Independent suspension

Hi Pete,

As I haven't been around the country before, I can't say what conditions I will encounter, but I don't plan to go on tracks that are only suitable for 4wd only. That's not to say I won't end up on them, hence I want to be prepared.
I will be towing with 4wd & don't want to miss something, because the van is not up to it.
But I guess I am thinking, off road that will have corregations but are used by others, sand, small creek crossings etc.
But I am not planning to trailblaze new tracks into the wilderness?

Thanks Wamuranman,

Comprehensive response those laws of physics certainly knew what traveling in the outback was all about huh?lol

I have been doing some research & I have seen the cruisemaster systems & in fact have requested more info from them. I think Jayco use simplicity, but they didnt get me to excited TBH? I didn't think of so many shockies though? Why so many, was it to do with van weight? The swan (currently) is approx 1300 kgs loaded?

Yes Tony I guess the 2 shocks for each wheel it is to do with the weight and the spread of that weight. Our van is not overly heavy for an off road van (about 2260kg tare) and all up laden weight of about 2700kg (but is rated to 3000kg). Also I think 2 shockies per wheel allows a margin if you have a failure of of one of them.
Another consideration I forgot to mention is ease of wheel alignments with different brands of IS...they can get out of alignment. But with cruisemaster it is easy to do yourself with a digital spirit level. Not sure how the other brands of IS compare on ease of wheel alignment.
Cheers

Andy, Great pics..............

One question, limit chains seem to be at limit, how does one get any down travel of the wheel? Or am I missing something?

A great looking conversion Andrew - can't go wrong with Withers. Once you get onto the gravel, those pesky stones will try and do their worst to those lovely Koni's (mono-tubes I presume).....suggest some sort of protection will pay off.

This is what you've got to look forward to on your camper John.
I'm not sure about the limit chains but the van is empty at the moment so there's quite a bit of weight to go into it and I would assume it will settle for a while as the suspension starts to move around.

Darian, are you happy with the suspension on your Trackmaster? That's a lovely looking van you've got there.
It was watching a single axle Trackmaster going across a paddock at Easter this year that hooked me on the SugarGlider suspension.
My van is work in progress at the moment but I agree with your assessment about protecting them.
Originally when I got it back from Withers I only specified 1 shock per wheel as it was what I could afford but seeing that empty shock mount bracket got the better of me while I was working underneath the van so I just had to do something about it.
I does look good with the 2 Koni's per wheel though.
Cheers Andrew.

I opted for the Trakair air bag suspension offered by Trakmaster (but hell... did it cost !)....its still trailing arm with twin shocks, but different geometry to the Sugar Glider. Re shocks - the resistance they provide would be matched by an engineer to the work envisaged I presume - adding an extra (probably a good idea) would probably require a lower resistance spec for each... just a thought. My Trakair has two fat shocks per axle but the suspension layout requires them to be installed at a disadvantage - if their position could be closer to 90 degrees to the axle travel, my guess is one would do the work.

Wise words Mark - even on my 'offroader', I've added rubber flaps underneath to deflect stones away from the suspension arms, shockers and some other bits (I use recycled conveyor belting).... the stone damage takes a toll.

Darian, would it be possible for you to take some pics of how you have added the rubber protection flaps to your Trackmaster?
Regards Andrew.

Hi Mark,

Yes I have thought about that issue, I am thinking a stone stomper & some mesh across the chassis, but looking at the underneath I thought the Jayco looked OK, though TBH I haven't seen other campers to compare. One good thing with the Jayco chassis, it looks reasonably easy to couple the mesh up to the chassis?

But I am looking at increased height more than ride comfort, I do think IS will be a vast improvement over the beam axle configuration anyway in keeping the trailer square on the road, but as everyone says, need to drive to the conditions, but the height is for bad patches of road, be it a creek crossing or just plain lumpy. I just feel the 14" wheels are to small & besides, I want matching tyres to suit my vehicle, to give me more spare tyres, if it is needed?

