Bath University / Bailey Towing Stability Website

[nrdonnelly]

I've just been sent this website by Bailey Caravans, which has a caravan loading simulation on it, to show the affect of loading a van badly. It's based on the test-bed that it had running at the NEC show. It's pretty good, but even loading the van really stupidly doesn't get you much of a telling off...

www.towingstabilitystudies.co.uk

Nigel Donnelly

Editor

Practical Caravan

 

[Surfer]

It is a load of codswallop written by amateurs with no experience of loading. Has any one got an overhead locker in their caravan that can easily accommodate a TV or a 20kgs article? The only way I could get all the stars was to incrfease my nsoewight to 90kgs! Try it if you don't believe me.

 

[nrdonnelly]

It seems a bit harsh to call the University of Bath's Department of Mechanical Engineering a bunch of amateurs!

The only way I could get all the stars with a proper noseweight was to leave some stuff out, but you can put things on top of one another to try and balance the weight. It's quite interesting, nonetheless.

Nigel Donnelly

Editor

Practical Caravan

 

[peter_5297]

It is a load of codswallop written by amateurs with no experience of loading. Has any one got an overhead locker in their caravan that can easily accommodate a TV or a 20kgs article? The only way I could get all the stars was to incrfease my nsoewight to 90kgs! Try it if you don't believe me.

From Peter W Jones

The Department of Mechanical Engineering at Bath University have done some excellent research which would have been of great benefit to caravanners if they had been given the details.

See in particular 1994 Fratilla (Phd thesis) on friction based stabilisers and Standen (Phd thesis 1999) on wind induced snaking. Further details can be found in my effort below at

www.caravanaccidents.wordpress.com

Bath's latest research is also excellent, but the Caravan Club magazine quoted Dr Darling of Bath University as not having thought it advisable to go over 50 mph when carrying out tests wih the latest "Bailey test bed". This small matter is very significant if read alongside my comments above concerning 50mph for caravans.

I also think that Bath University would have been better employed following up Standen's 1999 research on "Towed Vehicle Aerodynamics." We will not be safe (in my opinion) at over 50 mph air speed until we catch up with the USA and use their electric brakes and develop aerofoils to create down force (as on formula 1 racing cars). Standen proved in wind tunnel tests that such technology would make vans safer; he did not manage to come up with a practical solution. This is the most important item that needs further research.

 

[peter_5297]

It is a load of codswallop written by amateurs with no experience of loading. Has any one got an overhead locker in their caravan that can easily accommodate a TV or a 20kgs article? The only way I could get all the stars was to incrfease my nsoewight to 90kgs! Try it if you don't believe me.

From Peter W Jones

The Department of Mechanical Engineering at Bath University have done some excellent research which would have been of great benefit to caravanners if they had been given the details.

See in particular 1994 Fratilla (Phd thesis) on friction based stabilisers and Standen (Phd thesis 1999) on wind induced snaking. Further details can be found in my effort below at

www.caravanaccidents.wordpress.com

Bath's latest research is also excellent, but the Caravan Club magazine quoted Dr Darling of Bath University as not having thought it advisable to go over 50 mph when carrying out tests wih the latest "Bailey test bed". This small matter is very significant if read alongside my comments above concerning 50mph for caravans.

I also think that Bath University would have been better employed following up Standen's 1999 research on "Towed Vehicle Aerodynamics." We will not be safe (in my opinion) at over 50 mph air speed until we catch up with the USA and use their electric brakes and develop aerofoils to create down force (as on formula 1 racing cars). Standen proved in wind tunnel tests that such technology would make vans safer; he did not manage to come up with a practical solution. This is the most important item that needs further research.

 

[Lutz]

The results documented by Bath University don't surprise me one bit. By applying basic engineering judgement, one would expect to arrive at the same general conclusions.

Although something is to be said for electric brakes, I foresee major hurdles in their implementation. For one thing, current 13-pin connections between car and trailer would be inadequate to meet the power needs for electric brakes so new industry standards would have to be developed. Furthermore, the car and the trailer industry would have to agree on a common system of achieving the same degree of proportional braking between the car's hydraulic brakes and electric brakes of a caravan. To make sense and to allow any make of towcar to tow any caravan without modification, all car manufacturers would also have to agree on a common standard amongst themselves. I can't see this happening without an appropriate European directive, but experience has shown that developing such a directive is a notoriously slow process, so I can't see electrical brakes as a practical proposition within the next 5 to 8 years. Too much work has to be done beforehand.

