Need some help with shock logic for trailers

[dieselcruiserhead]

I'm building an imitation AT style independent A-arm suspension with airbags. I'm curious on thoughts about shock-logic for this type of suspension.

I noticed they use nitrogen charged Old Man Emu shocks for their airbag setups. My trailer won't weigh quite as much as an AT, I believe... My theory with the independent arms is that I will have noticeable sway versus a traditional leaf spring setup (possibly). It will need to tow solid as a rock on both the highway and trail of course and am concerned about sway as well.

I was thinking a reverse charged or something stiff static shock that allows quick uptravel would be best. So if you hit a rock or rut uptravel is quick, but restricting outward flow would be best for restricting sway for quick manevers...

Or, is it better to simply have a nice firm nitrogen charged shock such as what AT uses?

Thoughts?

 

[Tucson T4R]

My KK trailer has independant susupension and uses nitro charged Fox shocks, with limit straps in the coils and poly bump stops. The set up works perfectly for me and I never notice any sway problems.

 

[gabepari]

shocks

You don't want the rebound damping too high, or your suspension will "pack" on wash board roads. Very quick successive bumps will compress the suspension and your shocks may not allow the springs to extend quick enough for the next bump.

Gabe

 

[ntsqd]

While I concur with the above, rebound damping is traditionally higher than compression damping. For example look at the Bilstein std. valving for leaf springs: 255/70. The 70 is compression force needed to move the shaft at a certain speed and the 255 is rebound force needed for that same shaft speed. With air springs or coil springs those numbers will be too low as leaf springs have considerable internal friction, which is a form of self damping.

The reason that the valving is so far apart takes some mental gymnastics. It is not the initial impact that you feel in a bump. It is the spring pushing the axle back into position. Wasn't easy for me to swallow this, so consider what the axle assembly weighs and then consider what the sprung weight of the rig is. Picture it all going over a bump, which is going to instantly move the easiest, the axle or the whole rig?
Initially you will feel the actual impact, but the vast majority of the whole bump experience felt by the occupants happens after the tire has already cleared the bump. So the bulk of the bump experience is due to the spring valiantly trying to push the axle back into place. With no rebound damping this can happen very fast, but at the sacrifice of ride quality since it will also upset the sprung mass. So what is needed is just enough rebound damping to keep the ride quality in the comfort zone.

With a small trailer there aren't any occupants, so the temptation is to say that it can do with equal damping in both directions but that ignores the fragile items that might be loaded in the trailer. I believe that a trailer can stand stiffer valving than a vehicle carrying passengers can, but not by a lot if you plan to cook those eggs any way but scrambled.

As a point of reference, the TrailBlazer trailer on long term loan to me has rubber torsion axles and Rancho RS9000's under it. After testing the available settings, the 9k's are set to their highest rebound setting. Effectively they could've been 5k's, but at the time of the build (10+ years ago) little was known about what would be best.

 

[Martyn]

Andre

PM sent.

 

[dieselcruiserhead]

Thanks guys as always...

I have a set of Bilstein 255/70 that are also nitrogen charged. They seemed really stiff as I tested them but sounds like they would do the trick with the correct valving according to what you both are reporting. Only issue is these in particular are too long.

I also recently tossed a bunch of old shocks, I really wish I'd kept some of the stock FJ60 shocks I had lying around, perfect length and probably correct valving as well...

There are cheap M416 stock shocks but I think I will end up going nitrogen charged assuming the AT guys confirm (thanks again Martyn).

 

[GeoTracker90]

Another option?

If sway is the main concern to be addressed then what are your options for installing a sway bar? This way you would have a specific component for each of the concerns.

Shocks - to control compression and rebound.
Sway Bar - to control side to side sway or body roll.
Springs - to suspend the load.

Some light weight cars might be a source for an inexpensive sway bar (think wreacking yard), or for a little higher dollar option you might look at the sway bars that the rock crawling crowd use on their softly sprung Jeeps and buggies. Take a look at theCurrie Anti-Rock kit. This one retails for $390.00

If you don't want their full kit they do have parts piece meal as well:

32" Sway bar - $160 each
17" Steel Sway Bar Arm - $40 each
Poly End Caps/Bushings - $10.00 each
Frame Brackets - $80.00/pair

Provide your own end links and go for it. With this set up the arms have different mounting holes for the end links so that you could fine tune the action to suit your needs.

