Home built independent suspension?

[JeepDork]

Great point, and I agree as both wheels are connected one side will always affect the other. The amount it affects it is another story. I guess it really depends on cost at that point. I would imagine a true independent set up would perform better. If cost isn't a consideration then that might be the best bet. I think I fall into a happy medium somewhere. I would like the ability to adjust the damping with air bags, but still like the ease of only running leafs as a location point for the axle. As far as never being able to get the damping correct because of multi spring applications such as air bags and springs, I disagree. If that were the case looking for the perfect ride changes each time you change the weight of the cargo. Using air bags and the ability to change the pressure inside would give you the same result if you were correct. Load a sports car up with people it won't handle nearly as well as with just the driver. Using a lighter weight leaf with an airbag is a common practice for carrying different load weights. Such as adding air bags to and RV or pick up. Pickups ride considerably better these days than they did 20 years ago, yet the still have leafs in the rear.? This is because they use a softer main leaf and a helper of stiffer leaf together. Then if that isn't enough you can add airbags. So to say you can't get the damping correct isn't exactly right... or wrong. I don't think you can ever get it perfect but you can adjust it to help get the best ride/handling possible even in a trailer.

 

[ntsqd]

You must understand that my background has a lot of racing, with a fair amount of damping intensive desert racing, in it. As such my expectations of damping are more demanding than most. With the two different spring rate deltas with suspension position there is no way to have one damper meet all of those demands. Maybe it doesn't matter to most, but it does to me.

Linkage with a live axle on a trailer can be as simple or as complicated as you want to make it. My own thought is to go with a simple trailing arm design, one that takes the design points of the "truck trailing arm system" found under the rear of "Winston" Cup cars (as originated by GM under some of their mid-60's 1/2t pick-ups). Two links rigidly attached to the axle tube, with a lateral locator (panhard, Watts, WOB, trailing arm diagonal, etc.). Pay some attention to Anti-Dive geometry and move on to the next challenge. I have posted around here somewhere at least one conceptual model of this design.

 

[JeepDork]

Panhard bar with independent suspension? Huh? I think your over thinking this. It sounds like you know way more than I do about suspension. I think we are after different things here. I want the best possible ride too, but I think with a such a different amount of weight in a trailer you can't have one size fits all perfectly. I'd say a trip overnight vs. a week long trip to nowhere. The weight would be a lot different depending on the trip. I think the suspension will vary too.

 

[ntsqd]

No, I'm under-thinking this. True "ITS" might be cool and be bragging rights, but I'm not convinced of the need. There are exponentially more live axle'd trailers out there giving good off road service than there are "ITS"'d trailers out there.
Note that the images below aren't exactly what I described earlier, but they illustrate my ideas and since I found them in my CAD files ready to go I'm using them.
Trailing arm with a diagonal lateral locating link:

Trailing arm with a panhard lateral locating link:

 

[JeepDork]

why?

Why would you do all that? Suspension for a truck or car is different than that for a trailer. A trailer gets most of it's articulation from the hitch not the suspension. I guess that is the beauty of building your own trailer. You get to do it any way you want. I think your nuts for making it much more complicated than you have too. The reason to 4 link something off road is for better flex and and suspension travel. That isn't a concern when you only have one axle and a hitch. Damping is the important issue here.

 

[ntsqd]

If I went with the diagonal locator then I'd only have to design & build the structure around two attaching points rather than the 4 needed for leaf springs. It is quite a simple suspension design. Going with the panhard version ups that by one attaching point and it need not be quite as robust as the forward mounts. I have a version of it that has brake torque reaction built into both longitudinal locating links, but I doubt that's necessary and it is probably not desirable. The point isn't about articulation, the reason for only one torque reactor is to reduce "spring rate" in sway as an effort to further decouple the tires. It also reduces fabrication effort. Very few of those parts are not off the shelf bits. Those that are not can easily be made from off the shelf parts. The lone exception is the one bent tube and even that could designed in a way to remove that feature. So the whole assembly becomes very much a "tab A, slot B" assembly effort rather than an effort of 37 unique, ground-up parts that have to be made from scratch.

I know that the title of this thread is about true independent suspension as applied to trailers, but my current thinking is that doing that is over-thinking things. Hats off to those who want to try it, but I don't see much gain for the increase in effort required. If you search the topic further you will find lots of mention of the difficulty in making both swing-arms the same (or mirror images of each other). Why? Granted the tires are totally decoupled. Is that really all that important? Something simply fabricated, and easily field repaired, is much more important.

 

[JeepDork]

Something simply fabricated, and easily field repaired, is much more important.[/QUOTE]

I agree with you, and leafs are as about as simple as it gets. Try replacing your "Tab A or Slot B" in Kanab Utah in the middle of August. The local store probably won't have it. Leaf spring parts are everywhere. Glad we see things the same!

 

[ntsqd]

Try replacing a broken, typical trailer type main leaf on the south side of the San Ignacio lagoon near the infamous silt beds. Can't weld springs with any reliability like you can with structural steel. That was 35+ miles at ~5 MPH because any faster and the ratchet straps couldn't hold the spacer board in place (spare was too tall to fit). There was no spring in San Iggy that would work to make the trailer towable to the border. So if they still have it, that is how Rice & Beans in San Ignacio ended up with a Yanmar trailer-able emergency power generator.

