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85% Towing Advice


Towtug
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This may be a contentious issue, but Where did the magic 85% weight advice originate from?

Was it derived empirically, is it supported by data. I've asked the NCC because they use it and they didn't know.

It doesn't appear to be in an regulatory info either.

Manufacturers of towing vehicles spend alot on research to determine what the vehicle capabilities are and equally the government decides what we can tow licence wise.

So where does it come from and is it useful?

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Don't know the origins exactly, but it's to do with physics. You don't want to pull anything that can be the boss of the thing doing the pulling. If your van was heavier than your car, it would be in control in any severe situation, not the car. I guess the 85% has been worked out as the percentage weight difference whereby the tug has enough additional weight over the van as to remain in charge in most situations.

 

It is handy yes, although with experience people tow at higher ratios, it's always a good rule of thumb, especially for novice tuggers.

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But that doesn't hold true for Semi trailers or does it, obviously the imposed load on the tow vehicle is higher. And alot of Multi purpose vehicles have towing masses far in excess of the MRO.

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It seems to have come out of the NCCs desire to provide some guidance on the issue. The manufacturers and clubs all seemed to agree that 85% was a sensible figure to work around. I don't think there's a mathematical formula to support it, it's more tried and trusted experience.
If you think about it saying a caravan weighing 85% of a car when almost empty is OK means that when the car's normally loaded that % is much lower. So someone towing at 100% kerbweight is probably towing at 85% when loaded and if they tow at 85% kerbweight they're actually pulling at 70% with a load on board.

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I don't think the op is questioning the wisdom. ..just asking from where the figure was derived. Which is a fair enough.

 

If the NCC don't know then I would suggest its most likely been derived empirically over years. I don't recall(maybe wrong) it being discussed back in the 70s, and the Europeans don't know about it . ..so we can't blame the EU.

 

Having recently switched from a 96% setup to a 70% setup, there is no doubting which is best. I could quite imagine the 85% being arrived at by no other method than available cars and caravans weights at the time and a concensus.

 

Maybe nothing more than a Pareto 80/20 assessment?

Edited by ericfield
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I heard somewhere and I've no evidence of it and can't find any reference to it being wrote down,is that a man in a grey suit somewhere made the recommendation that the tow vehicle should be 10% heavier than the trailer it's pulling plus a 5% safety margin. Sounds login to me I suppose, I think it's a good guideline for people trying to match their car to a caravan or trailer.

Edited by F185

Compass Corona (Magnum) 524 (2003) -----------VW Passat Alltrack 170 BHP DSG

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It seems to be a totally arbitrary figure. It could just as well have been 90% or 80% (which was actually the legal limit in Germany for a time if one wanted to tow at 100km/h until it was raised to 100% because of lack of supportive data showing that accidents were any more likely at 100% than 80%).

 

I would challenge anyone to notice any difference in the way his outfit handles at 80% or at 90% compared with 85%, all other criteria remaining unchanged, although there is no denying that once one exceeds 100% things may start to become a bit dicey under adverse conditions. However, that only applies to caravans due to their big, boxy shape. One can happily tow other types of trailer over 100% if the limits specified by the manufacturer allow.

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I know the answer, I have consulted the google and the person who first said that the 85% rule applied was The Expert.

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I know the answer, I have consulted the google and the person who first said that the 85% rule applied was The Expert.

 

Which one? There are an awful lot of them around who call themselves that and chances are, that many won't agree among each other.

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I know the answer, I have consulted the google and the person who first said that the 85% rule applied was The Expert.

I know his wife - The Supreme Expert :)

Graham

Unless otherwise stated all posts are my personal opinion 

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I guess it is similar to the 7% for working out the nose weight and this goes right out the window with many caravans as the maximum weight on the hitch is limited to 100kg.

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It seems to be a totally arbitrary figure. It could just as well have been 90% or 80% (which was actually the legal limit in Germany for a time if one wanted to tow at 100km/h until it was raised to 100% because of lack of supportive data showing that accidents were any more likely at 100% than 80%).

 

I would challenge anyone to notice any difference in the way his outfit handles at 80% or at 90% compared with 85%, all other criteria remaining unchanged, although there is no denying that once one exceeds 100% things may start to become a bit dicey under adverse conditions. However, that only applies to caravans due to their big, boxy shape. One can happily tow other types of trailer over 100% if the limits specified by the manufacturer allow.

I agree with all of this and my own testing with a client sort of bears this out.

The reason for asking is that we have been experimenting with different axle configurations and wheelbases to get away from the traditional centre axle types. For example the American configuration placing the axles further back is quite good as it reduces the pendulum effect when towing and seems improve stability although the trade off is a higher nose weight.

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The Caravan Club has used various guideline percentages since the '50s, mostly varying as the method of "kerbweight" definition has changed but not changing the actual towing ratio using actual laden weights, originally based simply on collective wisdom from experienced caravanners.

 

The NCC, the caravan industry trade body, incorporated that guideline into their "Towing Code" - at one time it was worded as 85% for beginners and 100% for experienced caravanners - which introduces another can of worms, just what constitutes an "experienced caravanner" so later editions dropped the 100%

 

The problem with public forum pronouncements is that a towing ratio that's fine for an experience driver, with a lot of towing experience and with good mechanical empathy as well as an OCD attitude to loading just isn't fine for drivers at the opposite end of the spectrum.

