Wanted a dirt bike nut !!

[
EDIT put it {axle} central then its a matter of two facing forward or two facing the rear
to get some weight on the towball[/QUOTE] So your for centre of load area and work draw bar length from there ?

Also what about a small amount of toe in for tracking do you feel it will have any bearing on a single axle it certainly does on a tandem car trailer with a rally car and spares towed like a dream.

No if your making it from scratch you'll build it up side down and get the axle as plumb as possible.
other wise it will have a crab stance. Easy enough to flip over

3dogs the brain is going 100k/hr at the moment so everyone's input will help me over engeering it and make a nice trailer this is a project with my lads so we only want to do it once , as a dry old bloke said to me once some butchers ware a apron and some ware overalls that cracked me up .LOL

heres two of three I've made, the second I still use.
one is a SR5 extra cab the other is a J deck single cab [hilux]
string line and tape is your friend.
Chassis wise I'd suggest 40x40x2mm RHS fully welded.even 50x50x2mm rhs
once the springs are on you can flip and make the front stone/wind
storage area to protect bikes in transit. Yes over engineer it
As you know triangles are your friend, cant bend a triangle
I a big believer in triple draw bars with the centre one going the full length
It can be from smaller material, notch it in then you weld a plate on top to except which ever coupling you want.
I went the O'Brien coupling Aust made and easy to use, don't forget to use rated chain and shackles
Question ::: will you be running 6 stud rims and or brakes

Sorry mate had to run down and back to Brisvages for a course today from Mackay bit how its going but to answer the ? disk bakes and 6 stud and I'm with you running a centre chassis rail then side rails is the way to go .My uninformed view is that all stress is on the centre rail and the side rails only have to cope with flex so the load is centred on the centre rail and the design is specked to that , I hope you understand that and I think you do going on your designs .

Your A frame draw bar is the strongest as it gets the forces of tow to drag in a straight line.
Your wheels (the drag) ain't in the middle of the trailer, and you'll have very little trailer flex as that's what the suspension and coupling control.

The central bar just aids the A frame it load bearing, but for bikes woukd be way over kill..

I would have thought a bike cog would be as central as possible, so the rider has full control and not having to out weight it.

I like the 2 forward and 1 backwards idea, but most do all 3 forward with the middle one in front a bit to allow tie downs to clear the side bikes.

Also use fold flat tie down points. You'll want 5 for the handle bars and 3 for the tails. The 2 outside bikes will tie to a central point which will be under or behind the middle bike.

Also place these tie down points next to a cross rail, adds more strength.

If your looking at a light sheet I'd also add some smaller frame work into the chassis so the tie down ain't relying on the sheet to hole em down..

I've always used the simple 10% behind cog for axle position, but being a dedicated bike trailer you'll have to work that one out..

Have you worked out water and fuel weights yet?

Westy we had a bit of a think tank and no 2 son may have come up with the fluid solution .Use jerry cans that way if a rock or stick goes into a tank u will only loose 20 lts and move them around to balance so that's where we are going .

I like the jerry can idea better for sure.
A few fuel a few water.