 

[Bluewater Australia]

Granted the tires are totally decoupled. Is that really all that important?

Yes. I had the opportunity to follow two 4WDs, each towing a camper trailer, along a couple of hours' worth of very second-rate Tasmanian roads a few years back. Watching how those two trailers behaved - one with independant swing arms, coils and shocks, and the other with leaf springs, beam axle and no shocks, made it absolutely clear to me that when I got my camper trailer, it would have the former setup (except I chose air springs). Trailing behaviour, and how it translates to comfort and safety, were a world apart, and I could see that from 50 yards away easily. I have since been in plenty of 4WDs towing the latter setup, and knowing why they feel like they do, it honestly scares me that people tow these things at highway speeds.

Something simply fabricated, and easily field repaired, is much more important.

I'd have to guess that you're making a connection between simply fabricated and easily field repaired. I don't think they're necessarily the same thing. As much as every vehicle manufacturer in the world would love to be able to have their suspension simply fabricated, nobody's is because that's not the best way to make suspension, so they design them well instead and have few problems anyway. When things break, unless you have every spare part on board, it's not going to be an easy field repair regardless of what style of suspension it is. It's going to be what we call a "bush repair" - as in doing the best you can out in the bush. On the last two big trips I've had, there has been an accompanying trailer with leaf springs break a leaf. One time was a bush repair with a steel bar, the other was a bush repair with a block of wood. We got home, so that was OK. It didn't make me want to ditch my airbags and swing arms and get leaf springs.

 

[JeepDork]

Every repair is different and depending what you have with so is the solution. Now to take a trip with both suspensions right in front of you would be great. I'd love to watch how each handle the same terrain at the same speed. That is trip I wish I was on, but from what I have read from others I think I would agree with you on independent.

 

[ntsqd]

I don't dispute that ITS should be a better ride for the trailer, but you have to admit that the baseline trailer was ill-equipped to be taken where it was. I'll guess based on the description that it was the typical leaf sprung trailer suspension, which is designed for pavement travel nearly exclusively. In that use the typical short, stiff trailer leaf springs work fine, not exceptional and not without minor complaints, but well enough and with a reasonable service life. Taking them out of their intended environ exposed all of that design's short comings. Merely adding dampers and going to a more supple and likely longer spring would have made a huge difference in your observations. I wouldn't expect that it would work as well as a well laid out ITS, just that you wouldn't be so quick to condemn a live axle. It does seem ironic that most of the rigs towing these trailers have at least one live axle, if not two of them, and that is by choice.

Designing in reasonable Factors of Safety is obviously necessary, but designing in simple field repair options for the expected modes of failure is an important point that few seem to grasp. Where a simple fix for a failure mode can not be made the particular Factor of Safety should be increased. It is far better to put the work and effort into the design and fabrication up front so that field repairs, generally and if necessary, are simple procedures. If all of the parts are designed and built such that there are no failure modes the suspension will be excessively heavy and the trailer will be an ungainly poor choice. Tempering that temptation with reasonable fatigue service life expectations is also important to the design cycle. If you hit it *just* right the wear parts wear out at a rate that allows early detection so that field repairs are only the result of the part that slipped through the mfg's QA/QC system undetected.

 

[Bluewater Australia]

you have to admit that the baseline trailer was ill-equipped to be taken where it was

No argument. It was on a pavement road.

For the record my own vehicle has two beam axles and all leaf springs, and is one of the best 4WDs ever made. But that bears no relation to a towed trailer that weighs a quarter as much. Leaf-sprung trailers are built down to a price, end of story, as far as I have seen. That definitely means they have their place, but if cost is not the primary concern, there are better-designed options.

 

[JeepDork]

= that bears no relation to a towed trailer that weighs a quarter as much. Leaf-sprung trailers are built down to a price, end of story, as far as I have seen. That definitely means they have their place, but if cost is not the primary concern, there are better-designed options.

Yes I agree, completely.

 

[system-f]

There is a gentleman who I have wheeled with that has a trailer he tows offroad and on the highway. At first he used standard trailer leaf springs and the results were obvious, broken beer bottles in the cooler and an unacceptable ride for the items in the trailer. The next year he did a triangulated 4-link, left the axle solid but upgraded to an 8 lug 5200lb axle, and put some airbags plus shocks on. This trailer has now been over some of the hardest trails in Arizona and all beer bottles are completely in tact. Independent suspension on a trailer is not required to have a smooth riding yet extreme trailer. That being said an independent suspension trailer is cool and has its merits. No this is not my truck, but it is proof that you can do it with a straight trailer axle.

http://www.piratepathy.com/AZ_IX/reddog/slides/P1010115.html

 

[merlin44]

Seems like that would be easy to fix in the field. Maybe even a spare arm or two stored inside.
No panhard rod on this one? Does the axle want to wander back and forth any?

Looks like a very clean installation.

Just checked out the pics with the Blazer pulling it. My favorite is old Blazers. The trailer seems to work really good and follow well.

Need seatbelts on the trailer, though.

Billy