Sensitivity to load distribution

[David_from_Ashby]

I just thought I would share a finding from my last towing trip.

I have a 2001 Compass Corona 524 (MTPLM 1300kg), towed with a 57plate CR-V diesel. Whenever we went away on a trip, the van behaved impeccably, with no hint of unruly behaviour. Even passing trucks and coaches failed to upset it at all. We have the noseweight set at around 85kg, and everything is loaded sensibly inside, with most of the heavy stff going in the car.

Coming back, the same story , noseweight set to around 85kg, most of the kit in the same place as before, apart from a bag of dirty clothes which tends to live under the sink in the end washroom. It does not weigh a huge amount, but is usually a good black bin liner full, weighing perhaps 15kg (at a guess).

However, usually on a return trip, the van has always been somewhat more fussey, wagging its tail a bit more than is comfortable on the motorway. I have always "lived with this in the past, but on the last trip my son needed a loo stop, so I took the opportunity of moving the bag of clothes from under the rear sink to just in front of the axle, to see if it made any difference. What a difference it made to the stability. You would not think that 15kg's of load sat at the back of the (otherwise well loaded) van would affect stability so much.

That is a lesson well learned.

Thought it was worth sharing with everyone.

Cheers

David

 

[Dustydog]

Hi David

Remember your physics lessons at school and the laws of moments?

Like the see saw 15kgs at the very back of the caravan are using all the floor length rear of the centre wheels as a "crowbar" to accentutae that weight.

A very good point because our dirty washing tends to stay at the back.

I always empty all the water tanks too although sometimes not the toilet flush as opposed to the the toilet waste. Food for thought.

Cheers

Alan

 

[David_from_Ashby]

Hi Alan

I remember physics very well, this is my specialist area of expertise (multi body dynamics simulation). However, we are taking about 15 kg's of mass about 2.2 meters behind the axle being sufficient to significantly reduce the stabilty of an outfit whose combined weight is over 3200kg!

I always drain down the water tanks, but very rarely (if ever) the toilet flush tank, because it usually tries to flood the floor of the van when I take the drain bung out. I now take the approach of keeping the flush tank fairly near empty, and top it up with enough to keep it going a couple of days.

It makes me wonder how the likes of Fiamma can get away with selling bike racks designed to go on the back of a caravan. I dread to think what it must do for stability with a bike rack (say 5 kg) plus a coouple of mountain bikes (10kg each) slung behind the van. I'll keep mine on the car roof I think.

Cheers

David

 

[Lutz]

It's not only the noseweight which has a big influence but also the load distribtuion from side to side. A couple of weeks ago I read a test about a caravan (I can't remember which one) which had a difference of almost 100kg from side to side. The testers commented on it being much more susceptible to 'twitchiness' than comparable models which were better balanced. Perhaps such data should also be included in PC tests.

 

[ChattyCathie]

We have side/rear washroom and fill flush water to approx 3/4 full before we go away. Should we not be towing with this amount of weight?

Also our staorage space under the front seating is limited as the pump/battery are all under there, so we put bedding under the seating just to the rear of the axle. Under our rear seating storage is just 2 telescopic fold out chairs and rubber mallet, is this too much? it is the only storage erea left

Appreciate your comments

 

[David_from_Ashby]

Hi ChattyCathie

It is impossible to say whether your example is good or not. Only you can answer this. Does it feel stable on the motorway? Does the van move around when given a disturbance (passing lorry, cross wind etc)? If not, then you are most likely OK.

I just really wanted to pass on my findings, as it surprised me how little weight it needed in the back to have a noticable affect on stability.

It is kind of incidental to the stability issue I encountered, but I just bought a roof box for the car, and plan to put the bulky but not too heavy stuff in there (Awning, folding chairs/table etc), and the remainin heavy stuff will then go in the car boot. That will only leave the aquaroll/wastehog to go in the caravan, plus a bag of clothes/bedding under the front seats. This means the caravan is immediately usable when arriving on site without having to unload everything else. Ideal for a series of short stops on tour.

Cheers

David

 

[Lutz]

Not only is a roof box beneficial in improving the actual weight ratio, but it also has an aerodynamic advantage, too. By shielding more of the projected frontal area of the caravan it ensures that the dynamic noseweight does not reduce as much with increasing speed.

 

[ColinA]

David, how do you secure your aquaroll/wastehog in the caravan to stop them bouncing about and damaging things.

Colin.

 

[David_from_Ashby]

Hi Colin

So far we just sit them on the floor, aquaroll at the front, then the wasthog behind, and then (so far) the awning poles other assorted bags of "stuff"! We have not made any attempt to secure it and have had no problems with the load moving around at all.

With the previous car (Renault Grand Scenic) I had to carry the spare wheel in the van too, as it pushed the noseweight too high iof left in the front locker. That then sat on the floor above the axle. I use one of the plastic carpet protectors on the carpet up the centre of the van.

Occasionally one of the side dinnette cushions falls over in transit, but never the load.

