3-link (three link) w/air bag suspension questions

[1x1_Speed_Craig]

I'm first addressing the biggest priorities with my M101A3 trailer (wiring, coupler swap/mods, etc.), I want to give some thought to, and start researching, the future air bag suspension design. I already have a set of AT mounts, and should have some air bags shortly (currently a backorder issue...hopefully getting an update soon).

Given my intended use for the trailer (mainly road, with some light offroad), I have zero interest in getting into the complexities of scratch-building an independent trailing arm suspension set-up (I might consider it if I could just buy the kit from AT, but a retrofit by them isn't an option due mainly to geography, and also by cost). I'm not confident that I could get the alignment correct given the tools/knowledge I have. So, my intentions are to use a standard trailer axle from ABC Trailer Parts, and:

• Use the main leaf spring from the stock suspension (removing all other leaves) to locate the axle. I'll do this if there's adequate clearance, as it'll be the simplest approach.
  *OR*
• Build a 3-link set-up.

For the latter option, I have searched extensively here, and am looking for specific recommendations on:

1. 3-link design (e.g. - THIS THREAD was very helpful)
2. Product/component suggestions

Parts/component selection:

• BTF link mounts, or something similar. I have ordered from BTF before, and like their stuff.
  
  
• 2" or 2 1/2" Johnny Joint on the frame end. The stock M101A3 spring hanger has a ~2 1/4" ID, so the 2" model would work well for width. I thought I could use the existing leaf spring hanger to mount the front Johnny Joint, but I'll have to see what diameter main eye bolt the stock leaves use. The 2" JJ only uses a 7/16" bolt (too small, I believe), while the 2 1/2" JJ (which is 2 5/8" wide) uses a 9/16" bolt. A custom hanger bracket may be required regardless, but I'd really like to retain the stock hanger, as the tongue mounting bracket is also integrated into the stock cast spring hanger bracket (translation = simplicity).
  
  
• Panhard bar with poly bushings on both ends. I could make one, or just use an off-the-shelf trac bar from a Jeep or other application, and could use these BTF mounting brackets.
• Use the stock shocks already on the M101A3.

Questions:

1. Is 22-24" long enough for the trailing arm links?
2. Are there any real downsides to a 3-link on a trailer with this intended use?
3. What may I be overlooking with the design ideas?

Discuss, please, and enlighten me. :sombrero:


Craig

 

[1x1_Speed_Craig]

:sombrero:

 

[stonehenge]

may i ask why you arent using leaf springs? air bags bleed down and could cause a wandering trailer issue. how are you going to inflate and continue to keep inflated?

if doing a link style setup i would go with a "y" setup but your still going to need a trackbar to keep it from wandering.

also try www.etrailer.com they have a lot of great stuff and most (99%) is in stock with great prices.

 

[ntsqd]

The single leaf spring, augmented spring system is appealing from a cost perspective, but damping more than one type of spring at the same wheel is a nightmare. Even though it's only a trailer I still wouldn't do it or recommend it.

Were I building a 3 link for me I would do it like the attached image. Using an 'A' upper arm puts the Roll Center closer to the Center of Gravity (CG), so the CG has less leverage on the Roll Center. Meaning that the trailer will roll less in corners.

Using the above design I don't see a problem in using the 2" joints with their 7/16" bolts. When have a good idea of the total weight I would suggest running the shear number on those bolts, but mostly just as a precautionary measure.
To avoid aftermarket parts you might think about using large tie rod ends for the linkage joints. Doing so complicates the fabrication a bit, but gives you service parts that are replaceable nearly anywhere. Something like:
http://www.partsmike.com/store/store.php?crn=204&rn=1516&action=show_detail
or
http://www.partsmike.com/store/store.php?crn=204&rn=1110&action=show_detail

Nothing in the attached model is scaled to anything, but those lower links are 29" long. AFAIC the longer, within reason, the better.

I would highly encourage you to build a new frame and not try to make the existing frame work. It can be done, but by the time you've modified it to work you could have built a new one and had fewer design compromises.

