Towing - caravan bouncing car around on bumpy road

[tuffty]

Woah, had no idea I was opening such a hornet's nest! It's been interesting reading though.

Yes I can confirm the Milenco moves quite a lot as the spring is compressed. It was significantly higher than my towball when uncompressed and being slotted under the hitch, then lowered as I raised the jockey wheel. It was "roughly" the height of my towball when laden, but I can't say I was paying particular attention as I didn't realise it was such an issue (although makes perfect sense now I've heard it explained)!

Anyway - if nothing else, it reminds me of the Arnold Schwarzenegger "Bullworker" I had when I was 16 so can be used to get a bit of exercise when I'm away.... :lol:

Also yes I can confirm that I don't appear to have any dampers ... never even considered this would be an optional extra! I'll be scheduling another B-road drive in soon, this time with awning in the van after a trip to the weighbridge and without kids in the car so I can pay attention to exactly what is happening and whether I can live with it.

 

[Dustydog]

Tuffty
Fit a set of shock absorbers and have the caravan wheels balanced.
You will notice a vast difference

 

[Hutch]

Tuffty, here is a link to the Alko stabalisers, It would be the red ones your after, If you were to fit them, carry on towing for several trips and see how you feel. Also if as you said you moved items to the back of the van to get your "dreaded nose weight" to where your happy, you might be getting a bounce effect their.

http://www.al-ko.com/shop/uk_vt/products/caravan-accessories-1/shock_absorbers.html?price=423
Hutch.

 

[DrZhivago]

Deleted

 

[Parksy]

Parksy,
I must be reading the posts by the 'Prof' in a different light, the impression I get is that he thinks that the Milenco nose weight gauge is not fit for purpose because it is far better to use a set of bathroom scales, but we all know that bathroom scales are far from accurate, have you ever know anyone to get on to the bathroom scales when they are on a diet and say that can't be right, anyway personally I really to give a toss because I find my Milenco gauge is fit for purpose, and would I really want to go on Judge Rinder with the 'Prof', I doubt if I would get a word in,

The Prof posts a great many informative and valuable comments to this forum Camel, and I know that he has the best interests of caravan owners at heart.
At the bottom of every one of the Prof's posts his signature in effect states that his comments are opinions and that facts should be checked and verified.
Any form of 'calibrated' noseweight measurement using a commercially available measuring device is better than none at all, I'm sure that most of us would agree?

...It's highly debatable whether any method, including the Prof's bathroom scales, can guarantee 100% accuracy....

I have never claimed the bathroom scales method was 100% accurate, only that it was likely to be far more accurate than compressible stick nose load gauges. Which was a valid point becasue of the undisputed reports of highly inaccurate compressible gauges both in teh press and from forum members. - And I hate to see people spending out on in effective products.

I wasn't trying to attribute statements to you that you haven't made Prof.
Most of us will know that actual noseweight can vary when a caravan is on the move, I'm simply pointing out that no common commercially available measuring device can be guaranteed to provide 100% accuracy every time.

I'm loving reading this thread, very entertaining!

......As for caravan nose weights, scientific accuracy is just not necessary. There are too many other variables to worry about. Weigh scales and the Milenco, as long as you get the height right, are both good enough.

* For the more philosophically minded, the Milenco would make a good, visible, "layman's" demonstration of Heisenberg's uncertainty principle - which in simple terms says that you cannot measure something without interfering with its configuration to some extent, which therefore gives some uncertainty to the result.

Well said Dr Z!
While I'd never advocate intentional law breaking, the chances of a successful UK prosecution for excessive caravan noseweight alone when a commercially available gauge has been used are so small that there would be more chance of winning the national lottery and being struck by lightning on the same day! :lol:
We can only do the best that we can with what we have available, be that a proprietary brand of noseweight gauge or a set of bathroom scales raised to the correct hitch height.

Woah, had no idea I was opening such a hornet's nest! It's been interesting reading though.