Just another option.

Mike

 

[dieselcruiserhead]

Hey Mike,
That has actually crossed my mind considerably and I have a complete sway bar from a FJ62 land cruiser application that I think would work well and be relatively easy to install. It is a good question.

The good news is I won't be powdercoating anything so it will be relatively easy to add it after if I decide not to use it initially... Also the AT trails don't use a sway bar so I have considerably optimism without it as well.

 

[ntsqd]

Coming back to this after a long dormancy, found while looking for something else. Of course....
:sombrero:

Traditionally shocks are not used for static sway control, but they can not help but have an impact on transitional or dynamic sway.

Said differently, if you do not mind the trailer leaning over a bit while sitting still on a side-hill, but are trying to prevent the inertia from a side-hill bump from flopping the trailer over or to stabilize the trailer when at speed (sway, cornering, etc), then either a swaybar or appropriate shock damping will work. Since you already need to have shocks on the trailer, why not avoid the added complexity of a swaybar and linkage until proven necessary?

HTH

 

[R_Lefebvre]

I don't think a swaybar will help a single side bump induced flop. Yes, it increases the single side bump energy absorbtion, but then it just fires it right back down. I think the shock is the only thing absorbing the energy of a single side hit threatening to flop the trailer. You need a soft spring, soft bump, and lots of rebound damping to take it.

I'd like to mention my pet theory about airbags. I'm not sure if I'm right or wrong... but here we go:

It seems to me that most airbags are placed right on top of the axle, or even ahead of the axle, and they end up having pretty short travel, while still taking up a whole bunch of vertical space. Results in a long distance between the axle and the trailer floor, or not a whole lot of travel. Combine that with the very high rising spring rate as the bag compresses and it's a recipe for disaster.

I've seen large equipment trailers with very heavy suspensions that use a better setup. They use a solid axle, bolted to trailing arms. The arm extends behind the axle, which is where the bag sits. This allows a lot more room for the bag underneath the frame. You could use a taller bag for the same amount of travel, which would then have a much lower rising rate spring rate.

 

[ntsqd]

If the rebound damping is set right then the swaybar's additional energy absorbsion will be controlled on release. But to get there is very likely a whole lot more shock tuning complexity than is really needed on a trailer.
Some compression damping to control the transients on corners, single tire bumps etc. will be good, but too much will start breaking those breakfast eggs.

I like your air spring reasoning. I've not been comfortable with using them because of the rapid rising rates involved. Slowing down their motion ratio is a good idea.

 

[pyro4x4]

Why not go with simplicity and use a couple of little coilovers. That would eliminate the air pump, airlines, bags, wiring, and controls. There would be a lot less room for failure of some sort. Not to mention they are fully customizable and adjustable to your needs.

 

[R_Lefebvre]

Was talking to an OEM truck suspension engineer, his feeling is that swaybars will not help in the single sided bump flop scenario.

We just need bump absorbtion. Let the wheel come up. Yes, coilovers could solve a lot of problems. You could use soft springs, but the adjustable collar could accomodate load changes.

But the entire system is still more complex than leaf springs. I'd just prefer to build a leaf spring system with long soft leafs. I don't really buy into the independent suspension on a trailer thing. That's what I'd do if I did it again.

I think part of the problem we're having is the shock absorbers. We have competing needs. We need no bump damping for this, but then you have the packing issue on washboard. A more sophisticated damper might help.

I also wonder if an MCU bumpstop would help.

 

[ntsqd]

A couple of the MCU bumps off the front of a Dura-Max GM truck could probably carry the whole load!

 

[Metal Twister]

Not to change the direction here but this might be a way around sway bars?
Ive been thinking about the self leveling unit used on bus air bags. Its a very simple device that attaches to the frame. An arm extends from the body of the unit and attaches to the axle end which allows air in or out as the axle raises or lowers. One is put on each side allowing the trailer to self adjust while traveling. On the street it would work well as a sway bar replacement. I dont think it would be of any advantage off road with the exception of self leveling for tent use, or on dirt roads. It could easily be turned off while doing extreme off road. Any Thoughts? Yea or Nea?