 

[Surfer]

I notice that Peter W Jones, former school teacher, is back with his hypothesis on the aerodynamics of towing. If I recall correctly ths theory was totally ridiculed on another forum becasue there were too many flaws in the theory on caravan aerodynamics.

I also think that Bath University would have been better employed following up Standen's 1999 research on "Towed Vehicle Aerodynamics." We will not be safe (in my opinion) at over 50 mph air speed until we catch up with the USA and use their electric brakes and develop aerofoils to create down force (as on formula 1 racing cars). Standen proved in wind tunnel tests that such technology would make vans safer; he did not manage to come up with a practical solution. This is the most important item that needs further research.

Why woudl not be wanting to exceed 50mph or more on an A road as without a caravan it is dangerous due to the poor state of many of oour roads. Even if we wanted to travel at speeds in excess of 60mph why would aerofoils improve stability given that they create turberlance directly behind them due to the vacuum being formed?

There is no doubt in my mind that having the correct vehicle for the type of caravan you are towing is in the forefront of safety precautions when towing. Secondly is loading, thirdly is obviously caravan servicing to ensure that tyre pressures are correct, brakes okay etc. Some may differ on my first two points which is fine but if you have a large 4 x 4 and you are towing with an uneequal load you will probably get away with it however with a lighter car, your loading would be your number one priority. It seems that either way having a noseweight that is heavier than the noseweight of the car is safer than having a lighter noseweight than the nsoeweight of the car. I think I have to agree on thsi point as long as it is not over the top.

 

[Lutz]

Although aerodynamic improvement to caravans may reduce the effect of the noseweight decreasing with speed, it wouldn't help to solve the problem of instability due to crosswind. Caravans are very slab-sided and there is little one can do about that. Besides, in order to generate a downward force to assist maintaining the noseweight, the aerofoil would have to be at the front of the caravan, not at the back.

 

[nrdonnelly]

That's quite interesting - the problem is that increasing downforce would also alter the loading on the car. With an aerofoil on the front of the van, noseweight would go through the roof on the move, and putting it at the back could put you in negative noseweight territory. The only option would possibly be directly above the axle, but presumably the effect would be the same as putting loads more weight in the van. I imagine the biggest problem would likely be unpredictable on-road behaviour of the outfit, if the aerofoil positions were speed related. I asked a caravan designer about the possibility of using ground effect to suck a caravan to the road, in the same way as modern performance cars do, but the combination of the towcar disrupting the airflow and the low speed make this a no-go.

Fascinating subject though.

Nigel Donnelly

Editor

Practical Caravan

 

[Lutz]

The whole object of the aerofoil at the front would be to offset the reduction in noseweight that occurs anyway as speed increases. At speed, the 75kg that one normally has in the static condition, reduce dramatically.

If the aerofoil were designed correctly, this would be prevented and the vertical downward load on the towbar would not change.

 

[anotherDavid]

It is a load of codswallop written by amateurs with no experience of loading. Has any one got an overhead locker in their caravan that can easily accommodate a TV or a 20kgs article? The only way I could get all the stars was to incrfease my nsoewight to 90kgs! Try it if you don't believe me.

As it happens I have overhead locker in my Hymer that takes my flatscreen telly and it is used .

Dave

 

[Surfer]

Perhaps if deflectors were fitted to the rear and on the sides of the caravan they would reduce the drag but should not have an effect on the overall stability of the caravan.

 

[rob_jax]

Hi Lutz,

Totally agree with the premise of using an aerofoil to maintain a more constant nose-weight.

But an aerofoil would need to be mounted at the front - not necessarily my friend - inverting the aerofoil and mounting it on the rear, together with a better shaped airflow separation zone (rather than the flat brick shape) would provide a up force on the rear, pushing down on the coupling and maintaining the nose-weight.

And on this subject, I've seen several new caravans from different manufacturers with a lip spoiler on the rear - doing exactly what you don't want - further reducing nose-weight at speed.

Robert

 

[Lutz]

Maybe I have misunderstood you or you me but I am in full agreement with you, Robert. Only a downward acting aerofoil at the front of the caravan makes sense.

