85% Rule

[Chrisbee]

Just how relevant is the so called 85% rule? I've read all the ins & outs about weight distribution etc and understand the importance of correct loading but I am never going to tow my van at m.t.p.l.m with my car at kerbweight. As I carry my awning and the majority of kit in the car the car will be near to but not exceeding it's maximum weight but the caravan will be well shy of it's max weight. Are any % figures available for suggested safe towing in this configuration? And yes, I am within my train weight.

Chrisbee

Chrisbee

 

[Lutz]

I would say the so-called 85% rule is a good starting point although it is entirely arbitrary. There's no good reason why it shouldn't be an 80% or 90% rule. There is no data to suggest that things suddenly go wrong if you exceed 85%. It's more a sliding scale of safety margin and that, in turn, is also dependent on what type of trailer you are towing. However, caravans are by far and away the least favourable type of trailer. Therefore, by the time you reach 100% with a caravan the risk is certainly no longer negligible (always assuming that you are still within the manufacturer's towing limit).

As you correctly infer, the weight ratio is based on a fully laden caravan being towed by a next-to-empty car. If you are always going to be far away from that condition you can certainly afford to put up with a higher weight ratio than 85% but I'd always stay below 100%.

 

[Meister]

I really think that it would help a lot if we all stopped calling it the 85% RULE.

Lets call it what it is, the 85% GUIDELINE then perhaps newcomers wont be so confused as I was when I started caravanning last year.

Steve

 

[RogerL]

Steve's absolutely right - the more people call it an 85% rule, the harder it is to convey to people that it's just an 85% GUIDELINE - it's very good advice, based on experience, for beginners to caravanning.

In the same way, there's no such thing as a 100% limit - although new drivers ('97-on) who haven't passed the E towing test and drivers in Germany using Tempo 100 do have a 100% limit among their restrictions.

 

[Lutz]

You're right, of course, about there being no 100% rule as such except for conditions that you mention, but I would say that the safety margin does start to get rather small when you go much over 100% with a caravan. Don't forget that weather conditions, loading, noseweight, tyre pressures, etc. can have a cumulating negative effect. I certainly would be rather wary about towing at over 100% in gale force winds.

 

[GrahamSouthBrum]

Lutz said

"Don't forget that weather conditions, loading, noseweight, tyre pressures, etc. can have a cumulating negative effect. I certainly would be rather wary about towing at over 100% in gale force winds."

Lutz,

I would be worried about towing anything in gale force winds.

I was driving a rigid lorry with curtain sides the other week during the gales. I kept noticing that the speed kept on going up & down even though I was doing the same speed.

When I stopped at the next service area another driver confirmed my fears that one of the back wheels was lifting off the ground when the lorry was being hit by the gusts of wind.

If it can do that to a partly loaded lorry imagine what it would do to a caravan if loaded at 50% or even 100% of the towing vehicles weight.

 

[Lutz]

Lutz said

"Don't forget that weather conditions, loading, noseweight, tyre pressures, etc. can have a cumulating negative effect. I certainly would be rather wary about towing at over 100% in gale force winds."

Lutz,

I would be worried about towing anything in gale force winds.

I was driving a rigid lorry with curtain sides the other week during the gales. I kept noticing that the speed kept on going up & down even though I was doing the same speed.

When I stopped at the next service area another driver confirmed my fears that one of the back wheels was lifting off the ground when the lorry was being hit by the gusts of wind.

If it can do that to a partly loaded lorry imagine what it would do to a caravan if loaded at 50% or even 100% of the towing vehicles weight.

Of course, I was exaggerating a bit to get the point home.

 

[Steve L]

First of all, the 85% figure is just a guide which according to so called club experts should give you a safe match between tow car and caravan, to alledgedly give you trouble free towing.

However it is not LAW!!

What is law is, the figures quoted by your car manufacturer on the VIN plate attatched to the vehicle and the weight plate attatched to your caravan by it's manufacturer. Another legal point is the tow bar down force limit specified by the manufacturer, and the nose weight specified by the caravan manufacturer.

