Thread: Triathlon trailer

Joists were spaced off the floor sheets to prevent any culumlative error in the spacing. The joists, rails and chassis was all welded together.

I see where you're coming from Steve, and to be honest I did struggle a bit in the design of this.... well, in the sense of gaining rigidity. The chassis rails were based on car carrying floats that are a similar length rather than truss/sided car trailers. We did a similar one about 2 years ago from 100 or 125x50 and the main concern was the flex that was generated after the axles. We ended up trussing it and it came out very stiff after that and you wouldn't believe the way it has been loaded since! This one has gone to 150x50 and that straight away has made a significant difference in its stiffness, probably twice as stiff (confirmed with the hightly scientific jump on it and test the flex test), especially after the Xmembers were welded in and then again the floor joists. When I designed it, thinking that even the 150x50x4 might flex around a bit, the floor joists space the floor plate up another 50mm, and when the floor is welded in, I'm hoping the structure of the floor will allow the chassis rails to act in a way that is similar to a 200x50 section chassis. Likewise the floor is also being used as the triangulation for the chassis to prevent lateral movement when turning corners etc.

In addition to this, the box which goes on top of it hopefully will also generate stiffness, similar to how a monocoque operates. The sides of the trailer are 2m high and sheeted in 1.6 hot rolled MS, and I tend to think that this is structural enough to also hopefully isolate a lot of the bending loads (not torsional) from being transferred into the chassis rails.

Originally Posted by roverrescue

Load of 2T is load of 2T

A car trailer will generally have a top rail of something like 50SHS2 or so running the length of the chassis rails as a "tyre rail" this is essentially ladder stiffening and adds enormously to the vertical stiffness of the deck

The closest thing to this that I have worked on is an 8m boat trailer (overall trailer length 10m)
-main rails are 150RHS4 with a ladder truss of 16mm round bar spaced 150mm off the top of the rails for their full length (rungs every 250mm)
-It is a widely spaced triple with at least 3m of PFC to transfer the load to the chassis rails - (Simons loads its springs through 6 points direct to the rails = stress points)
-Overall load is greater than this thing but it still bends visibly when on road (lucky its simply a haul out trailer)

S

My money says that if hes got verticals every second floor joist and at the joist in the center of the axles then with some clever stressing during the construction of the stiffening diagonals and the roof he can shift some of the loading from the center out to the end like a center support wire stayed balance bridge.

Originally Posted by Blknight.aus

My money says that if hes got verticals every second floor joist and at the joist in the center of the axles then with some clever stressing during the construction of the stiffening diagonals and the roof he can shift some of the loading from the center out to the end like a center support wire stayed balance bridge.

Verticals are going in with each joist, spacings of about 600 to help prevent the sides from turning into Rolfe Harris' wobble boards while being towed. The joists have prestressed the chassis, and currently sits dead flat with the trailer supports at it ends only so I'm hoping this isn't too big a problem when it lands on its wheels. The box will go on like this though, so hopefully will be setup to keep the trailer flat. I also tend to think that it will do as you describe.

Having had a closer look at the pictures....

Id guess at

if you put your verticals in at the ends of where your planning on running the tubs (so the joint of the first complete joust either end of the suspension) and joined them with 3 Xs (one on either side and one bowed up in the middle to give the roof a little rise to aid run off and to prevent roof sag) put all your verticles on, weld diagonals at the 4 corners that run back to the top of the box over the axles, support the chassis on the 4 corners add about 1/3 of the predicted payload to the center then weld the diagonals onto the top corner of the box you'd very neatly stiffen it up ready to have the axles and wheels bolted under.

based on the wifes mammoth trailer...

to eliminate the wobble board

you can go verticals every 1200 and then using some light 15mm right angle tacked between the verticals horizontally (between 800 and 1200)and some good sikaflex or spot welds and the sheetings not going to move, especially not if you do your attachements from the center out. If I had to redo the wifes trailer again about the only thing ID do differently would be to stagger the horizontal supports between the verticals.

oh yeah,

whatever youre going to do, mock it up with dowels and string first. Its a pain to get everything in and then realise that the weld you want to do you now cant because youve already welded in a member and its blocking your access to the spot you want to weld....

First off - let it be known Im a shade tree hack
But I still think an engineer would baulk at the design (even though you dont need engineering signoff)
Relying on a 2m high twisting and turning sheet metal box to support the chassis seems fraught?

As you say those 28 welds across the top of each rail should have prestressed (cupped) the rails upwards - It is weird out how it sits flat supported at the each end and with the welds? I would have guessed it would have pre stressed a noticeable amount by now ie cupped convex up?
maybe for a test jack the chassis rails up at the mid point and put them on a pair of axle stands measure the deflection!

Also rather than welding the spring hangers direct to the underside of your main structural would there be value in making a sliding suspension frame that will remove point loadings (welds across the chassis) and also allow you to perfectly balance the rig once she is completed?

S

I just want to see the bloody great thing loaded up being towed

Made the roof frame this afternoon, and stuck it up on stilts.



and a few studs to hold the roof up and support the wall structure before checking the frame for square and welding up. .



1 of 16 wall sheets done!