 

[1x1_Speed_Craig]

may i ask why you arent using leaf springs? air bags bleed down and could cause a wandering trailer issue. how are you going to inflate and continue to keep inflated?

also try www.etrailer.com they have a lot of great stuff and most (99%) is in stock with great prices.

I'll see how the ride of the stock leaf springs is over a little time. I already own the air bags & brackets, but don't need to rush to install them. My old M-416 bounced a lot when empty. I'm not sure if this trailer will be better (probably will due to the heavier weight).

I have purchased a couple times from e-trailer...great experiences so far.

The single leaf spring, augmented spring system is appealing from a cost perspective, but damping more than one type of spring at the same wheel is a nightmare. Even though it's only a trailer I still wouldn't do it or recommend it.

The idea has been mentioned by a few people here, but I get the impression that I'd be the guinnea pig for actually trying it out.

Were I building a 3 link for me I would do it like the attached image. Using an 'A' upper arm puts the Roll Center closer to the Center of Gravity (CG), so the CG has less leverage on the Roll Center. Meaning that the trailer will roll less in corners.

Using the above design I don't see a problem in using the 2" joints with their 7/16" bolts. When have a good idea of the total weight I would suggest running the shear number on those bolts, but mostly just as a precautionary measure.
To avoid aftermarket parts you might think about using large tie rod ends for the linkage joints. Doing so complicates the fabrication a bit, but gives you service parts that are replaceable nearly anywhere. Something like:
http://www.partsmike.com/store/store.php?crn=204&rn=1516&action=show_detail
or
http://www.partsmike.com/store/store.php?crn=204&rn=1110&action=show_detail

Nothing in the attached model is scaled to anything, but those lower links are 29" long. AFAIC the longer, within reason, the better.

Another impressive on-the-fly design. I lack the engineering background, so I'm still learning from you and others here. Thanks for the input.

I would highly encourage you to build a new frame and not try to make the existing frame work. It can be done, but by the time you've modified it to work you could have built a new one and had fewer design compromises.

I find this comment interesting. I bought the M101A3 to save time, and it's a great set-up for my needs. At this point, a scratch-build frame isn't an option any longer. I guess I don't see how modifying this trailer would be that difficult or time-consuming relative to building one from scratch. Maybe you could enlighten me as to the compromises you might foresee.


Craig

 

[Martyn]

The specific air bags that Craig has are not prone to leaking. So I don't think that will be an issue at all.

The air bag is primarily providing the ride height in this set up. Craig needs to install bump stops and limit straps to keep the total motion under wraps, but I'm hoping that with an adjustable shock he'll be able to dampen the system without too much of an issue.

It is a grand experiment, and I think Craig is up to it. I prefer to see what he comes up with and then address any issues that arise, rather than cast doubt on it before he's even taken his first trip around the block.

Best of luck mate.

 

[1x1_Speed_Craig]

The air bag is primarily providing the ride height in this set up. Craig needs to install bump stops and limit straps to keep the total motion under wraps, but I'm hoping that with an adjustable shock he'll be able to dampen the system without too much of an issue.

Thanks Martyn. To clarify, I'm looking for the air bags to act as the actual spring, not just to provide the ride height.

There are bump stops are already in place (mounted on the axle). I thought the limited drop from the leaf spring/shackle movement would also act as a "limit strap", but I'll have to see. I didn't see any instructions/manual with the air bags, but I'm sure they have it online; I can get the max compressed/expanded measurements there, I'm sure.

It is a grand experiment, and I think Craig is up to it. I prefer to see what he comes up with and then address any issues that arise, rather than cast doubt on it before he's even taken his first trip around the block.

Best of luck mate.

It's not because of my credentials (office guy by day, hobbyist tinkerer by night/weekend) . That being said, I haven't been afraid to try new/different things in the past (building one-off motor mounts for Jeeps, making my own biodiesel fuel, etc.).


