Trailer "A" frames and the alternative.

mart · May 2, 2008

1. When making the “A” frame on a trailer, what is it that determines the length of it?

2. Are there any disadvantages/advantages in just using a single length of (50mm?) box section as an alternative to the “A” frame with one end welded to the chassis and this same piece of box also welded to the front section of the (bottom angle iron ) of the trailer frame? In fact I seem to recall that the commercially built trailers are like this.

rtbcomp · May 2, 2008

I think A-frames are used for strength - the triangle is a very strong shape so savings can be made on weight & cost of materials.

The small camping type trailers don't really need the strength so savings in manufacturing can be gained by using a single piece of steel.

I suspect the length is determined by how much clearance you need between the front of the box and the vehicle for safe cornering.

English Steve · May 2, 2008

Hi Mart,
As RTB has said, an A-frame usually allows a better strength to weight ratio, although there are commercially available goods trailer chassis with a single beam drawbar (generally known as T-Poles). AL-KO make a T-Pole chassis that you can simply bolt your trailer body onto.
If you are thinking of adjusting the length of a trailer drawbar, please bear in mind that it has a direct affect on the coupling load (noseweight). The longer the drawbar on a given trailer, the lower the noseweight will be and vice-versa.
The formula for the calculation is:

S=(BxGa)/A

where
S is noseweight
B is distance from axle centreline to theoretical centre of gravity of the load
Ga is weight of load
A is length of drawbar from coupling centre to axle centreline

I assume you know that there should always be a load on the coupling and so distance B in the formula will be ahead of the axle.

Hope this helps

Steve

peterd51 · May 2, 2008

Hi,

I had use of a trailer for many years when we took the kids camping...

it was a 'home-made' jobbie about 5' x 3' and was a decent size for the big frame-tent and all the camping gear. Clothes/bedding, etc, went in the car boot or on the back seat.

It had a single drawbar welded on to the trailer crossbeams but it also had a couple of angle-iron strentheners from about half-way along the drawbar going to the the front corners of the trailer at about 45 degrees...a sort of semi 'A' frame.

Apparently the guy that built it made a slight error in making the drawbar a bit too short.

The trailer followed the car beautifully at any sort of speed I wanted to travel but it was an absolute begger to reverse. The first hint that the trailer was drifting away from dead straight and it was pretty much impossible to recover it. I can reverse the caravan easier!

So I'd suggest you try to get the hitch more than five feet in front of the wheel-hubs, the longer the better, but as stated above if it's too long then you have other problems such as nose-weight.

Regards
Peter

English Steve · May 2, 2008

Mart, Having just re-read the original post, if you do decide to go down the T-Pole route I would strongly recommend that the drawbar is welded to the chassis at the axle beam - not just at the front.

Steve

rtbcomp · May 2, 2008

English Steve said:
Mart, Having just re-read the original post, if you do decide to go down the T-Pole route I would strongly recommend that the drawbar is welded to the chassis at the axle beam - not just at the front.

Steve

Yes, it's essential to weld it in two places at least. If you just weld it at the front there will be a vertical twisting moment on the weld.

snowcat · May 2, 2008

This is My old trailer built around 10 years ago, the axles were robbed from two maestro vans, the chassis were made from 2" box, and the A runs near too the axles, as said Ts always need bracing in several places, as said Ts need to be welded right back into the chassis, on heavy commercial trailers the norm is to build box chassis then weld another cross member 1/3 down (or often to a ram tipping point) then weld and gusset the front, for the stuff iv done this is normally some pretty heavy material......Car trailers are a compromise between weight vs strength,

BIG JAY VTR · May 2, 2008

not a very clear pic but this is how i built mine

it carries a decent weight and no probs with turning with the a frame type but i think i over engineer things as iv got 50mm centre box section and its some 45mm box making the a frame part all seam welded

MrFluffy · May 2, 2008

I inherited a single drawbar trailer that some genius had cut the drawbar down when it snapped from its axle mounting and welded it to the front of the (sheetmetal galv'd) trailer body.
Net result is it had trashed the main body of the trailer putting loads where they were not designed to be put.
Once Id finished sewing the body back together and straightening it up and put the drawbar back mounted on the axle again it was fine...

rtbcomp · May 2, 2008

A mate built a trailer with a chipboard box - and bolted the indespension units directly to the chipboard!

Luke · May 2, 2008

Jay, why didn't you profile the points of the A frame to meet the main draw bar properly?

Would have made the joints a lot stronger.

scarecrow · May 3, 2008

snowcat said:
This is My old trailer built around 10 years ago, the axles were robbed from two maestro vans, the chassis were made from 2" box, and the A runs near too the axles, as said Ts always need bracing in several places, as said Ts need to be welded right back into the chassis, on heavy commercial trailers the norm is to build box chassis then weld another cross member 1/3 down (or often to a ram tipping point) then weld and gusset the front, for the stuff iv done this is normally some pretty heavy material......Car trailers are a compromise between weight vs strength,

View attachment 3416

I thought most trailers were made with an angle iron chassis because they would be more flexible and less likely to crack around the weld

BIG JAY VTR · May 3, 2008

Luke said:
Jay, why didn't you profile the points of the A frame to meet the main draw bar properly?