Yes I can confirm the Milenco moves quite a lot as the spring is compressed. It was significantly higher than my towball when uncompressed and being slotted under the hitch, then lowered as I raised the jockey wheel. It was "roughly" the height of my towball when laden, but I can't say I was paying particular attention as I didn't realise it was such an issue (although makes perfect sense now I've heard it explained)!

Anyway - if nothing else, it reminds me of the Arnold Schwarzenegger "Bullworker" I had when I was 16 so can be used to get a bit of exercise when I'm away.... :lol:

Also yes I can confirm that I don't appear to have any dampers ... never even considered this would be an optional extra! I'll be scheduling another B-road drive in soon, this time with awning in the van after a trip to the weighbridge and without kids in the car so I can pay attention to exactly what is happening and whether I can live with it.

We've had many an interesting discussion on noseweight measurement, it's passed many a winter hour but basically we all agree that we should measure as accurately as possible, without knowingly exceeding the limits.
All aspects of caravan and towcar safety are worthy of discussion, this will lead to safer trouble free caravanning

 

[camel]

Parksy,
I know the Profs posts are very informative I have never said they are not, but I think there is nothing wrong with making a comment about a thread and god forbid if it goes against the grain, I'm very surprised that the Prof was never head hunted by the Practical Caravan channel to replace the late John Wickersham who gave good sensible advice about various caravan technical matters just Like the Prof does on here,

 

[ProfJohnL]

...
Most of us will know that actual noseweight can vary when a caravan is on the move, ...

Hello Parksy,

Just for "accuracy" hmy: the definition of the S value is teh "Static" vertical Nose Load, So what happens on the move is not relevant to the measurement of nose load.

It is perfectly true that when under way the actual load applied to the ball hitch will vary according to the accelerations caused by vibrations the hitch is exposed to. In fact the equivalent mass that produces the nose load does not change, but the vibrations and motions that affect the hitch will amplify the force produced by the mass.

Vibrations produce accelerations and the effective mass acting on the ball hitch will produce changes to the weight/thrust/load/force the mass produces on the ball hitch.

Its a simple formula Force = Mass x acceleration.

Even under normal towing the vibrations can produce accelerations in the order of 5G and more on poor surfaces.

This means the a nominal static nose load of say 100kg can easily produce a force equate to 500Kg or more on the ball hitch (pro-rata for other nose load values)

This also means that if a static nose load produced exceeds the S value, that over load is also magnified by the virbatory accelerations, and that may exceed the safety limits of the towbar design, that is why over loading is potentially dangerous

It does however mean that when the outfit is stationary without the steadies deployed, no damage will occur if people enter the caravan whilst its still hitched to the car.

 

[Hutch]

What the Prof is saying is that your caravan WILL bounce on a bumpy road.
Tuffty, check your caravan weights and tyre pressures, log off from PC and have a great week away.
Don't worry too much, we have all been there and been wrong, enjoy your Summer.
Come back to The PC forum tell us about your slight mistakes and Join the Woosie Club and have a meet up at Stratford on Avon.
Happy touring Hutch.

 

[Parksy]

Parksy,
I know the Profs posts are very informative I have never said they are not, but I think there is nothing wrong with making a comment about a thread and god forbid if it goes against the grain, I'm very surprised that the Prof was never head hunted by the Practical Caravan channel to replace the late John Wickersham who gave good sensible advice about various caravan technical matters just Like the Prof does on here,

I think that we are at cross purposes here Camel. :dry:

 

[Petemate]

