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Topic: Is the 85% rule AS relevant these days
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15/3/2013 at 5:36pm
Location: None Entered Outfit: Coachman Pastiche
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In theory, a heavy caravan towed by a light car is inherently unstable. The 85% guideline is a guideline for inexperienced towers.
Technology cannot be allowed to become a substitute for stability though. An inherently unstable that is covered up with a stabiliser and electronic aids remains an unstable outfit. There will become a point where the technology is defeated by physics and this is liable to happen very quickly.
Having said that, I always used to tow well under the 85% figure, at one point around 60%. Having then changed my caravan for a heavier one which had a wider axle, I found that despite coming up in excess of 70% the outfit felt more stable.
Having since changed the car the car for a lighter one, we are now up to 92% which was initially a concern. However, the lighter car is just as stable towing if not slightly better than the previous one was. I can only assume this is down to the lower centre of gravity and the tow ball being closer to the axle on a hatchback than it was on a 4x4.
My conclusion: While the weight ratio is important to stability, it is not the be all and end all. There are many other factors that will have an effect.
With all the best intentions of loading and checking the caravan's nose weight, how many actually consider the effect the weight in the car will have on the car's stability? How can an unstable car possibly become stable by hitching a caravan on the back?
For all the hype that the 85% guideline gets, along with correct caravan loading and nose weight, the weight distribution in the car gets very little attention. Large amounts of weight in the boot is effectively adding to nose weight although it is rarely taken into the equation.
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18/3/2013 at 7:23pm
Location: Rushden Northants Outfit: Mercedes 614D Conversion
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When I worked for a Scania distributor in the mid 1970's, the trucks for the UK market were fitted with 3-line braking for the trailer:
Red for compressed air
Blue for the service brake
Yellow for the separate control on the steering column which applied the trailer brakes independently from the footbrake.
LHD chassis that came in were converted to 3-line by me, so that operators based in the UK could run with either UK or continental trailers, the continentals used 2-line brakes. We worked out a better scheme than that originally fitted for the UK, using standard Wabco parts.
I still have my 1971 Scania Training School notes and some manuals from those days.
As far as the 85% rule goes, many Land Rovers are legal at more than 100%, 2.2tonnes for a Discovery and 3.5tonnes for a fully laden trailer. That's about 160%.
There's another issue, a centre-axle trailer or caravan has a big effect on the attitude and stability of the towing vehicle, and in extremis can exert sufficient leverage through the towbar to lift the back of the car off the ground.
We tow at 3.5 tonnes all the time as our display engine and caravan trailer is that much when loaded, but it is a turntable drawbar trailer and apart from pushpull forces, it doesn't place any weight on the towing vehicle other than the drawbar and coupling weight.
Peter
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