Caravan under Slung vs Over Slung Suspension

Submitted: Monday, Nov 10, 2014 at 12:36
ThreadID: 110097 Views:6796 Replies:7 FollowUps:5
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Hi All,

Just looking at changing my rolla rocker suspension from under slung to over slung as I need the van to be lifted 40mm to get it level to the vehicle as I cant get enough adjust out of my adjustable hitch. Also the extra height would be handy on the van for getting in & out of caravan parks.

My question is have any of you that have made this change noticed any change in towing stability as I am worried it may make it less stable lifting the van height.

The van ATM is 3050kg.

Thanks in advance for any feedback.

Regards

Richard
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Reply By: pop2jocem - Monday, Nov 10, 2014 at 12:56

Monday, Nov 10, 2014 at 12:56
Richard,

Our 22' van has an ATM of 2800 kg. Roller rocker suspension.

I did look at the option of over v underslung axles but decided to go with getting an extra 75 x 50 mm chassis extension which of course raises the whole van by that amount.
To me it also gives the existing 100 x 75 chassis member some more stiffness lengthwise. How much of an advantage this is I'm not sure, but having that extra ground clearance has certainly made the entry and exit of CP's and servo driveways a much less nervous operation.
I have not had any noticeable instability even with very strong cross winds or trucks passing in opposite directions or overtaking. Doesn't seem any different from original.

Cheers
Pop
AnswerID: 541494

Reply By: TomH - Monday, Nov 10, 2014 at 13:03

Monday, Nov 10, 2014 at 13:03
If you have a HR hitch why not turn the hitch over to get it to point up instead of down.

Will give you heaps of adjustment.

Or weld a piece of box on the A frame to lift it the 40mm
AnswerID: 541495

Follow Up By: Member - Richard L8 - Monday, Nov 10, 2014 at 14:54

Monday, Nov 10, 2014 at 14:54
Hi Tom

The HR hitch is as low as it can go already. I have a Disco 3 with a Mitchell hitch on it which is a high hitch for better 4WD departure angles & for ease of spare tyre removal when Caravan is connected.

I was leaning towards over slung modification as it will help clearance issues in caravan parks & servo's as well as the vehicle height issue also but worried about inducing stability issue.
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Reply By: gbc - Monday, Nov 10, 2014 at 14:03

Monday, Nov 10, 2014 at 14:03
Under slinging should raise it by more like 100mm. Ive done it on trailers and boats etc over the years. No appreciable difference to the tow characteristics from what I have found, though the mathematics show there is a small one.
AnswerID: 541506

Reply By: Patrol GU IV - Monday, Nov 10, 2014 at 14:47

Monday, Nov 10, 2014 at 14:47
If you have a standard adjustable hitch you can buy a longer drop shank as shown below :

http://caravansplus.com.au/catalog/product_info.php?products_id=9520
AnswerID: 541510

Follow Up By: Member - Richard L8 - Monday, Nov 10, 2014 at 14:58

Monday, Nov 10, 2014 at 14:58
Cheers for the link. I've been searching the net & hadn't came across these.

Regards

Richard
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Reply By: Ron N - Monday, Nov 10, 2014 at 15:06

Monday, Nov 10, 2014 at 15:06
Richard, if you only want 40mm total lift, then the better method of doing it, would be to install new (longer) spring perch brackets.

This would involve carefully cutting off the old brackets and welding the new brackets on.

A move from under slung to over slung springs is going to give you 100mm or more lift, as gbc has pointed out.
There's a substantial difference between 40mm and 100mm, and you will notice it.

The difference in ride height also makes for an increased step-up, into the van.
Doesn't seem like much, but you will notice it.
Try jacking the 'van and supporting it with jack stands at the two different heights, to see how it looks.

100mm lift will only make a difference in stability if you have soft springs.
If you have four spring roller rocker suspension, then it rates as a stiff suspension.

Cheers, Ron.
AnswerID: 541511

Reply By: Rustytuba - Tuesday, Nov 11, 2014 at 07:13

Tuesday, Nov 11, 2014 at 07:13
Fitting "straight" axles will lift it 50mm.

cheers

AnswerID: 541532

Follow Up By: Member - Bruce C (NSW) - Wednesday, Nov 12, 2014 at 14:48

Wednesday, Nov 12, 2014 at 14:48
I can never work out why they use drop axles.

To my thinking this places undue strain on the U bolts and springs as every time you go over a bump the axles would want to lag behind the U bolts and axle beam magnifying the stress and therefore creating needless stress. The drop axle therefore, in my opinion, makes an inferior solution. See pic.



Note that I am not a qualified engineer but I do have a pretty good comprehension of the stresses involved.

You still get this effect with straight axles but only slightly and nowhere near as evident as on a drop axle.

The only thing a drop axle offers that I can see is a lower centre of gravity.

At home and at ease on a track that I know not and
restless and lost on a track that I know. HL.

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Reply By: Member - Bruce C (NSW) - Wednesday, Nov 12, 2014 at 15:11

Wednesday, Nov 12, 2014 at 15:11
Hi Richard,
I have a 24 foot Jayco Sterling which had the axle beam fitted under the springs before I got it, by a previous owner, and I have no issues with that at all and would advise anyone wanting to do the same to do it as it does raise the clearance and does not effect handling as far as I can tell. Mind you I have not had it much above 110 and very rarely at that but usually travel around 90 with it with absolutely no problems.

We did the same thing on the mates 21ft Jayco and he reports no handling problems either and is very happy with it. He was having the same problems as you with ride height and is now very happy with the outcome. He was recently up in the NT and had it up to 130 with no issues just to see what the 200 series cruiser would do with the van on. Mind you there was not a bend in sight no doubt.

The nephew has a 21 ft Jayco and it also has the springs fitted on top of the axle beam as well for extra height and he is happy with his.

Another mate got a new Jayco Discovery and we did the same to that and he is also very happy.

My advice is do it if you are of a mind to, but,

As Rustytuba says, a straight axle is a better solution if you can afford the change over. A bit of money involved though I would say.

Cheers, Bruce.


At home and at ease on a track that I know not and
restless and lost on a track that I know. HL.

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Follow Up By: Member - Richard L8 - Thursday, Nov 13, 2014 at 08:36

Thursday, Nov 13, 2014 at 08:36
Cheers Bruce for the feedback. I going to change it over on the weekend ready for a trip to Batemans bay the following weekend. Looking at it last night it should be easy enough to change back if it does affect the handling.

Regards

Richard
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Follow Up By: Member - Bruce C (NSW) - Thursday, Nov 13, 2014 at 10:59

Thursday, Nov 13, 2014 at 10:59
Richard,
the simplest way is to drop your springs and replace them above the axle. Otherwise you are constrained by the wires to the electric brakes, if they are fitted. This is also a good opportunity to replace any worn bushes etc.

I don't think you will have any regrets doing the spring change over.
Neither the mates nor myself have any regrets, in fact I cannot tell the difference in handling.

Mind you I do not drive fast enough to really test it out.

A further piece of advice is that, should you find you have some loss of effectiveness on you electric brakes, do not be tempted to replace just the magnets, or just the shoes. It is by far cheaper to buy the whole brake setup as a complete unit for each wheel. That is 'backing plate, hub, shoes, new bearings and magnets as one single unit as it is about half price doing it that way.

The other thing is that the drums wear on the vertical face where the magnets run, as well as the magnets wearing, and this was one of the major causes of brake efficiency loss in my case.

Cheers, Bruce.
At home and at ease on a track that I know not and
restless and lost on a track that I know. HL.

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