In a "nut shell", if your caravan does not exceed the maximum tow capacity of the car and the combined weight of car, caravan, all passengers (including the driver), plus luggage, does not exceed the "Gross Train Weight" which is the highest figure quoted on the VIN plate of the car. Then the nose weight of the caravan (hitch weight)is below the lowest figure quoted for car, tow bar or caravan.

Compliance with the above renders your outfit road legal.

The biggest single factor of stability of any towing outfit is the loading of the caravan and getting the nose weight within the limit.

While putting a lot of luggage weight in the back of the car might boost the gross weight of the car, it does lighten the pressure of the steering (front) wheels on the road, which in extreme circumstances could lead to loss of control of the vehicle.

We normally carry two dogs in the boot of our estate car when towing and so end up loading the caravan with awnings etc anyway.

Towing above the 85% gide is not traumatic, and so long as you pay attention to your driving, you will be perfectly fine.

Over the 20+ years I've towed I can hoestly say that those drivers I see having trouble usually have got the loading and nose weights completely wrong, or they are rushing to get to site which means they take stupid risks assuming that the car is going to sort it's self out.

As with any driving these days, there are drivers and ther are "drivers"!!

Patience and courtesy have been thrown out the window these days.

Steve L.

 

[Dustydog]

Hi Steve

The sad fact is that the idiots who get it wrong do not subscribe to this forum and never read any of the caravan press.I know of one in our family.

My rig runs at 72% and I have no worries. Terri tows as well and she says most of the time she could easily forget the caravan was on the back. Three years ago we were towing on 90% and apart from the white man van slip stream were very comfortable.

Anything is possible but in fairness the public organisations have to be seen to be careful. Also I think the CC did lots of research with the Uni of Bath and it was that body that "suggested - recommended" the 85% GUIDELINE.

And yes there are loads of us who can tow at 150% , but so what. Be safe and enjoy your hobby.

Cheers

Alan

 

[Lutz]

The 85% figure was published long before the University of Bath ever got involved in investigating caravan safety. No real research was ever done in establishing the 85% and it is completely arbitrary. The risk of instability based on weight ratio is on a sliding scale with a very big grey area and so many other variables, so there is no logical reason why the figure should not have been 80% or 90%. Besides, as it only represents a worst case scenario that, in the majority of cases, is never encountered in practice, it can hardly be described as very scientific.

The main conclusion that the University of Bath came up with, however, was that stability really starts to suffer at speeds over 50mph, even for a properly laden outfit. Based on their findings, there would be more a case of limiting speeds of caravans to 50mph than to stick to an 85% weight ratio.

 

[Dustydog]

Hi Lutz,

Did the 85% guideline come from the CC?

Speed is interesting. For years I had a 50mph sticker on the back of the caravan, it's maximum allowable speed.

The Government then increased it to 60mph. Now I assumed that as caravan technology and cars improved 60 was seen as safe.

I agree stability must decay as speed increases but in all honesty I have never felt any stability issue cruising at 55- 60mph where legal.

I have been overtaken a number of times by tuggers who I swear blind were doing 70mph. Stupid!

For my outfit, 55 is a relaxed cruise. The diesel is churning over at her optimim revs for bhp & torque, the turbo is spinning nicely and my fuel consumption doesn't drop too much.

Cheers

Alan

 

[Steve L]

Here here Alan. If only everyone took the relaxed approach to driving and drove sesibly like you and I do then there would undoubtedly be a lot fewer accidents.

I tow a 20ft twin axle van at 100% of my Renault Laguna estate's tow capacity.

Like you by keeping the speed to 55/60mph (where legal) and driving defencively. I can get a very relaxing tow, arrive only 20 mins behind mad Harry on a 4 hour drive, but save a tenner in fuel consumption.

So long as the van has no less than 60Kgs and no more than 75Kgs on the hich, it tows like a dream. Also by using a 1970's wind jammer aerofoil on the roof of the car I get 5mpg more, which get's me nearly 30mpg towing and 47mpg solo for my gallon of lead free liquid gold.