 

Personally, I think 85% is a good starting point - once you've got experience, go upwards if you wish - but don't claim that 100% is ok because someone on an interweb forum said so!

2015 VW Touareg 3. 0 V6 TDI + 2013 Lunar Clubman ES

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For example the American configuration placing the axles further back is quite good as it reduces the pendulum effect when towing and seems improve stability although the trade off is a higher nose weight.

Presumably if you took that to the limit, and placed the axle right to the rear of the trailer / caravan (providing the towing vehicle could stand the strain) then there would be no pendulum action at all and you would have perfect stability at a much higher weight ratio. If you could then move the position of the hitch forward of the towing vehicle's rear axle then stability could be further improved. . . but wait . . . that's exactly what an articulated truck does, and explains their stability with a trailer that's much heavier than the towing unit! The only time there is an issue then is when the tyres loose traction on the road.

 

Anyone remember the Trapezium, and Geomatic hitches that both through a mechanical linkage moved the effectively trailer pivot point forward of the towing vehicle's rear axle?

 

I have safely towed trailers well in excess of this figure, while respecting the mechanical limits of both towing vehicle and trailer, but equally other trailers have been much lower in weight. Obviously the lower the ratio, the more 'control' the towing vehicle has over the trailer, should a sudden change of direction or speed be required.

 

As for the original question . . . I don't believe there is any magic mathematical formula for the 85% recommendation.

Gordon.

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I was led believe that the 85% advice came about in the 60's and 70's when the capability of caravans and cars was far less of the vehicles of today especially engine, brakes and suspension. Towing today is a little more complex as some cars with stability control towing a van with stabilty control can approach the 100% ratio with reasonable safety whereas other combinations may not be stable above 85%. This is probably why the 85% advice is still quoted.

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I was led believe that the 85% advice came about in the 60's and 70's when the capability of caravans and cars was far less of the vehicles of today especially engine, brakes and suspension. Towing today is a little more complex as some cars with stability control towing a van with stabilty control can approach the 100% ratio with reasonable safety whereas other combinations may not be stable above 85%. This is probably why the 85% advice is still quoted.

Now this gets to the crux of the matter, you see the perception is, and you have stated it, as do many trade organisations that you can approach the 100% (1:1 ) ratio with reasonable safety.

One of my clients uses a HILux It has a kerbweight of approx 2000kg and a towing mass of 2800 kg. He tows large tandem axle trailer to its capacity. With the Hilux at MRO the train weight is approx 4800 kg (a ratio of 1. 4:1) but it could be a 700kg heavier.

It tows easily and is stable and it is by no means isolated as a majority of dual purpose and light commercials are just as capable.

So what makes a caravan inherently less safe, is their a technical reason, is it a confidence or experience thing.

We have sort of agreed it is an arbitrary figure with its origins lost in the mists of time, but I struggle to see with the advances in modern vehicles ABS, EBS, ESC etc why caravans and the "industry" seem to want to treat themselves differently

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Such things aren't limited to caravans though. The speed limit for HGVs has been circa 56mph for decades, whilst the power of their engines, their efficiency, their braking and control systems have all improved immensely in that time, but nobody mentions upping their speed limit.

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Such things aren't limited to caravans though. The speed limit for HGVs has been circa 56mph for decades, whilst the power of their engines, their efficiency, their braking and control systems have all improved immensely in that time, but nobody mentions upping their speed limit.

This is governed by law though (although there is currently an industry wide consultation underway regarding an increase)

 

My point is the caravan industry imposes this on itself but doesn't appear to know why.

 

I raised this because I listened to an "industry expert at one of the testing centres" vociferously defending this last week but when questioned he couldn't explain why.

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Such things aren't limited to caravans though. The speed limit for HGVs has been circa 56mph for decades, whilst the power of their engines, their efficiency, their braking and control systems have all improved immensely in that time, but nobody mentions upping their speed limit.

 

But and a big BUT is with all this modern technology in cars/trucks etc its still Human error that causes most accidents and one reason why i object to lifting speed limit .

 

Ste

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I held the same view until I witnessed some wet testing a couple of years ago of a system similar to this.

 

Volvo Trucks - Emergency braking at its best!:

 

The pressure to increase speed goes hand in hand with technology driving safety. Just this week the government announced they would debate allowing autonomous car trials.

 

You never know in a couple of years you could be sending your caravan off on its holidays on its own, and catching the train down to meet it.

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Steamdrivenandy

 

This is the consultation on speeds if you wanted to take part

 

https://www. gov. uk/government/consultations/increased-speed-limit-for-heavy-goods-vehicles-over-75-tonnes-dual-carriageway

Edited by Towtug
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So long as not all outfits on the road are fitted with technical features such as electronic stabilisers raising the speed limit would not be justified (unless one has a scheme of identifying those which do by means of speed stickers on the back of the caravan, as is the case in Germany). However, even there, the stickers only allow 100km/h instead of the 80km/h that otherwise apply.

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