Hi Tuffty. I ashamedly admit that I did not read through the whole of the thread, However, I get the picture.
FWIW, we experienced similar symptoms to yours on the last trip out last year. We have had the car for three years and at four trips per year, we had got to know the traits of our outfit. Our car is a Saab Vector Sportwagon 2008, auto, 150 bhp Tid. (previous car the same but manual, also owned for three years and towing) The kerb weight is 1635, and our caravan, a Sterling Eccles Topaz, about 1275kg MTPLM.
Towing is extremely stable, and the capabilities mean that without paying attention to the speedo when I am not using cruise control I could reasonably easily lose my licence. Our loading is set to ensure the noseweight as recommended by the car maker at 75kg. We carry only the chairs and some clothing and sometimes the sat dish kit, as far forward as possible, in the car. The awning is positioned over the axle and there are the usual changes of clothing in the rear lockers. NO water in either the heater tank or the toilet.
After noticing the pitching last year, and it was not bad but could just be noticed, I decided to replace the rear shockers as the car had done 95k miles after last season's towing. Solo was OK, but my son who drives the car once or twice a week reckoned he could detect some slight handling issues. (he is an HGV driver, and used to rally, and currently owns a very nicely tuned EVO IX) So a couple of months ago I replaced those rear shocks, and I must say I did notice that the car seemed more firm. But what a difference on our first towing trip a few weeks ago. No pitching whatever on any class of road. Bear in mind that the Vector Sport Saabs have quite firm suspension, not controllable by the owner. They also sit slightly lower than standard Saabs, and this means that the 'van's nose is slightly down but that of course comes within the recommendations of "level or slightly nose down to aid stability" (no air lifting our caravan thanks lol)
Hope you are sorting it out Tuffty, it does make for a nice trip when the outfit is behaving itself!
Pete

 

[RayS]

All my working life has been based on measurement and control so I read all treads involving these topics with great interest and, sometimes, amusement, shock, and incredulity.

I am not going to enter the controversy about the merits of the vertical spring compression devices against the domestic bathroom scale but there is a simple method of checking either for sensitivity if not for absolute accuracy.
Take a water container - jerry can style is better than Aquaroll and place on the front of the drawbar - empty, with your weighing device of choice in place. Read you device then gently fill the container with a hose and see the change in reading. Using a 20 litre container should produce a change of give or take 20 kg - certainly to sufficient accuracy for the purpose. Even a 10 l litre container will give you some indication. The container needs to be - ideally - over the ball but if not as close to the front as you can get. The difference in lever arm will be that from ball to container compared to that from ball to axle - probably a matter of a few %.

One day I would like to splash out on the Reich device which measures load on all three wheels as not only should this show nose weight but also did to side balance which will be a second order factor in stability.
Given that many modern caravans have most of the heavy stuff down wone side - usually the offside - I suspect the type loading will be quite a way apart. Full water heater, boiler, fridge, cooker, store cupboard, possibly loo flushtank and container and in some cases batter and PSU as well -all commonly on offside.
I wonder if there is any data as to which wheel suffers more blow out failures ?

 

[ProfJohnL]

... and this means that the 'van's nose is slightly down but that of course comes within the recommendations of "level or slightly nose down to aid stability" (no air lifting our caravan thanks lol)...

The issue of towing attitude is open to considerable debate, suffice to say there is no conclusive evidence that a level or nose down offers ant net benefit to towing stability over the slight nose up that is possible within the construction and use requirements of hitch heights.

 

[Petemate]

I still personally prefer not to have a nose-up attitude, even slight. IMHO Prof this could lead to air pressure under the front of the caravan.

 

[ProfJohnL]

I still personally prefer not to have a nose-up attitude, even slight. IMHO Prof this could lead to air pressure under the front of the caravan.

Let me fist say there is nothing wrong in having a nose down attitude, but you stated basis for that preference is flawed.

If all that was involved was the underside of the caravan floor then it may be true.
Aircraft designers will tell you the major part of the lift generated by a wing is due to the air moving over the top of the wing not under the flatter bottom. But there are far more factors that will have a much bigger influence on the issue.

The most obvious one is the fact there is a tow car punching a complex hole in the air just ahead of the caravan. And you must not ignore the aerodynamic effects of the front top and back of the caravan. I dont suggest I can explain in detail, because each combination of tow vehicle and trailer will be different, but it is certainly the case tha the amount of nose up attitude that is permitted within the hitch height regulations is quite small and I seriously doubt most caravanners could detect it if all other things are equal, which in most cases they are not.

