How far out from the springs is too far

[MickinOz]

I'm getting ready to order a 77 inch axle. (tip to tip)
My calcs (repeated many times) say this will give me enough clearance for my extended floor width.
The springs mount directly under a 55 inch wide frame.
So each side the axle will stick out 11 inches.

I want to stick with the 1.53 inch diameter round axle if possible. I then wouldn't need to add new u-bolts, and weight increase would be minimal.
The 39mm/1.53inch axle is considered a 750kg / 1650 lb axle over here.

My target weight is about 900 -1000 lbs loaded, and current indications (estimating ply weights from the density labels on the sheets, weighing bits as I fabricate them, etc) are that I'll make it.
So the question is, "Is 11 inches of overhang outside the spring too much? Does it allow too much leverage, how much does it de-rate the axle?"
Opinions?

[RJ Howell]

That is very close to the outboard of my commercial trailer. Many of us are re-inforcing that section due to stress and axle issues. You ratio of trailer load to axle load is much better than mine. I'm 1 to 1, obvious reason for issue. You are in much better shape there. Your close to 40% under the axle rating.

I don't know the math, yet another ran some numbers on ours and stated we lost 12% of the rating due to the distance (mount to tire). How true is that? I really don't know. So using that as a base, then you'd be axle rate 1,450ish still being a solid 30% under you axle rating.

If you state the axle, I'm sure someone here can figure all the factors involved and give you a better answer.

[MickinOz]

Thanks for replying so quickly.
I don't really know much about the specs of the (edit: current) axle except it's 39mm diameter.
No idea what sort of steel it is.
It's not galvanised, which I believe is a good thing. There is some thought that pickling steel and dipping it in molten zinc can reduce the strength a little if it isn't done carefully.
As received, the tips of the axle were about 9 inches past the outer edges of the springs.
I'm looking to go another 2 inches.
There is an outfit over here called Spinny Things, who I've dealt with before.
They sell all things trailer related - springs, axles, hubs, couplings, etc.
Their technical guide is easy to find and very useful, but doesn't really touch on maximum overhang, outboard length, whatever it may be called.
I've emailed them the question this morning. hopefully they'll get back to me soon.

[Daz7]

Mick a lot will also depend on your wheel and tyre backspacing i.e. you'll have a shorter axle width with a bigger offset on your rims.

FWIW, I have similar issue to you and did a 50mm square axle with parallel bearings that is rated to 1600kg. As the teardrop will be around the 600kg mark I feel it should be ok.

[MickinOz]

All has been measured. I bought the wheels first, precisely so I could accurately measure off-set, back space with tyre mounted, etc. before changing axles.
Plenty people have been caught out doing it the other way round.
IMO 50mm counts as "Super Duty Plus" for a 600kg trailer.
You could run that sticking out 600mm either side.
I'm only looking to widen the gap between the wheels by 100mm, from the currrent axle, so the most I'm considering upgrading to is 40mm square (1000 kg rating).
In the old days, we would have run down to the steel supply shop, bought a piece of 39 mm ID tubing, cut the axle in the middle used the tube as a sleeve to get the right distance apart and welded it all back together.
I wanna be just a touch more professional than that.