1"x2"x.188" Steel Frame/Axle Concern

[RichAFix]

I have thought, rethought, and re-rethought out ever aspect of this thing about 4 or 5 hundred times, I am getting ready to jump into the build phase. I had my mind made up I was going to use 2"x2"x.125" square tube for the frame, but after seeing everyone talk about how overbuilt most of these end up I am leaning more towards 1"x2"x.188" C-channel for the frame. I don't worry about it being strong enough especially with adding a dado in the side walls, but is 1" going to give me enough beef on the frame to weld/bolt the axle to the frame? I plan on using a torsion axle (and there are going to be plenty of questions prior to ordering that, as I really don't understand that angle/ride height/tire size thing, but more on that when we get there). By going with the c channel rather than the square tube I save about 30 pounds and about half the cost of the steel. I would appreciate all thoughts/concerns/comments, don't hesitate to call me a knucklehead if you see something that doesn't seem right.

[rbeemer]

RichAFix,

You do not have to have the luan on the exterior of your floor. Also remember there will be cross members for your frame that will add stiffness to the c-channels for the sides. I think you should be fine with the thickness of the metal but I am no engineer, there are others that can give you a better answer.

[madjack]

...we use 1x2x3/16 "C" chanell for our frames and a single sheet of 11ply 1/2" cabinet grade ply for the floor...just the way we doit....
madjack

[RichAFix]

I guess the only reason I was thinking about the luan on the underside of the floor is that I was planning on putting insulation in the floor (cutting some of the plywood out and putting insulation in the holes). I was just thinking it would look cleaner and stay a little more sealed with another sheet of luan on the underside to protect from debris/water. Maybe just using .5" ply for the entire thing makes more sense. It would weight approx. the same as the 2 sheets of luan and the mattress would give a person enough insulation from the floor.

[brian_bp]

... I am leaning more towards 1"x2"x.188" C-channel for the frame.
... but is 1" going to give me enough beef on the frame to weld/bolt the axle to the frame? I plan on using a torsion axle...

The bracket which is attached to the main tube of a typical torsion axle (Dexter or AL-KO) has a main vertical plate, which bends 90 degrees to form a flat mounting area against the frame, and in some load ranges it bends back down and has another vertical part. The idea is to line up the main vertical plate with the strongest point of the frame, which in the case of the C-channel is whatever side is not open. The width of the C-channel's horizontal flanges (1" in this case) is not important, as long as the horizontal flange of the C-channel and the horizontal top of the bracket are in contact (not going different ways).

Here's how Dexter puts it:

The preferred arrangement should place the longest vertical section of the axle bracket directly under the most rigid section of the frame member.

For example, if using Dexter parts, and C-channel with the open side facing inward, you would want to order the axle with the brackets having the main vertical plate on the outside, and match the outside-to-outside width of the frame to the outside-to-outside width of the axle brackets. This is shown in this drawing which I clipped from the Torflex mounting section of the Dexter Application Guide:


The drawing also suggests reinforcement on the open side (boxing it in, in the area of the bracket) which seems like a good idea but I don't know if anyone actually does it. It shows directly bolted, but you can bolt via their side-mount brackets instead, or weld directly to the trailer frame, and the relative positions remain the same. If you're bolting to the C-channel, the drawing shows that if the channel is typical structural steel you need tapered washers; if the channel is a bent plate (constant thickness) you don't.

[brian_bp]

... I had my mind made up I was going to use 2"x2"x.125" square tube for the frame, but after seeing everyone talk about how overbuilt most of these end up I am leaning more towards 1"x2"x.188" C-channel for the frame.
...By going with the c channel rather than the square tube I save about 30 pounds ...

1"x2"x0.188" channel will have about 0.752 in2 of material (9 cubic inches per foot), while 1"x2"x0.125" box section will have 0.750 in2 of material... the same amount of steel, with more strength. But the channel is easier to bolt to.

The originally proposed 2"x2"x0.125" box-section would weigh about 3 lb per foot, and so the 1"x2"x0.188" channel will save 3/4 of a pound per foot... maybe 12 pounds for both main rails of a trailer with an 8-foot body (36 lb total instead of 48 lb). Is that a problem? The expected 30-lb weight savings must be based on the same material selection of the main rails, cross-members, and tongue... but those are all different situations which could use different sizes. I've never seen a real production vehicle (truck or trailer) with crossmembers the same size as the frame rails.

I would even consider 2"x2"x0.095" box section which would be stronger and much stiffer than the channel for virtually the same weight (0.760 in2 in cross section); I don't know if the wall would be too thin for reliable attachment of suspension, etc.

If anyone wants to check my weight calculations, which are by density and not from an authoritative steel product table, please do.

