Caravan tows with nose up attitude

[otherclive]

Collected the caravan last week after 2+ years without one. It towed very well, as I would expect of a lightweight unit. However, it has a nose up attitude when coupled up to the car which isn't a problem towing but could lead to problems with speed humps or ferry ramps. The rear barely clears the drive as I take it on to the road such that jack pads may not be feasible. The Alko chassis is lower than on "standard" UK vans which doesn't help that much. I found a thread on UK Caravantalk going back a few years where an owner had the same van as mine which had a nose up attitude when towed with a VW Caravelle. He fitted the A frame with a spacer plate that effectively lifted the tow hitch/handbrake assembly which is all mounted on a single plate at the apex of the A-frame. What wasn't stated was if it was an official Alko modification. Has anyone heard of such modification to lightweight vans or camping trailers? It's in with my local caravan Alko service centre at the moment being given a health check, so I had asked them to investigate. No surprise thus far other than the Whale water heater element is shot.

The cars tow kit is OEM and non-adjustable. And "regrettably" the Subaru self levelling suspension works far too well and pushing another 70kpa of air pressure into the rear tyres exacerbates the problem. There is no clearance in the wheel box for larger radius wheels/tyres either so I have run out of the easy options.

 

[Grahamh]

surely it would be easier to use an adapter/plate to lower the Subaru towball?

 

[Damian]

Having searched through all my Al-Ko information I can find no mention ever of a modification such as described for the Al-Ko chasis and hitch setup.
I must assume therefore that it was a non approved modification done by an owner.

However, as has been suggested, if your vehicle manufacturer has authorised its use a drop plate would be the sensible option to lower the towball.

 

[ProfJohnL]

I would add that all caravan manufacturers are fully aware of the EU regulations concerning the height of the coupled hitch, and will make their caravan's tow with proper ground clearances with the coupled hitch height of between 350 and 420mm, ground to centre of towball.

All normal domestic cars sold in the EU should be compliant with these regulations. Which means if If your vehicle's tow ball when hitched is outside of those limits then it's the car or its tow hitch that is non compliant.

 

[JTQ]

IMO it is not a desirable feature to tow nose up in that it adds to the aerodynamic lift reducing the nose weight. Having an adequate dynamic nose weight is a paramount characteristic in terms of towing stability. If nothing else I would contrive to run at as high a static nose weight as the kit allows so the dynamic nose weight is also as high as possible.

 

[otherclive]

Thanks for the replies.
The towball centre height is within the specified band and it's an OEM detachable swan neck. I am contacting Witter to see if it could be fitted with a flange fixed towball as the new removable flange towball fixing isn't yet available for the Subaru. But it would still depend on whether a fixed (or removable) flange towball would have lower centre height compared to my existing one. When I see current model Forester with fixed towball (Witter) I will wip out my tape measure!! I also will be contacting Alko DE direct to seek their advice.

Agree totally with the comments regarding the towing attitude regarding stability and nose weight. The car can take 80kg and the caravan 75kg, so I will be loading it up to its maximum and making sure that my load is low down and as near centre as feasible. I normally carry as much heavy gear in the car anyway. As I said in my post it's mainly speed bumps I am concerned about. Another 25mm would alleviate that concern.

 

[ProfJohnL]

IMO it is not a desirable feature to tow nose up in that it adds to the aerodynamic lift reducing the nose weight. Having an adequate dynamic nose weight is a paramount characteristic in terms of towing stability. If nothing else I would contrive to run at as high a static nose weight as the kit allows so the dynamic nose weight is also as high as possible.

Hello JTQ,

It is the traditional view that caravans should be towed with a nose down altitude. This is another example of a tradition being blindly accepted without rational forethought.

I seriously doubt that the degree of variation of attitude permitted in the hitch height regulations will make any significant difference to the towing characteristics of most outfits at normal towing speeds.

You suggest a nose up attitude adds lift, I assume your proposal is based on the more open wedge it makes with the road surface, well yes that might try to lift the front of the caravan more but you must also consider the top of the caravan, where the rear also widens it's angle of release and like an aircraft wing will try to lift, but because both these forces are acting in opposition at either end of the caravan the net effect is much reduced.

If you also consider the front face of the caravan, most caravans have a raked front, so any impinging wind will be have two resultant forces, some will attempt to tip the top of the caravan back, but a proportion will be reflected up and over the caravan which will produce a countering downwards reaction. If the caravan is already tipped backwards as a result of a nose up attitude, then a greater proportion of the head on windage will be reflected upwards.

It's almost impossible to predict the scale and proportions of these effects because when being towed you have a vehicle in front of the caravan which produces some very unpredictable air flows behind it, which will have effects on the caravan.