Thinking about how we drive and having that fantastic but rare ability of todays drivers, to anticipate what numpty is going to do next, is far more important than some mythical figure that does nothing but put false hope in a novice's heart.

Just look at the stupid things people do with sat navs. Before them we had to use our brains and read a map before starting the car.

Steve L.

 

[Lutz]

Fair comment, although the University of Bath's findings are based on scientific evidence that shows that there is a threshold speed below which anything that disturbs the equilibrium, such as a sudden crosswind or buffeting by a passing truck, is self damping, i.e. any sway will die down of its own accord. Above that speed, the forces that disturb the equilibrium are so large that the system is no longer capable of dampening the instability by itself. Unless there is some active interference to the system, such as slowing down below the threshold speed or applying the overrun brakes on the caravan, the situation will get out of control. The threshold speed is variable depending on the outfit including, but not exclusively, its weight ratio, and is normally somewhere around 50-55mph.

 

[Dustydog]

Lutz,

That's bery worrying if what you sayis true. I just don't understand why th espeed limit was upped to 60mph if the critical safety margin is eroded above 55 mph, mmm food for thought then?

Cheers

Alan

 

[Lutz]

I guess it was a compromise to allow caravans to travel at least at the same speed as lorries and therefore to improve the traffic flow. Besides, the decision was taken before the Univ. of Bath published their report. It is not without reason that the vast majority of European countries (except France and Belgium) have an 80km/h speed limit for caravans.

 

[Oldfogey]

I think you will find that all these figures, including those from Bath, assume a typical caravan matched with an appropriately sized car.

My setup is a bit non - standard, and I have never had any trouble with it, even in quite blustery winds on the Severn Bridge.

The van is a Sprite Compact pop-top, with a weight of 685kg, and all storage is at waist level or lower.

The tow car is a 1989 Range Rover, and all heavy gear (eg awning) is packed in the car with the rear seats folded, so that most weight is kept forward of the rear wheels.

The caravan, top lowered, is barely higher than the car, and the ratio is probably about 25%

A daft anomoly in the rules is that my wife, on her post '97 licence, can drive this rig, but if we got a bigger van, she would either have to take an E test, or, and this is the daft bit, WE WOULD HAVE TO GET A SMALLER CAR!

 

[Hedgehog1]

When towing my caravan I am within the 85% Guideline. Car kerb weight is around 1650kgs and MTPLM of van is around 1400kgs. I have no issues at all.

However, the max towing limit of my car is 2000kgs and I do attain this when towing my twin-axle Brian James trailer when loaded with my Subaru Impreza track car. Although I cant tell I am pulling a heavier load I have no issues with stability upto 60-65 MPH. Obviously the dynamics are differnt in so far as although the trailer/car combination is heavier I have a lower centre of gravity.

Its all relative!!

TONY

 

[ProfJohnL]

I think you will find that all these figures, including those from Bath, assume a typical caravan matched with an appropriately sized car.

My setup is a bit non - standard, and I have never had any trouble with it, even in quite blustery winds on the Severn Bridge.

The van is a Sprite Compact pop-top, with a weight of 685kg, and all storage is at waist level or lower.

The tow car is a 1989 Range Rover, and all heavy gear (eg awning) is packed in the car with the rear seats folded, so that most weight is kept forward of the rear wheels.

The caravan, top lowered, is barely higher than the car, and the ratio is probably about 25%

A daft anomoly in the rules is that my wife, on her post '97 licence, can drive this rig, but if we got a bigger van, she would either have to take an E test, or, and this is the daft bit, WE WOULD HAVE TO GET A SMALLER CAR!

Hi Oldfogey

I am sorry but I think you may have misunderstood the way the licence entitlements work, and I venture to suggest that your wife may not be legal to tow.

The B class only licence limits the driver to an outfit of less than 3500Kg Maximum Authorised Mass. This is not based on the kerbweight but on the sum of the MAM's for both the car and trailer.

