DVLA agreed to make this the basis for all licences so things are now more simple.
I would suggest that we change the wording on the FAQ Answer to:

The 85% figure is a recommendation, not a legal limit, given by caravan clubs to give good power to weight ratio for successful towing. The recommendation is aimed especially at those inexperienced in towing.

You may legally tow up to the car manufacturer’s towing limit as ascertained from the vehicle’s type or chassis plate (see the first question, above).

Does this make more sense? We will be reviewing our advice items as we revise our website in the course of the next few months. I expect that we will need to change a number of points that are a little out of date.

Thanks again for your input.

Kind regards,

Larry Lambert

Company Secretary
National Trailer and Towing Association Ltd”

**  For full details on licence restrictions, and all other key aspects of towing legislation, check out our earlier article; The Layman’s Guide To Towing In The UK.

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About Alan Young

MD and owner of the Woodhurst Group, including Praxis Accountancy Limited and Blue Sky Recreation Limited. Also Commercial Director of The Sky visor Group
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6 Responses to 85% Of Kerb Weight. Law, Guide Or Myth?

  1. Trevor Marron says:

    “Also worth pointing out, at this stage, that, whilst driving licence restrictions* (where applicable) work on plated maximum weights,”

    Which is where people also often fall foul of the law, As far as the law is concerned for licencing purposes an empty trailer is the same as a trailer loaded to its maximum permissible mass. So for example if someone who does not have a category E entitlement on their licence tows an empty braked trailer behind a big 4 x 4 or a transit sized van would be immediately over the maximum train weight of 3500Kg that their licence allows.

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    • Alan Young says:

      Perfectly correct, Trevor. That point is covered, in a lot more detail in the article linked to, in this one; ‘The Layman’s Guide To Towing In The UK’, which is why it wasn’t mentioned here, but it is a very pertinent point, and one which catches people out, on a daily basis.

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  2. Steamdrivenandy says:

    You don’t cover the point that one has to assume that when the 85% Guide was drawn up, whoever did it was knowledgeable enough to realise that an 85% kerbweight/MTPLM ratio would mean that in real life with a loaded car and loaded van the ratio would be nearer 70% or so and a 100% Guide would actually produce a ratio of about 85%.

    And whilst vehicle technology has moved on, there the roads are much more crowded, vehicles travel at generally higher speeds, the laws of physics and man’s reaction times and capabilities are still the same.

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    • Alan Young says:

      Thanks for the comment, however, I’m afraid you’re points are, somewhat, misguided.

      I didn’t cover the fact that the person, or persons who originated the guide were sufficiently knowledgeable to calculate the effective ratios correctly, as I don’t believe they were calculated correctly, even at the time. Whilst I have no doubt, the calculations were the best that were available at that time, that is, unfortunately, all they were. We have to bear in mind that, way back then, there were no alternatives, whatsoever. Manufacturers did not specify a braked towing limit, so the 85% guide was an attempt to apply a ‘one size fits all’ guide for those new to towing. The alternative to this guide was, well, nothing, and, unfortunately, that is about all we can say about the guide; “It’s better than nothing”. As my original article clearly demonstrates, one size most definitely does not fit all. I suspect that there is a much broader spectrum of vehicle specifications around today, than there once was, which is another reason the guide (whether we agree on its initial validity or not) is now completely out of touch with the modern towing scenario.

      Can’t say I’m with you on the second point, either, I’m afraid. Vehicle speed limits have not changed significantly since 1966, and, of course, the towing limit remains at 60mph. I very much doubt we were, as a nation, towing much below that, even in the 60s and 70s.

      Traffic volumes are, indeed, significantly, up on earlier decades, but this will have very little impact on the relevance of the 85% guide. The primary purpose of the guide was to minimise the risk of snaking. It has no bearing, whatsoever, on the vehicles ability to pull away, from a standing start, or its ability to stop, only its stability at speed. Whilst there may well be more large lorries around on the motorways, these days, that is probably the only factor likely to have any bearing, whatsoever, on said stability.

      I’m sure you will have seen at least some of the huge selection of videos online, demonstrating the potential causes of snaking. This is just one of those, chosen because it demonstrates the situation, perfectly, in just 26 seconds.

      Whilst I appreciate this is somewhat of an over simplification, and there are much more detailed studies available, you will note how the trailer goes from highly stable to dangerously snaking, out of control, simply by re distributing the weight. The key factor here is that there is NO increase in weight of the trailer, whatsoever, only a redistribution of the weight, which adds further doubt to the over simplified guide.

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      • Alan Murphy says:

        So can you help me clear this point up.
        It says if you passed your test after 1997 you can only tow a caravan up to 750kg but most caravans weight more than that so it all seems very confusing to me as a newbie caravaneer and auto license holder.

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      • Alan Young says:

        You appear to be misreading that, fortunately, Alan. It doesn’t, actually, say that, at all. It says you can tow a trailer weighing 750kg, with a vehicle of up to 3,500kg (so, a combined gross weight of 4,250kg). It also states (which I suspect is the part you have missed) that you can tow a much heavier trailer, provided the combined gross weight (vehicle plus trailer) doesn’t exceed 3,500kg. So, by way of example, from a purely licence perspective, it is perfectly legal to tow a 1,500kg max trailer, with a 2,000kg max vehicle, on a post 1997 licence. In fact, again, from a purely licence perspective, it is perfectly legal to tow an 1,800kg trailer, with a 1,700kg vehicle, also (as long as the the trailer is within the maximum braked capacity of the towing vehicle). Not recommended that you tow a caravan that is much heavier than the towing vehicle, of course, but perfectly legal, from a licence stand point.

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