Trailer Building Indepent Arm Building - Official Guide

[dieselcruiserhead]

I'm going back to my old thread and this clearly didn't work. I didn't anticipate the arm to want to pull the steel down as well, I thought it would try to push it up for some reason (though it makes sense now)...

Here are the arms separate from the frame for closer inspection (old pic).

 

[dieselcruiserhead]

Here are the mods I plan to do to the arm.. Big big thanks to MotoDave who is sending me about 6" of Delrin material that I'm going to get machined to slide into the tubes. Hopefully there is enough material for 1 or 2 spares as well "just in case."

And a new cross member:

And here is what it looked like before the illustrations...
http://www.expeditionportal.com/forum/showthread.php?p=348509#post348509

 

[dieselcruiserhead]

Please let me know your thoughts/input on these upgrades, before I do them! Trust me I have a thick skin on this/not offended. Likewise I planned to align it so I would just barely have toe-in so under load it should straighten out with minor arm flex. I doubt I'm going to be towing the trailer too much empty I anticipate...


Also interestingly enough all of these mods crossed my mind when I was putting it together. Such is life...

 

[Trailpsycho]

Let me first state that I have not (yet) built what you are attempting to build, nor am I an engineer. However, as a fledgling-novice-wanna-be fabricator and welder, I was suprised from the get go that you did not replace the cross member or at the vaery least, significantly gusset it and then box it in. I dont think there is much wrong with the arms or the bushings themselves. Its my opinion that the gusset from the tube to the arm is a solid idea/upgrade. As well, replacing the crossmember and plating it in to the frame by at least 8 inches each side (fore and aft) of its juncture with the frame would be adequate to remedy most of the problems you are experiencing.

Its obvious that you took alot of your design from AT, which is cool, they put in the time and know how to get a quality solution...gleen from it and make it on your own. The biggest discrepancy (and a major weak point) that I see in your design is the crossmember/mounts and how it/they relate to the frame.

While I think 3/16" is plenty thick in most cases, I would also suggest that instead of the L shaped tabs that you are currently using to attach the arms to the crossmember, that you use a section of rectangular tubing with a cut away so that there is a built in "gusset" into these mounts. I would also add a lateral-gusset to the crossmember. The same hardware and bushings should work fine...you would access them with a deep well socket however.

Just my 0.02; please let me know if I am missing something obvious.

 

[indiedog]

Andre, as per my original comments in your build thread, I still think you should make the arms wider at the pivot point. Considering the trailer wheel will push up, the outside edge of the pivot point will want to push up while the inside edge will want to pull down. So increasing the distance between the two (making the legs on the A frame wider) will decrease these forces by increasing the reaction leverage. Granted beefing up the frame may make it cope with the increased moment forces, but reducing them in the first place would be a good thing. And I know that makes for a lot more fabrication.......


The other thing you should consider is to move your airbag mounting point toward the rear as again the wheel force is using the bag as a pivot to lever on the bushings. From your pictures, if you cut off the frame mounting bracket and re-attach it so the bag is more vertical then it will connect to the swingarm further back and closer to the axle. Your bag will also then be more vertical.

One other thing I've learnt about independent swing arms is that the closer the arm is to horizontal the better it works. So if you install another fixed piece of tube between the frame and the pivot bushes that would improve things. Check out any of the Aussie camper systems and you'll see what I mean. Look at Ultimate Campers or Vehicle Components

Good luck with it and kudos for your bravery!

 

[James86004]

Considering the trailer wheel will push up, the outside edge of the pivot point will want to push up while the inside edge will want to pull down.

I agree with you, but something with the photo seems a little odd. I think the inside pivot should have a downward force, but the photo shows the metal is deformed up. That has been bothering me a lot.

 

[indiedog]

I agree with you, but something with the photo seems a little odd. I think the inside pivot should have a downward force, but the photo shows the metal is deformed up. That has been bothering me a lot.

James, I reckon you'll find that "kink" occurred because it's a free edge of a channel. So any compressive force along its length will cause that. And who knows what other loads have been exerted on the trailer in its usage.

EDIT; The fact that the swing arms are pushing up in the middle and pulling down at the outer edge means there will be a compressive force along the bottom edge of the cross member, so that's what I believe would have caused that kink.

EDIT EDIT; I think I got that wrong. The swing arms are actually pulling down the cross member in the middle so that would add tension to the bottom edge of the member. If you look at forces in the horizontal plane there is potential for the wheel to want to push backwards when hitting bumps and this would cause a horizontal bending force in the cross member. And on top of this the whole thing is dynamic so it would just take the right combination of possible forces during travel to cause damage.

