- a centre rail will add very little for structural rigidity to the box - you need triangulation!!!!! Tieing the box to the tub will help in its rigidity when static but as the draw bar IS going to flex it will actually put more load on the box and may lead to failure?
- the issue is going to come back to the draw bar ;) a static load of 350kg is nadda compared to a 749kg trailer flexing on three sections of 50SHS bouncing down the road. To test this flexing, remove the front box, fill the tub with a few hundred kg then get someone to tow the trailer whilst you drive alongside the trailer (somewhere safe and preferable mildly uneven road surface) You will be amazed at the amount of flex that will set itself up at the junction between drawbar and box in a dynamic situation.
Flexing in and of itself is not a bad thing (steel can be used in its elastic range), you can make a trailer too rigid BUT in my experience for offroad trailers its best to control flex in the suspension rather than at the aframe.
Persoanlly I would traingulate the box making it essentially a self supporting structure attach it to the draw bar with 30mm of clearance to the tub and then run the ladder frame under the SHS. Then after using it for a season and learning - Mk2 will use softer suspension :)
Steve