Would have made the joints a lot stronger.

to be honest i didnt think of that at the time,, would have looked better to,, will remember for any future trailers tho ,, thanks

rtbcomp · May 3, 2008

You could always weld a triangular plate over the joint to strengthen it.

snowcat · May 3, 2008

scarecrow said:
I thought most trailers were made with an angle iron chassis because they would be more flexible and less likely to crack around the weld

Most of the commercial builds Iv looked at are press braked profiles although I'm much more used to the other end of the scale, people like Herbst, A/S Marston, and Richard Weston all use box chassis , Ivor seam to roll a load of C section Chanel's, I'm actualy overhaul 40 yard bins at the moment which are all press braked C herringbones built around a basic box section , there also showing how price built they are by the 18 month failure rate.....

Although flex is unavoidable, I would have thought that stress reversal cracks and general fatigue in angle, would be more of a problem than HAZ cracks......

40' artic trailers run on special built I beams BTW, this must have something to do with weight vs strength id imagine, bulkers are also the same (i think), although there is some pure aluminum bukers about, running box chassis.....

just to add, I wouldnt bother makeing another trailer for the truck, the comercial built Ivor and betson, trailers are around £800 for the one my size.... but with sides ramps and galvanised......

BIG JAY VTR · May 4, 2008

thats happening soon as im putting a rail at the front to take the front wheel of a bike so it doubles as a bike trailer to

scarecrow · May 4, 2008

snowcat said:
Most of the commercial builds Iv looked at are press braked profiles although I'm much more used to the other end of the scale, people like Herbst, A/S Marston, and Richard Weston all use box chassis , Ivor seam to roll a load of C section Chanel's, I'm actualy overhaul 40 yard bins at the moment which are all press braked C herringbones built around a basic box section , there also showing how price built they are by the 18 month failure rate.....

Although flex is unavoidable, I would have thought that stress reversal cracks and general fatigue in angle, would be more of a problem than HAZ cracks......

40' artic trailers run on special built I beams BTW, this must have something to do with weight vs strength id imagine, bulkers are also the same (i think), although there is some pure aluminum bukers about, running box chassis.....

just to add, I wouldnt bother makeing another trailer for the truck, the comercial built Ivor and betson, trailers are around £800 for the one my size.... but with sides ramps and galvanised......

I'm no expert but I thought that the theory behind the flexible Chessie was to dissipate the twisting forces on the welded joints for but I could be wrong
I came across this web site while trying to figure out weight versus strength of different types of Chessie http://images.google.ie/imgres?imgu...w=108&prev=/images?q=I+beams+&um=1&hl=en&sa=G but it was getting a bit technical for me

snowcat · May 4, 2008

scarecrow said:
I'm no expert but I thought that the theory behind the flexible Chessie was to dissipate the twisting forces on the welded joints for but I could be wrong
I came across this web site while trying to figure out weight versus strength of different types of Chessie http://images.google.ie/imgres?imgu...w=108&prev=/images?q=I+beams+&um=1&hl=en&sa=G but it was getting a bit technical for me

I know large commercial truck trailers have huge amounts of twist in them (I beam section), I think yes your probably right, If you want light weight with lots of strength use flexible sections, however often on everything I build its just plain heavy with no flex anyway.........., as iv said I tend to look at what the big mfs are doing and "borrow" the Idea......

scarecrow · May 4, 2008

snowcat said:
I know large commercial truck trailers have huge amounts of twist in them (I beam section), I think yes your probably right, If you want light weight with lots of strength use flexible sections, however often on everything I build its just plain heavy with no flex anyway.........., as iv said I tend to look at what the big mfs are doing and "borrow" the Idea......

Common sense I presume getting back the original question on agricultural trailers you get A frame and straight draw-bars the straight draw-bars are made up of the same size channel as the A frame one but but it takes more material and work to attach the straight draw-bar as you have t put supports
out to the side of the trailer to help make the body more rigid the straight drawbar is easier to reverse with a tractor because you can lock tighter before the wheel hits the drawbar this should be a problem with a car it's just a matter of cost that most drawbar are Aframe

Trailer "A" frames and the alternative.

mart

Member

rtbcomp

Moderator

English Steve

Senior Bumpkin

peterd51

happy to be here!

English Steve

Senior Bumpkin

rtbcomp

Moderator

snowcat

back in black

BIG JAY VTR

Member

MrFluffy

Member

rtbcomp

Moderator

Luke

Forum Supporter

scarecrow

Outstanding in his field

BIG JAY VTR

Member

rtbcomp

Moderator

snowcat

back in black

BIG JAY VTR

Member

scarecrow

Outstanding in his field

snowcat

back in black

scarecrow

Outstanding in his field