 

[ntsqd]

I wasn't thinking of one long bushing, but rather one on each end of the tube. Sort of like UK's set-up only with one, long tube and two bushes instead of two tubes & 4 bushes. The full length tube is just for rigidity.

 

[UK4X4]

Your gusset 3 was added to my arms as well before they were finished, I did think of adding gussets in the arms themselves, but thought it was overkill

really though when you look at your pic's, those cross members
are just not strong enough to take the loads

I'll take a guess that your bush failed after the mount moved.

Your mounts are also not very long, mine were 4" front to back

Your airbag is also mounted a long way from the wheel, giving
the action more twist, they would be better being mounted closer to the
outside of the trailer.

Shocks yep- better mounted vertical- but then you loose space in the trailer
and holes to fill where they would come up through the floor.

Airbag movement is about 6-7 " full travel, the shock maybe moves 3"


"Lowering the mounts to give the arms a more horizontal position"

I looked at this but when I plotted out the arcs of travel I did'nt see much gain in anything.

And you have to add a large dangling support rail which would add more stress to the equation- more steel and give less ground clearance.

On the angled arm I get only 1/2 an inch or forward backward movement
1/4" either side of running height, so no issues for the bags being stressed

 

[ntsqd]

To my thinking some rearward travel in compression is desirable. On impact the tire moves away from the obstruction rather than no lateral movement or worse compressing into it. The input is still the same, but by the tire moving back the duration on the bump is increased while the abruptness of the bump is decreased. It is for this reason that the VW from beam/trailing arm suspension was popular in desert racing for so long. Only the desire for greater wheel travel than a trailing arm design can reasonably provide has driven the designers to A-arm designs.

 

[R_Lefebvre]

You are exactly right. What you are talking about is properly called "Kinematic Recession". It's a big deal in auto ride/handling engineering. No car can ride well without some KR.

I think using a single tube at the pivot is a mistake. Then the entire load will be on only two mounting ears instead of 4, and one really long pivot bolt. Stick with the 2 independent pivots. I think it would be fine. 90% of your problem is that weak crossmember, and the other 10% is the bushings. I don't know if I mentioned it, but I know I sure thought about it, that that cross member just wasn't going to be strong enough.

 

[ExpoMike]

As for bushing ideas, I would consider something like the Global West Del-A-Lum bushing idea. I have used these in a couple different performance car applications and they worked great. Pivot with the low friction like a bearing and about zero deflection but without the wear issues a metal to metal bushing might have. Here is a link to them, http://www.globalwest.net/Del-a-lum_suspension_Bushings_by_Global_West.htm

A little better pic

 

[R_Lefebvre]

Wow, Global West, does that take me back!

I haven't bothered greasing my delrin bushings, no issues so far. The thing about grease is that it will attract and suspend dirt in the grease unless they're sealed. That becomes abrasive and can lead to wear. I believe Delrin is meant to be unlubed when used for a bearing application.

 

[dieselcruiserhead]

Guys there are some great comments here, sorry I have been pretty busy this week with not a lot of time to pay attention to the specifics so far. I have the machinist and the delrin lined up (thanks again MotoDave). I don't want to re-do the arms so I'm going to extend them by using a longer piece of DOM but leaving the actual pivots the same minus all the additional gussetting of course. I think Rob is also right about the cross member being a major issue as well... I think this will hopefully fix it all I think mostly again because the girth of the steel I've used and will be adding is so significant so I shouldn't get any more tweaking...

thanks again guys, I'll read up tonight and post back shortly.. I was hoping to maybe do this sometime this week if I'm lucky and I'll document it well in my thread... Thanks, Andre

 

[UK4X4]

mm ok rocket scientists- delrin - phosfor bronce - unobtainium

jello - vasiline - or chain lube for lubricants

Issues basicly are

lower pivot point- the air bag is not positioned close enough to the tire
giving too much leverage.

The upper supports for the arms are mounted to a weak bent up C section.

The bush only gave up the ghost as things became un -true.

His arm design is fine- the dual mounts for each arm is fine.

The air bag needs moving closer to the wheel to minimise leverage
and a decent size crossmember will sort the issue.

Wether his poly bushing is chinese made and soft as a Jelly bean could be an issue - but the mechanics need sorting first.