Thread: Multi-purpose trailer suspension choices.

Hi all.
I'm planning on building an electric braked single axle, 7x4 box trailer at some stage in the future and I thought I'd run a few ideas by you clever cookies. The beast in question is intended to be a general purpose work-horse, or a camper when the need arises and I'm trying to minimise the trade-offs. I don't have the room to store two trailers, so I'll just take the camper top off when not required.


I've done more than enough (over)loading 1t of rock/sand/bricks into a standard 6x4 and now I'd prefer to do it legally and safely, so a frame and draw bar from 75x50x3 RHS with a suitably rated electrically braked axle is on the cards. There will be side tool boxes to the front and rear of the wheels, but I want to retain the standard basic box shape for load carrying, so that means no dedicated camper features like side drawers that run into the load area.


For my needs as a camper, the spring rates probably should be optimised for a load of maybe 400kg, or around half of that required as a load hauler to reduce ride harshness. I also like the idea of shocks as well for improved road holding and to keep luggage in one piece if I take it off road.


I think this guy makes a lot of sense when he talks about using a full size car leaf spring with shocks instead of smaller trailer springs and no shocks. The phenomenal distance he has travelled without damage due to vibration suggests to me that there must be something to this theory. Anyone have any thoughts about using the variable rate leaf packs from something like a one-ton ute in an attempt to reduce ride harshness with a lighter load when used as a camper while retaining some load carrying ability?


Well, that's enough topics to get the conversation started for now. Please fire away with your thoughts and suggestions.



Cheers,
Mick

Originally Posted by mikm

Hi all.
I also like the idea of shocks as well for improved road holding and to keep luggage in one piece if I take it off road.

Hey Mick

IMO dampers are not required as they are designed to dampen the oscillations of the spring to assist road holding and steering which is not required on a trailer. Most use gas over hydraulic shocks that stiffen the suspension. Perhaps a very light hydraulic damper might be okay, but I still dont think that important. Correct weight for the springs is the main thing.

Originally Posted by mikm

I think this guy makes a lot of sense when he talks about using a full size car leaf spring with shocks instead of smaller trailer springs and no shocks. The phenomenal distance he has travelled without damage due to vibration suggests to me that there must be something to this theory. Anyone have any thoughts about using the variable rate leaf packs from something like a one-ton ute in an attempt to reduce ride harshness with a lighter load when used as a camper while retaining some load carrying ability?

This setup has been going around for a while and there are probably just as many failures as those that shine. I would suggest that most failures are down to overloading, lack of maintenance and/or excessive speed for the conditions. I think a lot of the failures on the cape particularly is down to excess speed for the conditions - people trying to rush there and back.

I know of at least one CT that was built with rear springs from a falcon wagon from memory that was a failure and had to be re-sprung. I would run the ALKO outback 60mm x 7leaf springs which have a pretty good reputation.

As for the chassis, 75x50x3mm for drawbar is fine, 3mm is overkill IMO for the chassis and would stick with 2mm.

My bobcat trailer is only 2mm wall and carries 1600kg. http://www.woodworkforums.com/f184/p...-bobcat-72023/

Just my thoughts. Good luck and have fun.

Interesting point of view on dampers, Daz. I would have said "...they are designed to dampen the oscillations of the spring to assist road holding and steering (or tracking) which is definitely required on a trailer, as with all other road going vehicles" I read somewhere that trailer springs have some degree of inherent self damping which is probably why we're used to seeing trailers without shocks.

I assumed that you'd match a damper to the natural frequency of the suspension to minimise vibration in the trailer and to allow maximum tyre grip, but I suspect that may be easier said than done if variable rate springs were used.

I would suggest that most failures are down to overloading

Yeah, that wouldn't surprise me at all, actually. Given that car springs are optimised for a trade-off between handling, comfort and load, with load usually the least important consideration, I imagine it would be very easy to overload wagon springs if you add tent, gear, bikes, boat etc. I hear that one-ton twin cab utes ride terribly when empty, especially in the back, due to the stiffer spring rates required by heavier load ratings.

I agree with "Correct weight for the springs is the main thing", which is the main problem I have at the moment. I want to optimise for a CT load of around 500kg, but also have the ability to carry up to 1000kg with out killing myself or the trailer. So that's why I've been thinking about variable rate ute springs (and matching shocks), assuming that VR trailer springs aren't available?

ALKO outback 60mm x 7 leaf springs certainly look the goods, but don't you reckon that while fine for my load carrying intentions, the listed capacity of 1530 kg is a bit stiff for a CT, considering I'm aiming at only 700 kg or so total? Off the shelf trailer springs would make for a much easier build than adapting automotive springs, though.

As for the chassis, 75x50x3mm for drawbar is fine, 3mm is overkill IMO for the chassis and would stick with 2mm.

I thought that might have been the case too, but I threw together a quick spread sheet to calculate frame mass based on various RHS dimensions and found that for the design I have in mind, the difference is stuff all so why not over-engineer it? For example, the difference in frame mass between 75x50x3 and 75x50x2 is only 6.9 kg. I've seen a few 'heavy duty' 6x4 advertised with a 40x40 chassis. For my application, 40x40x3 vs 75x50x3 is only 7.7 kg lighter.