Nose level or down is another one of those "caravanning traditions" which under proper scrutiny do not stand up to logical, But if a caravanner were to try and adjust an outfit that has a legitimate nose up attitude, and force the nose down they may end up exceeding some other load limit.

 

[Petemate]

Nose level or down is another one of those "caravanning traditions" which under proper scrutiny do not stand up to logical, But if a caravanner were to try and adjust an outfit that has a legitimate nose up attitude, and force the nose down they may end up exceeding some other load limit.

Fair comments Prof! especially the last paragraph.

 

[tadhgocuilleain]

Hi Tuffty,

In my experience, pitching of the caravan is normal on bumpy roads, especially a long 'van like yours. I'm just back from a weekend of touring on mostly bumpy B roads around the southwest of Ireland. The speed limit was usually 80kph, but I often had to slow to 50 or 60. On bad stretches, I'd even have to go as slow as 30kph. I had to regularly pull in to let following vehicles pass, so as not to frustrate them.

Setting up the caravan properly will improve the drive, but a bumpy road will always be harder to drive than a smooth one. Tyre pressures make a huge difference. Check them cold on both the car and caravan, and set them to the manufacturer's specifications. I find a difference of even 1 psi between the caravan's wheels makes it bounce around a lot more.

If the caravan starts to see-saw, I find gently braking to throw more weight to the front settles it down. With a bit of experience, you'll be able to read the road surface and know what type of bumps you need to slow down a lot for and what type the caravan will glide over. Keep an eye out for scrapes/gouges in the asphalt - that's when you need to slow a lot because someone else has previously bottomed out and scraped the bottom of their car along the road. hmy:

I hope that helps,
Tim

 

[WoodlandsCamper]

I find a difference of even 1 psi between the caravan's wheels makes it bounce around a lot more.

I think you must be super-sensitive. 1 psi is neither here nor there and is about the tolerance of the measuring device. With TPMS on my car I noticed at the weekend that the difference in pressure on the car tyres between cold and hot was about 3 psi.

 

[tadhgocuilleain]

I think you must be super-sensitive.

Yes, I am. I drive about 100,000km per year, so I've become very sensitive to changes in the vehicle.

With TPMS on my car I noticed at the weekend that the difference in pressure on the car tyres between cold and hot was about 3 psi.

That's Boyle's Law. That's why I said to check the tyre pressures when cold.

 

[otherclive]

Parksy,
I must be reading the posts by the 'Prof' in a different light, the impression I get is that he thinks that the Milenco nose weight gauge is not fit for purpose because it is far better to use a set of bathroom scales, but we all know that bathroom scales are far from accurate, have you ever know anyone to get on to the bathroom scales when they are on a diet and say that can't be right, anyway personally I really to give a toss because I find my Milenco gauge is fit for purpose, and would I really want to go on Judge Rinder with the 'Prof', I doubt if I would get a word in,

I beg to differ re your comments on bathroom scales. Ours are within 0.25kg when weighing luggage prior to flights. We have recently been to India and the luggage allowance was 30kg each. My method is to weigh myself. then weigh myself holding the suitcase. I can then work out the suitcase weight. And after using six flights in three weeks (international and internal) the weight was within 0.25 of that record by the airline check in. So there are some accurate scales around even in the range 30-130kg!! Which should meet most caravaners needs.

 

[Raywood]

I think the point about bathroom scales is that to get the noseweight right you have to weigh at the height of the hitch when it is on the car. With a noseweight gauge you cannot do this in most cases as you are stuck with whatever size it is and it might be different to the tow ball height. With bathroom scales you can adjust the wood you use until it is about the right height. The problem is that most of us have no idea of how accurate our bathroom scales are so cannot be sure of that one either. As I see it you do the best you can and that is all you can do, it is not an answer if you get it wrong, but you should be near enough to get a warning rather than a prosecution if you are a bit high.

 

[ProfJohnL]

The problem is that most of us have no idea of how accurate our bathroom scales are....