 

[rob_jax]

Hi Lutz,

I was thinking that instead of a downforce on the front of the caravan - which would maintain the nose-weight as towing speed increased - but create turbulence along the caravan roof; it might be better to fit an inverted airfoil at the rear of the caravan - this would still create the same down-force on the tow-bar but could also have the benefit of cleaning up the aiflow over the rear of the caravan - and maybe reduce drag if combined with a careful reshape of the caravan rear - improving fuel consumption as well as maintaining nose-weight.

That statement is pure conjecture, but it would be nice to see a model made and tested...

 

[ValA.]

Not a 'tecchie' reply at all - but we find our Ford Mondeo Estate/Swift Speedbird combo tows much better when we have a cheap roofbox on the car. It isn't that loading is any different with the roofbox on (we don't need all that extra space any more now there's only two of us) but we keep it on just because of the improvement in stability when towing. (And of course we can fit in some wine boxes for the homeward trip!!!)

 

[Lutz]

The problem with increasing the noseweight at the front by using an aerofoil to raise the back (i.e. in effect, a see-saw) is that you would at the same time be decreasing the axle load and this could be to the detriment of stability.

 

[Lutz]

Val has a valid point. The presence of a roof box on the car increases the frontal area of the car to cover a larger fraction of the total frontal area of the caravan. Although this will increase the aerodynamic drag on the car itself, it results in a similar reduction of drag on the caravan (because it is in a larger wind shadow). This latter effect lessens the reduction of noseweight at speed. For this reason, by moving the drag further forward from the caravan to the towcar, there is something to be said for fitting a roof box.

 

[rob_jax]

Hi Lutz,

Using the see-saw idea - I don't believe it's an issue to use a rear-wing to transfer a speed related 10 kg - 20 kg - 30 kg load onto the tow-coupling. Compared to the static weight of the caravan, none of those weights should be onerous.

But it's also something that can't be reasoned by "common sense" and needs real investigation...

Robert

 

[Lutz]

On second thoughts you may have a point, Robert. Without anyone knowing for sure by how much the noseweight is reduced at speeds of around 60mph (it'll vary from model to model, too) it's difficult to give a qualified answer. You're right, the subject needs more investigation.

 

[Robert_374242026]

Casting my memory back many years , i think it was in the 60s that some caravanners use to have a airfoil mounted on the roof like a roof rack fitting which was supposed to deflect the weather etc. I wonder if that had an effect on the subject spoken about?.

 

[ProfJohnL]

I agree that a properly designed study would very useful to try and understand the dynamics of car & caravan combinations. Aerodynamics are of course important but other mechanical factors should also be factored in. The big difficulty I see is the fact that are a vast number of possible car and caravan combinations, and each one will present their own particular characteristics. Even the same combination but loaded or maintained differently will present changed characteristics. This would be prohibitively expensive to fund - so it is unlikely that a result will be produced that will allow accurate predictive modelling for all outfits.

It might produce a 'generic model' approach on how to assess and improve any given combination, which would be helpful.

 

[gary]

I agree with John, someone mentioned aerofoils and formula 1 cars, how many times have you heard the commentator mention he's losing ground-force or whatever from being in 'dirty air' from the car in front?

Well obviously a caravan is in dirty air from the car in front all the time! and how dirty depends on the shape of that car.

So surely then, for an aerofoil to work on any caravan it needs to be set up as a pair with a specific car or ultimately what's to stop it making things even worse??

 

[RayS]

This is a very interesting thread. i suspect however, that fitting of aerofoils etc. for maximum benefit would require simulation of each car / van combination individually, plus taking into account the load distribution and thus would not really be on, except perhaps for record attempts or similar.

The aerodynamics of an additional roofbox are similarly variable. I've tried a Thule Weekender on top of Disco / pastiche and MB270 / Burstner outfits. On the former there was no detectable difference. On the latter, the box completely wrecked the airflow patters, evidence by the fact the van got absolutely filthy front and back, whereas it normally stays reasonably clean even in rainy conditions, and the mpg suffered by about 10%.

There is also the problem that the towing speed is often at the same order of magnitude as any wind, but the direction of the wind relative to caravan direction is of course variable, so what may be stable / improved aerodynamics with a head wind may not be so with a three quarter rear wind.

I keep telling myself that the standard towing ourfit cannot be much worse than the 2.5 x3.5m "headwind" of a large motor van.