Cheers

David

 

[Garfield]

Perhaps adding to the single vs twin axle debate, in our Lexon 640 t/a we have to put the spare wheel under the side bunks behind the axle and stow the awning and poles in the rear bathroom, well behind the axle to get us down to 75kg noseweight. For some, a sure recipe for instability, especially as the missus had stowed a load of heavy items in the top lockers but the outfit rolls along without a twitch up to and beyond legal limits.

I'm a little unconvinced with the practice of loading the car up to improve the weight ratio as an overloaded car will handle like a pig without a caravan on the back.

I believe that cars and caravans have their own natural snaking frequency like a metronome and if the two are the same frequency you get harmonics and a serious snake. Just like adding /removing weight on a pendulum, by moving weight from trailer to car or vice versa or redistributing the weight, you can change the natural snaking frequency of car and trailer

 

[Lutz]

Of course, transferring weight from the caravan to the car has its limits. For a start you mustn't exceed the car's maximum permissible GVW.

Although you have a point concerning the natural frequencies of car and caravan, one cannot assume that if both are of the same weight, then both will have the same natural frequency. The car has its wheels more or less at the four corners whereas the caravan has its in the middle. The caravan is also a big box compared to the car and it probably has a higher centre of gravity. The suspension systems of the two vehicles are specific, too. The polar moments of inertia of car and caravan, which are a major determining factor, will always be different. Not only weight but also relative size also plays an important r

 

[David_from_Ashby]

Hi Lutz

I agree about the complexity needing some serious modelling. That is my thing and if I ever get a quite spell, I may try some experiments and see which factors have the greatest affect on stability. Watch this space.....but don't hold your breath

David

 

[ColinA]

Hi Colin

So far we just sit them on the floor, aquaroll at the front, then the wasthog behind, and then (so far) the awning poles other assorted bags of "stuff"! We have not made any attempt to secure it and have had no problems with the load moving around at all.

With the previous car (Renault Grand Scenic) I had to carry the spare wheel in the van too, as it pushed the noseweight too high iof left in the front locker. That then sat on the floor above the axle. I use one of the plastic carpet protectors on the carpet up the centre of the van.

Occasionally one of the side dinnette cushions falls over in transit, but never the load.

Cheers

David

Thanks david I was allways worried it would get thrown around

Colin

 

[Garfield]

I'll leave the mathematic modelling to someone else but if the outfit is unstable for no apparent reason then swapping some weight about should take a tad less time, if the theory has any substance then even moving weight from the car to the caravan could improve an unstable outfit.

A good set of sports dampers on the car to kill the sway in the first place would be better.

Has PC mag ever conducted a test where an outfit that is naturaly or deliberately unstable has various mods done to see the effect on stability ie

Fit larger wheels with low profile tyres

Fit stronger / sports dampers to the car

Fit dampers to the caravan

Fit low profile or wider tyres to the caravan

Compare stabilisers

Develop a definitive and quantifiable measure of stability that rules out human opinion ( we all know there are cars that have their supporters rather like the wife who supports the husband that beats her senseless)

 

[Steve L]

HI David.

I can certainly share your experience with weight

We have a twenty foot twin axle van which we tow behind a 1998 Laguna estate 2 Ltr.

The nose weight has to be set at close to the 75Kg limit otherwise we get a lot of yawing (up and down motion of the hitch).

The use of a roof box for aero dynamics will help a little and of coarse offer extra storage, however you will notice an increase in fuel consumption.

On the other hand, I use a 1970's wind jammer aerofoil on my laguna which does quite the opposite. I get 5mpg better fuel consumption using it with the caravan in tow, and can get 7mph better mean towing speed. That is, the speed at which the outfit seems most happy at and further depression of the accelerator has little affect.

Another benefit is that the airflow is not only deflected up over the caravan, but also deflected around the sides, thus creating even airflow which seems to help stabilise the outfit.

I did have to modify the deflector to fit my roof rail cross bars, but that is within the scope of any competent DIYer.

I found my deflector at a car boot sale for the princely sum of

 

[DevonshireDave]

HI David.

I can certainly share your experience with weight

We have a twenty foot twin axle van which we tow behind a 1998 Laguna estate 2 Ltr.

The nose weight has to be set at close to the 75Kg limit otherwise we get a lot of yawing (up and down motion of the hitch).

The use of a roof box for aero dynamics will help a little and of coarse offer extra storage, however you will notice an increase in fuel consumption.

On the other hand, I use a 1970's wind jammer aerofoil on my laguna which does quite the opposite. I get 5mpg better fuel consumption using it with the caravan in tow, and can get 7mph better mean towing speed. That is, the speed at which the outfit seems most happy at and further depression of the accelerator has little affect.

Another benefit is that the airflow is not only deflected up over the caravan, but also deflected around the sides, thus creating even airflow which seems to help stabilise the outfit.

I did have to modify the deflector to fit my roof rail cross bars, but that is within the scope of any competent DIYer.

I found my deflector at a car boot sale for the princely sum of