Why do some vans have twin axles and others single?

[Buckman]

I don’t quite see what E&P self levelling has got to do with it. Noseweight is noseweight. So long as it’s always measured under the same conditions as when hooked up to the car it doesn’t make any difference.
I wasn’t advocating a low noseweight. I was only questioning why it needs to be as high as you suggest if everything else is set up properly. The regulations state that the manufacturer must guarantee a minimum noseweight of at least 4% of the weight of the trailer. Consequently, it must be possible to ensure adequately safe conditions when towing a 2000 kg caravan if the noseweight is 80kg. It should not be necessary to have 145kg on the towball.

You are correct about R&P making no difference, however the layout of the caravan will affect its characteristics. Our island bed is behind the axles and we have an upgraded heavier mattress.

Also with the E&P you have heavy duty steadies in the front plus of course the boiler etc. The A frame is also longer than many caravans all adding to increased nose weight. If nose weight is at about 100kg the outfit can feel very twitchy no matter how well you load it.

Manufacturers recommend between 5-7% of the MTPLM is the correct nose weight. Like many others with the same type of caravan we have found that about 140kg is ideal for a comfortable tow and it is our choice.

 

[Lutz]

Its to do with tyres. Tyres on larger single axle caravans can only just carry the weight which the tyres were designed for, so if they make a caravan which is slightly heavier they have to go to a twin axle.
Which is why what you can carry in a single axle caravan is so low.

I don’t know where you got that information from, but tyres are readily available with load ratings well exceeding the needs for the very heaviest of single axle caravans rated at 2000kg. Above that there are no single axle caravan chassis available anyway so it’s the chassis that’s the limiting factor, not the tyres.

You are correct about R&P making no difference, however the layout of the caravan will affect its characteristics. Our island bed is behind the axles and we have an upgraded heavier mattress.

Also with the E&P you have heavy duty steadies in the front plus of course the boiler etc. The A frame is also longer than many caravans all adding to increased nose weight. If nose weight is at about 100kg the outfit can feel very twitchy no matter how well you load it.

Manufacturers recommend between 5-7% of the MTPLM is the correct nose weight. Like many others with the same type of caravan we have found that about 140kg is ideal for a comfortable tow and it is our choice.

If anything, a longer A frame would reduce the noseweight, not increase it.
That aside, both the Lexus that I owned previously and my current BMW had/have an 80kg noseweight limit for a towload limit of 2000kg, so I don’t know where you got the 5-7% figure from that you quote as a manufacturer’s recommendation.

 

[Buckman]

If anything, a longer A frame would reduce the noseweight, not increase it.
That aside, both the Lexus that I owned previously and my current BMW had/have an 80kg noseweight limit for a towload limit of 2000kg, so I don’t know where you got the 5-7% figure from that you quote as a manufacturer’s recommendation.

In Swift owner;s manual plus other sources.

 

[Dustydog]

Why is it more difficult with a TA?

Provided you take the measurement in the way I have been at pains to point out over the years, the most significant variables are accounted for.

Why should variations creep in? Can you explain please?

In answer to your question Prof firstly after 45 years of towing may I say I am fully conversant with the correct method of measuring nose load in terms of the final height once attached to the towing vehicle. The TA has an additional problem that until you can measure it’s nose load , know where the suspension will settle you really don’t know straight off where the towing height will be. Reich take these factors into account by selling two different guages SA & TA. Experience, previous experiment, resolved the issue for me.
Self levelling suspension solves the height problem but not everyone has it , I think Buckman does.

I can do no better than quote the following immortal words from one of our seasoned contributors:—

The science and mathematics of a single axle caravan are relatively easy but to summarise :-
For a single axle caravan, the lower the hitch is the greater the noseweight will be, conversely the higher the hitch is raised the nose weight will reduce, a single axle caravan has a fairly progressive rate of change of nose weight with hitch height.

But the twin axle arrangements are substantially more complex and may not follow the same simple pattern of weight transference. In fact you can expect several Kg nose weight change with only a small change in nose height, which it is why it is particularly critical to make the nose weight measurement at the working height.

 

[Buckman]

Well said DD.

 

[otherclive]

Or as explained so succinctly by Reg in 2010, leads me to wonder how I managed to survive and tow a caravan too.

For 2 wheel (single axle) only:-
1 - Measure distance from centre of wheel to centre of coupling ("A") and multiply by required nose weight ("NW")
2 - Measure HORIZONTAL distance from centre of jockey wheel to centre of coupling ("a") and subtract from "A" then divide into "1" . Note that I have stressed that it MUST be the horizontal distance.
i.e.
"A" x "NW
---------------- = Nose Weight at Jockey Wheel.
"A" - "a"

An example - If "A" = 5.0m, "NW" noseweight at towball = 80Kg and HORIZONTAL dist from towball to centre of jockey wheel "a" = 20cm.