Craig

 

[stonehenge]

The specific air bags that Craig has are not prone to leaking. So I don't think that will be an issue at all.

The air bag is primarily providing the ride height in this set up. Craig needs to install bump stops and limit straps to keep the total motion under wraps, but I'm hoping that with an adjustable shock he'll be able to dampen the system without too much of an issue.

It is a grand experiment, and I think Craig is up to it. I prefer to see what he comes up with and then address any issues that arise, rather than cast doubt on it before he's even taken his first trip around the block.

Best of luck mate.

I wasnt casting doubt, i was simply asking a question. I have been building custom vehicles for 26 years and i have yet to see/find/work with an airspring/airbag that doesnt bleed down over time. I was and am curious as to why an airbag over a traditional spring, torsion or even a coilover for that matter. no harm no foul.

Anytime someone experiments with something it is always interesting to get an opinion as to why, it helps me and others to learn. there was nothing nefarious about my question.

 

[1x1_Speed_Craig]

I wasnt casting doubt, i was simply asking a question. I have been building custom vehicles for 26 years and i have yet to see/find/work with an airspring/airbag that doesnt bleed down over time. I was and am curious as to why an airbag over a traditional spring, torsion or even a coilover for that matter. no harm no foul.

Anytime someone experiments with something it is always interesting to get an opinion as to why, it helps me and others to learn. there was nothing nefarious about my question.

They're all fair questions in my book. :sombrero:

I'm not sure of the Firestone air bag model. The 3/8"-fitting model that AT has been using/selling for years is experiencing a switch-over to an equivalent 1/2"-fitting model, which is why it took me a while to get the air bags. I'm plenty comfortable with the brand/style choice given the long-term use by AT, and many other people here. If they were some "Acme-brand" air bag, I may be more cautious.

stonehenge - I just realized I didn't answer your question above about how I'd inflate the air bags. I'm an avid cyclist, and always have a floor pump handy. To keep things simple, I'd have the Schraeder valves mounted on the tongue, and use the manual floor pump to inflate the bags individually. I'd rather have that simplicity than rigging up some 12V pump, but we'll see.

I'm still interested in further researching the 3-link route if that would be better. Again, I'm no engineer (I think I have said that once or twice ).


Craig

 

[bluejeep]

I put bags on my trailer that I haul the jeep around on - best thing I ever did. No bouncing like with metal springs, and that's whether I'm loaded (well, I mean the trailer is loaded) or empty. Just adjust the air pressure. Smoothest riding trailer ever.
I have onboard air for my jeep tires and also in my tow vehicle so that's 2 sources to fill the bags. I use a 120V compressor plugged into a AC/DC inverter. They run at much lower rpm, and mine has 100% duty cycle. Handles the bags fine, up to max 100 psi.
My air plumbing has a valve so while filling, the bags are connected, then I turn the valve so the bags are separate. Else when cornering you may squish all the air out of the bag on the outside of the corner and push it to the inside bag. Less stable.

 

[1x1_Speed_Craig]

I put bags on my trailer that I haul the jeep around on - best thing I ever did. No bouncing like with metal springs, and that's whether I'm loaded (well, I mean the trailer is loaded) or empty. Just adjust the air pressure. Smoothest riding trailer ever.
I have onboard air for my jeep tires and also in my tow vehicle so that's 2 sources to fill the bags. I use a 120V compressor plugged into a AC/DC inverter. They run at much lower rpm, and mine has 100% duty cycle. Handles the bags fine, up to max 100 psi.
My air plumbing has a valve so while filling, the bags are connected, then I turn the valve so the bags are separate. Else when cornering you may squish all the air out of the bag on the outside of the corner and push it to the inside bag. Less stable.

Good feedback - thanks. Are these "helper" air bags (along with the steel springs)? What's the rest of your suspension set-up.

I'd have two (2) Schraeder valves - one for each side. With a pressure gauge on my pump, I'll probably just keep it simple vs. having the extra valve for equalization of pressure. I'd definitely have them work independently of one another when in use.

