M416 Build

[indiedog]

Hi Andre. A long time ago in a former life I studied structural engineering. I know enough to get myself into trouble! Out of curiosity I did some calculations to see what the loads on your trailing arms would be. I had to make a lot of assumptions about layouts and loads but it was interesting to do. You'd be suprised what the results were and how much the positioning of the airbags and other things make. Here's a very quick summary.

Assumptions; Load/airbag = 100kg, airbag positioned 75% between hinge and axle, ignore tongue load and only consider load over airbag as direct down force, distance between hinges 250mm (10in), airbag 50mm (2in) inside outer hinge, wheel load 200mm outside outer hinge.

Looking at the forces only along the length of the trailer, the wheel pushing up essentially pivots around the airbag to create a resulting downforce on the frame at the hinges. A load of 100kg down at the airbag means there is a 25kg total downforce exerted by the swingarm at the hinge end. Remember this is distributed over both the inside and outside hinges. Now let's look at the forces across the trailer. Again as the wheel force is eccentric it will be trying to not only be pushing upwards, but trying to rotate the whole system. Taking the moments around the outer hinge (essentially eliminating it from the calculations), the force required to keep things in equilibrium at the inner hinge is 80kg! So the swingarm is pushing down here with a weight of 80% of the load. As the total force at the hinges is down at 25kg, that means the outer hinge has an upload transmitted by the swingarm of 55kg. So over a distance of 250mm the forces in the cross member supporting the swingarm change from up 55kg to down 80kg. This is a lot of bending stress for a 100kg load!! Now this only considers a static load, no bouncing over dirt roads, no lateral forces, etc etc and then put in a realistic load of say 400kg/airbag! So things can only get worse from here.


If you adjust the airbags to be 90% of the way back along the swingarm, the total load at the hinges reduces to 10kg. The distribution of the load between the hinges stays the same. This reduces the loads to 40% of having the airbag at 75% distance! A big difference. Now I know there are other things that will influence the outcome here such as the positioning of the load in the trailer, the dimensions of the swingarm etc, but the basic principles will stay the same.

So, while not being either an engineering or suspension expert, I'd say some good ideas for home made trailing arm suspension are;

- Keep the airbag as close to being in line with the axles as possible
- Make the distance between the hinges on the swingarm as wide as possible to help reduce torsion effect
- Position airbag and outer hinge as close to outside of trailer as practicable to give more direct loading

So out of that I'd say you really do need to beef up the cross member holding the swingarms and look at repositioning the airbag. Hope that is of some use. And if there are any engineers/suspension experts that wish to confirm/shoot down the above I'm only too happy to hear it. Now I need to wake up....

 

[dieselcruiserhead]

Very good comments. I'm going to pull everything apart to see how it appears, particularly the upper airbag mounts. I'm pretty doubtful it will involve an airbag repositioning but I've already picked up the steel for the new cross member...

 

[dieselcruiserhead]

Hey Adam,
Thanks for your comments as always man... Yes its a good question on the weather stripping. I do think its fairly important, I think I mentioned it earlier but I experienced rain and lots of dust. It does help...

I'm looking forward to seeing how you like your trailer as well and seeing some photos/trip reports

Cool to hear your impressions and feedback after that trip Andre. I still haven't had a chance to really test mine yet. Of course I still need to build a lid for mine but that will come. I think my trailer is very light all things considered and I'm guessing it's about 500lbs empty, but I haven't weighed it yet.

After reading your thoughts, I'm sure I'll need to address the lack of weather stripping around my tailgate. I'll have to put some thought into that. More and more I think I'm really glad I built from scratch and kept things simple. My trailer isn't the eye-candy that an AT is and nothing as cool as a Campa trailer or Conqueror, but it's strong and functional. I'm just excited to get out and use it. Thanks again for a very detailed build thread and for sharing your results after using it. Helpful for sure.

 

[R_Lefebvre]

So out of that I'd say you really do need to beef up the cross member holding the swingarms and look at repositioning the airbag. Hope that is of some use. And if there are any engineers/suspension experts that wish to confirm/shoot down the above I'm only too happy to hear it. Now I need to wake up....

Well, the design is way too complicated to be analyzed accurately in a linear fashion as you have done. However, the basic ideas are all sound, and the "direction" of your conclusions seem right. The further away the bag is from the pivot, the better the situation will be. In fact, I'd prefer the bag were actually at or behind the axle. As it is now, the bag positioning magnifies all the forces in the suspension. There's more forced on the bags than the weight of the trailer would exert by themselves, and the control arms have a twisting force in addition to the vertical loading.

It's not really anything Andre did, it's inherent in this suspension design. It requires everything underneath to be really heavy duty.

 

[Martyn]

Andre

What we found was that the stock upper air bag mounts are not heavy duty enough for the suspension system and conditions. That's why we had our own heavy duty design made.