5m x 80Kg
-------------- = 83.33 Kg at Jockey wheel.
4.8m

As was previously said the jockey wheel is best in the most forward position and the nose weight should always be checked in that position.
I do not have any experience with twin axles but I would imagine that the same calculations would apply measured from the front axle.

 

[Lutz]

In Swift owner;s manual plus other sources.

Now I understand. You were referring to caravan manufacturers' recommendations, not car manufacturers' recommendations. Just shows that caravan manufacturers aren't really in touch with actual conditions as I'm sure that Lexus and BMW aren't the only car manufacturers that limit the noseweight at 4% of the allowable trailer weight.

Throughout this thread I get the impression that some contributors are just digging a bit too deeply into the subject than absolutely necessary. I raised and lowered the hitch by about 10cm of my single axle caravan (overall length 7.8m) and although 10cm is quite substantial, the noseweight didn't change by more than 4kg either way. The conclusion that I draw from that is that it's not so important to get hitch height absolutely right. Maybe the picture is a bit different if the caravan is considerably shorter or if the load distribution within the caravan is different. I can't speak for a twin axle as I've never had one.

 

[ProfJohnL]

Its to do with tyres. Tyres on larger single axle caravans can only just carry the weight which the tyres were designed for, so if they make a caravan which is slightly heavier they have to go to a twin axle.
Which is why what you can carry in a single axle caravan is so low.

Im sorry, but that not correct.

The choice of single axle or twin axle is purely a marketing decision made by the caravan manufacturer.

There are plenty of tyres the manufacturer could choose which can carry much higher loads than caravans, so tyres are not the limiting factor.

The manufacturer will decide at the time of design what weight the caravan should be. and the number of axles is purely a marketing decision and what type of customer the caravan will be aimed at.

Because TA's tend to be more expensive to manufacture, they tend to be fitted to models aimed at more affluent customers.

In virtually all cases the maximum permitted weight of the caravan is dictated by the chassis - and not the number of axles.

 

[ProfJohnL]

In answer to your question Prof firstly after 45 years of towing may I say I am fully conversant with the correct method of measuring nose load in terms of the final height once attached to the towing vehicle.

The TA has an additional problem that until you can measure it’s nose load , know where the suspension will settle you really don’t know straight off where the towing height will be. Reich take these factors into account by selling two

As you know the "S" value of both the tow vehicle and trailer relates to the static measurement, taken when stationary.

Assuming you measure the working loaded hitch height and you recreate that height when measuring the trailers nose load, then you get the real nose load applied by the trailer.

The process is exactly the same for SA or TA, so that height variable is accounted for.

Reich take these factors into account by selling two different guages SA & TA. Experience, previous experiment, resolved the issue for me.

It seems the Reich solution is just an preinstalled offset to the readings, and as such it may make a reasonable approximation of the effect but its not specifically tuned to each caravan outfit.

The Quote you included from a previous thread is correct- Thank you.

 

[Buckman]

In answer to your question Prof firstly after 45 years of towing may I say I am fully conversant with the correct method of measuring nose load in terms of the final height once attached to the towing vehicle. The TA has an additional problem that until you can measure it’s nose load , know where the suspension will settle you really don’t know straight off where the towing height will be. Reich take these factors into account by selling two different guages SA & TA. Experience, previous experiment, resolved the issue for me.
Self levelling suspension solves the height problem but not everyone has it , I think Buckman does.

I can do no better than quote the following immortal words from one of our seasoned contributors:—

The science and mathematics of a single axle caravan are relatively easy but to summarise :-
For a single axle caravan, the lower the hitch is the greater the noseweight will be, conversely the higher the hitch is raised the nose weight will reduce, a single axle caravan has a fairly progressive rate of change of nose weight with hitch height.

But the twin axle arrangements are substantially more complex and may not follow the same simple pattern of weight transference. In fact you can expect several Kg nose weight change with only a small change in nose height, which it is why it is particularly critical to make the nose weight measurement at the working height.

DD as you are aware, no matter what you say even if you are correct, you will never win!

 

[Dustydog]

Assuming you measure the working loaded hitch height and you recreate that height when measuring the trailers nose load, then you get the real nose load applied by the trailer.

Thanks Prof👏Exactly what I said from the start. It may take several attempts at measuring to get the height and NL correct with a TA!!

