Daft question about axles

[saint-spoon]

What difference does having a twin axle make apart from the obvious of being able to carry more weight? I know it must sound daft, it does to me and I am an engineer, however I am intrigued to know what practical differences it make to the caravan.

Please no one point out that you have twice as many tyres.

 

[WoodlandsCamper]

There's a discussion here on the topic.

 

[ProfJohnL]

What difference does having a twin axle make apart from the obvious of being able to carry more weight? I know it must sound daft, it does to me and I am an engineer, however I am intrigued to know what practical differences it make to the caravan.

Please no one point out that you have twice as many tyres.

Hello SS.
Strange as it may seem, a twin axle caravan of the same length as an SA may not have any major weight advantage. The reason is the the A-typical suspension on caravans is truly independant. Consequently if one of the axle becomes unloaded say by traversing a speed hump, virtually the entire weight of the trailer is supported by just one axle. Thus the specification of each axle has to be able to carry the MTPLM. This also means that each tyre should be specified as if just one axle is being used.

Consider a TA trailer being towed with its hitch at its upper height limit. More load will shifted to the rear axle, and the leading axle will be carrying less, switch the nose height to its lowest limit and the relative axle loading will occur.

The situation is different for large multi-axle lorries, as they use tandem axles which are interlinked to provide load sharing. meaning that if the wheels have to traverse an discontinuity the trailer wheels conform and the load sharing links keep the relative differential loading on the axles to a minimum.

I am not entirely sure why caravan manufacturers opt for TA, arrangements. It is perhaps more of an aesthetic platitude. It certainly complicates the nose loading arrangements as the load transfer with changes to hitch height are far more severe than with an SA trailer.

TA's are much more difficult to man handle.

Yes they do have twice as many tyres as well as hubs and maintenance costs.

And the jury is out about difference is towing stability.

They generally cost more to purchase than an SA caravan of same size.

 

[Dustydog]

Hi Prof
I'm surprised at some of your comments.
I need to check my TA and make sure each individual axle is rated at more than 1700kgs. The axle loading plate is attached to each axle facing rearwards and positioned in the centre.
My tyres run at 29 psi, less than half that of the similar SA model which is around 65 psi.
My tyre loadings are 93. The SA 104.
I must confess it never crossed my mind that each axle and set of tyres on a TA must be capable of carrying the entire MTPLM.
I will check next week . Live and learn.

 

[ProfJohnL]

Hello Dusty,

You might find this article explains the points with some diagrams

http://caravanchronicles.com/guides/understanding-the-dynamics-of-towing/
This document was suggested in a previous thread some time ago. There are some inaccuracies in it, but it does provide a good general insight into the issues.

 

[SirRogerFFS]

Saint -Spoon.My Last two vans have been twin Axles.Firstly I find them more stable when towing.You dont get too much slip stream from other vehicles either.Now if you look In Practical Caravan Mag you will be able to check out the lay outs where I feel there is a bigger choice .I left SWMBO to choose our latest van as she has better eye for detail.You have mentioned about motor movers well I have been lucky so far and not had too much problem parking up on site. I will say that when you are reversing with a T/A the van I find doesnt respond as quick as S/A..Go to a dealer and have a look at them ours has a huge bathroom on the back, plenty of room .By the way there are no daft questions on here sometimes in jest you may get a daft answer but generally speaking there is a lot of knowledge on this Forum

 

[ProfJohnL]

............ I will say that when you are reversing with a T/A the van I find doesnt respond as quick as S/A................

This comment has been made on a number of occasions. The rate at which a trailer will respond to turning when reversing has nothing to do with number of axles but it is related to distance from the tow ball to the effective centre of turn. The longer this distance is the few degrees of rotation the caravan will turn for the same vehicle input.

It is generally the case that TA caravans are longer than SA's which means they will also generally have have a greater hitch to centre of turn.

 

[seth1]

Another plus with a t/a is they put less ground pressure to the erm...........ground.

 

[paws]

I have never used a twin axle caravan, so have no experience of them, but might it make changing a wheel a little easier in the event of a puncture?
I am thinking of getting an ordinary jack underneath....
Regards
paws

 

[Dustydog]

The TA does not freely rotate about the mid point of the two axles in the same way a SA does. The tyres definitely " scrub" which imo slows down or delays the turn. It's almost as if it is fighting you to stay straight.
I agree with Prof's point about length but in practice I suspect a SA and TA of the same length will not turn in the reverse at the same rate.
On Paws point I think jacking up a TA is far harder because they are heavier and to line up the Al ko wheel locks requires both wheels on the n/s airborne.