Craig

 

[Martyn]

Thanks Martyn. To clarify, I'm looking for the air bags to act as the actual spring, not just to provide the ride height.

There are bump stops are already in place (mounted on the axle). I thought the limited drop from the leaf spring/shackle movement would also act as a "limit strap", but I'll have to see. I didn't see any instructions/manual with the air bags, but I'm sure they have it online; I can get the max compressed/expanded measurements there, I'm sure.

It's not because of my credentials (office guy by day, hobbyist tinkerer by night/weekend) . That being said, I haven't been afraid to try new/different things in the past (building one-off motor mounts for Jeeps, making my own biodiesel fuel, etc.).Craig

Craig

I don't see a problem using the air bag as a spring either.

Ride height on these bags in 5.5 - 6.5"

The bags are strong enough to act as limiters, but they allow too much movement and will create sway side to side. The air bag system is going to react a lot faster to movement than the old leaf spring system. An if Newton is to believed every action has a reaction.

Finally it's the fact you are a hobbyist that makes this adventure all the more exciting.

 

[stonehenge]

Good feedback - thanks. Are these "helper" air bags (along with the steel springs)? What's the rest of your suspension set-up.

I'd have two (2) Schraeder valves - one for each side. With a pressure gauge on my pump, I'll probably just keep it simple vs. having the extra valve for equalization of pressure. I'd definitely have them work independently of one another when in use.

Craig

what pressure do you think you will run? empty vs loaded?

Can you use a "sway bar" to control the roll?

 

[bluejeep]

Good feedback - thanks. Are these "helper" air bags (along with the steel springs)? What's the rest of your suspension set-up.

Nope, not helpers. Full scale Firestone bags. I have a solid axle, with trailing arms going forward from the axle to the trailer frame. Each arm is roughly an H shape with the inner vertical being somewhat angled inward to resist lateral forces. That's instead of including a panhard bar. I originally had a Y shaped trailing bar to the middle of the axle much like the design shown in this thread. That worked well also. When I 'redid' my trailer tho I wound up with clearance issues between it and the trailer bed at full compression.

The axle connects to the arms via std axle u-bolts. These arms go beneath the axle and extend past the axle to the rear where the bags sit on top of them. The top of the bags then connect to the trailer frame. Not exotic, but functional.

Wish I had a pic/camera - sorry

I am going to add a sway bar (left over from an 8.8 axle swap on the jeep) to see if that does anything. I think the vast majority of the Jeep leaning that I see in the rearview mirror is actually from the jeep suspension, not the trailer, but since I have the parts handy I'll do a bit of fab and see if there's any positive change.

 

[Martyn]

I wasnt casting doubt, i was simply asking a question. I have been building custom vehicles for 26 years and i have yet to see/find/work with an airspring/airbag that doesnt bleed down over time. I was and am curious as to why an airbag over a traditional spring, torsion or even a coilover for that matter. no harm no foul.

Anytime someone experiments with something it is always interesting to get an opinion as to why, it helps me and others to learn. there was nothing nefarious about my question.

Your right they all bleed down over time and require the same maintenance as tires do. I guess I'm talking in degrees, these bags will stay inflated at the correct ride height for a week, rather than for an hour.

The big difference between air bags used on a trailer and airbags used on a car is that the % of load increases is huge. My Grand Cherokee has a load capacity of 55% of it's total weight, where as our Chaser trailer has a load capacity of 285% of it's total weight. This means that the maintenance on a trailer air bag has to be much more frequent to accommodate the change is load. This huge % change in load is why leaf springs and coils are less than ideal when used in a trailer, and yet are applicable for a car or SUV.

I don't mean to cast aspersion on your line of questioning. It's more of a philosophic matter, I believe that we stifle innovation when ever we tell people something can not or should not be done. Worst case scenario is that Craig's system doesn't work out as he intends, but we'll all learn something from him attempting it.

I believe there is a good chance it will work out fine. It may need adjustments and modifications, and I'm willing to help Craig solve any issues that arise, as I'm sure you are.