 

[Hutch]

Now I understand. You were referring to caravan manufacturers' recommendations, not car manufacturers' recommendations. Just shows that caravan manufacturers aren't really in touch with actual conditions as I'm sure that Lexus and BMW aren't the only car manufacturers that limit the noseweight at 4% of the allowable trailer weight.

Throughout this thread I get the impression that some contributors are just digging a bit too deeply into the subject than absolutely necessary. I raised and lowered the hitch by about 10cm of my single axle caravan (overall length 7.8m) and although 10cm is quite substantial, the noseweight didn't change by more than 4kg either way. The conclusion that I draw from that is that it's not so important to get hitch height absolutely right. Maybe the picture is a bit different if the caravan is considerably shorter or if the load distribution within the caravan is different. I can't speak for a twin axle as I've never had one.

The main reason that nose weight changes with the raising or lowering of the tow hitch is mainly, in shorter caravans, with higher weights , ie, microwave, and other things stowed high up over the axle.
But we are normally talking about + or- 5 kg.
It normally doesn't effect much except in caravan forums. For debating.

 

[GaryB]

When we had our twin axle I measured the height of the car's tow ball unhitched and used a noseweight gauge at that height. I don't know how accurate that was but in the four years we had it it towed perfectly and I concentrated on the enjoyment of using it rather than overly worrying about anything else.

 

[RogerL]

When we had our twin axle I measured the height of the car's tow ball unhitched and used a noseweight gauge at that height. I don't know how accurate that was but in the four years we had it it towed perfectly and I concentrated on the enjoyment of using it rather than overly worrying about anything else.

It won't have been absolutely accurate as the towball will (usually) drop slightly when the noseweight is applied - but as upthread, the difference isn't enough to worry about.

Early in our caravanning times, I would meticulously measure the noseweight every time before hitching up - and it was always different each time during a touring holiday - I don't bother now unless we've changed the caravan or fundamentally loaded differently.

 

[littlebasher]

It won't have been absolutely accurate as the towball will (usually) drop slightly when the noseweight is applied - but as upthread, the difference isn't enough to worry about.

Early in our caravanning times, I would meticulously measure the noseweight every time before hitching up - and it was always different each time during a touring holiday - I don't bother now unless we've changed the caravan or fundamentally loaded differently.

I thought i would get smart and buy one of those Reich digital noseweight scales that sits between the towball and hitch.

It seems to read something different every time !

 

[Dustydog]

Some years ago a regular Forumite posted this . Maybe one day it could be a reality?

 

[Lutz]

Obviously a retouched photo as the front axle has no apparent steering and the hitch is of the overrun type which is prohibited the caravan doesn’t have its own fully independent braking system.

 

[Dustydog]

Obviously a retouched photo as the front axle has no apparent steering and the hitch is of the overrun type which is prohibited the caravan doesn’t have its own fully independent braking system.

Yes, it’s a spoof but an Artist’s impression 🤪

 

[Lutz]

I don’t see the point of such a configuration. What’s the benefit? I only see disadvantages.

 

[otherclive]

I don’t see the point of such a configuration. What’s the benefit? I only see disadvantages.

Could it not be intended as a joke?

 

[Dustydog]

I don’t see the point of such a configuration. What’s the benefit? I only see disadvantages.

Lutz,
it was done a long time ago as a laugh, a joke, a spoof , nothing particularly technical more just a light hearted piece of fun😎

 

[Jcloughie]

4 corners works for these guys.




😀

John

 

[Lutz]

Could it not be intended as a joke?

I suppose in this instance it was a joke, but I have read posts where the subject was actually taken seriously.

4 corners works for these guys.

View attachment 4848

I wouldn't want them on the pitch next to mine though. "Please reverse in between the two posts, sir".

 

[ProfJohnL]

Just to muddy the water a bit more,

Apparently this is a model of a wartime German Commanders caravan

It was not designed to be towed behind a car.

But it reminded me of an experimental towing system where basically a small SA trailer was modified by removing its load carrier and replaced with a 50mm ball directly over its axle, to which any traditional caravan could be coupled.

I do not know why it was tried, but I suspect that it could not be approved in the UK, becasue of the braking system.

 

[RogerL]

Just to muddy the water a bit more,

Apparently this is a model of a wartime German Commanders caravan

It was not designed to be towed behind a car.

But it reminded me of an experimental towing system where basically a small SA trailer was modified by removing its load carrier and replaced with a 50mm ball directly over its axle, to which any traditional caravan could be coupled.

I do not know why it was tried, but I suspect that it could not be approved in the UK, becasue of the braking system.

It also couldn't be approved in the UK as it would be a trailer towing another trailer - which is only legal for "showmen"