[RichAFix]

The originally proposed 2"x2"x0.125" box-section would weigh about 3 lb per foot, and so the 1"x2"x0.188" channel will save 3/4 of a pound per foot... maybe 12 pounds for both main rails of a trailer with an 8-foot body (36 lb total instead of 48 lb). Is that a problem?

Okay your first post made sense to me, but not sure if I follow you on this part. My 30# savings is based on side rails, front/back/one cross rail and the a frame part of the tongue. I still have a 2"x3"x.125" tongue, which may be overkill, but I have had a trailer that failed at the tongue so I prefer that a bit stronger for my own peace of mind. My thinking (and I am far from an engineer, so please don't hesitate to correct me) is that the frame part of the trailer is getting a lot of strength from the floor and side walls that will be bolted to it. My biggest worry on this whole thing is that I don't want to attach the axle until I know the weight so I can move it to give me the best tongue weight. With that in mind, I will pretty much have the thing done when it is time to attach the axle. I don't want to weld (prefer to bolt) the axle as the trailer will be painted and the tear will be on top, so I don't want any surprises at that point. Does this make sense? If I need more steel, not a problem, even the weight isn't the end of the world to me (but I am trying to shave off as much weight on each step as possible as I really want to keep this thing under 1000#'s, so I can pull it with our car, not the truck), I just need to know before I get to that point as it will be too late to do anything when I get there.

[angib]

I think your 1"x2"x3/16" channel is plenty strong enough as the main frame of the teardrop. But I have two areas of concern:

- If you are proposing to use the 2"x3"x1/8" as an A-frame, then that is certainly strong enough - it would pass the Australian rules for a trailer of about 3,000lb! But if you are thinking of using the 2x3 as a single tongue (which it is quite strong enough for), you will need to have a big spacing between the crossmembers it's attached to - otherwise the tongue won't fail, but the crossmember will.

- A 1" flange is very narrow to bolt an axle bracket to, whether it's torsion or leaf-sprung. For example, the Dexter #9 bracket is 2" wide and assumes the use of a 1/2" bolt, as shown below - now cutting a 1/2" hole through a 1" flange is taking away quite a lot of its strength, not to mention trying to squeeze a 3/4" AF nut onto a 13/16" wide flange.



Andrew

[RichAFix]

- A 1" flange is very narrow to bolt an axle bracket to, whether it's torsion or leaf-sprung. For example, the Dexter #9 bracket is 2" wide and assumes the use of a 1/2" bolt, as shown below - now cutting a 1/2" hole through a 1" flange is taking away quite a lot of its strength, not to mention trying to squeeze a 3/4" AF nut onto a 13/16" wide flange

javascript:emoticon(':applause:')

Thank you, that is exactly what I was worried about, your explanation makes sense to me. I think I will just go back to "plan A" and use the 2"x2"x.125", at least for the frame sides. Of course that makes bolting the axle on impossible as the frame would be enclosed and I wouldn't be able to get to the inside of it.

[Nitetimes]

Of course that makes bolting the axle on impossible as the frame would be enclosed and I wouldn't be able to get to the inside of it.

Not at all. When you order the axle order the weld on brackets too. They look like a piece of angle (bent steel tho) pre-drilled to fit the axle mount holes. Weld them on and bolt the axle to them.

[brian_bp]

Yes, the narrow (1") flange of the proposed channel does make direct bolting impractical; since the Dexter diagram showed it, I mentioned it more for completeness than a serious suggestion to bolt on the axle in this case. In addition to the sheer lack of material in which to drill a big hole, the bolt holes in the Dexter bracket are centred on the bracket, and thus much more than 1/2" from the aligned faces of the channel and bracket.

The direct bolting approach is only practical if the frame is at least as wide as the bracket, which is 2" (dimension G in the drawing supplied by Andrew) for a Dexter #9.

The Dexter bolt-on bracket option (which they call a side mount hanger) is welded to the frame, and thus just like directly welding on the axle bracket, from the point of view of the frame. As I mentioned earlier, the position of the axle's bracket (and thus the width to order) is the same whether or not this additional bracket is used. Personally, I would use the side mount hanger, but I'll admit that it would probably never be needed because the axle will last as long as the original owner keeps the trailer.

[brian_bp]

...I think I will just go back to "plan A" and use the 2"x2"x.125", at least for the frame sides. Of course that makes bolting the axle on impossible as the frame would be enclosed and I wouldn't be able to get to the inside of it.