 

[ProfJohnL]

Hello Dusty,
Tyre scrub will have some impact on the path the caravan will try to take, but under the control of a car the forces will will not be enough to make any practical difference. Please also note that I stated the effective point of turn which will usually reside somewhere between the axles as both sets of tyres are likely to scrub a bit, but which will scrub least will be the pair with the highest loading and that is determined by a combination of how the caravan is loaded AND the angle the chassis makes to the ground. So it could be different with different tow vehicles.,

 

[David_5127637]

I have just changed my twin axle caravans. I have now had three over the last twenty plus years prior to which I had single axle but only 14 ft long. A caravan is like a clock pendulum swinging about a pivot - the tow hitch. My last caravan at 6.56 meters length was a Bessacar 500GL of 1998 vintage - a twin axle. This van was totally stable and never moved out of line behind my Jeep Grand Cherokee Overland tow-car. For me twin axles are safer. The points raised previously are valid - they need a wider turning circle (which is directionally stable), there are four wheels to maintain rather than two, there are on site levelling problems transversely. For me though, it is about tyre rubber in contact with the road, stability (especially with a puncture) and peace of mind.
Sadly I have had to get a bigger caravan to be able to have twin axles (yes, it too has ATC anti-snake system) but in the end you go with what you feel happier with. For me that is twin axle.

 

[limecc]

Comparing our old 1500kgs single axle to our current 1700kgs twin axle is no contest in terms of stability. I'd never go back.

Unfortunately I have to carry a trolley jack to line up those wheel locks and it cost me extra to get AWD motor movers but for me it is a price worth paying. I always struggled to top up 65psi tyres but that is in the past along with worry about blowouts.

Some other things, I find it pitches differently when towing. Slower but with more force when it does. Nose weight seems less critical. I don't need to put steadies down to walk in the van. When jacking it up no fear it will pivot and come off the jack.

 

[David_5127637]

Another convert to the twin axle club - great. My previous caravan, a 1998 Bessacarr 500GL, had twin axles with a shipping length of only 6.3 meters. It was superb to tow. When will manufacturers stop putting these long caravans on single axles? I know they wish to avoid the extra weight but the improved stability is sure worth it.
My current Bessacarr twin axle van is 7.99 meters shipping length - quite a brute!

 

[ProfJohnL]

Comparing our old 1500kgs single axle to our current 1700kgs twin axle is no contest in terms of stability. I'd never go back.
.................I always struggled to top up 65psi tyres but that is in the past along with worry about blowouts.

Some other things, I find it pitches differently when towing. Slower but with more force when it does. Nose weight seems less critical. I don't need to put steadies down to walk in the van. When jacking it up no fear it will pivot and come off the jack.

Hello Limecc,

You mention blow outs. There is no reason for a single axle caravan to be more susceptible to blow outs than a TA provided the tyres have been inflated correctly, and ignoring faulty tyres. Obviously if a SA has a blow out the there is a significant risk of damage to the caravan body and floor. The damage issue may be reduced for a a TA but care must be taken if you do have a blow out, to stop as soon as possible because when the blow out occurs, the load that tyre was carrying will be transferred to the remaining tyre probably doubling its load, yet its pressure will not have increased to cope with it. If it run for any time and being under inflated for the load its carrying will mean the side walls will be over stressed, over heated and weakened raising its probability of short term failure.

I am pleased you find your TA is more stable than your previous SA. Its easy to make the assumption the apparent improvement is down to having the second axle, but I can't necessarily draw the same conclusion. There are other factors that affect stability and you have already mentioned two of them, but possibly without realising their importance.

The extra mass and length of your new caravan affects the way it responds to the forces of towing. You have already commented on the pitching which is the rotation around the axle in the vertical plane, but it will have a similar effect on the Yaw motion in the horizontal plane. This gives the impression that a caravan maybe more stable, but the down side is when it does start to go, it needs more energy to be dissipated to bring it under control.

You say "Nose weight is less critical". Here I totally disagree with you. Regardless of the number of axles on your trailer Nose load is very important. You must have enough nose load to ensure the trailer maintains a positive pressure on the tow vehicles tow ball. If the nose load goes negative (which it can with a TA because of the interaction of the two sets of suspension) it will change the driving characteristics and ride of the tow vehicle each time the nose load direction changes.

In other threads I have described the importance of measuring nose weight correctly, especially for TA's partly because of the point above.

 

[limecc]

In practice it works differently regarding the yaw. Twin axles are naturally self correcting they only want to go in straight lines. This works for you when at speed and against you when manouvering. Despite the extra length of this van to act like a sail, it's far less susceptible to instability caused by side winds and white-van-man passing at 90mph. That's my observation based on practical experience. Out of curiosity have you ever towed one?

When I say nose weight is less critical, I mean it's easier to get right. I have a square area inside the van where I can place heavy items, but with single axles it's always a balancing act like a see-saw.

A cheap yet valuable contributor to stability for single and twin axles alike is the addition of shock absorbers and I fitted these and over-specced the tyres I purchased recently.

I forgot to add another twin axle benefit - braking and grip. There is an extra set of anchors and double the contact area with the road.

There are corner steady ground clearance issues with any long van. I used to dread crossing the road from the waiting area at Cowes ferry on the IOW. This may be marginally aided if a second axle means it can be closer to the rear of the van.