ATM, I am concerned if the 100mm chassis will handle it, but I did notice that Andrew seems to have beefed his up, so that still seems the way to go?

The other concern now is that someone mentioned compliance issues, need to sort through that first?

Cheers

Tony, when I 1st started looking at my project I looked at what was underneath most of the "offroad" vans on the market.
They generally comprise of 6" A frames and chassis' and my Galaxy had 4" A frame and chassis.
For the anticipated weight of the van and considering it was a single axle, Withers engineering were recommending using a 6" deep crossmember so it was a "no brainer" for me to uprate the chassis before I had the suspension fitted.
So I cut off the springs and welded a 3" X 2" X 3mm thick riser tube going from the end of the A frame to the rear of the van and removed the barrup strap under the A frame and replaced it with a 2" X 2" X 3mm thick RHS underneath the A frame.
This now gave me a 7" deep chassis and 6" deep A frame and being laminated they are actually stronger then standard 6" X 2" RHS.
With suspension and chassis mods and an increase in tyre size from 195R14 to 205R16 I've lifted the van considerably which is what I was looking for in the 1st place.
For compliance, I'll be taking it for a weighbridge certificate and presenting the van to an approved engineer so it can be certified for the increase in the ATM and have the van issued with a secondary compliance plate.
Regards Andrew.

Thanks again Andrew,

You have certainly reinforced my thoughts. I will be getting the suspension supplier to fit the gear & the chassis issue is solved with your mods.

Cheers

Andrew,

This was part of Neville Withers response to my enquiry?

Hi Tony

With the van being fitted with the sugar glider suspensions the van will be raised about 75 mill which will give room to run bigger wheels which can be easily obtained.

The caravan chassis does not have to be strengthened because the soft suspension with the long coil springs and shock absorbers will give a very soft ride and will not hurt the van or cause the furniture to be separated from the floor.

The only issue you need to be concerned with is whether your axle rating is above the weight of you van when fully laden.

But I do like the idea of strengthening the chassis, so I will make appropriate enquiries.

Cheers

Mate you are converting from a system that supports the load on 4 points on the chasis to a system that supports on two.

AND

you are converting a van intended for road use to one intended for off road use.

think about it

cheers

Andrew asked for a couple of pics (quick snaps off the smartphone)......I've fixed the flaps with cable ties (cheap and easy to replace) - the suspension is painted - the chassis is a full hot dip - I don't want to drill either. While under the van doing routine maintenance, I can easily slide the odd replacement cable tie through - I've only done a dozen or so in 2 years.

Thanks Darian, that gives me some ideas on how to protect the arms and shock absorbers.
Regards Andrew.

I guess it's simple, I did it because I could and it's what I wanted.
My van had slipper springs and dragged its bum around the country, the tyres were hitting the inside of the wheel arches and according to friends who followed my van it "bounced" down the road while under tow.
It was already at the weight limit for the axle group and I needed to do something so I could visit places that were otherwise out of bounds for me.
To uprate the axles to a 2 tonne anticipated ATM then then hubs, 12" drums, backing plates, offroad magnets etc need to be bought and that's a $1050 bill in itself whether it is leaf spring suspension or independant.
That's buying top line components not imported.
I was looking for a compliant suspension and what I've done fits the bill perfectly for what I want to achieve.
A friend of mine has done exactly the same thing to his "shackle" leaf sprung Goldstream Explorer van and in his words "it's transformed the van".
Strengthening the chassis? Yep I did that. Strengthening the A frame? Ditto.
This project has been pretty well thought through, down to the point of counter weighing the van to find out exactly what weight is on either wheel across the van and on the towball so I can position weight to its best advantage.
Yes it's been a learning curve, but as I said previously we are basically happy with the van we have and I now have a suspension system that will handle most things I can throw at it.
Regards Andrew.

It's rare that somebody spends a stack of money and then declares it has made no difference at all, so I'm always dubious of such statements. However I do know that all of my leaf sprung trailers seem to bounce far more over cattle grids than my neighbours independently sprung box trailer.