Hello Ray
I fully accept that most people do not know how accurate their bathroom scales are, but do you know how accurate your nose load gauge is? Its far harder to check a nose load gauge than a set of bathrooms scales, becasue in much the same way that Camel has identified, you can at least load the scales relatively easily with known weights , for example weigh an empty 25 litre plastic bottle . then fill it with 25Lt then check the scales again. It should show 25kg + the weight of the empty bottle. Or use a new bag of garden compost, etc

The chances are the accuracy of the bathroom scales will be an order of magnitude or more better than the generic compression nose load spring - if you have a means of actually testing it!.

 

[DrZhivago]

I think the point about bathroom scales is that to get the noseweight right you have to weigh at the height of the hitch when it is on the car. With a noseweight gauge you cannot do this in most cases as you are stuck with whatever size it is ...

Whatever method you use the hitch should be at the height it is on the car on level ground. As you say, this is easier with bathroom scales - because they are thin and you can shim them up to the right height (steps plus wooden blocks). But as I said half-jokingly in an earlier thread, if a noseweight gauge is too short you can block it up anyway, and if it is too tall you can make a hole in the ground or even saw off a bit of its length (seriously, if possible). A good noseweight gauge ought to have a screw height adjustment ; I don't know if there are such things, and don't care as I'll stick to bathroom scales.

I said on level ground just then. That is just as important as getting the height right, or rather it is part of getting the height right. So if you match it to car tow-ball height on a sloping field or driveway it will be wrong. If you are pointing downhill it will read too high and vice-versa.

You can still do it on a slope though : one day, measure and make a note the slope of the caravan floor with a small spirit level (say 1 ft long) while hitched up to the normally loaded car on level ground. Eg, the floor might be nose-down by a quarter-inch over the length of the spirit level. Then when measuring nose weight "in the field", do so with this same true inclination of the floor set with the aid of the spirit level , irrespective of the slope of the field.

 

[tadhgocuilleain]

I think you are all reading way too much into the height of the caravan towbar when checking the nose weight. If measuring in laboratory conditions, it's necessary but not in practical conditions.

The height will make the greatest difference on a short, two berth caravan. Let's say the distance from the hitch to the axle is about 3.5m. Let's go mad and say there's a one foot (30cm) height difference. That means the caravan will be tilting down (or up) at about 5 degrees. Therefore, the measured value will be about 99.6% (Cosine 5°) of the true value. For a measured value of 90kg, the real value will be 90.34kg. If you put a litre of milk in the boot it will have three times more of an effect and I don't think anyone will prosecute you over 340g.

Tuffy's caravan is significantly longer, so the height of the hitch will make even less of a difference. The Milenco gauge he has will be perfectly accurate for practical purposes.

 

[Hutch]

I gave bitten my tongue a long time about this nose weight/ tow ball height height.
You will only get a big tow hitch difference. If you have a very High , C of G , imediaetly above the axle . . And that has to be a a lot. . The one to two inch difference in tow height differencec is not enough to give you a Kilo difference to the normal person on their caravan. .

 

[DrZhivago]

Let's say the distance from the hitch to the axle is about 3.5m. Let's go mad and say there's a one foot (30cm) height difference. That means the caravan will be tilting down (or up) at about 5 degrees. Therefore, the measured value will be about 99.6% (Cosine 5°) of the true value. For a measured value of 90kg, the real value will be 90.34kg.

You have neglected the height of the caravan's centre of gravity above the axle. If it is 500mm (I don't know anything better) then with a change of hitch height of 150mm (the Milenco spring range I understand) and an axle-to-hitch length of 3500mm, then the CG will move horizontally relative to the axle by 150*500/3500 mm = 21mm.
If the caravan weighs 1250kg, then this generates a moment of 1250*21 = 26,250 kg.mm
This will cause a change of reaction (ie weight) at the hitch point of 26,250/3500 = 7.5 kg
A change of, or error, of 7.5kg is certainly the sort of figure that people here are concerned about. For other caravan weights and axle-hitch distances you can simply factor the above figure.

I think in your calc you are talking about the shortening of the horizontal axle-hitch length due to the inclination. Yes, that would be tiny and I have neglected it.