Nice work on the bob cat trailer . I've read that thread with interest a few times now.

Good luck and have fun.

Thanks mate. I'm enjoying myself so far and I haven't built anything...

I know I'm probably pushing the proverbial up hill with a chopstick trying to satisfy both needs, but I want to consider all possibilities before I shelve anything on the wish list just yet.

Cheers,
Mick

Hey Mikm, is that weight correct at 6.9kg heavier for the whole frame. If so I am going to have to eat some pie from the humble pie company .

Can you show us dummies how you worked it out.

The ALKO off road springs have the dampening springs (the wavy ones on top) that are supposed to help dampen them. I think they counteract the spring action and fight against it so to speak. You can remove a leaf or two to soften them if required.

Will look forward to the build. Wheres Yonnee?

Ooops
I re-ran the numbers and found a fault with the calculation. Still not too much to worry about, however.

Assuming 8000 kg/m^3 for steel and 7.16 m of frame material (not including any bracing), I get 40.9 kg for 75x50x3 (area of 75x50x3 is: 0.075 x 0.05 - [0.069 x 0.044] m^2. Multiply by length and density to get 40.9 kg) and 27.72 kg for 75x50x2, a difference of only 13.17 kg.

Take a look at the pictures to clarify what I'm talking about. Ignore the drawbar and front spring hangers. The second pic shows the floor bracing which I expect to come in under 20kg.

The ALKO off road springs have the dampening springs (the wavy ones on top) that are supposed to help dampen them.

I wondered about those - so that's what they're for. I wonder if a flatter bottom leaf(s) could be substituted to give me a variable ratio, damped spring?

Assuming two sides and one front rail of your 100x50 chassis is 7.25 m of steel, the difference between 2 and 3 mm wall thickness would be around 50 (3 mm)- 34 (2 mm) = 16 kg. Continuing my rough guessing, 11.25 m of 50x50 bracing is 51 kg for 3 mm and 35 kg for 2 mm. All up, the difference between 2 and 3 mm wall thickness for your frame and bracing would have been around 32 kg. Not that I have anything for 3 mm or against 2 mm, I just thought you might be interested in the figures.

PS: Comparing measured to predicted mass of a bit of 50x50x2.5, I get density of steel a little lower at around 7500 kg/m^3, so the figures above are slight over estimates.

Originally Posted by dazzler

Where's Yonnee?

Here... Hey Dazz.


There's always been an arguement when it comes to trailer springs. And there's never been any one solution, only comprimises.

These comprimises are due to the fact that trailer specific suspensions have such varying load demands, it's impossible to have a system that is capable of being soft and supple when the trailer is empty, but can then have the capacity to handle a load three to four times the tare weight, without costing as much as a small car.

Alternatively, designing your own system from car bits, such as GoJeep's, has some merits in that you are always carrying some spare parts for your tow vehicle, but you're limiting yourself to having to source those replacement parts from a dealer or a wrecker at greater expense than any trailer specific parts, which are carried in many more places. You're also having to do some homework as to the actual weight range that the car spring is designed to work in, and make sure the trailer will be within that range. The thing about a Camper, is that the Tare to Gross weight isn't going to vary much more than a few hundred kilograms, whereas a box trailer can go from 350Kg empty, to 1400Kg fully laden, and that weight range for just about any spring is near impossible.

As for shock absorbers on a trailer...

Originally Posted by mikm

I read somewhere that trailer springs have some degree of inherent self damping which is probably why we're used to seeing trailers without shocks.

Correct! The friction of the leaves rubbing together as they move does have some self damping, plus the fact that they're quite a short spring, means they don't have much oscillation in the first place. But they are simple (the design's been around since the horse & cart), and made by the thousands, so are cheap. Space on a trailer has never really been accomodating for shock absorber either. For them to work at their optimum, most require installing upright (or close to it), which if you designed a trailer with shock absorbers and mounts in mind, you would greatly reduce the load carrying area of the trailer.


And then there are some that believe that if you install shock absorbers onto your existing trailer, you will increase its carrying capacity...

And if I have to explain that to anyone, then maybe trailer building isn't for you.


You could build it for maximum capacity, buy a good compressor, and whenever the trailer is empty, drop the trailer's tyre pressure to 15psi. It's about as close to having your cake and eating it as you'll get without spending a small fortune. Or... if you have a reasonable budget, then you could look at air suspension. Comfort and Capacity, but at a Cost.

I'm intrigued by your chassis setup in the second picture. You would use less steel by having all cross braces rather than the one cross and two longitudinally. Roughly you have a 4' cross and two 7' lengthwise, 18' in total with approximately 1.333' gaps. If you had four evenly spaced cross rails, your gaps would only be 1.4' and you'd use only 16' of steel.

Nice work as usual, Yonnee.

There's always been an arguement when it comes to trailer springs. And there's never been any one solution, only comprimises.