Whilst "ride comfort" seems unimportant in a trailer, in fact every jolt is causing wear and stress to every item, component and joint on the trailer, and is also being transmitted back through the hitch to the tow vehicle. So where possible I do believe a smoother riding trailer is preferable.

I wonder if there are any parabolic springs available in suitable sizes for camper trailers? Parabolics have much the same travel as standard spring packs, but seem to give a smoother ride whilst retaining the same "simple" beam axle/leaf spring suspension design.

Yeh well any propely designed suspension would be better than a frankly slip shod suspension that allows the wheels to rub on the body.

This is the simplest thing to get right when building a trailer.....fitting an axle of the correct length......but I am gob smacked how many "trailer manufacturers" can't even get this right.

What hope is there that they can select the correct spring rate or do any meaningfull suspenion design.

A trailer bouncing down the road is no real problem ( remember what I said about expectations) as long as the frequency is low.

My 40 year old box trailer bounces over large bumps, but because it has long springs its peroid of oscilation is low when loaded and it presents no problem.......but this trailer is unusual, because it has long, soft springs.


As I said, it is the short hard springs that are responsible for the misbehaviour of a great many trailers.

As I say if you are serious about a leaf spring trailer you will not use trailer springs, you will get them fom some sort of suitable passenger car or light commercial.

Then we get to the issue of shock absorbers.

just adding shock absorbers to a leaf sprung trailer will radicaly change how it behaves....and as someone else has mentioned you will need far less damping on a leaf spring, particularly if the spring rate is properly matched to the load.

As for the availablity of parabolics, I run parabolics on the ligheter of my two boat trailers, purely on the basis of corrosion.

If you select a vehicle that has aftermarket paraboloics available, of course you can fit them to your trailer.


Then there is the issue of correctly mounting springs, very few trailers do they bother makig the spring hangers long enough to allow the spring to run its full travel, and they almost never fit bump stops.

All these parts and the design lay out can be got off the donour vehicle.....look at the rear axle ratings of variuos vehicles and most of the work is done for you.

Wnat to carry about 1.5 tonne, look at a hilux or navara, up in the 2 tonne bracket look at the 70 series.

If you want a soft compliant suspension that still carries a bit of load..look at one of the vans.

cheers

Hi Joe,

I have swung back to that conclusion myself, I was convinced I need a bigger van for this trip, but I have just got back from a couple of weeks in it & think I can make appropriate improvements thus saving quite a bit of money in the process. Its still got more room than much bigger vans.

I know they are not made for off road, but some height & driving to conditions should see me through.

I have a few emails away to the suspension people & will start getting some estimates.

Many thanks to every one for there thoughts.

Whilst I haven't actually weighed it, I would be fairly certain that there wouldn't be any great increase in weight between my CruiseMaster system and an equally rated beam axle and leaf springs. The CruiseMaster is fabricated from about 3m of 50x50 (from memory) RHS with a coil spring and solid stub axle and is rated to 2300kg. A 2000kg rated beam axle would be perhaps 1.9m of 50x50 bar (solid), plus the leaf springs, and to get to 2300kg might require a 60mm solid round axle. On the other hand, the 1200kg CruiseMaster uses the exact same arms, and so is relatively heavier when compared to a 45mm (or less) solid axle, though I'm not sure it would actually be heavier.

The normal mounting for a leaf sprung axle is on top of the spring pack, so the spring pack and U-bolts hang below the axle. Whilst I can't speak for other brands, on the CruiseMaster there is nothing below the axle other than the end of the arm on which it is mounted (round on square, if you see what I mean) which is about 10mm. Obviously the drop axle version is different and the axle is raised to the top of the swing arm by design, giving perhaps a 60mm protrusion.

To start with these is no reason whatsoever that the axle can not be mounted under the springs......toyota hiluxes come that way as do a great many trucks.
Two of my current trailers are sprung over.
My larger boat trailer is sprung under with 13 inch wheels when you order the same trailer with 14's it comes sprung over

Running spring over axle will result in at least an extra 2 inches of body height at no cost whatsoever....AND...actually spring over axle makes more engineering sence, because the spring sits on the axle, and the whole weight of the trailer is not borne by the U bolts.