 

[Raywood]

I tend to agree with the Prof on the nose up attitude not being that important but the fact the rear end is very low is a worry. I do not know the layout of your entrance so cannot be sure if this might be replicated on the road. If you do hit this problem at speed it seems to me that you might damage the rear end of the caravan so care is needed.
The problem with lowering the towball which might be the answer if it can be done is that whatever you use must be type approved. A DIY solution therefore is likely to be illegal but if Witter or another towball manufacturer can supply something to do this then you should be safe.

 

[otherclive]

When I collected the van from Portsmouth I took it along the A27 and A36 and whilst on the M27 I deliberately sat for a spell at 50mph to see what effect there was when being overtaken by HGV and coaches, and it was absolutely fine. On the M27 at 60mph (gps) it was steady too. Didn't get the chance to see what it was like going downhill on a motorway or A road at normal speeds but that will come when I take it over to Wales.

So its the lowish rear end that is my principal concern. The house drive is level as I enter the road but there is the normal downward slope associated with the lift off of the road. Before I turn to the left, or right, the car is at 90 degrees to the direction of the road and so is going up the road's camber as I make the turn to the right or left. The vans away for service at the moment so when I get it back I will check clearance going onto and off of the road. Then when the outfit is fully loaded for its "maiden" voyage via a Welsh weigh bridge in Brecon I will again check it. For the house drive I can always motor move the van onto the road and hitch up parallel to the pavement. We have some nice speed bumps in our town comprising the standard transverse humps and some of those nice truncated pyramids that wear out the insides of your tyres. I avoid them both like the plague even on a good day, and taking caravan through the narrow streets with the medieval bridge isn't much fun either.

 

[JTQ]

This is another example of a tradition being blindly accepted without rational forethought.[/b]

.

All I want to say in response to that is it is a sad, disappointing, wrong and somewhat offensive presumption, quite irrespective of any technical merit further debate might yield. It certainly makes one uninterested in that further debate on a subject where one has the tools to contribute.

 

[chrisn7]

I didn't see anything said about a traditional view, and I don't recall any such view being commonplace. Older caravans did, before we all became obsessed with noseweight, sometimes have their single axle slightly aft of the transverse centre of the van, as this was considered to promote stability - again before stabilisers of became common. It was the stability that was sought rather than any nosedown attitude that may have ensued.

 

[DaveA1]

Just a thought. Have the right profile of tyres been fitted, is the profile to low??

Think I'll get shot down in flames. I'll get mi coat :dry: :dry:

 

[Jeff_Lon]

Hi. Seems that your towball is too high. Have you got the correct towbar fitted for your model of car. Can a drop plate be fitted? A nose up set-up can lead to instability. Far better a slightly nose down attitude.

 

[ProfJohnL]

Hi. Seems that your towball is too high. Have you got the correct towbar fitted for your model of car. Can a drop plate be fitted? A nose up set-up can lead to instability. Far better a slightly nose down attitude.

Hello Jeff.

Can you substantiate your claim about nose up leading to instability?

 

[Raywood]

Hi. Seems that your towball is too high. Have you got the correct towbar fitted for your model of car. Can a drop plate be fitted? A nose up set-up can lead to instability. Far better a slightly nose down attitude.

Hello Jeff.

Can you substantiate your claim about nose up leading to instability?

I think the idea is more theoretical than real life.

 

[ProfJohnL]

Hello Ray,

Its circumstances like these that breed misconceptions which are often expressed as traditions. The problem is traditions are often believed even if there is no substantive or even contrary evidence to support them, which is why relying on traditions is potentially quite dangerous.

The EU sets out regulations for the height of loaded tow balls for domestic cars of between 350 and 420mm ground to teh centre of the tow ball.

But the EU also sets out the limits for the height of the trailer's coupling for when the bed of the trailer is set horizontal of between 385 to 455mm.

So if a trailer whose horizontal coupling head is only 385mm is coupled to vehicle with a tow ball at 420mm, the trailer will assume a nose up attitude. This is perfectly legal, and because it is a prescribed range of coupling heights, trailer manufacturers should have ensured there is no unreasonable detriment to the trailers handling abilities provided an adequate nose load has been set.

I hope it is obvious that within the prescribed ranges of ball and coupling heights the trailers can assume a variety of angles of attitude. The shorter the distance between the hitch and the axle, trailer the bigger the variation in angles that might arise, and conversely the greater the distance, the smaller the range of angles.

To put this into perspective for a caravan, The shortest caravan might have a ball to wheel distance of only 2.8M. The permitted hitch height variation will only give an angular variation of 1.4 degrees! And all other caravans will be less.