Pretty much what I expected unfortunately. Given the above, I assume you haven't seen/used/heard of variable rate springs on a trailer either?

And then there are some that believe that if you install shock absorbers onto your existing trailer, you will increase its carrying capacity...

Yep, there definitely are some bunnies out there (excluding air shocks, of course).

You'd want to be careful to avoid blowouts with under inflated tyres, but providing speed and weight were restricted, it would lower the overall rate.

Thanks for your input.

Originally Posted by Yonnee

I'm intrigued by your chassis setup in the second picture. You would use less steel by having all cross braces rather than the one cross and two longitudinally. Roughly you have a 4' cross and two 7' lengthwise, 18' in total with approximately 1.333' gaps. If you had four evenly spaced cross rails, your gaps would only be 1.4' and you'd use only 16' of steel.

Good point.

Originally Posted by mikm

Pretty much what I expected unfortunately. Given the above, I assume you haven't seen/used/heard of variable rate springs on a trailer either?

I have seen some pretty weird and wonderful (and some not so wonderful) stuff on trailers over the years, but I've yet to see much in the way of a variable rate spring. It certainly provides some food for thought, and is probably worth asking the question of a company such as Industrial Springs in Adelaide or Moorabbin Spring Works here in Melbourne. I know the latter repair and modify trailer springs, including adding to and re-cambering springs and I would be interested in their response as to whether it's possible to grab say a 60mm wide, 9 leaf eye-to-eye 'Off-Road' spring rated to 1500Kg the pair, and re-camber the top three or four leaves slightly to give you around the 400Kg mark when empty, and as you added the weight, they flatten out to their original specification.

Yep, there definitely are some bunnies out there (excluding air shocks, of course).

Mmm... to a point. You're probably not looking at too much of an increase against the cost of a pair of air shockers, but I will stand corrected on that one. But only just... (and you're still going to have to make them fit...)

You'd want to be careful to avoid blowouts with under inflated tyres, but providing speed and weight were restricted, it would lower the overall rate.

There's the thing. If you didn't have the weight in the trailer, ie; they didn't look flat, then you shouldn't have a problem towing at highway speeds. It's when they're under inflated for the weight they're carrying that you have a problem. So, irrespective of the weight of and in the trailer, if they look flat, then you'll get the sidewall flex that will kill a tyre at speed.

Replies to Yonnee:
- I'll make some enquiries and post what I find out.

- Yeah, air shocks weren't of interest to me here for all those reasons, particularly mounting considerations on a trailer. Another thing I don't like about them in cars is the extra load that wasn't accounted for by the manufacturer can cause failure of the shocker mounts. While a solution to a problem, they're not the best way to skin that cat.

- Agreed. I've had 45 to 50 psi in the rear tyres of a ute with a big load to keep the tyres from getting too hot and it works fine. Different story if they're left at a more usual 32 or so.

Thanks for your input, fellas.

Did those figures seem reasonable, Dazzler?

Why not build the trailer to carry the max payload you want on it then when you want it as a CT change the springs over.

Not a lot of work to do and you get the best of both worlds at nominal expense.

I changed two springs on a car trailer a few weeks ago in less than an hour.

Had a twin cab 4x4 a few years ago, the veichle rarely carried loads and on rough roads would try to overtake itself. Solved this by installing a 3mm spacer between #2 & #3 springs, trialed this between 3&4 but was still too harsh so lifted it up one. The spacer was about 100mm long with feathered ends. Initial ride was through the top 2 springs then as the spring compressed the others in the stack would take up. The spacers were left in the springs for the 2 years that I owned the veichle, worked brilliantly.
I am about to build a new boat trailer with independent suspension using leaf springs with a similar variable rate system.
I obviously take no responsibility for some moron taking themselves or others out by taking any notice to anything I have said!

Originally Posted by zuffen

Why not build the trailer to carry the max payload you want on it then when you want it as a CT change the springs over.

Not a lot of work to do and you get the best of both worlds at nominal expense.

I changed two springs on a car trailer a few weeks ago in less than an hour.

While not a big deal for a one off job, zuffen, it would get real old real quick when I want to go camping on the weekends but carry a load during the week. Thanks anyway.

Originally Posted by Stringy

Solved this by installing a 3mm spacer between #2 & #3 springs, trialed this between 3&4 but was still too harsh so lifted it up one. The spacer was about 100mm long with feathered ends. Initial ride was through the top 2 springs then as the spring compressed the others in the stack would take up. The spacers were left in the springs for the 2 years that I owned the veichle, worked brilliantly.

Thinking out aloud, that's not such a silly idea. Rather than fabricate a spacer, if you pulled the spring apart and replaced the shortest bottom leaf back in between the 2nd & 3rd leaves and reassembled the spring, you'd probably achieve what you're after.

I am about to build a new boat trailer with independent suspension using leaf springs with a similar variable rate system.

Is the suspension something you're fabricating, or an 'off-the-shelf' system? I'd be interested in seeing your ideas. There's a few systems and concepts floating around, some work, and for various reasons some don't as they haven't been completely thought through.