Running spring over puts the nits and threads of the U bolts out of the way on top

the other issue with independent suspension is they do not distribute the load across 4 widely spread points on the chasis like leaves do.
The longer the leaves the better.

Thus significantly stronger chasis long members are required and thus an increase in weight.......and I doo mean a significantly stronger chasis

BTW a 50mm round axle is good for 2 tonnes with 16 inch wheels and a 60mm round axle can be good for 3 tonnes

cheers

Yes, the axles can be mounted under the springs, but most aren't so the assumption that Ross made that IS gives less ground clearance is wrong. In fact almost ALL "definitive statements" are wrong, or at least not applicable in all situations. I have a Hilux (spring over), a bunch of Land Rovers (including aftermarket parabolics, spring under), beam axle trailers with spring over and under configurations, dual axle trailers and I'm currently building an IS trailer. They all have pro's and cons, and there are good and bad designs of each variant!

The stresses applied by suspension to the chassis are not always obvious. An overly stiff leaf spring applies far more shock load over the entire trailer than a suitably damped coil sprung system, so the overall weight (if we regard weight as a function of strength) requirement may well be less for a coil sprung chassis. Additionally, whilst it is true that at rest the coil spring transfers the majority of it's load through one point (the spring base), once on the move the shock loads are shared with the shock absorber and also the swing arm pivot - 3 points spread over 780mm on my system, much the same as most standard leaf springs, slightly shorter than offroad "rebound" leaves (often 880mm or so eye to eye, plus shackle offset).

In fact until the IS system hits the bump stops the spring base can never (within practical considerations) take more load than required to compress the spring, and because of the shock absorber converting force to heat, the energy reaching the chassis is greatly reduced. In a well set up leaf spring, there is also a damping effect (friction creating heat in the springs) but as most spring packs aren't well designed and are often largely seized solid, they tend to transfer far more shock load to the chassis. The reason a (poorly designed, as most are) leaf sprung trailer bangs and bounces is that MORE force is exerted on the trailer. The poorer ride quality of the leaf spring is a direct indicator that more force is being applied to the chassis, so I'm not convinced of the logic that an IS system requires a stronger chassis.

Having said that, it is also true that any retrofitted system that does not use the original mounting points (including longer leaf springs) may well require additional strengthening because the "new" mounting points were never designed for the point stresses involved in suspension mounting. This applies to ALL spring bases, whether leaf or coil, and ALL shock absorber mountings, whether with leaf or coil springs.

And please don't get me wrong. A well designed and well set up leaf spring can be a very effective system, though I think we agree that most such systems are neither well designed or well set up! But I believe that IS is still an improvement on it. Whether that improvement is required or cost effective is up to the individual to decide.

Ya drawing some pretty longs bows there.

we keep hearing people comparing thier coil indemendent systems to the worst of the leaf spring systems......this is hardly fair or reasonable.

There is no reason a leaf system has to be overly stiff and there is no reason it can not be fitted with adequate shock absorbers.

Thus most of the arguments against leaf sprung systems are spurious because they are only valid in relation to badly desigend leaf systems

Then we start looking at the physics involved in the in the coil independent system.

AT no point does the front pivot point in a trailing arm system support any significant downward load.


Likewise you can not rely on shockabsorbers supporting any load what so ever.....in fact the load on the shockie mounts will constantly be changing from up to down.
In fact under certian circumstances the shockie will be placing more load on the springs and the spring cups as it damps the suspension in downward travel

So we are back to a system that supports almost its entire load on the spring cups.

saying that most leaf spring systems are sprung under, is both incorrect and irrelivent.
Likewise saying that most leaf spring packs are seased solid is plainly rediculius.

the simple fact that a leaf sprung system distributes its load across 4 widely seperated points is fundamental to the design of the beam or truss that is the long side of a trailer.

