Frame Design: Strong Enough?

[Evan Gaffney]

The NEWB is back! Now that you guys have got me pretty well straightened out on the whole axle/brakes issue (I think I'll be using a de-rated 3500lb torsion axle with 10" brakes), I'm working on frame design, and I need to make the frame and body of the trailer at light as possible because I'll be pulling with a wussy focus. It seems to me like the three easiest places to save weight in a TD are the frame, the floor, and the side walls. My sidewalls will be only an inch think (3/4 inch ply and foam center, and 1/8 luan skins). I may even ditch the outside luan skin and just cover the ply and foam in frp??? I have some ideas on the frame, but I was hoping you guys might help me fill in some blanks.

My idea is to do the outside rails of the frame in 2x2 angle, so that the wooden frame of the floor sits down inside the angle iron (if the floor is sturdy, should eliminate the need for too many steel crossmembers).

The tongue and the horizontal crossmember it terminates at would be 3x3 square tube and the two diagonal braces would be 2x2 square tube.

Here's a crappy drawing, but it is to scale somewhat. I'm shooting for 5-6 feet of tongue beyond the front of the frame. Is that enough or too much? The goal is to keep the overall length under 17.5' so that the trailer will fit into any garage that my Galaxie will.



Questions:
-Is this frame concept strong enough for a 5'x10' teardrop?
-What thickness angle and tube can I get away with?
-Will I have to beef up the parts of the side rail where the axle mounts? How?
-Should I move the crossmember further back?

Thanks,
Evan

[bobhenry]

When I drew this I thought you said the tongue member was 6 foot.

When I reread it ,you stated it was 6 feet beyond the front member so my dimensions are off but here are my thoughts right or wrong.



A 1500 pound tear should have about 10% of the total as tongue weight as a rule ( not mine).

Thats 150 pounds ! My picture only shows shows 80 #

As the ball trys to lift the tongue weight see what happens at the other end. "For every action there is an equal and opposite reaction"
In this case we have introduced a mechanical advantage of three to one so our opposite reaction is tripled. I learned this lession with the barn build. The donor trailer had the single center member bent down almost 2" from the previous owner overloading it. I replaced the tongue on the barn trailer and went clear to the rear which let me do this......



and this! Note [ not legal in all states ]



In the bottom half of the 1st picture I have lengthened the under frame portion of the tongue to eliminate the mechanical advantage. The down force is now shared by 2 points ( the white dots) representing two crossmembers. I am hoping one of the engineer types can calculate the new downforce that these two members will face. I am sure its NOT 40# and 40# but more like 20# in the front and 60# in the rear but I can be way off base.
I know there is also some contributort strength added by the diagonals as well but that gets too fuzzy for my little bitty brain.

This is just my two cents worth ( and that is probably overpriced )

[Evan Gaffney]

I see what you mean about the leverage. I guess if the tongue is 5 or 6 feet, I'll shoot for 4 or 5 feet on the other side and then I'll weld extra good. The diagonals will be extended too, that way they spread some of the load to the outside of the frame and not just the crossmember.

[bobhenry]

I have witnessed the aftermath of two tongue failures and they
arn't pretty (and real expensive if you are on the road).
It definatly is not a place to scrimp by to save a few pounds.

[aggie79]

Evan,

An item to consider in using angle iron in the orientation you propose is how you will seal the gap between the side of the angle iron and the floor to keep it from being a water trap.

I think you could shorten your tongue length some. The overall length of our 4' tall x 5'wide x 10' long teardrop is 14'. Our a-frame tongue is 4'-6" long with the front of the teardrop shell cantilevering about 6".



BTW, are you sure you want/need a 3500# torsion axle? I have 2000# #9 Dexter that is derated to 1500# and it is way more substantial than anything a teardrop would encounter (other than perhaps if plan to do serious off-roading with it.) I'm not sure that a 3500# torsion axle can be derated to the 1500-1700# capacity for a typical teardrop.

Take care,
Tom

[Forrest747]

my 2 cents

As far as tongue length i was told half the width of the tow vehicle plus 6 inches. so take the wider of the two and apply the formula. that is if the hitch is in the center.

i went with 2X3 for my tongue and 2 inch square tubbing for the body of the frame. it weighed in with wheels and axle at 400 pounds if i recall.

a 2 inch angle iron should be enough and where your axle goes then just add some square tubbing with a little extra on each side.

QUOTE "My idea is to do the outside rails of the frame in 2x2 angle, so that the wooden frame of the floor sits down inside the angle iron (if the floor is sturdy, should eliminate the need for too many steel crossmembers). "

I would set the floor on top of the angle iron with the horizontal on top this will allow you to put any crossmemebers underneath. and if you make them out of the same angle iron you would just need to trim a little at the ends. JMO someone that has done more frames out of angle iron will know more than me.

Dont forget gussetts

To save weight you may want to go with 1/2 thick plywood. I am kicking myself for not doing that. or at least do a cookie cutter out of 3/4, thinking i should of done that as well.

Also below is a link to dimensions to a 10 foot trailer. the lower non tilting is a good point to help plan.



Here is what my frame is. i did a 5X10 like you are planning.




http://www.loadtrail.com/detail.cfm?model=SE03&desc=Single%20Axle%202%2C990%20Lb%20w%2F2.5%22%20Angle%20Iron%20Frame
Coping angle iron for joints http://www.youtube.com/watch?gl=AU&feature=related&hl=en-GB&v=2tEsWkooS28

[bobhenry]

Now thats a good lookin' frame !



It is also a good representation of what we are talking about !

[angib]

The Australian trailer rules say that the tongue must be able to carry a vertical (or horizontal) load at the coupler of half the trailer's weight. Yes, half. Remember that we aren't concerned with the static load but the dynamic one when both the tow vehicle and trailer hit bumps.

Now the conclusion I've reached is that half the trailer weight may be right for a used-and-abused utility trailer travelling on Australian washboard roads, but a teardrop travelling on American roads needs only half that strength, but that is still a quarter of the weight of the teardrop, 200-400 pounds.