Not necessarily impossible. Someone (I think in fiberglassRV.com) described his approach, in which he used a "nut plate": a steel plate just narrow enough to fit inside the frame tube with two nuts welded at holes at the same spacing as those in the axle bracket. He slid this in from the frame tube end (which must then be open, unlike my travel trailer) and ran the bolts up through the axle bracket and holes drilled in the bottom of the frame tube into the nuts.

This still puts a couple of 1/2" holes in the bottom of the frame, which is generally a structurally bad idea even in a 2" wide frame; however, at the point where the axle is supporting the frame the bottom is under compression stress (not tension like the top), so holes filled with bolts are not so bad.

In practice, I think it makes more sense to weld on the axle, in the case of box-section (tube) frame rails. To make the axle removable, I would weld on the side mount hangers and bolt the axle to them, as Nitetimes described. By the way, they're called "side mount" because they bolt to the sides of the axle brackets: they are still on the bottom of the trailer frame.

[brian_bp]

... My 30# savings is based on side rails, front/back/one cross rail and the a frame part of the tongue. I still have a 2"x3"x.125" tongue, which may be overkill, but I have had a trailer that failed at the tongue so I prefer that a bit stronger for my own peace of mind...

I did my calculation and comments assuming that the tongue would be an A-frame of the same material as the frame rails, and both that tongue and (especially) the cross-members don't have to be as strong as the main rails.

It seems that the plan, instead, is to build an A-frame of the same material as the main rails (originally 2"x2"x0.125" box, then proposed 2"x1"x0.188" angle) and a central tube of 2"x3"x.125". Either a central tube or an A-frame would be appropriate; both seems to me to be entirely pointless. I understand wanting to avoid a similar failure to the past experience, but adding an entire A-frame, instead of correcting the straight tongue design or switching from straight to A-frame, is the part which seems like overkill to me. This has come up in discussion here before.

As Andrew mentioned, a single-tube straight tongue carries all the load to the first couple of cross-members, which then need to be very strong. Avoiding the use of a straight tongue like this means that the cross-members could be lighter, which was my point earlier - not everything has to be as large as the main frame rails.

That central tube would be about 75 cubic inches of steel (if it were 5' long, for a tongue length of 3' plus another 2' to the next crossmember)... the same as ten feet of C-channel frame rail. That's about 20 lb of extra steel, making it pointless to split hairs on the main rail dimension choice.

Why not just use an A-frame, perhaps of the same material as the main rails for convenience, and save weight by omitting the pointless straight tongue and perhaps using lighter cross-members? If the C-channel is not strong enough to serve as an A-frame tongue, the 2"x2" box could be used instead.

Again, I don't have any reason to believe that the C-channel would not be perfectly adequate... just that using it for the main rails doesn't save a meaningful amount of weight compared to the originally proposed 2"x2" tube.

[Alphacarina]

- A 1" flange is very narrow to bolt an axle bracket to, whether it's torsion or leaf-sprung. For example, the Dexter #9 bracket is 2" wide and assumes the use of a 1/2" bolt, as shown below - now cutting a 1/2" hole through a 1" flange is taking away quite a lot of its strength, not to mention trying to squeeze a 3/4" AF nut onto a 13/16" wide flange

Thank you, that is exactly what I was worried about, your explanation makes sense to me. I think I will just go back to "plan A" and use the 2"x2"x.125", at least for the frame sides. Of course that makes bolting the axle on impossible as the frame would be enclosed and I wouldn't be able to get to the inside of it.

You could use the 1 X 2 'C' channel and then box the back side of it with another piece of the same material welded to it 3 or 4 inches in front of and behind where the suspension bolts to it - That would give you a 2 X 2 boxed tube to bolt the suspension to . . . . and you wouldn't be adding any unneeded strength or weight on the rest of the frame where it isn't needed

Don

[Esteban]

RichAFix, it looks like you could save a little weight by using 1/8" plywood, instead of 1/4", for your inside cabin walls. !/4" luan plywood for the floor seems like it might be flimsy. Maybe you could end up with about the same overall weight by using 1/8" plywood for your inside wall skins and stiffen up your floor with 3/8" plywood instead.

I used the Dexter low profile side mount axle brackets for my frame, like Andrew's illustration. The brackets are welded to the bottom of the 2" tubing. The axle is then bolted, not welded, to the brackets. If the axle ever needed to be changed it'd be easy to unbolt it.

My 4' long A-frame tongue is made from 2"x3"x1/8" tubing. It's much stronger than the Aussie Rules would require for my 5'x10' trailer with an all loaded up for camping weight goal not to exceed 1500 lbs. My main frame is made from 2"x2"x1/8" tubing. Internal cross pieces are 2"x2"x1/8" angle. It's all very rigid. Probably could have used 14G tubing for the main frame to lighten it up some more.