So we have a 8 foot long box trailer supported on 4 foot long springs ( well designed spring packs with friction pads and a progressive designe like you would get off a hilux not the crap you will pay $50 for at the trailer shop).
the main chasis overhangs 2 feet either side of the spring hangers, likewise the span between the hangers is supported no more than two feet from a spring hanger.

Install an independnet system of any type and you have now doubled the overhang from the main load point that is unsuported.

this is realy basic beam and truss theory, doubling a span or overhang requires significantly more strength.

Perhaps this is why so many of these camper trailer sre so rediculously heavy....come on now 2.5 to 3 tonnes for a tent screwed to a box trailer.......someone is not engineering smart here.

But that is typical of the trailer industry...very little real engineering goes on.

cheers

"At no point does the front pivot support..."

The coil spring provides no forward-backward location of the swing arm. The shock absorber is at an angle, and heading backwards from the moving end of the swing arm, forming a "V" shape. If an upward force is applied to the swing arm, and resisted by the shock absorber, that upward force is split between the shock absorber mount and the front pivot mount. If you remove the coil spring you will find exactly the same thing happens even without the spring being present. If you push up on the point of the "V", the force is split between the two ends. The fact that one side of the "V" slowly decreases in length does not affect where the force is going. The amount of force that is split between the shock absorber mount and the swing arm pivot depends on the resistance of the shock absorber. If the shock absorber was removed (or given zero resistance), then all the force goes to the spring and no force is split over the two other mounts. If the shock absorber is replaced by a solid rod, then 100% of the force is split between the swingarm mount and shock absorber mount, and none goes to the spring - effectively we have created a completely inflexible "leaf spring". The forces are upwards and forwards (at the front) and upwards and backwards (at the rear), the same as for a leaf spring. In the real world, it will depend entirely on the resistance of the shock absorber to upward deflection. With a slow deflection the shock will provide minimal resistance, but there will also be very little force above the "at rest" level, so the trailer will rise. With a fast deflection the resistance will increase and the concepts described above will come in to play. However it is also important to note that upward damping is far less than downward damping, the system allows the swingarm to travel upwards (so the trailer doesn't get bumped) and then damps the downward movement. In a leaf spring this is not the case, and the built in damping causes greater shock loads to reach the mounts because the leaf spring itself resists bending. Again this is minimised in a perfectly set up leaf spring, but those are (as we agree) rare. The load is more centralised with a coil spring, but the degree of "shock" is greater in a (typical) leaf. There are other factors at play here. The resistance of the spring to compression determines how much force is exerted on the chassis, and that changes through the range of suspension movement and the design and spec of the spring, and if either spring runs out of travel (or increases its spring rate radically) the result is a thump. In reality either system with the right spring can produce low shock loads on the chassis if they stay within their travel and/or have a suitably soft rate increase with compression. Again, the issue is that most leaf spring systems are not well designed or well maintained.

"so many camper trailers are heavy"

I agree. But the mindset is very basic engineering using heavy sections. I used to build spaceframe chassis "performance" vehicles which were very light. But there was a huge amount of cutting and welding, not to mention some complex design work to make sure that they were stiff enough and strong enough. And they were tricky to repair. A trailer on similar principles would be much lighter. But would it be repairable in the outback using a sledge hammer and 2 x 12V batteries? And could it be designed and made by a typical trailer company? At a realistic price?

Again, I do agree that a well designed and set up leaf spring is a good system. But such systems are rare.

You can go on about 'v's and loads transfered....the fundamental fact is that no independent suspension system distributes the losd widely on the chasis thus requiring a substantially stronger chasis...and if indeed any engineering design is employed and very different arrangement of members in the truss or beam.

leaf springs remain the lightest trailer frame option, A because of the widely distributed load and B because there is tortortional load on any members in the chasis.

Look at the pictures in reply 3

note the additional overlay of a second layer of RHS, the additional braces and the large crossmember that carries the trailing links.