We also used a steel alloy to make the pivot rods out of. I don't know the exact alloy mix or strengths, but after long term prototyping the rods had to be made out of a new alloy that was three times the strength of the original.

The first trailing arms we made bent on the initial test drive and needed a major redesign before we considered them off-road worthy.

So in a nutshell everything the engineers are telling you is correct.

 

[R_Lefebvre]

As simple as a "Chromoly" (4130, 4140, 4340) for the rods? That's not too hard to find, and I can't imagine you went to any kind of custom alloy.

 

[dieselcruiserhead]

Martyn when you say pivot rods you mean the bolts that the a-arms attach to the frame with?

thanks,
Andre

 

[indiedog]

Well, the design is way too complicated to be analyzed accurately in a linear fashion as you have done. However, the basic ideas are all sound, and the "direction" of your conclusions seem right. The further away the bag is from the pivot, the better the situation will be. In fact, I'd prefer the bag were actually at or behind the axle. As it is now, the bag positioning magnifies all the forces in the suspension. There's more forced on the bags than the weight of the trailer would exert by themselves, and the control arms have a twisting force in addition to the vertical loading.

It's not really anything Andre did, it's inherent in this suspension design. It requires everything underneath to be really heavy duty.

Rob thanks for the feedback. Given that I did my calc's on the back of an envelope, in my head, and that the engineering was studied in the early 80's (while also studying drinking and girls), linear and moment analysis is about as much as I can handle. There's waaaay too much going on in that sort of suspension layout to be fully analysed that way! But I was sure the basic principles were sound.

What I have definitely picked up though in recent times is that chassis frames are NOT just something you can throw either an axle/leaf spring or trailing arm/airbag onto. Both systems support the chassis totally differently along with the forces exerted. And I'm not commenting specifically on Andre's work here, it's a general observation on this and other sites. It seems that many people are attempting to construct their own trailing arm suspensions so possibly it's worth a dedicated thread to give a better understanding of what needs to be done differently to make sure these things are safe at least? I know engineers should really certify these things but that's not going to happen. Thoughts?

 

[R_Lefebvre]

I agree completely. And I'm no expert in this either, though my training is about 15 years more recent. I was going to attack it the same way you did, and probably get the same result. I'm just pointing out that a system this complicated really needs a computer analysis. Or, lots of testing as it seems AT has done.

Yes, the frames need to be very different. It seems to me that trailing arm/airbag, or even Torflex axles require much more chassis strength than a leaf spring, since they both concentrate the loads, and also add some twisting into it too.

Leaf springs are... elegantly simple. There's a reason they've been around so long, despite being Byzantine technology.

 

[Martyn]

Martyn when you say pivot rods you mean the bolts that the a-arms attach to the frame with?

thanks,
Andre

Andre we use rods instead of bolts. Your set up uses bolts on both sides of the arms, we use a single rod.

 

[Martyn]

As simple as a "Chromoly" (4130, 4140, 4340) for the rods? That's not too hard to find, and I can't imagine you went to any kind of custom alloy.

It's not a custom alloy for sure.

We have the rods made for us by a machine screw company. The alloy we used first time around was easy to get from suppliers. The stronger alloy was a special order.

 

[dieselcruiserhead]

This is all stuff I wish I had thought or more knowledge of truthfully - everything as far airbags to mounting. They are nice and work well, I'm going to have to evaluate further when I start taking it apart. I noticed no further major tweaking once fairly heavily used off road, the tweaking I'm assuming from both loading and from warpage of the arms when welded. They are difficult for sure. I definitely second the notion of hte difficulty of fabbing independent arms. They are pretty but definitely not professional/engineering designed. I don't think I ever mentioned it though but the air bag is placed square between mounting points but the load is not accurate because the wheel mounts on the one side. It is really interesting (and slightly unfortunate) to have this discussion now...

 

[indiedog]

It is really interesting (and slightly unfortunate) to have this discussion now...

Andre, apologies to have opened a bit of a can of worms. However I'm sure you don't want to continue to have failures and not know why. It's no doubt better to re-assess things now than when possibly stuck with a broken suspension in the middle of nowhere! And hey, my observations could just be wrong!

Again I'm incredibly impressed with your build and you've gone places I would not have considered! Happy trails mate.

 

[R_Lefebvre]

It's not a custom alloy for sure.

We have the rods made for us by a machine screw company. The alloy we used first time around was easy to get from suppliers. The stronger alloy was a special order.

I would think that if long rods aren't needed, Andre could simply use Grade 8 fasteners, and they will prove to be stronger than anything they will be fastening.

Once Andre assesses the damage, I'm sure we can help fix it up.

 

[dieselcruiserhead]

If it helps, they are grade 8! I'll keep everyone posted, again it will be a couple weeks. My gut wrenches at it but worse comes to worse just fab up a new frame... Doh...