Scooterotica

Coming back from Christchurch to Auckland a 2000km round trip mine sheared on the main highway. Luckily a few bruised ribs and a munted engine casing was the only damage but could have been a lot worse. I have got a broady layshaft on order.

So which type are actually snapping. We might see a pattern to avoid. Has anyone snapped a thicker S1/2 type?

It's not the thick bit where they are snapping Lamaddict, it's the end part where the axle actually goes through the hub. They are all the same size at the breakage point.
I know for a fact the axle that snapped on mine was from an utouched motor. Torqued up correclty and everything. One place I do not use an airgun is the back hub nut.
Just a thought, but I wonder if the braking axles had the o-ring groove cut to deep? Worn out tooling and such...

Never had a problem with MB stainless rear hub nut and locking washer. I've had a standard rear hub nut let go on a longish trip on my Annhilator TS1. So I switched to MB stainless, like this one that Taffspeed put on my GP200 10 years ago:

with the evidence I have seen, I would suggest that the layshaft fails at the weakest point of the layshaft in 99.9% of cases. The area in question is where the threaded part of the layshaft meets the main body. I would think a bigger radius in most cases would reduce the risk of failure.
The locking nut serves just to stop the nut from undoing. The type of failures I have seen suggest the layshaft has been objected to big forces/moments bumping roads etc "direct shear stress". I don't believe we are looking at torsional stress failure due to engine torque.
As the threaded part of the shaft is continually under stress anyway due to the torqued hub nut, material selection is critical.
Steel isn't just steel all have different yield points. Just my opinion.

What I find odd, good but odd, is that the shafts always seem to break just AFTER peeps have been doing good speeds, have slowed down and then are pulling away again.

the cs locking ring as described on page 1 of this post