Look in the pictures in follow up 8 of 8.....note the tall long sections and the double layer RHS and the diagonal braces.

Also look at the suspension geometry..there is no way the trailing link attachments are carrying any significant vertical load.

The massive size of the trailing link carrier is required to cope with the tortional loads locating the wheels horisontally.

It is plainly clear the vast majority of the load is borne on the airbag mounts.

Look at the pictures in relpy 12 of 13

Look at the massive size of the compomnents in the trailing link suspension compared to the surrounding chasis.

There is no possible argument that a significantly stronger chasis is required to support the sorts of independent suspension being offered.

And any notion that independent suspension is lighter in its self simply cant be maintained.

Note that one poster says fitting independent suspension added arround 100KG to his trailer......that is no small potatoes...that is arround 2 steel bullbars, 3 electric winches with steel cable , 4 x N70 batteries or arround 120 liters of fuel in plastic jerries.

With almost everybody pushing close to their GVM, this is a very real issue.




Of course there is an inbetween option, and that is a tranverse arm independent suspension resting on more or less conventional leaf springs like was fitted to the rear of the honda Coup 9.

Where the single beam axle is simply replaced by a pair of plain light transverse arms anchored to the opposite chasis rail.

It is simple light, offers improved centre clearance and maintains the same 4 point support of a leaf spring system and does not require an increased chasis strength or weight.

BUT the problem is that it is, far too simple, looks far to simple, can not be protected by design regestration.....any half decent boilermaker could build one..quicly and cheaply.

In fact is has been used and proven in off road trailers over may decades.

So we are never likly to see it promoted.

cheers

True, but unfortunately that argument runs along the same lines as criticising "worst case" leaf spring systems. You are arguing that a well designed leaf spring system works well (which it does), and stating, quite rightly, that pointing out the shortcomings of badly designed leaf springs is not relevant. However the examples you mention are nearly as far from "well designed" independent coil sprung systems as the rusted up slipper leaf is from a 4' PTFE laminated leaf spring.

The system that I am currently working with uses the same swing arm from the 1200kg model through to the 2300kg model (and in fact on to the 4 wheel versions). From an engineering viewpoint that is dire - if it is specified correctly for 2.3T, it is at least double the required strength (in simple terms) for a 1.2T installation. The result is that the unsprung weight on a 1.2T trailer is grossly higher than needed, and total weight is also too high.

Again, this is another case of poor design and engineering, but that does not write off independent suspension, nor make it an inherently worse option. It means that like 99% of leaf springs systems, it was not the best possible design, but instead was a compromise to allow a single component to be sold in to a great many applications. That, sadly, is the reality of modern commerce - quality is no longer a driving force!

Adding 100kg by installing independent suspension is equally questionable. We can, of course, add as much weight as we like by keeping adding steel. But that doesn't mean it's actually required. The doubling up of the RHS is another example. That is not a function of the suspension design, it is a function of selling a "weld in" component. It is easier for the installer to simply weld on a preconstructed length of 50x50 RHS than it is to weld 4 plates in exactly the right places to fit the swingarm and align it correctly.

The transverse arm suspension is indeed a good design, though it does not gain quite the centre clearance available from a swingarm system, assuming that the tow vehicle is not the limiting factor. However in a leaf spring installation it too suffers from the largely inherent problem of spring friction. Whilst the leaf spring divides the load equally between two points, even with lubrication and tip pads it still suffers from internal friction that increases the force applied to the chassis as a shock load. A coil spring does not, and so transfers far less shock to the chassis.

Overall, as I said previously, any suspension system can be made to work well if designed correctly for the job and well maintained. Equally any system can perform poorly if badly executed.

The whole shock load thing due to leaf friction is being grossely overstated.

And is totally spuirious in justifying retrofitting a single point of support where two points of support previoulsy existed with out additional reinforcement.