Now, I do not have the figures for the Range Rover, but the kerbweights seem to be typically around the 2500Kg, and I am sure they are permitted to be loaded with more than 400Kg pay load. This is would give a MAM for the RR of 2900Kg Add to this the MTPLM of you caravan (645Kg?) that is more than 3500Kg

If so, you wife is not entitled to tow your outfit.

Please check you limits, and the wording of the licence entitlement.

 

[ProfJohnL]

When towing my caravan I am within the 85% Guideline. Car kerb weight is around 1650kgs and MTPLM of van is around 1400kgs. I have no issues at all.

However, the max towing limit of my car is 2000kgs and I do attain this when towing my twin-axle Brian James trailer when loaded with my Subaru Impreza track car. Although I cant tell I am pulling a heavier load I have no issues with stability upto 60-65 MPH. Obviously the dynamics are differnt in so far as although the trailer/car combination is heavier I have a lower centre of gravity.

Its all relative!!

TONY

Hello Hedgehog,

You are correct that the dynamics of a caravan are very different to a car on a trailer.

I am not sure if the CoG will actually be lower on your car trailer when it is loaded, but certainly the aerodynamics will be far more favourable than a caravan. The slab sides and fronatal area of a caravan make a wonderful sail, and it is hardly surprising that they can get blown about by side winds and bow waves etc.

 

[Oldfogey]

I think you will find that all these figures, including those from Bath, assume a typical caravan matched with an appropriately sized car.

My setup is a bit non - standard, and I have never had any trouble with it, even in quite blustery winds on the Severn Bridge.

The van is a Sprite Compact pop-top, with a weight of 685kg, and all storage is at waist level or lower.

The tow car is a 1989 Range Rover, and all heavy gear (eg awning) is packed in the car with the rear seats folded, so that most weight is kept forward of the rear wheels.

The caravan, top lowered, is barely higher than the car, and the ratio is probably about 25%

A daft anomoly in the rules is that my wife, on her post '97 licence, can drive this rig, but if we got a bigger van, she would either have to take an E test, or, and this is the daft bit, WE WOULD HAVE TO GET A SMALLER CAR!

Just dug out the handbook, and I have the following numbers;

Kerb Weight: 1723Kg

Maximum Vehicle Weight: 2504

Maximum Payload; 780

So -

Max Vehicle Weight 2504

Caravan Max weight 685

------------------------

Total Train Weight 3189.

Thus, as I understandf, legal on 'B' licence - but if the van was much bigger she could only tow with a smaller car!

 

[Oldfogey]

The question of towing a car on a trailer is interesting, as I may soon need to tow another Range Rover on a trailer behind mine.

RR Weighs 1723 Kerb weight

Max GTW with a braked 4 wheel trailer is 6504Kg

Assume the trailer weighs 1000Kg.

Train Weight would be GVW of Towcar 2504 + Trailer 1000 + 1723 on trailer.

Total 5227.

Quite a load - but within legal spec.

 

[Lutz]

Of course, the studies carried out by the Univ. of Bath only reflect the conditions of towing a big boxy trailer like a caravan. Other types of trailer such as car transporters are a completely different kettle of fish. That's why the 85% recommendation, for example, only applies to caravans.

 

[Stephen_374341239]

Hi there

85% is a Guideline.

What you need to keep in mind is you need the car to pull the caravan and to be able to stop. NOT the van pushing the car.

Safe towing.Or it will all end in tears.

Steve

 

[Chrisbee]

Surely braking power is taken into consideration by the manufacturer when they set the towing limits of the car?

Chrisbee.

 

[Lutz]

Good grief! I hope that the brakes on your car don't have to do the stopping for the caravan as well. The overrun brakes should be perfectly able to stop the caravan by itself. If all is set up correctly, a car towing will actually stop quicker than solo (because the caravan is transferring a downward load on the towball while braking and thus increasing the rear axle load on the car).