If this was an aircraft which relied on attack angles for generating lift this might be significant, but for a shoe box behind a brick brick The chances that variations as small as these will produce any dramatic change in the towing characteristics is minute compared to all the other factors are equal.

Consequently nose up or down within the tow ball limits is not going to make any perceptible difference, except in scientifically constant conditions which bear no relationship to real life.

 

[Mel]

I couldn't argue with your Physics Prof. But caravans towed nose up look soooo inelegant. B)
Mel

 

[Jeff_Lon]

No ProfJohn. I can not substantiate my post with facts and figures. But having been towing caravans for 32 years I can comment that it has always been regarded as 'bad form' to tow with the caravan nose up. I'm sure the Caravan Clubs would not recommend the practice. It has always been thought that air pressure getting under the front of the caravan can reduce weight on the towball and could therefore cause instability. QED

 

[ProfJohnL]

No ProfJohn. I can not substantiate my post with facts and figures. But having been towing caravans for 32 years I can comment that it has always been regarded as 'bad form' to tow with the caravan nose up. I'm sure the Caravan Clubs would not recommend the practice. It has always been thought that air pressure getting under the front of the caravan can reduce weight on the towball and could therefore cause instability. QED

Regardless of how long you may have been towing (and incidentally I have been towing for 45 years with experience of many different combinations) that does not necessarily mean what has become a traditional perception is actually valid.

Why is it considered 'bad form'? what evidence is there to support that view? I think the clubs ought to speak for them selves and justify their position with evidence. Nothing you have posted demonstrates the reasons for these views.

There is published evidence that confirms that nose load is a significant factor in maintained trailer stability. However there is no published evidence that shows that nose up, level or down inferences any greater trait of stability. There are lots of article that suggest nose down attitudes should be adopted but most simply refer back to the "traditional" position without exploring the facts.

I have found no published evidence that categorically demonstrates a nose up attitude within permitted design constraints of the construction regulations gives rise to poorer stability - provided all other factors and especially nose load are adequate.

The perception that a nose down attitude is better is I suspect more of a cosmetic opinion which has been banded around for so long that it has assumed the status of an accepted tradition, without a substantiated basis.

The issue of air pressures is vastly complicated, becasue every possible combination of tow vehicle and trailer will be unique, however there may be some identifiable trends such as vehicles generating lower pressure underneath than above, but there are bound to be some examples that buck the trends.

I am not arguing that nose down is wrong, I am arguing that it is wrong to suggest limiting caravanners options to a nose down situation, when in fact there is nothing wrong with considering a level or nose up attitude, within the constraints of the construction regulations and an adequate nose load being applied.

Always challenge "traditions" they can so often be wrong or unnecessarily limit people's real options.

 

[Martin24]

Sorry prof but tradition or not tradition slight nose down towards level is the accepted way of doing things around here following a lifetime of caravanning not just coming into it later in life therefore I am not challenging it because it's right :woohoo:

Mel I just so agree with you it looks so much nicer :silly:

 

[Raywood]

The nose up attitude would in theory cause the front of the caravan to lift but given the weight of the caravan it is theory rather than practice and as the prof says in real life it is not a problem.

 

[WoodlandsCamper]

Wouldn't the van only tend to lift if nose up if there was free undisturbed air in front of it (like an aeroplane)? Because it is actually being towed then the only air to get under the van to lift it is that air that has already been under the car and has been laminated. Also, with the van being higher than a car wouldn't the air going over the top of the car hit against the backward curve of the front windows and try to push the van down?

 

[ProfJohnL]

Wouldn't the van only tend to lift if nose up if there was free undisturbed air in front of it (like an aeroplane)? Because it is actually being towed then the only air to get under the van to lift it is that air that has already been under the car and has been laminated. Also, with the van being higher than a car wouldn't the air going over the top of the car hit against the backward curve of the front windows and try to push the van down?

Hello Woodlands

You have made some making perfectly sensible points, and it further verifies the nature of the issue as being far from simple.

As any self respecting aerodynamics engineer will attest, you cant just look at one particular aspect of Aero package you have to look at the way it interacts with other forces that will be brought to bear before you can start to understand and work out the total effects.

If we look at what the traditional view suggests, it implies that increased air pressure under the front of the caravan will attempt to lift the caravan. That is perfectly reasonable but limited suggestion, but when you consider that a caravan is in fact a see-saw about its axle line, and that about half of the caravan floor sits behind the axle, it must also be the case that the increased pressure under the front, would also exert a similar thrust on the rear portion of the floor, which will counter the thrust at the front!

Couple that revelation to the probability that the air entering the void under teh caravan is for the reasons you expressed already likely to be at a lower pressure, the case of caravan nose up attitude generating significant lift is highly unlikely.