BTW.....what "shock load" is not transmitted thur the leaf is transmitted thru the shock absorber back to the single point of support.....sorry there is no free lunch

The fundamental principle of load distribution on the beam /truss remains.......it can not be explained away...and regardless the same or similar forces are in play in a similarly compliant suspension

On the matter of clearance....almost without exception the independent suspension systems suffer badly in clearance close to the hub


Look at the pictures, some of them show large lumps of metal almost as low as the brake backing plate...these present more oportunities for hang up that any plain beam axle would.

Remember we are trained and it is good practice to know where our diff pumpkins are and to place our wheels accordingly........what these independent systems do is place hang up obsticles exacly where there are clear spaces in tthe track of most 4wd tow vehicles.

Sorry but none of te commonly available independent suspensions impress me at all, they all look crude, have short crude single arm trailing links that place whatever spring is used in a tight arc or curve and present as many clerance and hang up problems as they claim to solve.

When someone produces a dual wishbone or dual paralalogram trailing link suspension that places alll suspension componenst above the axle line, I'd say there has been a step forward.

But at the moment the advantages "as sold" simply do not exist in my view and I see nothing that would convince me that they perform even as well as a properly designed leaf spring beam axle system...especially in a single axle configuration

cheers

Shock load is a function of suspension compliance. The suspended component moves in response to a force, and the more compliant the suspension the less force is transmitted to the chassis. A coil spring has little internal friction and is very compliant, whereas a leaf spring has far more friction resulting in a higher resistance to bending. When exposed to a constant load, both springs will be compressed to a given point based on that load. However when exposed to a sudden shock load, the resistance to bending (friction) causes the leaf spring to transmit more of that load to the chassis than the coil spring does.

This is the reason why a rusted up old leaf pack may still compress when a static load is applied, but will give a very harsh ride as it resists rapid compression and transfers the shock loads to the chassis. It is also the reason why people who want the best performance from leaf springs use UHMW tape between the leaves in an attempt to reduce friction - the leaves slide over each other allowing the spring to respond more quickly, and why parabolic leaf springs (with no leaf to leaf contact) behave far more like coils and give a far smoother ride.

The energy involved in a wheel hitting a bump is constant no matter what the suspension type. The force applied to the chassis is not constant. The idea of suspension is to absorb very rapid movement and control it, rather than passing it on to the chassis. If the spring absorbs 100% of the energy, and releases it in a controlled manner, we have a very smooth ride. If the spring absorbs 50% of the energy we have a more "bumpy" ride. The inherent friction of a leaf spring means that it passes more energy to the chassis than the coil spring.

Conversely the friction in the leaf spring means that it does a better job of controlling the release of that stored energy, whilst the coil spring releases it more quickly causing the chassis to bounce and rebound. That aspect of the coil spring is solved by using a shock absorber to control the release of that stored energy. In "normal" leaf spring packs the shock absorber (where fitted at all, for example in cars) is of much reduced resistance to allow for the springs internal damping. However where the leaves are replaced with parabolics, or interleaf friction reduction is used, the shock absorbers have far more work to do.

I think someone needs to declare their interest.

Are you or are you not directly or indirectly connected with a manufacturer or marketer of independent suspension systems?


Your blinkered anti leaf spring rhetoric tell me you are.

cheers

No, absolutely not. I have no links with any industry related to this thread whatsoever.

As for "anti leaf spring rhetoric", I think you may need to reread my comments with a more open mind. My comments about both systems are unbiased, I have stated that both have pro's and con's, and that both can be made to do a good job if they are correctly sized and adjusted for the job at hand. I have agreed with you that the majority of suspensions systems, both leaf and independent, are far from "well suited" and are in fact often grossly over-engineered or poorly implemented.

My view that independent suspension is preferable is simply based on many years of using vehicles and trailers with a variety of suspension systems over a variety of terrains in a variety of countries. Throughout those experiences, my overriding impression has been that coil sprung systems in general do a better job of diminishing the shock loads transferred to the chassis, whether that chassis is a trailer or not. They are not a universal solution, and for high capacity trailers with unbreakable loads (for example a hay trailer) leaves work better as it is far easier and cheaper to use high tonnage leaf springs than equivalently rated coils. But a trailer to transport computers will use coils or airbags to protect the load as much as possible from shocks.

It is also my view that ride quality is in fact a very important aspect of lightweight trailer design, because whilst there are no passengers the laws of physics tell us that shock loads create far more stress than static (or nearly static) loads. The suspended structure and it's contents has a far easier life if such loads are minimised. Force = mass x acceleration, and given a constant mass we can reduce the force by reducing the acceleration. To do this we need to absorb energy in a spring rather than transmit it to the chassis, and coil springs are better at this than leaves. Any energy absorbed by the spring is not transmitted to the chassis, reducing stress. It is also not transmitted through the hitch to the towing vehicle, reducing stress on that vehicle and making a more comfortable ride for its passengers.

Whilst it is possible to improve leaf springs by using parabolics, UHMW laminated leaves or even the old Jaguar approach of grease/oil flooded leaf packs in a leather "bag" to prevent the ingress of dirt, experience tells me that the coil spring is in fact a far simpler way to produce a compliant suspension system.

In fact it is clear that coil springs provide a better ride, because almost every passenger vehicle in production uses coil springs (or airbags) and independent suspension. Any argument as to their efficacy in this regard is pointless. Even a direct comparison between an old beam axle Hilux and a new IFS version demonstrates the superior ride. The only argument is to whether they are suited to a lightweight trailer, and whether "ride quality" is important in a trailer. I recall you have stated that you don't believe it is. On the other hand I think that reducing force and therefore stress on a trailer and its contents is beneficial for a number of reasons.

Taking all that into account, I believe that a well designed independent system is preferable. But that's just my opinion, and I have nothing to gain or lose if others agree. It is simply a discussion.

Bantam, have you ever towed a trailer over distance with independent coil suspension and have you compared it with a leaf sprung trailer...... if you have what was the outcome?

Our Ultimate has sugar glider suspension and it's unbelievable over rough stuff our bike trailer has leaf springs, our old TrakShak had leaf with shockers and the independent coil spring setup leaves them for dead.

The same reason independent coil suspensions is far superior then leaf on motor vehicles and 4x4 for 95% of applications.

where those leaf sprung trailers properly designed or where they straight from the trailer parts catalogue and designed by a welder.

Look at what people are talking about in 4WD land......the overwhelming preference for "seriuous" off road work is still the beam axle, mostly leaf sprung, there are very few " serious 4wds" that are running independent suspension.

I agree that it is probably easier to buy a passable independent sprung trailer, but that has very little to do with the real benifits and the potential of both systems if equally implimented

AND the fact remains, an independent suspension requires a different and definitely heavier chasis design.

AND most importantly, if retrofitting an indepndent system where a leaf system was.....it would be foolish indeed to do so without reinforcing and bracing the chasis....particularly if the existing trailer was not specificaly intended for off road use.

cheers

Serious off-road work is a very different thing to a trailer. The "most serious" off road vehicles are not even road legal (actually most modified vehicles aren't actually legal in our over-legislated country), and are designed to do a very specific task, travelling quite slowly. That requires massive suspension travel. Less serious off-road vehicles are mostly just copies made by kids who have read a magazine but have no idea (I once had one of them mock me for suggesting that three lockable diffs was the best solution for traction, "3 diffs, haha, you moron there are only two diffs" - he was clearly an expert!)

Trailers are designed to do a bit of off road work and a lot of high speed driving. The loads involved with crawling along a track at walking pace are very different from corrugations at 80-100kph, or hitting a bump at 80kph. As you have said (I think it was you, sorry if not!) droop/flex isn't important in a 2 wheel trailer, as the entire trailer drops. Again, suspension must be suited to the usage, and what suits a serious offroad truck is not always the same as for a trailer. The only real requirement for trailer suspension is to reduce shock loads to the contents. And again, I feel that coils do a better job of this.

I do wholeheartedly agree that an onroad trailer may well need work to become and offroad trailer, but that will vary from case to case.